Reports abound celebrating the successful touchdown of the Phoenix lander on the northern lowlands of Mars. This is a region where water (locked away in the form of permafrost) is thought to exist quite close to the surface, and Phoenix has a nifty robot arm which can (hopefully) dig down to it and retrieve samples for on-board analysis. Obviously we’re all interested to see what it finds with regards to the possibility of ancient or modern Martian life; but this is only part of a more comprehensive analysis of the subsurface around the landing site, which should help to answer questions about the geological development of Mars. From the University of Arizona’s Phoenix page:

Goal 3: Characterize the geology of Mars

As on Earth, the past history of water is written below the surface because liquid water changes the soil chemistry and mineralogy in definite ways. Phoenix will use a suite of chemistry experiments to thoroughly analyze the soil’s chemistry and mineralogy. Some scientists speculate the landing site for Phoenix may have been a deep ocean in the planet’s distant past leaving evidence of sedimentation. If fine sediments of mud and silt are found at the site, it may support the hypothesis of an ancient ocean. Alternatively, coarse sediments of sand might indicate past flowing water, especially if these grains are rounded and well sorted. Using the first true microscope on Mars, Phoenix will examine the structure of these grains to better answer these questions about water’s influence on the geology of Mars.

Just as the Earth’s surface can be divided into high continents and sunken ocean basins, Mars is also divided – the southern Martian hemisphere has quite high topography and is mostly heavily cratered, suggesting that it is quite old, where as the northern hemisphere is low, flat, and much less heavily cratered, suggesting a more recent resurfacing. It appears that early on in Martian history a deep hole formed in the northern latitudes of Mars – explanations range from a massive asteroid impact crater to a basin formed by tectonic rifting – and it has since been partially filled in by later sediments. The question is, were these sediments blown in by aeolian processes on a dry Mars? Or could they have settled out from lakes and oceans on a wet Mars? Knowing the answer to this question will help us understand exactly what Mars was like earlier in its geological history, and hopefully Phoenix will help us to answer it.