The Mars rovers Spirit and Opportunity are acting as ground-truth operators, remotely manned by the researchers to explore the Martian terrain. A lengthy process for determining the rover landing sites, conducted by the Mars landing site steering committee co-chaired by Dr. Grant, culminated in the selection of Gusev Crater and Meridiani Terra. Speculation about the potential for ancient water processes at these two locations based on the analysis of remotely sensed imagery drove the selection of these two locations.
Since landing in January 2004 the Spirit rover has been trekking across the geomorphically diverse floor of Gusev crater. Exploration of the Gusev Plains reveals a flat terrain covered in basaltic rubble and pock-marked with secondary craters ranging in diameter size from less than 1 to 200 m1.
Though small, shallow craters called "hollows" out number the larger craters, such as Bonneville and Missoula craters, they all share the morphological features of smooth, low-sloping walls, variably infilled interiors, and raised rims2. This impact modified terrain experiences low rates of wind erosion and no evidence for erosion or deposition by water has been found.
Husband Hill, SE of the Gusev plains, represents an older geological feature within the crater compared to the surrounding Hesperian-aged plains. Husband Hill is characterized by exposed bedrock and an absence of thick regolith3, opposite of the Gusev plains. Additionally, the hill experiences higher rates of eolian erosion on the order of meters to tens of meters compared the tens of centimeters typifying much of the plains3. Some limited alteration of a few local rock outcroppings is observed, but there is no evidence for surface water in eroding the current landscape.
Home plate, a plateau 2-3 m high located within the Inner basin of Columbia hills and SE of Husband Hill, reveals a terrain more modified by explosive volcanism than impacts4. Rock outcrops differ slightly from the basaltic rocks strewn across the previous Gusev locations due to their higher amounts of trace elements (Cl, Br, Zn, and Ge)4.
The lower coarse grained unit, showing granular textures toward the bottom of the image and massive textures with rectilinear fracturing toward the top. Also shown is a feature interpreted as a possible bomb sag (arrow). The bomb is about 4 cm across. False color image obtained using Pancam's L2, L5, and L7 filters on sol 751.
Bomb sags typically form when materials ejected from explosive volcanoes, called bombs, land on nearby beds of tuff or ash and create noticeable sags in the layering. Bomb sags are found on Earth in pyroclastic hydrothermal environments and the features found along Home Plate may be analogous and may have been formed in materials that were wet at time of emplacement. However, formation in the dry sediments due to gas compaction4 cannot be ruled out.
Opportunity's expedition around Meridiani Terra provides evidence of an ancient landscape shaped by wind and occasional water. Today, however, the terrain is primarily modified by impacts and experiences moderate rates of wind erosion. At Meridiani the surface is younger than at Gusev and is Amazonian in age. Moreover, the bedrock is more sulfate-rich making it more susceptible to weathering than the rocks at Gusev. Although dry today and for much of Mars history, Meridiani's sulfates require and ancient landscape shaped by the wind and at least occasional wet periods that likely included contributions from near surface ground water and shallow temporary pools of water at the surface.
1. Grant, J.A., R. Arvidson, J.F. Bell III, N.A. Cabrol, M.H. Carr, P. Christensen, L. Crumpler, D.J. Des Marais, B.L. Ehlmann, J. Farmer, M. Golombek, F.D. Grant, R. Greeley, K. Herkenhoff, R. Li, H.Y. McSween, D.W. Ming, J. Moersch, J.W. Rice Jr., S. Ruff, L. Richter, S. Squyres, R. Sullivan, C. Weitz, 2004, Surficial deposits at Gusev Crater along Spirit rover traverses, Science, 305, 807-810.
2. Grant, J.A., R. Arvidson, L.S. Crumpler, M.P. Golombek, B. Hahn, A.F.C. Haldemann, R. Li, L.A. Soderblom, S.W., Squyres, S.P. Wright, and W.A. Watters, 2006, Crater gradation in Gusev crater and Meridiani Planum, Mars, J. Geophys. Res., 111, doi:10.1029/2005JE002465.
3. Grant, J. A., S. A. Wilson, S. W. Ruff, M. P. Golombek, and D. L. Koestler (2006), Distribution of rocks on the Gusev Plains and on Husband Hill, Mars, Geophys. Res. Lett., 33, L16202, doi:10.1029/2006GL026964.
4. Squyres, S.W., O. Aharonson, B.C. Clark, B. Cohen, L. Crumpler, P.A. de Souza, W.H. Farrand, R. Gellert, J. Grant, J.P. Grotzinger, A. Haldemann, J.R. Johnson, G. Klingelhöfer, K. Lewis, R. Li, T. McCoy, A.S. McEwen, H.Y. McSween, D.W. Ming, J. Moore, R.V. Morris, T.J. Parker, J. Rice, S. Ruff, M. Schmidt, C. Schröder, L.A. Soderblom, A. Yen (2007), Pyroclastic activity at Home Plate in Gusev Crater, Mars: Science, 316, 738-742.