A recent paper has presented an alternative to water as the prime liquid medium responsible for the above channels and streamlike patterns. In this view, CO2 (carbon dioxide) is proposed to exist in liquid form and in flowing upon expulsion at the surface brings about the erosional features described as fluvial. A variant of this suggests that liquid water at the times in the past when Mars was warmer may have contained a significant amount of dissolved carbon dioxide ("soda water") that increased its ability to erode.
So, as of mid-2006 what can be said that reasonably affirms the presence of water now and in the past in martian history? On the next two pages (19-13a and 19-13b) we will learn of the direct observation of sediments that normally require the involvement of water. The discovery of water in polar ice, probably in subsurface lower latitude materials, and in the thin atmosphere all point to survival of small amounts of water today. This water may just be that released by occasional volcanism or by shock-evaporation of incoming comets. The various riverine landforms shown above on this page seem to point to greater amounts of water in the past. One school of thought concludes that past atmospheres were more dense and had a greater water content. Ancient martian atmospheres have probably been progressively depleted of gaseous and liquid molecules, including water, by thermal activity, by gravitational loss, and especially by the relatively weak but potent solar winds.
The martian surface is amazingly varied, with landforms of diverse genesis, some probably related to water action, as you have just seen, and others to tectonic forces, volcanism, and wind, being given descriptive names. Here are some typical examples:
The mishmash of intersecting linear features, called grooved terrain, in this case may be a complex surface of eroded ash deposits or possibly joint enlargement of a now buried remnant of a volcanic lava unit.
Variants of grooved terrain are known as sulci (singular, sulcus). Here is a closeup example seen in a thermal image made by THEMIS:
Similar terrain occurs in the slopes beyond Olympus Mons where the features present are called part of this volcano's aureole. The criss-crossing grooves and ridges seen here are almost certainly tectonic in nature:
The next pair of images show fretted terrain, found usually near cratered terrain, consisting of separated higher mesa-like units, bounded by scarps and set within lower smooth plains. This terrain is usually associated with features suggesting the action of ice (glaciated) and may also be influenced by incomplete dissection of older landforms by water and/or wind. Here are three examples:
The next image portrays etched terrain which consists of shallow depressions likely developed by wind scouring and deflation of easily erodable unconsolidated surface materials.
This MRO image shows another variant of etched terrain (also called sculpted terrain), in which the depressions may be caused either by wind deflation or by subsurface sag:
Other landform types given distinctive names (see the map near the top of page 19-11) include: furrowed terrain, knobby terrain, channeled terrain, and layered terrain. Examples of several of these are shown elsewhere in the Mars subsection. Here are two images that show typical knobby terrain:
Below is an example of a peculiar terrain found mainly in Hellas Planitia. It is termed colloquially "taffy-pull" terrain. It's formative nature remains uncertain but one interpretation includes the possibility of erosion of hard and soft layers of sediment-like material; this does not quite explain the flow patterns in apparent channels.
Some of these exotic terrains can also be called enigmatic. Lets illustrate this by looking at some images that center on what was called "White Rock" after its discovery in Mariner 9 images. The feature is a light-toned landmass, strongly embayed, that rises above the floor of the crater named Pollack (seen here in a MGS MOC image) in the southern highlands at a low latitude:
In a Viking black and white image, this feature, which is approximate 12 x 12 km in dimension, indeed has a higher albedo than the crater floor (very dark) and thus stands out as an off-white feature. Seen in a Viking color image (not shown): 1) White Rock has the same reddish surface coating that most of Mars has, and 2) its "whiteness" is largely due to contrast with the floor; its gray tone level is similar to surface materials beyond Pollack's rim.
One of the early interpretations considered it to be ice preserved as a patch in the crater. One investigator proposed this feature to have been ice extruded from depth, much like salt forms in domes and may reach the surface. This was discounted by radiometric measurements that indicated too high a temperature and later measurements that ruled out H2O and CO2. Interest was renewed in White Rock from MOC images taken onboard the Mars Global Surveyor. Consider this next pair that zero in on the several prongs of the feature:
In the upper right of the above image is a cluster of the teardrop-like features. When enlarged, these seem to show thin layers but their shape may be attributable to yardang sculpturing by wind:
This image concentrates on several white prongs and the terrain in between. Of interest are the series of thin, arcuate equi-spaced lines in the dark areas between the ridges in the lower of the paired images. These seem to be controlled by the ridges. One interpretation is that they are regular dune-like markings that may result from martian winds that are directionally channeled by the ridges - but this is speculation.
Mars scientists now interpret this feature to be dissected lake beds, if Pollack was once filled with water. Others propose volcanic ash deposits that collected inside the crater and are being systematically removed (by wind?); no nearby volcanic vent is evident that would account for this. Still other explanations have been proposed. No consensus explanation has been reached at this time.
There is still much to do to properly categorize and explain the surface features on Mars. The sheer variety of landforms and related phenomena will require continued intensive study. New spacecraft with higher resolution imagers and other instruments are clearly called for. To anyone reading this who has not chosen a profession, the writer (NMS) strongly recommends a hard look at becoming a martian planetologist - probably of a "life's work" scope.
An excellent review article, "The Unearthly Landscapes of Mars" by Arden Albee, appearing in the June 2003 issue of Scientific American offers further insights into the surface features of the Red Planet.
As you shall see on the next three pages, the 21st century promises to bear witness to continuing and expanding exploration of Mars, by orbiters, landers, rovers, and, likely, humans.