One of the themes that constantly crops up in papers on Mars is water. Did it exist? If it did, is it responsible or is something else responsible for the gullies, deltas, valleys, and other features that look like (and are thus named like) formations here on Earth.
One of the more often argued over structures is Valles Marineris. Watch this cool fly over!
It looks like it is analogous to the Grand Canyon and was formed through flowing water (of some salinity) flowing/flooding across the surface. At the same time, scientists have argued that perhaps this is a wind or maybe even lava feature (although this later is far more rarely argued). The way to settle this argument is to look for a specific type of rocks (hydrous minerals) in the valley that could only be formed in liquid (in aqueous conditions). If this is a water feature, those minerals should exist in the valley and other “water features” in higher concentrations then we see them else where.
The two Mars rovers have been able to survey the minerals in their two landing sites, and Gustav crater is definitely someplace that once had liquid. Landing a rover in Valles Marineris is, um, a bit more challenging and won’t happen anytime soon. To get the data we need, geophysicists instead turn to spectroscopy. It is possible to tell from what specific colors of light Mars surface absorbs and reflects what minerals are present.
Using a comparison of data between MOLA (which gives highly accurate surface topology) and OMEGA (which gives really nice spectra information), data presented by J.-P. Bibring shows that Sulfates and hydrated phyllosilicates are found in Valles Marineris and other “water features” including Juventae Chasma.
At the same time, using this technique they found that Mawrth Vallis, a “river” bed with estuary are also filled with high-calcium pyroxene (a rock type not formed with water), indicating that this is a lava cut. On the banks of this “river” bed, there are hydrated minerals! This says the lava flowed in over an area that had once had water.
The general picture, traced out in gorgous images I will try and embed later, described water flowing in the earliest days of the Northern Highlands, with volcanism coming later and emplacing ontop of the hydrated minerals. In many cases these deposits will make it next to impossible to look for fossil life on “river” beds, but the banks, where hydrated materials are on the surface, just may give us some hope. Mawrth Vallis is a possible landing site for either MSL or ExoMars.