Planetary Systems

Terrestrial Planets - Mercury, Venus, Earth and Moon, Mars (MSR 2021-2031)

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 much of the equator. The drawing depicts

Figure 07-07a Venus [view large image]

a typical Venus landscape with barren, craggy rocks seen through thick carbon dioxide air under yellowish clouds, containing sulfuric acid droplets.

An article titled "Phosphine gas in the cloud decks of Venus" reveals the detection in 2020 (Figure 07-07aa). Phosphine (PH₃) is produced during a reduction reaction with electrons transforming to the hydrogen atoms. It requires a lot of energy in the process.

Phosphine is a constituent of the Earth's atmosphere at very low concentration probably from decaying organic matter. It is also found in Jupiter's atmosphere with the source in the planet's hot and high pressure interior. Venus lacks such high energy condition to form phosphine, thus another explanation for its presence is required. The paper announcing the discovery suggests that the phosphine "could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life". In addition, the process would have to be replenished as the phosphine would not persist in the Venusian atmosphere; being subject to ultraviolet radiation, it would eventually be consumed by water and carbon dioxide. For this reason phosphine has been proposed to be a usable biosignature for astrobiology. PH3 is associated with anaerobic ecosystems on Earth, which may be indicative of life on anoxic exoplanets.

Figure 07-07aa Venusian Phosphine

Figure 07-07aa shows the Venusian thick clouds in two bands of ultraviolet light. It is taken by the Venus-orbiting Akatsuki, a Japanese robotic satellite that has been orbiting the cloud-shrouded world since 2015.

• 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]

• Moon -- 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
Figure 07-08a Impact Theory	Figure 07-08b Lunar Phases [view animation, and chart]	illuminates different features differently. A full lunation takes about 29.5 (solar) days, just under a month. Hit STOP button of the browser or Esc to see a particular phase, reload to see animation again.



It is announced on mid November 2009 that Lunar scientists in NASA have detected water for certain near the north pole of the Moon, after the impact of a NASA projectile kicked up water vapor along with a plume of dust. Followings is an explanation for the origin of water on the moon and elsewhere in the universe:
1. Both oxygen and hydrogen are very reactive chemical elements. They combine to form water (H₂O) readily when meeting each other.
2. Soon after the moment of Big Bang up to the present, the material world is mostly in the form of hydrogen (99%).
3. Only at the dying moment of heavy stars, oxygen and other heavy elements are created in the supernove explosion.
4. Thus, water is ultimately created at the moment of planetary formation, when things are mixed together from the remnant of Big Bang and from the debris of dying stars.
5. Model study of planetary growth and development shows that water in terrestrial planets (including the moon) was swept out by energetic electromagnetic radiation emitting from the young Sun. Water (in the form of ice) was retained only beyond the "snow line", which is a virtual line near the asteroid belt where water freezes out. It is suggested that the water on Earth and Moon was re-acquired from asteroids visiting the inner part of the Solar system later. Other theories include delivery by comets and combining hydrogen in solar wind with oxygen on the moon.
At about the same time of the above-mentioned announcement, water has been confirmed on the asteroid 24 Themis. The ice there may have originated below the surface and been exposed by impacts.

There would not be any eclipse if the moon is not here. Then there would be solar and lunar eclipses every month when the moon comes between the sun-earth and behind the earth (Figure 07-08c) if the plane of the lunar orbit coincides with the ecliptic. In reality, the lunar orbital plane is inclined about 5^o to the ecliptic, it pass through the ecliptic only twice a month at a pair of points

		called the nodes (N1, N2 in Figure 07-08d). Eclipse would occur only when the Sun-Moon-Earth or Sun-Earth-Moon (~ 15 days later) line up at the node(s) roughly in a straight line. Such occurrence happens about once every six months.
Figure 07-08c Solar & Lunar Eclipses [view large image]	Figure 07-08d Lunar Orbit in Helio- and Geo-centric Coordinates [view large image]

The pattern of eclipse is further complicated by the motion of the nodes (caused by the Sun's gravity) in 18.6 years cycle called saros. It shortens the interval between successive eclipses by an average of about 10 days every six months. In addition, the prediction has to provide location, time, duration of the event, and whether it is total or partial. The easiest way to find the information is to visit NASA Eclipse.



The occurrence of eclipses can be explained even better by considering the paths of the Moon (blue) and Sun (red) as shown in Figure 07-08e. The map depicts the mid-latitude sky with the summer night in the middle and the day light on both sides. It shows the lunar phases for the month of June 2012. The Moon in the sky can be located by drawing a line from the phase picture for the day straight up to the blue curve. The time of its appearance is specified by the corresponding RA

Figure 07-08e Sun Moon Paths in June 2012 [view large image]

(Right Ascension) in unit of hour with a range of 24 hours. Actually, the night sky progressively moves eastward with each passing day at a rate of about 4 min/day. Its size also changes according to the four seasons.


The occurrence of a partial lunar eclipse on June 4, 2012 is explained in the followings:

1.   The paths of the Sun and Moon crosses at midnight June 3, 2012. However, these two celestial objects do not quite align at this point as indicated in Figure 07-08e. 2.   A near alignment occurs 24 hour later causing a partial lunar eclipse. For a total lunar eclipse to occur it requires RA(moon) = RA(sun) + 12h, and Dec(moon) = -Dec(sun) in Geocentric Coordinates. Such conditions are only approximately fulfilled (see Figure 07-08f) resulting in only a partial eclipse. 3.   Lunar eclipse always occurs at full moon. Simple minded consideration suggests that a solar eclipse would occur at new moon about 15 days later when the Moon is at a location between the Sun and Earth. Such event will not happen on June 18 because they will be drifting off from the N1 node (Figure 07-08d).

Figure 07-08f Lunar Eclipse on June 4, 2012 [view large image]

4.   An annular solar eclipse did happen 15 days earlier on May 20 when the alignment was nearly perfect with RA(moon) RA(sun), and Dec(moon) Dec(sun).


For a long time, astronomers could not understand why the moon would not settle down to an orbit on the ecliptic plane by tidal action. The absence of monthly total solar eclipse is explained by a computer simulation published in 2015. As shown in
Figure 07-08g :

	(a) Originally, the moon's orbit was in the Earth's equatorial plane. (b) The orbit expanded by tidal interaction with the Earth and was perturbed by large objects at the same time. (c) The population of these objects depleted over a period of about 107 years and eventually tilted the orbital plane by 5 degree.
Figure 07-08g Lunar Orbital Tilt [view large image]	Such interpretation also explains the relatively higher abundance of platinum and gold in the Earth's upper layer. It was delivered by a few impactors from those intruding objects according to the above scheme. See "The Moon's Tilt for Gold".





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 diameter up to 2 x 10^-5 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 diameter of several 10-5 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
Figure 07-09b Life on Mars [view large image]	Figure 07-09c Bacteria	living organism, and all the biosignatures in ALH84001 have been proven to be 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 depend on whether the supposed biosignatures could have been produced by non-living processes.

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

The search is now on for some abiotic way to make the tunnels.

A research team at NASA's Johnson Space Center has re-examined the ALH84001 meteorite using new technology. Their report in November 2009 indicated that there is strong evidence to support the existence of life on ancient Mars. The carbonate mineral globules containing the ALH84001 microscopic fossils (Figure 07-09e) were formed in temperature conducive to life contrary to the most strenuous counter-arguments that they were created in high-temperature environments. The Johnson team also claimed that biomorphs (a new phase coined by Richard Dawkins to describe the complex life form selected non-randomly from random mutation) have been detected in the Nakhla and Yamoto meteorites (NASA keeps a collection of 34 meteorites from Mars).

Figure 07-09e ALH84001 Globules [view large image]

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 07-09f). 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 07-09g 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 07-09f 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 without the protective shield.
Figure 07-09g Martian History [view large image]	Figure 07-09h Watermarks on Mars [view large image]

The evidences include (Figure 07-09h):


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. Martian Ocean - A 2010 study shows that of 52 deltas along Mar's northern lowlands, 29 deltas, or 55% seem to feed the same body of water (at about the same height) - the Martian ocean. The locations of the end points of the valleys also seem to be consistent with a possible coastline. The problem is to explain the 45% not conformed to the same height. One possible explanation is a large-scale deformation of the planet, or the sea level had changed over the eons.
4. 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.
5. Festoons - The rover Opportunity found a rock that was marked with festoons, which are formed by waves washing over sandy sediments.
6. 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 07-09i 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 07-09g). 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 07-09i 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 07-09j).

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

Figure 07-09k 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 07-09h. It is also found that the current distribution of water ice may

Figure 07-09k 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 07-09l, artist's rendition) touched down safely at the edge of the polar region (Figure 07-09m, 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 07-09l Phoenix Lander [large image]	Figure 07-09m 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 07-09n). 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 07-09n Ice on Mars	Figure 07-09o Martian Soil [view large image]	interacted with water in the past. Figure 07-09o 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 oC and dust storms and clouds obscured an enfeebled sun, the Phoenix Lander finally ceased to communicate on Novermber 3, 2008 (Figure 07-09p). 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 07-09p End of a Mission

		temperature ranging from -21 oC to -96 oC. The salt is not the common table salt (NaCl) but rather in the form of 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 07-09r) suggest the possibility that Martian microbes could be dredged up from underground lake.
Figure 07-09q Salty Water	Figure 07-09r Mud Volcano [view large image]

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 07-09s depicts ancient rivers ferried clay-like minerals (shown in green) into the

Figure 07-09s 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 07-09u 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 07-09t Glaciers on Mars [view large image]	Figure 07-09u 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 07-09g, 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 07-09v 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 07-09v Carbonate Minerals on Mars

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

Table 07-02 A History of Martian Water

		methane could also come from chemical reactions in which buried volcanic rocks rich in the mineral olivine 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
Figure 07-09wa Methane on Mars, Local	Figure 07-09wb Methane on Mars, Global	sunlight degradation. Figure 07-09wb shows the global emission of methane gas in Martian summer. The data were obtained spectroscopically using large ground based telescopes.

Curiosity also found an organic compound called chlorinated methane, but it is not sure whether the carbon in the sample is from the Martian soil, or contamination from Earth, or just from cosmic dust. In short, the scientific progress is painfully slow. The answer to the question of life on Mars would have to go through a flow chart (similar to the one for the discovery of the Higgs particle). The result can be determined only from such process of elimination by observational data. A negative answer is also considered as an achievement in science, but right now there is no definite answer to this question of

Figure 07-09x Curiosity Rover [view large image]

whether there is life on Mars. A March 12, 2013 announcement confirms that Curiosity has found an environment conducive to life by analyzing the drilled rock sample (see insert in Figure 07-09x) containing neutral pH clay minerals. The water, that formed the clay, would be the kind suitable to drink by human.

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 07-09y, a). The bright patches darkened in the weeks following initial observations as the freshly exposed ice vaporized into the thin martian atmosphere (Figure 07-09y, 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-ice. This ice is a relic of a more humid climate from perhaps just several thousand years ago.

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

		2. Martian Ocean - Other studies found that an ocean covered 1/3 of the Martian surface some 3.5 billion years ago (Figure 07-09g). Specifically they studied 52 deltas and roughly 40000 river valleys (Figure 07-09h). It is found that these geologic features are all at a similar elevation, implying that they were connected to the Martian ocean. The total amount of water is estimated to be about 1/10 of the current volume of Earth`s ocean.
Figure 07-09za Car- bonate Mineral	Figure 07-09zb Silicate in Martian Crater [view large image]	3. Hydrated Silicate inside Craters - Studies of 91 impact craters (Figure 07-09zb) where asteroids had exposed ancient material several miles

		The Mars Reconnaissance Orbiter also discovered hydrothermal mounds with mineral (such as hydrated silica - SiO2nH2O, which can trap and preserve life by wrapping it inside - much like the amber) deposited on a volcanic cone more than 3 billion years ago (Figure 07-09zc). It could be the relic left over from a period in the Martian history when the planet began to turn dry and cold. Life flourishes in various forms around hydrothermal mounds on Earth. Martian creatures could exist similarly there clinging to life in the dying gasp.
Figure 07-09zc Hydrated Silica on Mars [view large image]	Figure 07-09zd Mars 2014 [view large image]	Meanwhile in July 2104, the US Geological Survey has unveiled new Mars maps (Figure 07-09zd). See more in "New Global Geologic Map of Mars" by USGS; also "Organics on Maars".

21 September 2014. Figure 07-09de shows the October 2014 images of its atmosphere according to the atomic carbon, oxygen and hydrogen spectrographs. While the relatively heavier carbon and oxygen atoms remain in layer close to the surface, the hydrogen atoms appear to be leaving the planet's atmosphere in clumps and streams that reach about 10 Mars radii into space. The hydrogen comes from water vapour that breaks apart in the upper atmosphere meaning that part of the water is drifting into space, leaving a mostly dry, and frozen wasteland. The red color of Mars is the result of oxidizing the iron atoms on the surface (courtesy of the free oxygen from the dissociation of water).

Figure 07-09ze Mars Atmosphere, Losting [view large image]

Figure 07-09zf Missions to Mars, 21st Century [view large image]	Mission Home Page [click icon]

In the summer of 2020, 3 space-crafts were launched on their perilous journeys toward Mars.
1. USA (Perseverance) - This is their 5th rover to find evidence of past life and collect samples to be returned to Earth later.
2. China (Tianwen-1) - Their first exploration of Mars with an orbiter, lander and rover. The mission will study the planet's atmosphere, internal structures and surface environment - including search for presence of water and sign of life.
3. UAE (Hope) - The orbiter from United Arab Emirates will track daily weather variations and changing seasons for 2 years.

Figure 07-09zg Three Missions to Mars, 2020 [view large image]

After all that's been said and done about life on Mars. A comparison of the prebiotic condition between Mars and Earth as shown in Figure 07-09zh can reveal further information about the possibility.


• It shows that the condition between the 2 planets is very similar from their formation about 4.5 up to 3.5 GYA.
• Mars suddenly dried up at about 3.5 GYA as indicated by craters (from LHB) on river channels (see "Why Did Mars Dry Out ?"). The "dry-up" is relatively swift with a time scale of about 0.1 billion years. Ultimately related to its smaller size, which leaded to sequence of processes eventually striped off its atmosphere and water splits into hydrogen escaping to space while the free oxygen combines with iron to form ferric oxide (Fe₂O₃) in red color. (see "This Is Why Mars Is Red And Dead", and "MAVEN").
• Life emerged at about 3.6 GYA on Earth.
• Thus, Mars barely have enough time to produce life.

Even if it managed to overcome the adverse condition, life would be very primitive at the stage of prokaryote eking out an anaerobic and autotrophic living (do not breathe oxygen and made own food in the dark). They could hardly make it to the stage of cyanobacteria as they would live underground with no sun light to run photosynthesis. As it is still occurring on Earth today, prokaryotes in anoxic sediments produce methane, i.e., CH4 (see "Prokaryotic Metabolism"). Such gas has been detected and mapped on Mars (see Figures 07-09wa and 07-09wb), but is dismissed by scientists as usual because it can be produced by other means.

Figure 07-09zh Mars/Earth Comparison, Prebiotic [view large image]

Of course, the scenario would be different depending on the "dry-up" time (see "Major Events of Life on Earth" to match and select).

	By early 21st century, technology has advanced to the point which would enable the return of Mars sample to Earth. In April 2018, a letter of intent was signed by NASA and ESA that may provide a basis for a Mars sample-return mission. In July 2019, a mission architecture was proposed to return samples to Earth by 2031.
Figure 07-09zi MSR Concept	See "Summary of MSR"

Very briefly, the NASA-ESA scheme consists of three stages :



Mars 2000 - This is the initial phase for gathering samples, which are left on the ground for retrieval later by another rover. This is an ongoing phase runs by the Perseverance rover launched on July 2020 and and landed on February 2021. Its mission will last for about 2 (Earth) years, i.e., in 2023. Sample Retrieval Operation - A lander will be launched in July 2026 and will land near the Mars 2020 rover in August 2028. Another rover from which would collect the samples left behind by Mars 2020 using robotic arm and delivering them to the lander. It will use its robotic arm to take the sample tubes from the rover and load them into the sample Return Capsule which is inside a rocket's payload. Once loaded with the samples, the Mars ascent rocket will launch with the sample return canister in spring 2029. Sample Return orbiter - The Earth-return orbiter will be launched in October, 2026 and arrives at Mars in 2027. By July 2029. the orbiter will retrieve and seal the canister with the samples using a robotic arm to place the sealed container into an Earth-entry capsule, and return it to Earth during 2031.

Figure 07-09zj [MSR video]

Other nations are also planning their own MSR schemes. A Chinese version requires only one launch to achieve all the above-mentioned stages using a very powerful rocket.

See pictorial illustrations in Figure 07-09zi, a video about the concept (and a caption about the various events) in Figure 07-09zj.

		celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere. The Committee on Space Research (COSPAR) is tasked to set up policy for implementation of planetary protections. It meets every two years, in a gathering of 2000 to 3000 scientists to develop recommendations for avoiding interplanetary contamination. Five categories have been defined for different degrees of protections according to different kind of space missions (see Figure 07-09zk). The MSR samples will be managed under the umbrella of "Extraterrestrial sample curation". It is estimated that more than 90% of the samples can be analyzed in uncontained laboratories exempted from Sample Safety Assessment Protocol (SSAP) testing (Figure 07-09zl).
Figure 07-09zk COSPAR Categories	Figure 07-09zl MSR Sterilization [view large image]

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