Planetary Systems

Contents

Birth of a Planetary System¹

It is very difficult to observe planets outside the solar system because they can be seen only by the reflecting light. It is just too dim and too far away to be detected by the telescope. Recently, its presence has been deduced from the small perturbation on the movement of the central star, and HST (Hubble Space Telescope) was able to obtain pictures on the newborn planetary systems in the Orion Nebula as shown in Figure 07-01a. The photo on top shows a newborn star (the red dot) surrounded by dark, dusty disk of orbiting gas. The bottom photo shows the edge-on view of another star forming region. Some five billion years ago the solar system probably looked similar. Then, over the course of a few hundred million years, the dusty material clumped into the nine planets orbiting the Sun today.

Figure 07-01a Birth of Planetary System

In the spring of 2005, an image of an extrasolar planet was finally captured by the combined effort of VLT, HST, and the Subaru Telescope (see Figure 07-01b). The mass of the planet is about two times that of Jupiter. It is about 100 times farther from the young star GQ Lupi than Earth is from the Sun. The star GQ Lupi is part of a star-forming region about 400 light-years away. It is about 1 million years old with 70% the mass of the Sun. The planet is only 156 times fainter than the star, because it is still very young and hence still forming, still contracting with a temperature of about 3000oF. This system resembles in some respects our own solar system in its formation years. Despite the observational difficulties, astronomers have found about 150 extrasolar planets over

Figure 07-01b Extrasolar Planet

the past decade. It seems to indicate that formation of planetary system is a rather common phenomena.

Formation and Evolution of the Solar System²

			The solar system, it is thought, began as a subcondensation in an interstellar cloud of gas and dust, from which probably hundreds of other stars also formed. To begin with, this presolar cloud was spheroidal, slowly rotating, and quite large, with a diameter of perhaps one
Figure 07-02a Disk Formation 1 [view large image]	Figure 07-02b Disk Formation 2 [view large image]	Figure 07-02c Disk Formation 3 [view large image]	or two light-years (Figure 07-02a). As it con- densed, the gas in the cloud's equatorial plane moves inward more slowly because its rotation starts to balance the gravity, causing it to

The gas and dust in the protoplanetary disk formed small bodies between 1-10 km in diameter. These bodies are known as planetesimals. Initially they formed small fragments of solar dust up to about 1 cm in diameter by the processes of cohesion (sticking together by weak electrostatic forces) and by gravitational instability. Larger bodies formed later by collisions at low speed which caused the material to stick together by gravitational attraction. Support for this view of the process of accretion comes from a region on the edge of the solar system known as the Kuiper Belt, where it is thought that the accretive 'mopping up' had failed to complete and the raw materials are still around as comets. The final stage of accretion has been described as 'runaway accretion'. Planetesimals were swept up into well defined zones around the sun close to the present orbits of the terrestrial planets. The process led eventually to a small number of large planetary bodies. Evidence for this impacting process can be seen in the early impact craters found on planetary surfaces.

Two key factors determine what kind of planet a protoplanet will become: its mass and its distance from the central star. Planets of low mass cannot retain hydrogen and helium, the lightest and most abundant gases, especially if their temperature rises to the point at which the lightest molecules escape. When the planets were in their early accretive phase, the mass that agglomerated before the Sun began to shine helped determine how well the planet could retain its hydrogen and helium. The other crucial factor, the distance of the planet from the Sun, also influenced the escape of hydrogen and helium from the planet's gravity, because inner planets become hotter and so have more difficulty in retaining the lightest gases with a given amount of gravitational force. These considerations explain well the overall structure of the solar system. The four small, inner planets were unable to hold on to any free hydrogen and helium with which they may have started out. However, the four gas giants, lying much further out from the Sun and therefore having much lower temperatures, not only retained their light gases but, through their powerful gravitational pulls, continued to draw in more material after the Sun had turned on.

Figure 07-02d is an artist's conception of the formation of a planetary system. The first three lower inset boxes zoom in from the spiral arm of the Milky Way to a star-forming region such as Orion, and then to a newly-forming star with its gas disk. The upper picture shows that the disk has become thin and is beginning to break into rings of gas and dust. The dust rings will condense into rocky "planetesimals" that will eventually merge to become planets, as shown in the inset at the lower right. Jets of gas flow out from the newborn star in the polar directions.

Figure 07-02d Formation of Planetary System

[view large image]

A more elaborate scenario divides the planetary formation into 8 stages:



Stage 1 - Interstellar cloud of gas and dust collapses into cluster of stars, a process that takes 100,000 to a few million years. Surrounding each star is a rotating disk of leftover material. Stars between one million and 3 million years old have gas-rich disks, whereas those older than 10 million years have meager, gas-poor disk, the gas has been blown away by the newborn star. This span of time delineates the epoch of planet formation. Figure 07-02e shows the view of a protostellar object called HH-30, it reveals an edge-on disk of dust encircling a newly forming star. Light from the forming star illuminates the top and bottom surfaces of the disk, making them visible, while the star itself is hidden behind the densest parts of the disk. The reddish jet emanating from the inner region of the disk is possibly directly from the star itself.

Figure 07-02e Star Birth [view large image]

Stage 2 - In about one million years, the disk will separate into two parts divided by a "snow line" at a distance of 2 - 4 AU. It divides the planetary system into an inner, volatile-poor (lack of low boiling point substance) region filled with rocky bodies and an outer, volatile-rich region filled with icy ones and thus marks the boundary between the rocky planets and gas giants. Figure 07-02f depicts the processes leading to the production of the "snow line". The micro-size dust grains eventually grow (by gravitational attraction and adhesion) into kilometer-size bodies called planetesimals.

Figure 07-02f Stage 2 [view large image]

Stage 3 - Within one to ten million years, the planetesimals collide and adhere in runaway growth into moon- to Earth-size bodies known as embryos. Each embryo's feeding zone is a narrow band centered on its orbit. Its growth stops once it has acquired most of the residual planetesimals in the zone. Many of the aspiring planets will be removed in the process of settling down into a stable configuration (Figure 07-02g).

Figure 07-02g Stage 3 [view large image]

		Stage 4 - The formation of gas giant such as Jupiter is critical in the history of planetary system. If such a planet forms, it shapes the rest of the system. But for that to happen, an embryo must accumulate gas faster than it spirals inward (Figure 07-02h). Stage 5 - Meanwhile in many system, a giant planet forms and then spirals almost all the way into the star. The reason is that gas in the disk loses energy to internal friction and falls in, dragging the
Figure 07-02h Stage 4 [view large image]	Figure 07-02i Stage 5 [view large image]	planet with it. Eventually the planet get so close that the star exerts a torque on its orbit, stabilizing it (Figure 07-02i).

• Stage 6 - The first gas giant paves the way for others. The gap it clears out acts as a barrier to prevent material flowing

  		in from the outer reaches of the planetary system. Thus, the material accumulates on the outer edge of the gap, where it can coalesce into a new gas giant. Stage 7 - The inner planets assemble in a later stage from 10 to 100 million years because the embryos cannot grow by swooping up gas (no more gas left) but must collide with one another. This would not happen until their orbits are disturbed and intersect. The embryos agglomerate into Earth-size planet, which then returns to a
  Figure 07-02j Stage 6 [view large image]	Figure 07-02k Stage 7 [view large image]	stable orbit via interaction with the remaining gas and leftover planetesimals.

  Stage 8 - Near the end of the planetary development from 50 to 1000 million years, a few remaining processes are still running to fine-tune the system: the disintegration of the star cluster, which may destabilize the planets' orbits gravitationally; internal instabilities that develop after the star clears out the last of its gaseous disk; and the continued scattering of leftover planetesimals by the giant planets. At the end of these mop-up operations, the planetary orbits will be adjusted to the more stable configuration (Figure 07-02l). In some systems, epic collisions of full-fledged planets can occur late in the development stage.

  Figure 07-02l Solar System [view large image]

Solar System Data³

	Figure 07-02l is a schematic diagram of the nine planets. Figure 07-03 depicts the nine planets and moons to scale. Table 07-01 below is a fact sheet about these planets and other planetary objects in ascending distance from the Sun, where Me = Mass of the Earth = 6x1027 gm. De = Diameter of the Earth = 1.3x109 cm. Distance from Sun to Earth = 1 AU = 1.5x1013 cm.
Figure 07-03 Planets [view large image]

Table 07-01 Solar System Data

The Sun⁴

All the solar energies are produced at the core within a zone from the center to about 20% of the solar radius, where the density increases to about 160 times the density of water and the temperature reaches 15 million oK. The energetic photons lose energy as they diffuse outward in the radiative zone, which occupies about 50% of the solar radius. The energy is then transported by means of convection out to the photosphere where the radiation becomes mostly visible light. Data for the Surface Temperature and Atmospheric Composition of the Sun in Table 07-01 are referred to the photosphere, which is the visible surface radiating the continuous spectrum. The atmosphere consists of an inner layer called chromosphere and an outer layer called corona where the gaseous density becomes more tenuous but the temperature increases to more than one million degree K and radiates mainly at extreme ultraviolet and x-ray wavelengths. Figure 07-04 shows the structure of the Sun in details.

Figure 07-04 The Sun [view large image]

Terrestrial Planets⁵

The terrestrial (inner) planets are composed mostly of rock and metal. As shown in Figure 07-05, Earth is the largest of the inner planets, followed by Venus, Mars, Mercury and the Moon. The interior of the Earth consists of an Fe-Ni core below a mantle of silicate rock. A comparison shows that Venus and Mars have a comparatively similar rock-iron distribution. The Moon has a much smaller core, whereas Mercury, although it is small enough to fit in the Earth's core, has a relatively large iron core. Mercury and the Moon have a large surface temperature variation between night and day. It is the result of these objects' small mass, which can barely retain a thin atmosphere. The Moon is the only celestial object that has been visited by human.

Figure 07-05 The Terrestrial Planets [view large image]

Mercury -- Mercury is the innermost planet; it never wanders more than 27o from the Sun. Therefore, it can be observed only at dawn or dusk. As a result Mercury remains largely unexplored until the Mariner 10 flew by in the 1970s. It was discovered that the surface has heavily cratered regions and gently undulating plains - very similar to the Moon (see Figure 07-06a). It possesses a large and high-density iron core, powerful magnetic field (second only to the Earth's among the terrestrial planets), and tenuous atmosphere composed by trace of helium, hydrogen, and oxygen. The composition of this thin, temporary atmosphere varies

Figure 07-06a Mercury [view large image]

with time as gases are lost and replenished, because Mercury's gravitational pull cannot hold on to the gases. The artist's drawing shows the glare of the sun dominates the scorched landscape of Mercury.

The January, 2008 flyby of the MESSENGER spacecraft has captured the most recent image of Mercury (Figure 07-06b) revealing many previously unknown or unconfirmed features. There is a large impact basins created during the early history of the solar system by the impact of a large asteroid-sized body. The orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like

Figure 07-06b Impact Basin [view large image]

planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled.

• Venus -- In terms of mass, size, and proximity to the Sun, Venus is almost the Earth's twin. However, unlike the Earth, it has no detectable magnetic field. It spins very slowly on its axis in a retrograde direction (opposite to the direction of the Earth's rotation and to the direction in which the planets move around the Sun) in a period of 243 days. This "spin-flip" might have been caused by close encounter with another celestial body. The most unusual feature is its thick atmosphere, which reflects about 76% of the incoming sunlight and much of the rest is trapped under layers of clouds. The "greenhouse effect" maintains a sizzling surface temperature of 480^oC over the whole planet, with very little difference between day and night. Details of the surface have been hidden under the clouds until radar-mapping satellites

were placed in orbit around the planet. From 1990 to 1994 the Magellan space probe mapped the entire surface of Venus at high resolution by peering through the clouds with radar. It revealed a planet that has experienced a cataclysmic upheaval about 300 to 500 million years ago. There is no consensus on the cause for such global resurfacing event. Figure 07-07a shows a vast system of highlands and ridges running across

Figure 07-07a Venus [view large image]

much of the equator. The drawing depicts a typical Venus landscape with barren, craggy rocks seen through thick carbon dioxide air under yellowish clouds, containing sulfuric acid droplets.

• Earth -- See Topic 09.

Figure 07-07b Earth, New World [view large image]	Figure 07-07c Earth, Old World [view large image]	Figure 07-07d Earth, Whole [view large image]

• Moon⁶ -- There are many hypotheses to explain the origin of the Moon: the fission theory (the Moon had been ejected from the Earth); the co-creation theory (the orbiting Moon formed together with the Earth); the capture theory (the Moon is a foreign body captured by the Earth); and the impact theory (see Figure 07-08a), which suggests that the Earth was struck by a massive body, possibly as massive as the planet Mars, shortly after its formation. The Moon was formed by accumulating the orbiting debris through much the same processes as the planets themselves. According to data collected by the Apollo and other Lunar missions, the most favored hypothesis is the impact theory, which accounts for the lower proportions of iron and volatiles in the lunar rocks. If the impacting body had been moving in or close to the ecliptic plane (as the other planets do), the debris would have ended up orbiting the proto-Earth close to that plane, thereby accounting for the significant tilt of the Moon's orbit relative to the plane to the Earth's equator. The impact may also have been responsible for inducing the tilt in the Earth's rotational axis.

		Serenity had since been restored to the system. The Moon is the most luminous celestial body at night inspiring many folklores in different cultures. Actually, the Moon's appearance changes nightly. Figure 07-08b shows the changing face during a lunation, a complete lunar cycle. The Moon always keeps the same face toward the Earth. Its apparent size changes slightly, though, and a slight wobble called a libration is discernable as it progresses along its elliptical orbit. During the cycle, sunlight reflects from the Moon at different angles, and so illuminates different features
Figure 07-08a Impact Theory [view large image]	Figure 07-08b Lunation [view large animation]	differently. A full lunation takes about 29.5 days, just under a month. Click the STOP button (the on the toolbar) of your browser to see a particular phase.





Mars -- Mars is usually depicted as a version of Earth in its extreme - smaller, colder, drier, but sculpted by basically the same processes. The Viking (1976) and Mars Path-finder (1997) images from the surface look eerily Earth-like (Figure 07-09a). Researchers compare the Martian equatorial regions of Mars to the American Southwest, and the Martian polar regions to Antarctica. Image on the right shows a view from the Spirit Rover on January, 2004.

Figure 07-09a Mars [view large image]

Careful examination shows that present-day Mars differs from Earth in a number of broad respects:
1. It is enveloped in dust - very fine grained material that has settled out of the atmosphere. It is everywhere except the steepest features.
2. Mars is extremely windy. Spacecraft have seen globe-encircling dust storms, huge dust devils and dust avalanches - all wrought by the wind.
3. Mars produces amazing variety of weather and climate cycles, many of which are similar to those on Earth (since the Martian day is almost the same as an Earth day, and the tilt of Mars's rotation axis is very close to that of Earth's), many like nothing on Earth (because the atmospheric pressure is less than 1% of that on Earth, and it lacks the precipitation and oceans that are so crucial to weather on Earth). Although the noon temperature can briefly rise as high as 20^oC, it drops rapidly to around -70^oC at night. Temperature over the winter pole can be as low as -138^oC - low enough for carbon dioxide to freeze out of the atmosphere.
4. Liquid water is unstable at the Martian surface. Only water ice persist at some depth within the Martian soil during all or much of the year. Liquid water can sometimes leak onto the surface and produce fresh gullies that look like water-carved features on Earth.



Whether or not bacterial lifeforms exist on Mars now, or have existed in the past, is a matter of ongoing debate. Soil samples scooped up by the Viking spacecraft seems to indicate no evidence of biological activity now. The idea that life may have existed on Mars in the past received a boost in 1996 with the tentative identification of nanofossils (with a size up to 2 x 10^-2 cm) in the Martian meteorite ALH84001 (Figure 07-09b), which consists of rocks that are 4.5 billion years old. It may have subsequently been permeated by water about 3.6 billion years ago. These time scales coincide

		closely with the formation of the Earth and the origin of life on Earth. Figure 07-09c shows fossil imprints of filamentous bacteria (with a size of several 10-2 cm), whose cells have been preserved between layers of sedimentary rocks 3.465 billion years old in north western Australia. The similarity between the two samples is remarkable. But eventually, the general size is determined to be many hundred times too small for a viable living organism, and all the biosignatures in ALH84001 have been proven to be
Figure 07-09b Life on Mars [view large image]	Figure 07-09c Bac- teria [large image]	reproducible by non-biological means. Thus, the search for life on Mars remains inconclusive.

The search for life on Mars took another turn in 2004 with the report about another Martian meteorite known as Nakhla (Figure 07-09d). It fell to Earth on June 1911 in Egypt, but the analysis of its content was not initiated until 1998 following the furore over ALH84001. Examination reveals small tunnels virtually identical in size and shape to those in Earth rock bored by bacteria or archaeans. Analysis also finds carbon-rich material originated on Mars, and evidence of contact with liquid water. Critic notes that the tunnels are highly ordered in contrary to those made by Earth bacteria (see Figure 07-09d). Just as with ALH84001, settling the debate will large largely

Figure 07-09d Nakhla Meteorite [view large image]

depend on whether the supposed biosignatures could have been produced by non-living processes. The search is now on for some abiotic way to make the tunnels.



After a seven month voyage of nearly 500 million kilometers through interplanetary space, the robotic explorer "Spirit" has reached the surface of Mars on the evening of January 3, 2004. Its mission is to look for clues in rock and soil about whether the past environment at this part of Mars was ever watery and suitable to sustain life. A few weeks later, on January 25, 2004 the rover "Opportunity" landed in a small crater at another part of Mars. This particular landing site was chosen because it is believed to be full of iron-bearing hematite (Fe₂O₃). The semi-precious mineral usually forms on Earth in the presence of water. Then NASA scientists announced a month later that "Opportunity" have found hard evidence of liquid water once existed on Mars. The rover will continue to explore its surroundings and try to determine the nature and extent that water molded the region. Three days later the other rover "Spirit" has found evidence of past water activity in a volcanic rock on the other side of Mars. But the amount of water at Spirit's site is much less than what is indicated at Opportunity's. On March 2004, NASA reported that the rocks robotically examined by the rover Opportunity were formed at the bottom of a salty sea.

In July, 2004 the ESA's Mars Express have recored radiation indicating ammonia gas in Mar's atmosphere. Since ammonia can survive for only a few hours in the Martian atmosphere before breaking down, it must be constantly replenished from one of two possible sources: active volcanoes, of which none have been found on Mars, or microbes.

As early as 1979, the Viking Lander 2 had already captured an unusual image of the Martian surface sporting a thin layer of seasonal water ice (see Figure 07-09e). By 2006, information gathered by the Global Surveyor orbiters,

and the Mars Exploration rovers has enabled scientists to arrive at the tentative conclusion that liquid water not only existed on Mars, it once covered large parts of the planet's surface, perhaps for more than a billion years. Figure 07-09f depicts the four phases of conjectural Martian history from 4.6 billion years ago to the present. Phase 1 portrays a barren landscape bombarded by huge asteroids. Phase 2 is an episode of Earth-like conditions with liquid water filling some of the basins and carving enormous river valleys. Phase 3 turns to drying out and cooling down as water freezes, seeps under the ground, or evaporates into space. Phase 4 shows the arid

Figure 07-09e Thin Layer of Water [view large image]

and inhospitable Mars as we see today. According to this scenario, the organsims on Mars would be rather primitive during the half billion years or so of habitable episodes

		as it took 3.5 billion years on Earth to develop complex life forms (in the Cambrian explosion), and another half billion years for the evolution into intelligent beings. Astronomers believe Mars's magnetic field dissipated because the planet's molten iron core solidified. The solar wind may have stripped away much of the Martian atmosphere and sent much of the water into space
Figure 07-09f Martian History [view large image]	Figure 07-09g Watermarks on Mars [view large image]	without the protective shield.

The evidences include (Figure 07-09g):


1. Valley networks - High resolution images taken by orbiters has identified valley networks arranged in dense, branching patterns, and the lengths and widths of the valleys increase from their sources to their mouth. Moreover, the sources are located along the ridge crests, suggesting that the landscape was molded by precipitation and runoff.
2. Delta-like features - The best and largest example, photographed by the Mars Global Surveyor, is the river delta at the end of a valley network that drains into a crater. Like the Mississippi River delta, the fan shaped structure suggests that it grew and altered its courses many times, most likely responding to changes in the flow of its ancient river sources.
3. Erosion and sedimentation - Calculations based on new orbital imaging data have determined that the rate at which sediments were deposited and eroded in the first billion years of the planet's history may have been about a million times as high as the present-day rate. Such enormous amounts can be explained only by flowing water.
4. Festoons - The rover Opportunity found a rock that was marked with festoons, which are formed by waves washing over sandy sediments.
5. Hydrated minerals - Some minerals such as clay, hydrated iron oxides, and carbonates can be formed only in the presence of water. The rover Spirit has found many kinds of oxidized iron minerals, and hydrated sulfate minerals (such as the hematite grains nicknamed blueberry), while Opportunity has made equally amazing finds of hydrated rocks. Meanwhile, high-resolution orbital mapping data and close-up surface studies from the rovers have revealed abundant deposits of clays in many regions. Only carbonates are absent in the surveys; it is thought that the acidic water in the 3rd phase of the Martian history may inhibit the formation of carbonates on the surface.

Using a spectrometer, which collects 544 colors, or wavelengths, of reflected sunlight to detect minerals on the surface of Mars, NASA's Mars Reconnaissance Orbiter observed opal on Mars (see Figure 07-09h in a 2008 report). The hydrated, or water-containing, mineral deposits are telltale signs of where and when water was present on ancient Mars. The minerals are widespread and occur in relatively young terrains with an age of about 2 billion years ago (much younger than those indicated in Figure 07-09f). The discovery has important implication to life on Mars, since the longer liquid water existed on Mars, the longer the window during which Mars may have supported life.

Figure 07-09h Opal on Mars [view large image]

years ago. The Global Surveyor previously spotted tens of thousands of gullies that scientists believed were geologically young and carved by fast-moving water coursing down cliffs and steep crater walls. Then scientists decided to retake photos of thousands of gullies in a search for evidence of recent water activity. Two craters in the southern hemisphere that were originally photographed in 1999 and 2001 were examined again in 2004 and 2005, and the images yielded changes consistent with water flowing down the crater walls, according to the study (Figure 07-09i).

Figure 07-09i Water on Mars [view large image]

Figure 07-09j shows an updated version of the Mars history as the result of more data and further analysis. It suggests that instead of a watery past over billions of years, there are many episodes of flooding when volcanic activities thawed frozen reserves of underground water and drove it upwards to the surface. These events may not have lasted more than a few tens of thousands of years, but they have left ample evidence of water on the surface as shown in Figure 07-09g. It is also found that the current distribution of water ice may

Figure 07-09j Mars History, Updated [view large image]

be related to the wobbling of the rotational axis by the gravitational tug of Jupiter in 10 million years cycle.

		After a nearly 10-month voyage from Earth to Mars, the Phoenix Lander (Figure 07-09k, artist's rendition) touched down safely at the edge of the polar region (Figure 07-09l, actual image) on May 25, 2008. The area was chosen because it's suspected of harboring as much as 80% water ice by volume within just one meter of the surface. Its primary mission is to look for signs of water - liquid, ice or vapor - in the ground and atmosphere and possible traces of organic and biological material. It can measure
Figure 07-09k Phoenix Lander [large image]	Figure 07-09l Phoenix Landing Site [large image]	salts, pH levels and individual chemicals, but cannot analyse the building block of life, such as proteins, or DNA.

		of Figure 07-09m). On June 26, 2008 members of the Phoenix Mars Mission Team revealed that the Lander has found evidence of mineral nutrients essential to life in Martian soil. The sample of Martian dirt contained several soluble minerals, including potassium, magnesium and chloride. It is the type of soil common on Earth similar to those in the backyard. The Martian soil has a very alkaline pH of between 8 to 9 suitable for growing asparagus. There is clear indication that the soil has
Figure 07-09m Ice on Mars	Figure 07-09n Martian Soil [view large image]	interacted with water in the past. Figure 07-09n shows some Martian soil sprinkled from the lander's Robot Arm scoop onto a silicone substrate, which was then rotated in front of the microscope for photo taking.

As temperatures plummeted to nearly -100 °C and dust storms and clouds obscured an enfeebled sun, the Phoenix Lander finally ceased to communicate on Novermber 3, 2008 (Figure 07-09o). The mission was plagued with problems related to its instruments, it began to redeem itself toward the end when it sent a strong signal for calcium carbonate, a mineral that typically forms in the presence of water. A separate, weaker signal in the soil may indicate a different type of carbonate, or even an organic molecule. There is more work to be done on data collected during the last few months.

Figure 07-09o End of a Mission

		perchlorate salts, which likely include magnesium and calcium perchlorate hydrates. It is conceivable that microbes could be living happily several meters underground away from the harsh ultraviolet light. Certain bacteria on Earth can exist in extremely salty and cold conditions. Several recent (2009) observations of possible mud volcanoes on Mars (Figure 07-09q) suggest the possibility that Martian microbes could be dredged up from underground lake.
Figure 07-09p Salty Water	Figure 07-09q Mud Volcano

More evidences for water on Mars were gathered by NASA's Mars Reconnaissance Orbiter in mid 2008. The data have revealed that the Red Planet once hosted vast lakes, flowing rivers, and a variety of other wet environments that had the potential to support life. Study shows that vast regions of the ancient highlands of Mars, which cover about half the planet, contain clay minerals, which can form only in the presence of water. Volcanic lavas buried the clay-rich regions during subsequent, drier periods of the planet's history, but impact craters later exposed them at thousands of locations across Mars. The image in Figure 07-09r depicts ancient rivers ferried clay-like minerals (shown in green) into the

Figure 07-09r Water on Mars, More Evidences [large image]

lake, forming the delta. Clays tend to trap and preserve organic matter, making the delta a good place to look for signs of ancient life.

		Figure 07-09t is a 3-D map superimposes gamma-ray data from Mars Odyssey's Gamma-Ray Spectrometer onto topographic data from the laser altimeter onboard the Mars Global Surveyor, which can detect elements buried as much as 13 inches (1/3 meter), below the surface by the gamma rays they emit. The Red-to-yellow colors on the map mark the gamma ray emitting potassium-rich sedimentary deposits in lowlands. Such great concentration of these elements in
Figure 07-09s Glaciers on Mars [view large image]	Figure 07-09t Sedi- ments on Mars	the lowlands is interpreted as evidence of water moving them from the highlands to the lowlands.

Thus if Mars had an exclusively acidic environment during the period of 3.5 - 2.5 billion years ago as suggested in Figure 07-09f, all the carbonate minerals before this time would have been dissolved without a trace. However, an article at the end of 2008 reports that the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) has found carbonate minerals (see Figure 07-09u in colors) formed in bedrock layers more than 3.6 billion years ago. Therefore, different types of watery environments must have existed. The greater the variety of wet environments, the greater the chances one or more of them may have supported life. NASA's Phoenix Mars Lander discovered carbonates in soil samples. Researchers had previously found them in Martian meteorites that fell to Earth and in windblown Mars dust observed from orbit. However, the dust and soil could be mixtures from many areas, so the carbonates' origins have been unclear. The latest observations indicate carbonates may have formed over extended periods on early Mars in very specific locations, where future rovers and landers could search for possible evidence of past life.

Figure 07-09u Carbonate Minerals on Mars

Table 07-02 below summarizes observations of Martian water over the years up to 2008.

Table 07-02 A History of Martian Water

		interact with water. Another possibility is that the methane is escaping from buried clathrates, deposits of methane ice formed long ago by one of the other two mechanisms. The plumes are produced in high concen-tration (60 parts per billion) at a handful of hotspots hundreds of kilo-meters across. The methane plumes bloom and dissipate in less than a year - a fast process comparing to the time scale of sunlight degradation.
Figure 07-09v Methane on Mars, Local	Figure 07-09w Methane on Mars, Global	Figure 07-09w shows the global emission of methane gas in Martian summer. The data were obtained spectroscopically using large ground based telescopes.

from 0.5 - 2.5 meters. The craters did not exist in earlier images of the same sites. Some of the craters show a thin layer of bright ice atop darker underlying material (Figure 07-09x, a). The bright patches darkened in the weeks following initial observations as the freshly exposed ice vaporized into the thin martian atmosphere (Figure 07-09x, b). One of the new craters had a bright patch of material large enough for one of the orbiter's instruments to confirm it is water-

Figure 07-09x Relic of Ice [view large image]

ice. This ice is a relic of a more humid climate from perhaps just several thousand years ago.

Asteroid Belt⁷

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Although it seems to be a very dense belt in the schematic diagram of Figure 07-10a, spacecraft that have flown through this zone have found that it is really quite empty and that asteroids are separated by very large distances. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that

Figure 07-10a Asteroid Belt [view large image]

exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is the Barringer Meteor Crater near Winslow, Arizona (Figure 07-10b).

		planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers across -- less than half the diameter of our Moon. Figure 07-10c shows some asteroid orbits, all of which are close to the planetary plane, in the same direction as the
Figure 07-10b Meteor Crater [view large image]	Figure 07-10c Orbits [view large image]	planets. Asteroids in the Main Belt take about 3 - 6 years to complete a revolution. They spin as they revolve in just hours.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of the Earth. Asteroids that are on a collision course with the Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials (Figure 07-10a). Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

On September 2005, the Japanese spacecraft Hayabusa arrived at asteroid Itokawa and stationed itself only 20 kilometers away (see Figure 07-10d, not in proportion). Although a long term goal is to find out how much ice, rock and trace elements reside on the asteroid's surface, a shorter term goal is to determine the mass of the asteroid by measuring the attraction of the drifting spacecraft. In November, a small coffee-can sized robot is scheduled for release and is expected to hop around the asteroid taking pictures. Also in November, the spacecraft will fire pellets into asteroid Itokawa and collect some of the debris in a capsule. In December, the spacecraft will make its journey back to Earth and will deliver the capsule in 2007 June.

Figure 07-10d An Asteroid [view large image]

The return trip has been delayed because of communication problem (resolved in January 2006), and other issues. The spacecraft was seriously injured. Nevertheless, the project team is trying its best to bring it back by 2010.

Jovian Planets⁸

As shown in Figure 07-11, the giant outer planets consist mostly of hydrogen and helium gas and liquid, which surrounds a core of iron and rock and possibly a smaller amount of methane, carbondioxide and water ices. Jupiter is the largest planet, closely followed by Saturn. Uranus and Neptune are in comparison much smaller, although still significantly larger than any of the terrestrial planets. Jupiter is a "failed star" - it would have become a star igniting nuclear fusion at its core if its mass is about 80 times higher (the lowest mass limit for a star to form is about 0.05 MSun).

Figure 07-11 The Jovian Planets [view large image]

• Jupiter -- Like the Sun, Jupiter is composed mainly of hydrogen and helium and a rocky-metallic core as shown in Figure 07-11. It has a powerful magnetic field (more than ten times than the Earth's) generated by circulating liquid metallic hydrogen instead of molten iron. Electrically charged particles from the sun are sucked into the magnetic whirlpool creating an eerie auroral display with its multicolored curtains (Figure 07-12a). Jupiter's disc displays parallel bands of cloud (due to its rapid rotation) with continually changing detailed structure. The only really long-lived feature is

  		the Great Red Spot, a rotating weather system measuring some 25,000 km by 12,000 km, which has been lasted for over three centuries. Jupiter is surrounded by an exceedingly faint system of rings and a family of 16 satellites. Four of the Galilean satellites (discovered by Galileo in 1610) are about
  Figure 07-12a Jupiter [view large image]	Figure 07-12b The Galilean Moons [view large image]	the size of the Earth's moon or Mercury. Followings is a brief description of these Jovian moons in order of ascending distance from Jupiter:

  
  
  

  Io is the nearest moon to Jupiter. It is subjected to tremendous tidal force from Jupiter and the three other moons outside. Incessant volcanic activities have turned the materials inside out many times. The surface is pockmarked with craters and lava. The colorful image is produced by sulfur, which takes on various hue at different temperatures (see Figure 07-13). Jupiter's magnetic fields interact with Io's ionized gases from volcanic eruptions. It creates a torus of plasma carrying an electric current of five million amperes

  Figure 07-13 Io [view large image]

  (see Figure 07-12b). The drawing shows Io after sunset with a volcano erupting in the distance. The gasses it has released into the thin atomosphere are glowing in a brilliant auroral display.

  
  
  

  The effect of the tidal force on the second nearest moon Europa is not as severe. However, it has melted the water under the ice shell. Europa's icy countenance resembles a cracked eggshell (see Figure 07-14). Reddish material has oozed out of fractures opened up by Jupiter's gravitational forces. It is thought that the subsurface ocean might harbor life. Although only a theory, this scenario is bolstered by the discovery of life-forms on the Earth's ocean floor that exist in total blackness, sustained entirely by chemical rather solar energy. Bizarre tube worms, crabs, clams and other animals and plants live around warm-water vents in deep-sea midocean rifts, relying on the sulfur and oxygen in the mineral-rich water for the energy required to support them. Drawing on the right depicts an Europa covers with icy crusts, which squeezed up a pingo in the distance. The lower image is an orbital view of Europa. It clearly reveals how the ic-crust surface has been shattered by fractures into iceberglike blocks and plates. Tannish-

  Figure 07-14 Europa [view large image]

  brown stains around many fractured areas suggest organic material in the water that erupted from below through the cracks.

  In a sequel to the novel 2001: A Space Odyssey - 2010: Odyssey Two, there is a passage about the destruction of a Chinese spaceship landed on Europa. While it was pumping water from the "canal", its floodlights induced a fatal attraction of a phototropic creature. ...
  
  Europa and Ganymede were selected by NASA and ESA in 2009 as the next major mission to explore the solar system beyond Mars instead of Titan, an equally intriguing moon of Saturn. This is a logical choice as Europa may contain water underneath the surface while Titan could offer only liquid methane. The Europa Jupiter System Mission will launch two orbiters, one built by NASA and the other by ESA, in 2020, with a scheduled arrival time in the Jupiter system of 2026. The NASA orbiter will study the icy shell of Europa, while the European part of the mission, called Laplace, will investigate Ganymede.
  
  Ganymede is the largest moon in the solar system (slightly larger than Mercury). It is a cratered world with a mainly water-ice surface darkened by dirt (see Figure 07-15). Some of the impacting comets and asteroids punched through the light brown soil-ice surface composite and exposed brighter, cleaner ice underneath. But Ganymede has more than just craters. Large areas of ribbed and ridged icy material are spread like continents over the satellite. These regions,

  which look like glacial flows on Earth, have fewer craters than other sectors and are clearly younger. In 1996, the Galileo spacecraft revealed that this moon has a magnetic field of its own. Normally, planetary scientists consider such a discovery to be proof of a hot interior with a partially molten iron core. But some researchers have alternately suggested that the field arises in the salty waters of the buried ocean, closer to the surface. In

  Figure 07-15 Ganymede [view large image]

  either case, it suggests that Ganymede has participated in a process called tidal heating, in which gravitational forces, associated with Jupiter and its other nearby moons, slightly deform Ganymede and heat up its interior.

  Image on the right shows Ganymede's icy crust. As it shifts and splits apart, water wells up and forms fresh, light-toned swaths of surface. Gradually, the terrain becomes a mixture of old and new features. resembling a giant jigsaw puzzle.
  
  

  Callisto is the outermost the Galilean moons and the only one far enough from Jupiter's intense radiation belts to be a possible landing site for Earth explorers unprotected by shielding. It is one of the most heavily cratered object in the solar system (see Figure 07-16). The crater-scarred surface of Callisto appears much as it must have looked four billion years ago, soon after the solar system formed. Callisto's rock is primarily water ice with lunar-like dirt mixed in. At the surface temperature of -145oC,

  Figure 07-16 Callisto [view large image]

  water ice acquires the stiffness of rock, rather than the more plastic characteristics of glaciers on Earth. The drawing on the right shows a remote Jupiter hovering over a cratered landscape on Callisto.

  Many artist's drawings in this section are taken from "The Grand Tour" by Ron Miller and W. K. Hartmann.
  
  
• Saturn -- Nearly twice as remote as Jupiter, Saturn is the second largest planet in the Solar System. With a density just 0.7 times that of water, it is by far the least dense of the planets. Although it has a slightly longer roatational period than

Jupiter, it bulges more markedly at the equator. Saturn's composition and internal structure are believed to be broadly similar to those of Jupiter, but its cloud belts and weather systems are much more muted in appearance than are those of Jupiter. Saturn's most distinctive feature is its system of rings. The rings are composed of billions of individual particles of ice and ice-coated rock, ranging in size from 1 cm to tens of meters (lower left, Figure 07-17a). The rings are extremely thin and flat; they are about 300000 km in diameter, but their thickness is probably less than 1 kilometer and may be only a few hundred meters. Other objects around Saturn are

Figure 07-17a Saturn [view large image]

the 18 satellites with diameters ranging from 20 km to Titan's 5150 km (larger than Mercury). The satellites are believed to be composed of mixtures of rock and ice.

Titan is unique among planetary satellites in having a substantial atmosphere, composed mainly of nitrogen (90-95%) together with methane (~ 5%) and small quantities of other gases. It has a surface pressure 50% greater than the pressure of the Earth's atmosphere. The temperature at Titan's surface is about -178o C, since the boiling and melting points of methane (CH4) are -162oC and -182.6oC respectively, it is conceivable that this substance can exist simultaneously as a solid, liquid, or gas. Although radar measurements show that at least parts of Titan's surface must be solid, its is possible that significant areas are covered by oceans (or lakes) of liquid methane, ethane (C2H6, BP: -88.2oC, MP: -183oC),

Figure 07-17b Saturn & Titan [view large image]

or a mixture or both. Figure 07-17b is an artist's view of Titan in 1944 with Saturn and its rings and moons in the sky. The blue sky envisioned in this drawing has been replaced by orange, smoggy clouds following new discovery in the 1980s.


After a seven-year voyage, the Cassini orbiter entered Saturn's orbit in July of 2004. It will then began a four-year mission that includes more than 70 orbits around the ringed planet and its moons. Pointing its various instruments at carefully calculated scientific targets, Cassini will collect detailed data on Saturn, its rings and the moons orbiting this gas giant. Figure 07-17c is an image recorded as the Cassini spacecraft approached its first close flyby of Saturn's smog-

		shrouded moon on October 26, 2004. Here, red and green colors represent specific infrared wavelengths absorbed by Titan's atmospheric methane while bright and dark surface areas are revealed in a more penetrating infrared band. Ultraviolet data showing the extensive upper atmosphere and haze layers is seen as blue. Sprawling across the 5,000 kilometer wide moon,
Figure 07-17c Titan [view large image]	Figure 07-17d Titan Surface [view large image]	the bright continent-sized feature known as Xanadu is near picture center, bordered at the left by contrasting dark terrain. On the morning of January 14, 2005 after

being launched from the Cassini spacecraft, the Huygens probe reached the surface of Titan. The pictures in Figure
07-17d were taken as the Huygens probe neared the surface. It shows a landscape with highlands, drainage channels, shoreline, and flood plain (b). The picture in orange colour is the horizontal view (c). Another view from the landing site captured a flat landscape scattered with icy rocks (d). The image of a cluster of lakes that contained liquid (most likely a mixture of methane and ethane), was captured on July, 2006 (e). The terrain seemed strangely familiar, not unlike places on Earth, Mars and the Moon -- suggesting a commonality in the solar system.


By July, 2008 NASA scientists have concluded from data collected by the Cassini spacecraft that at least one of the large lakes observed on Saturn's moon Titan contains liquid solution with methane, other hydrocarbons and nitrogen. This makes Titan the only body in our solar system beyond Earth known to have liquid on its surface. Figure 07-17e shows the methane/ethane cycle. Methane is released into the atmosphere from Titan's interior through volcanic action, and evaporates from the lakes of hydrocarbon (identified by Cassini). Chemical reactions in the atmosphere

Figure 07-17e Hydrocarbon Lake [view large image]

convert it to ethane; complex organic aerosols consisting of carbon, hydrogen and nitrogen. Ethane and methane partly condense, forming clouds and hazes that precipitate, replenishing the lakes and bearing many organic species in solution.


It was announced in March, 2006 that the Cassini space probe has found evidence of geysers erupting from under-ground pools of liquid water on Saturn's moon Enceladus (Figure 07-17f), which has a diameter of about 500 km.

	High-definition pictures beamed back from the probe showed huge plumes of ice coming from the fractures on moon's south pole. The picture on the right of Figure 07-17f shows streams of ice and water vapor pouring off Enceladus into the E ring. The average surface temperature on this moon is about -200oC. It is estimated that the
Figure 07-17f Enceladus, Geyser [view large image]	depth of the water pool is only tens of meters - easily accessible by explorers. The energy source to keep a liquid ocean deep under the frozen crust may come from radioactive heating and/or tidal heating (by stretching and squeezing a solid object).




Lapetus was first discovered by Giovanni Cassini using a telescope in 1671, he could only see half of the moon. The Cassini spacecraft flyby in 2007 reveals that half of this peculiar moon of Saturn appears as dark as asphalt, the other half, as bright as snow. It is suggested that dark organic-rich gunk, probably from another moon, spatters the side facing forward (as Lapetus always shows the same face toward Saturn). It also shows an equatorial ridge extending across and beyond the dark, leading hemisphere of Lapetus gives the two-toned Saturnian moon a distinct walnut shape (Figure 07-17g). One of the scientific goals is to determine the composition and distribution of surface materials on Lapetus -- particularly the

Figure 07-17g Lapetus [view large image]

dark, organic-rich material and condensed ices.

The Spitzer Space Telescope discovered a new ring of Saturn in 2009 (Figure 07-17h). The extremely sparse nature of the ring was responsible for the failure of its detection by visible light. With the availability of the Spitzer Space Telescope's Multiband Imaging Photometer, which uses a cryogenic system, such weak infrared signal has finally been detected. The new ring extends from approximately 7.7 to 12.4 million km with a vertical thickness of 2.4 million km. It is very close to Phoebe (Figure 07-17i) at a distance of about 13 million km from Saturn. Following the explanation for

		the origin of the other rings, this ring was formed similarly from material ejected by impacts (with comets for example). Both Phoebe and the new ring revolve in a highly inclined orbit (a characteristic of the outer moons) of 27o and travel in a retrograde manner (moving backward). Thus, it is also suggested that the darker color in Lapetus' leading hemisphere, and the reddish deposits on Hyperion are the results of collisions with the ice and dust in this new ring (albeit these two moons are way outside the ring at about 3.6 and 1.5 million km from Saturn respectively) much like the bugs hitting the windshield.
Figure 07-17h New Ring	Figure 07-17i Phoebe [view large image]

Uranus -- Although Uranus is substantially smaller than Jupiter or Saturn, it has four times the Earth's diameter and 14.5 times the Earth's mass. A distinctive feature of the planet is its extreme axial inclination of 98o; its rotational axis lies almost in the plane of its orbit. Therefore, it has a most peculiar pattern of seasons, with each pole experiencing about 42 years of continuous sunlight followed by similar period of darkness. Uranus is surrounded by a system of at least ten narrow dark rings revolving at the same kind of tilted axis). The individual rings are composed primarily of meter-size lumps of material that are as dark as coal. The planet has sixteen or more satellites, which also revolve around Uranus' tilted equatorial plane. Some of these satellites are shown in Figure 07-18.

Figure 07-18 Uranus [view large image]

• Neptune -- Neptune's deep atmosphere consists primarily of hydrogen (80%), helium (19%), and methane (~1%), together with other hydrogen compounds. As with Uranus, the absorption of red light by atmospheric methane is responsible for the planet's distinctive bluish color (see Figure 07-19a). The strength of Neptune's magnetic field

at the cloud tops is about twice the surface field strength at the Earth's pole. The magnetic axis is tilted to the rotational axis by an angle of 47o, and, rather than passing through the center, it is offset to one side by about half the radius of the planet. As with Uranus, it suggests that the magnetic field is generated by circulating currents in the ice-rich envelope, not the planet's core. Neptune is surrounded by five faint rings and has eight satellites.

Figure 07-19a Neptune [view large image]

Drawing on the right shows thick clouds in the atomosphere of Neptune with Triton glimmer in the light of a tiny, chilly sun.

One of the satellites is Triton (Figure 07-19b), which was studied in detail by Voyager and proved to be a fascinating world. It is a highly reflective body and it is also the coldest body with a surface temperature of -236^oC. Its icy surface has an intriguing variety of terrain, and its polar cap, composed of frozen nitrogen, is streaked with dark material deposited from plumes of nitrogen ice, gas, and hydrocarbon-rich material ejected by erupting "geysers" that burst

through the icy crust. Unique among major planetary satellites, Triton revolves around Neptune in a retrograde direction (opposite to the direction of the planet's rotation). It is gradually spiral in toward the planet, so that, in about a hundred million years' time, it will either collide with Neptune or be torn apart by gravitational tidal forces and scattered around the planet to form a spectacular ring system.

Figure 07-19b Triton [view large image]

Pluto⁹, Dwarf Planets, and SSSB

inclination angle of 17o; and the spin axis has a tilt of 120o relative to the ecliptic plane. Pluto's atmosphere is extremely tenuous. It may exist as gas only when Pluto is near its perihelion (the nearest approach to the Sun), where it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. For the majority of Pluto's long year, the atmospheric gases are frozen into ice.

Figure 07-20a Pluto [view large image]

		September 2008, also in Figure 07-20d) and UB313 (nicknamed Xena, Figure 07-20b). Meanwhile, the universe unfolds itself as usual; it is indifferent to labeling. On 14 September 2006, the trouble-making UB313 that forced astronomers to reconsider Pluto's planetary status has finally received its official name - Eris. The name, taken from a Greek goddess of discord and strife (her golden apple ultimately ignited the Trojan war, Figure 07-20c), is most appropriate for this celestial object.
Figure 07-20b Eris (UB313) & its Moon [view large image]	Figure 07-20c Mythological Eris [view large image]

2. A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite (of a planet). 3. All other objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies” (SSSB).

Figure 07-20d TNO, KBO [view large image]

Figure 07-20d shows the artist concepts of the three dwarf planets and some SSSB, which are also referred to as Trans-Neptunian or Kuiper Belt Objects (TNO, KBO).

Comets¹⁰

Comets are formed of rocky material, dust, and water ice. A few have highly elliptical orbits that bring them very close to the sun and swing them deep into space, often beyond the orbit of Pluto. The most widely accepted theory of the origin of comets is that there is a huge cloud of comets called the Oort Cloud11 (see Figure 07-21a), of perhaps 1000 comets orbiting the sun at a distance of about 50,000 AU (in the Solar System halo). These comets are near the boundary between the gravitational forces of the sun and the gravitational forces of other stars with which the sun comes into interstellar proximity every several thousand years. According to the theory, these stellar passing perturb the orbits of the comets within the Oort cloud. As a result, some may be captured by the passing star, some may be lost to interstellar space, and some of their orbits are modified from a relatively circular orbit to an extremely elliptical one coming close to the sun. These are the long period comets with period more than 200 years.

Figure 07-21a Kuiper Belt and Oort Cloud [view large image]

		Another reservoir of comets is the Kuiper belt12 (see Figure 07-21a), a disk-shaped region about 30 to 100 AU from the sun beyond Neptune. This is considered to be the source of the short-period comets. The orbit of a Kuiper belt object is sometimes perturbed by gravitational interactions with the Jovian planets causing it to cross Neptune's orbit, where eventually it may have a close encounter with Neptune, either ejecting the comet or
Figure 07-21b Comet Hale-Bopp [view large image]	Figure 07-21c Comet Structure [view large image]	throwing it deeper into the solar system. These are the short period comets with period less than 200 years.

		by Stardust when passing within 500 kilometers. Clearly visible are numerous craters and hilly terrain. Stardust has also captured particles from the coma and will jettison them to Earth in 2006. Analyses of the images and returned particles will likely give fresh information about our Solar System back near its beginning, when Comet Wild 2 formed. Discovered in 1978, Comet Wild 2 takes 6.39 years to orbit the Sun, traveling nearly as close to the Sun as Mars is and as far away from the Sun as Jupiter. The space capsule that contains dust particles from the comet has returned to Earth on January 15, 2006.
Figure 07-22a Stardust [view large image]	Figure 07-22b Wild 2 [view large image]	Detailed analyses of the comet dust in the aerogel collector (Figure 07-22a insert) reveal that there is a vast array of different of particles - some from the heart of the Solar System,

with crater-like features. Trapped ice seems to be below the surface, possibly containing the primordial ingredients of the solar system. Click here to see the impact as observed by the HST. Further analysis shows that the comet's dust and ice grains form a fluffy structure of fine particles held together loosely by a weak gravitational pull. Materials in the debris include tiny grains of silicates, iron compounds, complex hydrocarbons, and clay and carbonates thought to require liquid water to form. The existence of a substantial amount of organic material

Figure 07-22c Tempel 1 Impact [view large image]

means that the comets might have brought such material to Earth early in the planet's history at a time when asteroid and meteor strikes were common.

Synchronous Rotation

		The satellite always presents the same face to the reference frame of the central body, i.e., it doesn't seem to be rotating (Figure 07-23a). This situation comes about when the satellite moves too close to a massive body. As shown in Figure 07-23b, the tidal force distorts the satellite into bulges, where the two ends experience difference force and hence
Figure 07-23a Synchornous Rotation [view large image]	Figure 07-23b Tidal Dragging [view large image]	difference torque, resulting in the retardation of the spinning rate. The time scale T for development of the locking can be expressed approximately as:

1. The initial spinning rate is taken to be one revolution every 12 hours (the rotational periods for most asteroids vary between 2 to 24 hours). The locking time T is proportional to the spinning rate, thus a rapidly spinning satellite would take longer time to be locked in.
2. Value of the rigidity of the satellite is taken to be 3x10¹⁰ Nm^-2 and 0.4x10¹⁰ Nm^-2 for rocky and icy objects respectively. This parameter is also proportion to T.
3. The dissipation function of the satellite is assumed to be 100. It governs the rate at which mechanical energy is converted to heat. This parameter is again proportion to T.
4. The density of the satellite is assumed to be about 3 gm/cm³. It is inversely proportional to T.

Table 07-03 Some Tidally Locked Systems

Solar Neighborhood¹³

		speed of about 20 km/sec. Further out still, if the Solar system is itself moving supersonically relative to the interstellar medium, there may be a large bow shock as shown in the illustration. Figure 07-25 shows some celestial objects in the spiral arms of the Milky Way within 6000 ly of the Sun. Table 07-04 lists a few of the prominent objects just beyond the solar halo.
Figure 07-24 Solar Boundary [view large image]	Figure 07-25 Solar Neigh- borhood [view large image]

Table 07-04 Solar Neighborhood

Extrasolar Planets

		In the summer of 2003 astronomers have discovered a planetary system more similar to our own Solar System than any known previously. The bright star HD70642, visible with binoculars toward the constellation of Puppis about 90 light years away, was already known to be a star like our Sun. Now a planet with twice Jupiter's mass has been discovered in a nearly circular orbit at approximately
Figure 07-26 Another World [view large image]	Figure 07-27 Ancient World [view large image]

Meanwhile it is discovered that a planet, a white dwarf, and a neutron star orbit each other in the giant globular star cluster M4, some 5,600 light-years away. The most visible member of the trio is the white dwarf star, indicated in an image from the Hubble Space Telescope (see Figure 07-27), while the neutron star is detected at radio frequencies as a pulsar. A third body was known to be present in the pulsar/white dwarf system and a detailed analysis of the Hubble data has indicated it is indeed a planet with about 2.5 times the mass of Jupiter. In such a system, the planet is likely to be about 13 billion years old. Compared to our solar system's tender 4.5 billion years and other identified planets of nearby stars, this truly ancient world is by far the oldest planet known, almost as old as the Universe itself. Its discovery as part of an evolved cosmic trio suggests that planet formation spans the age of the Universe and that this newly discovered planet is likely only one of many formed in the crowded environs of globular star clusters.

		Meanwhile the infrared image of another extrasolar system has been captured in 2008 by the Gemini Observatory on Mauna Kea, Hawaii (Figure 07-28b). The parent star for this planetary system is similar in mass but a little cooler than the Sun because it is much younger (only a few million years old). Spectra data reveal the nature of the companion planet, which has a mass about 8 times that of Jupiter, and lies roughly 330 times the Earth-Sun distance from its star (about 10 times the distance from the Sun to Neptune). It is
Figure 07-28a Extra- solar Planet 1	Figure 07-28b Extrasolar Planet 2	still red hot (1500oC) with heat generated during its formation by gravitational contraction (see little red object at 11 o'clock). This system is 500 light-years away toward the constellation Scorpius.

At the end of 2007, the number of discovered extrasolar planets has increased to 270. The number is going up week by week. The detection methods (Figure 07-29) now include: The Doppler effect as the star wobbling toward us and away - This is the leading method accounting for 90% of the discoveries. However, it yields only a minimum mass unless the planet's orbital inclination can be determined by some other mean. The dimming of star light by the transit of planet in front of the star - Such method can yield very accurate mass estimate. But only a small fraction of all planets transit their parent star's disk along our line of sight. Gravitational microlensing of the star light by the planet - This method is capable of finding Earth-mass planets, although the chance of such alignment is rather rare. Direct-imaging - This technique requires taking a planet's image (see Figure 07-28). It is a difficult task because of the glare of the star.

Figure 07-29 Methods of Detection

		the Sun-Jupiter distance). It is shown as a little red dot in Figure 07-30 with the camera's coronagraph to reduce the over-whelming glare from the star. The other direct image in Figure 07-31shows three exoplanets (b, c, d red dots) each 5-13 times the mass of Jupiter, in orbit around HR 8799, a star 130 light years from Earth. The arrows in the image indicate the direction and magnitude of the planet's velocity. Actually, these claims
Figure 07-30 Exoplanet Fomalhaut b	Figure 07-31 Exoplanets HR 8799b, c, d	have yet to be examined further to see if they are really exoplanets or brown dwarfs, which are failed stars just over 13 Jupiter masses.

		The Kepler space telescope was launched on March 6, 2009. Its mission is to detect exoplanets using the "transits" method, which is most suitable for finding earth-like planets as mentioned earlier. Kepler will search 100000 pre-selected Sun-like stars 180-920 parsecs away, sending back data to Earth every 30 days. Of the 342 exoplanets detected to date (March 2009), only 58 transiting planets have been found as shown in Figure 07-33. Most of the exoplanets have been detected through the radial velocity method.
Figure 07-32 Kepler Space Telescope [view large image]	Figure 07-33 Detected Exopanets

1. HD 168443 - Tidal forces could melt ice on a possible moon of this massive planet. 2. HD 16141 - A possible moon's arid landscape attests to this gas giant's proximity to      its Sun-like star. 3. HD 209458b - This jupiter-like planet is so close to its parent star that its heated      atmosphere (including water vapor) is simply expanding away into space. 4. PSR B1257+12 - The first exoplanets discovered, these three worlds orbit an

Figure 07-34 The Other Worlds [view large image]

energetic pulsar. 5. HD 38529 - It takes 6 years for this gas giant to orbit its distant sun, which harbors      another closer-in planet.

The discovery of 32 exoplanets was announced at the European Southern Observatory/Center for Astrophysics in October 2009. The detections were made possible by using the tactic of HARPS (High Accuracy Radial Velocity Planet Searcher in the La Silla Observatory, see Figure 07-35), which can detect slight wobbles of stars as they respond to tugs from exoplanets' gravity. The instrument can measure movements as small as 1 m/s (2 mph) - a slow walking pace. With the discovery, the tally of new exoplanets found by HARPS is now at 75, out of about 400 known exoplanets.

Figure 07-35 HARPS in La Silla Observatory [view large image]

References:

1. Planetary System, New Detection Technique
2. Evolution of the Solar System -- http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/ECText/ch11_txt.htm
3. Solar System Data -- http://hyperphysics.phy-astr.gsu.edu/hbase/solar/soldata2.html
4. The Sun -- http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html
5. Terrestrial Planets -- http://www.geocities.com/CapeCanaveral/Launchpad/1364/Terrestrial_Planets.html
6. Terrestrial Planets, The Moon -- http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
7. Asteroid Belt -- http://www.solarviews.com/eng/asteroid.htm
8. Jovian Planets -- http://www.geocities.com/CapeCanaveral/Launchpad/1364/Gaseous_Planets.html
9. Pluto -- http://seds.lpl.arizona.edu/nineplanets/nineplanets/pluto.html
10. Comets -- http://www.ifa.hawaii.edu/faculty/jewitt/comet.html
11. Comets, Oort Cloud -- http://www.windows.ucar.edu/tour/link=/comets/Oort_cloud.html
12. Comets, Kuiper Belt -- http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs&Display=OverviewLong
13. Solar Neighborhood -- http://www.geocities.com/CapeCanaveral/Launchpad/1364/Gaseous_Planets.html

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