4.6 Landing robotic geologists on Mars
In 2003, two robotic geologists were launched towards Mars. Their names: Spirit and Opportunity. The Spirit rover landed on 4 January 2004 in Gusev crater, a site chosen because it may have been a drainage basin or ‘catchment area’. The Opportunity rover landed two weeks later in Meridiani Planum, a large area where clay minerals had been detected from orbit. Carrying the same instruments, the two rovers were tasked with finding out if Mars had ever been habitable, i.e., hunting for evidence of water within the rocks.
The evidence of water that the rovers returned was plentiful but it is impossible to cover everything here. Instead, we can look at the important highlights, starting with…blueberries!
Features called ‘blueberries’ were found by the Opportunity rover. Figure 33 shows that these are not fruit but are, in fact, small, cm-sized, almost perfectly spherical features made mostly of the iron oxide mineral haematite. As you learned earlier, iron oxides are an excellent indicator that water has once been present.
A second highlight came from the discovery of bright white material (Figure 34) at a site named ‘Gertrude Weise’. This material was only uncovered because Spirit’s wheel malfunctioned and the rover dragged it along, creating a trench in the soil. This gave an opportunity for the rover’s instruments to analyse this uncovered material, and it was identified as almost pure silica (SiO2), another product of the interaction of water with rocks.
A third highlight was the discovery of calcium sulfate dispersed in the soils and in thin veins in rocks (Figure 35). Calcium sulfate can form three different minerals: anhydrite, bassanite and gypsum. These differ from one another depending on the amount of water they contain: anhydrite has no water (as represented by its formula CaSO4), bassanite (CaSO4.0.5H2O), and gypsum (CaSO4.2H2O) have some water, in differing proportions.
The Opportunity rover still holds the record for the longest distance travelled on Mars, taking over 11 years to reach Endeavour crater (Figure 35) where clay minerals had been found by orbiting spacecraft. Its reward for driving over 45 km (more than marathon distance) was more exciting findings.
It found clay minerals in a small incision (called ‘Marathon Valley’) in the crater rim, the chemistry of which suggested they had to have formed in the presence of hot water. As mentioned earlier, impact events might heat groundwater, producing hydrothermal systems. The hot water would alter the minerals to secondary minerals, which are then detected by spacecraft. The chemistry of the clay minerals detected (specifically the presence of nickel) suggested that the temperatures had once been very hot, probably several hundred degrees Celsius.
It is important to remember that, although the evidence from Spirit and Opportunity, coupled with the evidence from earlier missions, was categorical about water once being present. However, in the time these spacecraft were operating at Mars, others were being launched with the same broad goals: to find evidence of water. And so the story continues…