In planetary sciences it is often said that much is to be learned from comparative planetology, studying the similarities and differences in the properties of different planets. Mars has a lower atmospheric pressure than Earth (7 mb, cf 1000 mb), lower temperatures (-50oC – +50oC, cf -20 – + xx) a CO2 atmosphere instead of a 75% N2 and 25% O2, and so on. Planets also change their surface conditions. Neither Mars nor Earth had the same conditions 3 Ga agi that they have now. We try to infer similarities and differences in conditions by examining their surfaces. In western Oklahoma we saw several structures that have counterparts on Mars
Mesas on Mars
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows two large and many small mesas composed of frozen carbon dioxide on the south polar cap of Mars. MGS has observed the south polar cap through three whole summers, and MOC images have shown that the scarps on these mesas retreat an average of 3 meters---some retreat faster, some a bit slower---per martian summer. The south polar cap is the most rapidly-changing landscape on Mars. These mesas are located near 86.5°S, 358.5°W. The image covers an area approximately 2 km (1.2 mi) across and is illuminated by sunlight from the upper left.
This April 2003 Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows mesas and troughs in the northern Nilosyrtis Mensae region of Mars. The larger mesas are capped by boulders or small knobs which are probably the eroded remnants of a layer of rock. The picture is located near 37.8°N, 282.5°W and covers an area 3 km (1.9 mi) wide. The scene is illuminated by sunlight from the lower left.
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows several dark-toned mesas surrounded by light-toned sedimentary rock outcrops in Aram Chaos, a large impact basin--over 200 km (more than 125 mi) across. These mesas are remnants of a once more extensive rock unit. The image is located near 2.0°N, 20.2°W, and covers an area about 3 km (1.9 mi) wide. Sunlight illuminates the scene from the left.
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a mesa left standing when erosion created the Granicus Valles system, located west of the Elysium volcanoes near 27.8°N, 224.3°W. Dark dots at the base of the mesa's slopes and on the valley floor are large boulders. The image covers an area about 3 km (1.9 mi) across. Sunlight illuminates the scene from the lower left.
GULLIES ON MARS
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows gullies with banked and somewhat sinuous channels and inner channels cut into the wall of a south middle-latitude crater near 46.6°S, 175.7°W. Banked channels are among the key evidence suggesting that some martian gullies involved flowing fluids with all of the physical properties of liquid water. The image covers an area about 2.3 km (1.4 mi) wide, and is illuminated by sunlight from the left.
Many craters and troughs at polar and middle latitudes on Mars have gullies carved in their walls. These gullies may have formed by running water; others have suggested alternative, exotic fluids such as liquid or gaseous carbon dioxide. This view of martian gullies was acquired by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC). The image shows gullies in the wall of an old meteor impact crater near 39.0°S, 200.7°W. Sunlight illuminates the scene from the upper left.
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows gullies and debris aprons in a crater near 38.5°S, 174.5°W. Gullies such as these may have formed by running water, carbon dioxide, or dry mass movement processes. Most investigators of martian gullies consider that water, whether fresh or briney, may have been involved. This picture covers an area about 3 km (1.9 mi) across. Sunlight illuminates the scene from the upper left.
SAND DUNES ON MARS
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a field of low-albedo (i.e., dark) sand dunes in a crater in Noachis Terra. Dunes on Earth are usually light while those on Mars are usually dark. This contrast results from a difference in the mineral composition. Earth dunes often contain abundant quartz, which appears light, while martian dunes typically contain minerals and rock fragments abundant in iron- and magnesium-rich minerals, which are usually dark. This dune field is located near 41.7°S, 319.8°W. The steeper slopes on these dunes, known as slip faces, point toward the lower left (southwest), indicating the dominant winds come from the northeast (upper right). This picture covers an area about 3 km (1.9 mi) across. Sunlight illuminates the scene from the upper left.
Sometimes, pictures received from Mars Global Surveyor's Mars Orbiter Camera (MOC) are "just plain pretty." This image, taken in early September 2000, shows a group of sand dunes at the edge of a much larger field of dark-toned dunes in Proctor Crater. Located at 47.9°S, 330.4°W, in the 170 km- (106 mi-) diameter crater named for 19th Century British astronomer Richard A. Proctor (1837-1888), the dunes shown here are created by winds blowing largely from the east/northeast. A plethora of smaller, brighter ripples covers the substrate between the dunes. Sunlight illuminates them from the upper left. Other dunes in Proctor Crater were highlighted in August 1999 in: "Dune Activity in Proctor Crater," MOC2-170.
The most exciting new aspect of the Mars Global Surveyor (MGS) Extended Mission is the opportunity to turn the spacecraft and point the Mars Orbiter Camera (MOC) at specific features of interest. Opportunities to point the spacecraft come about ten times a week. Throughout the Primary Mission (March 1999 - January 2001), nearly all MGS operations were conducted with the spacecraft pointing "nadir"---that is, straight down. A search for the missing Mars Polar Lander in late 1999 and early 2000 demonstrated that pointing the spacecraft could allow opportunities for MOC to see things that simply had not entered its field of view during typical nadir-looking operations, and to target areas previously seen in a nadir view so that stereo ("3-D") pictures could be derived.
One of the very first places photographed by the MOC at the start of the Mapping Mission in March 1999 was a field of dunes located in Nili Patera, a volcanic depression in central Syrtis Major. A portion of this dune field was shown in a media release on March 11, 1999, "Sand Dunes of Nili Patera, Syrtis Major". Subsequently, the image was archived with the NASA Planetary Data System, as shown in the Malin Space Science Systems MOC Gallery. On April 24, 2001, an opportunity arose in which the MGS could be pointed off-nadir to take a new picture of the same dune field. By combining the nadir view from March 1999 and the off-nadir view from April 2001, a stereoscopic image was created. The anaglyph shown here must be viewed with red (left-eye) and blue (right-eye) "3-D" glasses. The dunes and the local topography of the volcanic crater's floor stand out in sharp relief. The images, taken more than one Mars year apart, show no change in the shape or location of the dunes---that is, they do not seem to have moved at all since March 1999.
Picture D is a sub-frame taken from near the center of image M03-01521. Like Picture C, it is oriented with north toward the lower left. This area shows the thinly-bedded lower units of the Gale Crater mound. The lower part of the mound has hundreds of thin (2-5 meters; 2-5 yards thick) beds of similar thickness and properties---in this regard, the lower units are similar to the beds observed elsewhere on Mars, such as in southwestern Candor Chasma. The most striking feature in this sub-frame, however, is the area labeled "filled channel." This is interpreted to be a channel that was cut into the layered rock some time in the past. Perhaps it was cut by running water. Later, the channel was filled and then completely buried by additional sediment. At an even later time (closer to the present, but still very ancient), the material that buried the channel was stripped away, leaving a filled channel that, at its lower end (from center toward lower right) actually stands as a ridge higher than the surrounding terrain. This channel attests to the possible erosion of the layered rock by running water. It also indicates that there was a period in the past when the rock was eroded before being covered-up again. Such evidence and interpretations are pieces of the story of this area.
Picture F is an interpreted cross-section through the part of the Gale Crater mound that is visible in MOC image M03-01521. The lower part of Picture F is the image, M03-01521, with each different rock unit (some have many, thin layers, others have few layers, others erode differently or have different brightness, etc.) shown by a different color. The cross section uses the MOLA topographic profile that was acquired by MGS at the same time as the MOC image. The MOLA data give elevations for the area between the two straight black lines running lengthwise across the MOC image. Where the MOLA profile intersects the contact between each colored unit, the position of this unit in the cross section can be inferred. These data and the observations presented in Pictures D and E show that the Gale Crater mound preserves a complex history that includes the formation of many layers in the lower part of the mound, a period of erosion and cratering on these lower layered units, then deposition on top of these materials by younger, brighter, and not-layered (i.e., massive) units.
Toward the end of its Primary Mapping Mission, the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) acquired one of its most spectacular pictures of layered sedimentary rock exposed within the ancient crater Becquerel. Pictures such as this one from January 25, 2001, underscore the fact that you never know from one day to the next what the next MOC images will uncover. While the Primary Mission ends January 31, 2001, thousands of new pictures---revealing as-yet-unseen terrain on the red planet---may be obtained during the Extended Mission phase, scheduled to run through at least April 2002.
The picture shown here reveals hundreds of light-toned layers in the basin named for 19th Century French physicist Antoine H. Becquerel (1852-1908). These layers are interpreted to be sedimentary rocks deposited in the crater at some time in the distant past. They have since been eroded and exposed, revealing faults, dark layers between the bright layers, and a long geologic history (of unknown duration) recorded in these materials. Sets of parallel faults can be seen cutting across the layers in the left third of the image. Sunlight illuminates this scene from the top/upper right.
Additional examples of layered sedimentary rock can be seen in earlier MGS MOC releases:
What we have found at the Opportunity site (Meridiani Planum) is a thick sequence of evaporites. These are salts that form when lakes or seas evaporate, and leave behind these beds of evaporites. The evidence is unambiguous.
Bringing Mars Home
Edited testimony of Michael Carr, President's Commission on the Moon, Mars and Beyond
SS: If you look at Pot of Gold (Spirit MER, Columbia Hills), it's got these planar sheets of apparently resistant stuff running through it, at different angles. So suppose somewhere up the hill, I can find a rock that's Pot of Gold-like, but it's got a clearly resistant feature, like one of these planar things, that's big enough that I can actually isolate it with the Mössbauer; and then it's got some of this matrix material that hasn't just crumbled away and left the stalks behind; and I can get the Mössbauer on that; and I can measure those two things.
Now suppose I find that hematite is concentrated in the hard resistant planar feature, but is low or absent in the other stuff. Then I have learned something about the heterogeneity of the hematite; and then I have taken my supposition that the morphology of this thing is related to differential cementation and I've actually made a measurement that supports that. Until I've got a measurement that supports the supposition, I don't want to draw conclusions from the supposition.
Spirit Explores the Columbia Hills
Interview with Steve Squyres by Henry Bortman