During their missions, the Mars rovers Spirit and Opportunity acted 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 Planum. Speculation about the potential for ancient water processes at these two locations based on the analysis of remotely sensed orbital imagery drove the selection of these two locations.

During its mission, the Spirit rover trekked across the geomorphically diverse floor of Gusev crater (Fig. 1). 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" outnumber 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.

Exploration of Home plate, a plateau 2-3 m high located within the Inner basin of Columbia hills and SE of Husband Hill (Fig. 2), revealed 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.

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 some surfaces are Amazonian in age. Moreover, the uppermost 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 sulfate-rich deposits 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.

Much of Dr. Grant’s research of Meridiani Planum using data from the Opportunity rover focused on using eroding impact craters to understand the processes and amount of erosion that has shaped the surfaces over time. This work details the important role of wind processes in shaping much of the landscape^5,6, but study of the older, Noachian Endeavour crater (Fig. 3) late in the mission revealed an important role was also played by water in the early erosion of the landscape⁷. More details on crater erosion in Meridiani can be found by linking to the section related to degradation of Victoria crater.

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.
5). Grant, J. A., R. E. 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.
6). Grant, J. A., S. A. Wilson, B. A. Cohen, M. P. Golombek, P. E. Geissler, R. J. Sullivan, R. L. Kirk, and T. J. Parker (2008), Degradational modification of Victoria crater, Mars:  J. Geophys. Res., 113, E11010. https://doi.org/10.1029/2008JE003155.
7). Grant, J. A., T. J. Parker, L. S. Crumpler, S. A. Wilson, M. P. Golombek, D. W. Mittlefehldt (2015), The degradational history of Endeavour crater, Mars, Icarus, 280, 22-36. https://doi.org/10.1016/j.icarus.2015.08.019.