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The search for water on Mars
The search for water on Mars

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4.4 The 1990s revival

1997 was a big year for Mars exploration. NASA’s orbiter, Mars Global Surveyor, reached the planet and the Pathfinder lander, with its shoebox-sized Sojourner rover, arrived on the martian surface.

Mars Global Surveyor was equipped with instruments including a high-resolution camera (that captured images of what appeared to be gullies formed by flowing water) and a laser altimeter (to measure the shape and elevation of the surface in detail) (Figure 21). This topographical information has revolutionised our understanding of Mars’ geomorphology, and provided essential information for engineers designing spacecraft for missions to Mars.

This figure is a map of Mars showing surface topography. Latitude and longitude are labelled, with the equator running across the centre of the image, horizontally. The map is colour coded. Blue and green areas are low elevation, red and brown areas are high elevation. The image can be divided into three areas. Across the top quarter (north) there is an area of low elevation, represented by blues and greens. Across the bottom quarter (south) there is an area of high elevation represented by reds and oranges. Between these areas there are regions of varying topography. Towards the left hand side of the image is a distinct red-brown region, which has some white patches. This represents very high elevations, in this case Olympus Mons volcanic region. To the right of the image is a distinct blue circular region, which represents a low lying impact crater.
Figure 21 Colourised Mars Orbiter Laser Altimeter (MOLA) topographic map of Mars.

Despite the successes of Mars Global Surveyor, in the search for water at that time, the Pathfinder mission took centre stage. This was mainly because it carried the first martian rover – the Sojourner rover (Figure 22). Sojourner did not directly contribute to the investigation of water on Mars but was an important demonstrator of instruments that would be used on Mars in the future. The Pathfinder lander, however, made a number of important discoveries.

This figure is a photograph taken by the Pathfinder langer of the Sojourner rover on the martian surface. The rover has four wheels with a flat platform on top. At the bottom of the image, a ramp and a deflated airbag are visible. The surface is mars is orange in colour, with fine soil and boulders and pebbles of various sizes.
Figure 22 Mosaic image of the Sojourner rover after leaving the landing platform. The ramp is visible on the bottom left of the image, a part of the deflated airbag on the bottom right. Image credit: NASA.

Pathfinder was fitted with an instrument with which to measure temperature variations at its landing site, Ares Vallis. Although it observed early morning water ice clouds in the lower atmosphere, even the warmest temperature it recorded was −8 °C, seemingly too cold for pure liquid water to exist on the surface. Two other clues, however, suggested this had not always been the case. Firstly, it detected that airborne dust was magnetic. This was speculated as being because it contained the mineral maghemite, a magnetic iron oxide. Importantly, maghemite forms from the weathering of iron in rocks.

Secondly, you will see when looking at Figure 23, taken at the landing site that Pathfinder also found evidence of flowing water in Mars’ past. This evidence included rounded rocks (labelled with red arrows) that were likely to have been shaped by the forces of water, possibly during a flood. The pale areas on Figure 23, indicated by the white arrows, are also believed to be deposits left behind by evaporating water, or aggregates of material fused together by the action of water. In contrast, the blue arrows indicate rocks with sharp edges that were probably ejected by nearby impact craters and/or ancient volcanoes.

This figure is a photograph taken by the Pathfinder mission showing the martian landscape. The surface is a red-brown colour and is strewn with boulders of varying shapes and sizes. Red arrows point to rounded rocks which were likely shaped by the forces of water. White arrows point to lighter colour areas where the surface dust is thinner. Blue arrows indicate rocks with sharp edges that were likely ejected by nearby impact craters and/or ancient volcanoes.
Figure 23 View to the northeast of the Pathfinder lander showing a variety of rocks recording past geological processes on Mars including water activity. Image credit: NASA.

Although the presence of rounded pebbles (which you can see more closely in Figure 24) is good evidence that there was once water, the pebbles were found buried in loose sand, making it hard to clearly discern their relationships to each other. It was therefore not possible to absolutely confirm a watery origin and the search for compelling evidence of liquid water continued!

This figure is a photograph taken by the Pathfinder lander of the martian surface. It is a greyscale image. It shows fine grained dust that contains small rounded pebbles and larger rounded boulders.
Figure 24 The area called ‘Cabbage Patch’ by the Pathfinder mission science team, which shows rounded pebbles that could be formed by the action of flowing water. Image credit: NASA.