 |
Study
of Mars
Landing Sites |
 |
| |
Study
of Mars
Landing Sites |
|
We find evidence that this may be an ancient lakebed floor, covered by evaporite deposits, which was covered over and preserved from a period 4 to 4.5 Ga ago, but which was exhumed and exposed only in the last few My. By this mechanism, the original deposits are uniquely well preserved. Our evidence includes saturation densities of extremely old, degraded craters (from the original lakebed?) and a second population of small, sharp craters representing no more than a few My of accumulation (formed on the lakebed after it was exposed?).
The is the first area of probable evaporite mineral deposits found on Mars by the Mars Global Surveyor (MGS) Thermal Emission Spectometer (TES), and was selected at the Houston October 1999 landing site meeting to be on the shortlist of two candidate sites for the 2001 lander. Edgett and Parker, 1997 ("Water on early Mars: Possible subaqueous sedimentary deposits covering ancient cratered terrain in western Arabia and Sinus Meridiani, GRL 24, 2897-2900, Nov 1997) have mapped a smooth plains unit which turns out to coincide almost exactly with the hematite mineral signature found by TES. Several channels drain into this area. The area appears to be an ancient basin. Our working hypothesis is that it is an ancient lakebed, and that the hematite deposit involves a layer of evaporite minerals left on the ancient lake floor.
We have studied high-resolution MGS images from the Mars Orbital Camera (MOC), from the point of view of surface morphology and crater populations. As shown in the two MOC frames in FIGURE 1 (MOC image m4-0568) and FIGURE 2 (portion of MOC image m0-1661) much of the area is covered by a formation, unique in our experience, where extremely shallow circular features, usually defined by patchy rim deposits of different albedo than the background. These appear to be ancient degraded or silted-in craters; as we will show, they fall near the crater density defined as the saturation equilibrium level by Hartmann (1984, Icarus) and Hartmann and Gaskell (1997, MAPS). Some of these shallow craters have dune deposits in their floors. A second, distinct population of smaller, sharp-rimmed, bowl-shaped craters is present, which appear to be undegraded.
Figure 1
Figure 2
FIGURE 3 (portion of MOC image m7-1457) shows another type of surface found in the basin. This surface shows small-scale ridged units that resemble dunes. Ken Edgett (Houston meeting) interprets these not as loose sand but as an indurated unit produced by erosion. Whatever the source of this unit, it has few craters, and appears to cover the older unit saturated with soft, shallow craters. We suggest that it is the remnant of the unit which is currently deflating, and exhuming the old lakebed surface.
Figure 4
FIGURE 4 shows a summary of crater counts by Daniel Berman and William Hartmann from the MOC frames and a Viking mosaic of this area. Our counts and interpretation is continuing, but we post this preliminary work because of its current interest. The data on various MOC frames repeatably shows the old, soft craters lying near the saturation curve (solid, straight, upper line). The population of young sharp craters falls near the isochron defined for 1 My (see Hartmann, 1999, MAPS paper on cratering and other pages of our MGS web site).
The heavy solid bent line in FIGURE 4 is the upper limit defined for most other regions of Mars, and we believe it to be a steady state curve for the Mars surface, representing a balance between crater creation and obliteration by infill (Hartmann, 1999, MAPS). It is important to note that the hematite area's old crater population is denser than this, following the saturation line. This at once implies that it is older than the other surfaces, and indeed must date back to the period of intense cratering around 4 to 4.5 Ga ago.
The young craters follow an isochron for age about 0.7 to 2 Ma. We interpret this as the date of exhumation of the surface. Indeed, the exhumation is not yet complete, as shown by the dune like units still covering part of the area.
Our interpretation is that an ancient, cratered basin filled with water (or the craters may have formed under shallow water). Evaporation led to the hematite deposit, but the area was quickly covered by silt or other sediments. For roughly a million years ago, the area has been deflating.
Indeed, it is just this unusual recent exposure that explains why TES has been unable to locate very many evaporite deposits. Impact gardening destroys the pristine deposits on Noachian age surfaces that have been constantly exposed. To find evaporites, one must find the rare areas that where ancient lakebeds have been recently exhumed.
As a landing site, this site seems to offer a chance to study preserved, ancient lakebed deposits. FIGURE 5 shows a map of the general area, including locations of some of the MOC frames.
Figure 5