The MOC onboard the Mars Global Surveyor has been returning some provocative high resolution images that, after much interpretive debating, many geoscientists now believe offers supporting evidence of either water escape in the recent past or even indications of water still emerging. This MOC view below (5 m resolution) indicates channels coming from material in or below the edge of the southern ice cap. The stream outflow, if that is a valid identification, in each rill ends up with a small fan of deposits at its terminus - again, a feature consistent with fluid transport.
Gullies, or rills, also are noted in the walls of large craters, such as seen below. These may be caused by extended outflow of warmed subsurface ice which upon melting flows out at the walls. Other examples (second image) are considered by some to be streaks of dust avalanches, in which surface dust is mobilized, perhaps aided by water flow:
This Mars Global Surveyor image shows something unusual - what appears on the right to be a wedge of frozen ice cascading downslope. This has been interpreted to indicate that water moves at shallow subsurface depths and here extrudes at a canyon wall where is freezes (much like the ice sheets seen in roadcuts during winter). If so, this further supports the argument that the rills or gullies also seen in this image are water-generated.
A MGS MOC image analysis team has accumulated visual evidence that some active process - the best "guess" favors running water - is taking place today on Mars. Before-and-after images show light-toned deposits along gullied crater walls that were definitely not there on the earlier date:
A color view of the Centauri Montes crater deposit indicates that it is much like similar deposits caused by water on Earth:
Early surmise held these deposits to have formed by release of liquid water. However, modeling has shown a much likelier explanation is formation by landslides that release dry powdered soil whose tone is less dark than the thin surface layer.
Water seems to exist on Mars in four plausible settings: 1) in small amounts in the atmosphere; 2) in polar ice caps; 3) in subsurface ice; and 4) as yet unproved, liquid water probably derived from melting of that ice along crater walls; even if so released, this water does not survive long in the fluid state but evaporates and/or freezes.
This next pair of images offers solid proof that new small and local markings develop even today on Mars. A single black streak appears on the left (1998) image of part of a crater wall. On the right arrows point to new black streaks that appear between then and 2001. These are interpreted as related to water outflow.
The next three images (NASA JPL and Malin Space Science Systems [MSSS]) demonstrate how planetary geologists go about interpreting and reasoning to conclusions using imagery returned from martian satellites and probes. The argument to be devised uses Martian Global Surveyor data to examine the region illustrated in this topographic map:
The Cerberus Fossa (channel) includes this 100 m wide, 10 m deep straight feature that, by comparison to similar areas on Mars, is almost certainly a tectonic graben, bounded by faults on either side expressed as steep walls. Emanating from the upper wall is what appears to be lava flows spread over both the flat land beyond the fault and into the graben floor.
Water is presumed to have excaped from the Cerberus Fossa during, or at different times from, periods of volcanic activity. The amount of water needed to develop the valley is estimated to be about that found in Lake Erie today. Based on crater counts and other data, geologists believe that the principal valley-forming events may be as recent as 10 million years ago.
The Athabasca Valles extends southwest of the Fossa. It resembles a stream-cut valley similar to some on Earth. Within it is strong evidence of stream flow in the form of the streamlined, tear-drop shaped flatlands within it (see above on this page), such as shown here:
The Ma'adim Vallis (white arrow) is a 2.1 km (6900 ft) deep cut into the Southern Highlands that runs from a topographically lower area to the Gusev crater (see illustration on previous page). H. Irwin III, G. Franz of the National Air and Space Museum (NASM) and others have published evidence that the low area was once filled with water in several lakes whose extent, if on Earth, would extend across Texas and New Mexico. Their starting point is the low dark areas present in the MGS MOC image of this region. Using MOLA data to determine elevations, they have reconstructed these lakes (not now existing) by coloring the topographic lows blue against a color image of the region:
In their model, at some stage in Mars' past, the water from the central lake burst from its confines and rushed through the already forming valley, thus deepening it rapidly (analogous to the Scablands of the State of Washington; see page 17-4).
A new line of evidence for water and/or ice on Mars has come from MGS images of what is considered to be analogs to phreatomagmatic "rootless" volcanoes on Earth (especially in Iceland). Consider this image (MSSS) of a swarm of cones in the Cerberus Plains near the martian equator:
The origin of these miniature cones (typically, about 50 meters at their base) requires water to have been released at earlier times that becomes trapped near the surface (perhaps at depths of 3-5 meters). At the cold martian temperatures, this water is preserved as ice. Then, later eruption(s) of volcanic flows heat the water to steam, causing a pressure buildup that ejects material locally to build up the cones. The implication of the presence of these cones on what may be a young surface is that ice entrapment may occur within suitable martian terrains and could thus be available as a source of water during human exploration of this planet.
Some Mars scientists are arguing for the presence of water as ice and/or "snow" deposits that form both varying and persistent deposits in areas of lower latitudes. This is related to the migration of water vapor and CO2 during the changing martian seasons. Perhaps the most provocative indicator of subsurface water that outflows in non-polar regions of Mars is shown in this next illustration:
This feature has been given two interpretations: 1) it is an ice glacier (note the parallel ridges); or 2) it is a rock avalanche. Its geomorphic expression as seen from above seems better fitted to the first explanation.
The Laser Altimeter (MOLA) on the Mars Global Surveyor yields maps and profiles that show martian relief. The southern hemisphere is, on average, 10 km (6 miles) higher than the younger northern hemisphere. This is brought home by this block diagram that shows variations in topography alone the 0° latitude running from the North Pole (left) to the South Pole:
The maximum relief on Mars ranges to 30 km (19 miles), determined by Valles Marineris as the high point and the Hellas Basin the low point, about 1.5 times greater than that on Earth. The three white round areas are the Tharsis volcanoes and the offset white one is to the left. The brown rounded area above these is Alba Patera. Valles Marineris is obvious. The blue-green circular feature below is Argyre Planitia. The large elliptical blue region in the red southern hemisphere is a huge topographic depression about 2100 kilometers (1300 miles) wide known as the Hellas Basin.
One of the most dramatic of these MOLA data plots presents an exaggerated profile across Olympus Mons and Arsia Mons/Alba Patera (which is actually higher than Olympus).
Still another example of a MOLA-based topographic map is that centered on the Herschel crater east of Hesperia Planum:
The topographic map of Mars, combined with gravity and other data, allow calculation of the general thickness of the martian crust, as shown in this global map:
Thicknesses of the crust at major locations are estimated as shown in this Table:
Note the fairly strong correlation between topography and thickness, emphasized here by using the same scheme of reds/orange for higher elevation/greater thickness and blues for lowest elevations/minimal thickness in the two relevant maps.
Another exciting and informative scene taken by the Global Surveyor combines a color view of the North Pole Ice Cap with topographic data obtained from a series of Laser Altimeter passes to produce this three-dimensional view:
The size and amount of ice (which the spectrometer confirmed was largely water ice) was less than anticipated from pre-Surveyor observations. The cap is roughly 1,200 km (746 mi) across (crudely circular) and as much as 3 km (1.9 mi) thick. With an average thickness of 1 km (0.62 miles), the volume of ice is about 1.2 million cubic kilometers (288,000 cubic miles), roughly half that of the Greenland Ice Cap on Earth. The cap's general surface is quite smooth, but the ice cap is cut by large, deep (up to 1 km; 3,280 ft) steep-walled canyons and troughs, which may have come from cracks enlarged by wind and possibly meltwater. The amount of water located today at the North Pole, and lesser quantities now at the South Pole, does not seem sufficient to have once made an ocean over parts of Mars. It instead may be the remnants of any seawater that could have existed in the Mars past. Any relationship of this surviving water to the river-like channels on Mars is still speculative. Water may be widespread even now but would be subsurface. Detecting that water is one of the objectives of future Mars missions.
MGS's Thermal Emission Spectrometer (TES) can produce images similar to those made using visible light (as an example, check the image of Ma'adim Vallis on the previous page). Other THEMIS (longer) IR wavelengths form images which clearly differentiate hotter (whitish) from cooler (dark) surface materials. This image shows a thermal pattern in which the blackish part indicates a smooth, dust-covered surface and the light blotches represent blocks and debris in some cases associated with craters:
When TES surveys the same scene during martian daylight and again at martian nighttime, the differences in tone reveal information about the thermal inertia of the materials involved. Here are two views of a martian crater.
The ejecta blanket holdss more heat during the day and loses more during the night, owing to its porous nature. The surrounding rock/soil remains warmer during the night.
TES can identify some individual minerals and determine in a semi-quantitative way their proportions in the surface rocks (believed to be primarily basalt and andesites). The mineral Pyroxene (an iron-magnesium silicate) occurs in rocks found in Syrtis Major. Note its variable distribution, with the largest amounts present in the darker areas of this region.
MGS sheds new light on the famous "face" on Mars first noted in a Viking image and seized upon by zealots favoring a lost civilization on the Red Planet as an artifact carved (like the Sphinx) into bedrock. The left view is the Viking image; the center (MGS) view shows the feature to have several mini-mountain peaks partly responsible for the fortuitous shadowing that highlights the face; the right view is a negative of the center view that recreates the shadowing.
A more recent image of the "face", taken by Mars Odyssey (see page 19-13a) reveals an even more uncanny likeness to a face:
In this image, the rock promontory that looks like a nose even has a "nostril". Surrounding the "face" is a blanket of material that resembles hair. Judge for yourself what this feature might really be.
Perhaps another view will help in your surmise. This is a perspective image made from Mars Express data:
This view indicates the face to be an eroded mountain. The term "massif" applies to this and similar features. When seen as a vertical image looking straight down, the height of such a massif is not particularly impressive. But when seen in perspective, this topographic type comes across as a dominating landmark. Consider these two Mars Express images of Ausonia Massif; the first looking straight down; the second as a perspective view:
The MGS on Odyssey had an instrument capable of detecting weak magnetic fields. At the present time no active magnetic field exists. But earlier spacecraft suggested that one once existed. MGS provided this data set shown as three maps of the field strengths in different directions:
It is still somewhat uncertain whether this is an active magnetic field (implying some surviving liquidity of the core, within which magnetic fields are generated) or is a fossil (remanent) field. Opinion currently favors the latter; there are strong negative fields associated with the Argyre and Hellas Basins, which suggests an already frozen field lost strength from the impacts that caused these two structures. If true, then the protective cover afforded by a magnetic field that captures solar wind and other radiation did not last long. But prior to that an active field would help to maintain a thicker atmosphere which in turn might have supported water vapor that could produce rain, streams, and primitive life.
An April 1999 report by Mars Global Surveyor investigators has created a flurry of excitement about an aspect of the martian crust which may be a counterpart to the mobile segments of terrestrial crust that are involved in the concept of Plate Tectonics - a cornerstone in our understanding of the operation of the Earth's outer layers embodied in the idea of "continental drift". The Mars Global Surveyor magnetometer has picked out a series of magnetic stripes on Mars that systematically reverse their polarities. Thus:
As more data accumulated, a nearly complete map of magnetic strengths has been made. Again, the stripes are the most obvious feature. One interpretation is that they are related to a time in the past when Mars had some mode of plate tectonics, with crust forming, rising, and subducting.
These martian strips are found mainly in the older terrains of the Southern Hemisphere. They trend at high angles to the polar axis. The implication is that this crust was formed by outpourings of lava from analogs to the terrestrial oceanic ridges that push the plates towards subduction zones (no such features have yet been recognized on Mars nor are there changes of mountains that result above these zones). This also suggests a much stronger magnetic field during early martian times; this would indicate a (partially) molten core. The full meaning of this new discovery is yet to be assessed.
Martian exploration reached a new plateau when landers allowed on site close-up inspection of martian terrain and rocks. Launched on December 4, 1996, JPL's Pathfinder landed on July 4, 1997, in the Ares Valle near the earlier Viking 1 site . This shorter transit time (seven months) resulted from a better alignment of Mars and Earth in their orbits. Pathfinder used a cluster of small inflated bags which, together with a parachute, cushioned the descent and deployment onto the martian surface. Here is a mock-up version of the equipment as exhibited at the National Air and Space Museum. Note the small Sojourner rover.