Arguably, the most important results of the present crop of missions to Mars have been the discovery that in the past, the Red Planet was a wet planet. The discovery has taken the form of certain water shaped or water formed minerals, and once we knew what we were looking for, maps of the extent of the Martian ocean could be drawn up. They showed extensive regions of former wetlands in the southern part of the planet, with outcrops of water borne material popping out of the red dust. But nothing had been spotted in the lava strewn north.
However, help was on hand. Underground water ice was discovered on Mars in areas where meteorites punched through the topsoil. In the past, the planet has been bombarded with a whole zoo of rocks from outer space, leaving holes peppered throughout both hemispheres. In the north, these have broken through the lava layer, revealing past sediments, much like cores taken from arctic ice on Earth, or layers studied here by geologists.
The ESA probe Mars Express found the first hints of something when its OMEGA camera zoomed in on impact craters. But the hints were a little too subtle for the tool, so the CRISM camera on NASA’s Mars Reconnaissance Orbiter was called into action at the 91 identified sites. It was able, through archived data, to confirm the presence of minerals related to water. Further investigations showed that the water may have existed for 10-100 million years, a brief flash of geological time, and the lack of any indication of water modification of the lava layers above suggests that for the four billion or so years later, the north was dry.
On the other hand, a wet early Mars would’ve been a good place to start up a microbe or two, and we do need sites to send rovers over…