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Planetary Systems

Terrestrial Planets - Mercury, Venus, Earth and Moon, Mars

Inner Planets 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]

Opal on Mars 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]

It was reported in December 2006 that new images, taken by NASA's Mars Global Surveyor before it lost contact with Earth, show changes in craters that provide the strongest evidence yet that water coursed through them as recently as several
Water on Mars 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]

Mars History, Updated 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.

Phoenix Lander Phoenix Landing Site 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.

In mid June 2008 the Phoenix lander has uncovered some lumpy substances while digging into the Martian soil as shown on the lower left shadow region of the trench (in the June 15 image of Figure 07-09n). It could be either ice or salt. If they were ice, the newly exposed chunks would gradually sublime and disappear (ice doesn't melt in Mars's thin atmosphere it vaporizes). If the chunks were some sort of salt deposit, they would stay put. When the clumps completely disappeared over the course of a few days, it becomes perfectly clear that this is ice (see the disappearance in the June 19 image on the right
Ice on Mars Martian Soil 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.
It is announced on August 1, 2008 that laboratory tests aboard NASA's Phoenix Mars Lander have identified water in a soil sample. The lander's robotic arm delivered the sample to an instrument that identifies the water vapors produced by the heating of samples. It has also detected perchlorate ions (ClO4-) from the soil. Although some Earth bacteria use perchlorate as an energy source, too much of it can be toxic to life - it is very reactive chemically. Since the Cl atom in perchlorate ion has a valence of +7, a lot of energy is required to strip away the 7 electrons from its outer shell. It reacts readily with other substances to recover the electrons. The perchloric acid (HClO4) is a very strong acid, stronger than the sulfuric acid (H2SO4). The perchlorate salt KClO4 can be used as low explosive when mixed with glucose (C6H12O6). It emerged in 2009 that perchlorates could have far-reaching consequences on Mars for another reason: their ability to keep water liquid far below 0oC. Concentrated solutions of magnesium and sodium perchlorate can stay liquid down to -72oC and -37oC respectively. It explains many of the mysterious signs that suggest water is leaking out from below.
End of Mission 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

Further analysis in 2009 shows that the substance on the leg of the Mars Phoenix Lander (Figure 07-09q) is salty water and could be present at other locations on the red planet. The saline water (like anti-freeze) can stay in liquid state even in the frigid Martian
Salty Water Mud Volcano 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 Water 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.

In addition to the visual images, more evidences about water on Mars have been collected toward the end of 2008 in the form of radar and gamma ray signals. The image on the left of Figure 07-09t is a photo of some unusual grooved, flat, and shallow craters. Radar images from the Mars Reconnaissance Orbiter bolster an exciting hypothesis that those craters are huge glaciers of buried ice covered by Martian dirt. The drawing on the right of Figure 07-09t portraits the "would be" appearance of the glaciers if the dirt is removed.
Glaciers on Mars Sediments on Mars 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.

The chemical formula for carbonate is CO32- - the product of dissolving carbon dioxide in water:
CO2 + H2O 2H+ + CO32-.
It combines with calcium, iron or magnesium to form carbonate minerals such as CaCO3.
Carbonate mineral breaks down in reaction with acid such as HCl, e.g.:
CaCO3 + 2HCl H2O + CO2 + CaCl2.
Carbonate Minerals 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.
Year Observation
1965 Mariner 4 fly-by shows arid, crater-pocked planet lacking canals and seas
1972 Mariner 9 reveals abundant channels carved by water in ancient past
1976 Water vapour measured by Viking 2 confirms north polar cap is water ice, not frozen CO2
2000 Mars Global Surveyor (MGS) finds young gullies - the first hint of recent water flow
2003 MGS finds fan-shaped sediment deposits indicating long-term water flow
2004 Opportunity rover finds minerals that formed in liquid water
2006 MGS detects changes in gullies, suggesting present-day water flow
2008 Phoenix becomes first probe to taste Martian ice, and detects perchlorates
2008 Mars Express radar reveals buried glaciers near equator
2009 Mars Reconnaissance Orbiter found frozen water hiding just below the surface (see below)
2012 Curiosity rover found rounded pebbles related to flowing water
2015 NASA confirms the presence of liquid water on the surface of Mars

Table 07-02 A History of Martian Water

Plumes of methane have been identified on Mars in 2008 (Figure 07-09wa). On Earth, methane is mostly biological in origin; on Mars, it could signal microbes living deep underground. However as with other circumstantial evidences, the proof is not conclusive. The methane could also
Methane on Mars, Local Methane on Mars, Global 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 sunlight degradation.

Figure 07-09wa Methane on Mars, Local

Figure 07-09wb Methane on Mars, Global

Figure 07-09wb shows the global emission of methane gas in Martian summer. The data were obtained spectroscopically using large ground based telescopes.

The Curiosity rover (Figure 07-09x) landed on Mars in August 6, 2012 has failed to detect methane in measurements up till November 5, 2012. However, a NASA spokesman has created a media frenzy about the discovery of life on Mars by saying that the mission would be for the history book. Actually a report by the Curiosity team a week later (in the beginning of December, 2012) indicates that it has found the usual compounds such as minerals, glass, water (in much lower concentrations) and perchlorate (also identified by the Phoenix Lander). Curiosity also found an
Curiosity Rover 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 whether there is life on Mars.

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

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.

In the summer of 2009, NASA's Mars Reconnaissance Orbiter has revealed frozen water hiding just below the surface of mid-latitude Mars. Instruments on the orbiter found bright ice exposed at five martian sites with new craters that range in depth from 0.5 - 2.5 meters. The craters
Relic of Ice 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]

Three scientific papers in June 2010 alone report strong evidences of water on Mars :

1. Carbonate Mineral - Analysis of the Spirit Mars Rover`s data collected in 2005 reveals that a rock outcrop contains large amount of magnesium-iron carbonates (about 16-34% by weight, see Figure 07-09za). Since such minerals usually originates in wet conditions but dissolves in acid. Therefore, the ancient water on Mars could not be acidic, which implies the environment was more favorable for life then previous thought.
Carbonate Mineral Martian Crater 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
below the surface, indicates that hydrated silicate minerals is wide spread in both the northern and southern hemispheres. Silicate minerals were found previously in the south, the search is more difficult in the northern lowlands until recently because younger volcanic activity has buried the older surface more deeply there. Since these minerals can form only in wet environments, it is concluded that the planet was altered on a global scale by liquid water more than 4 billion years ago. In Figure 07-09zb, CRISM is the visible-infrared spectrometer aboard Mars Reconnaissance Orbiter, and OMEGA is another spectrometer on Mars Express.

Hydrated Silica Mars 2014 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".

NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft (launched in November 2013) began orbiting the planet on
Mars Atmosphere, Losting 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 0709zf presents the missions to Mars in the 21st century. The corresponding home pages have good educational material for up-to-date information on Mars in particular and astronomy in general.

Mars Missions

Figure 07-09zf Missions to Mars, 21st Century
[view large image]

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