Ice in the underground
A striking network of small, ramified valleys in the right half of the picture (color image) testifies to water that has flowed from the edge of Argyres over the surface into the interior of the basin. It came either from rainfalls in the early days of Mars or from melted glacier ice. These drainage networks still document the water-rich past of Mars today.
For a long time, there has been intense debate about whether there was actually a significantly warmer and humid climate in the early Martian period that enabled a water cycle, even with precipitation and a network of water bodies on the surface, over a longer period of time - or only episodically. It is also conceivable that the thin atmosphere with hardly any greenhouse effect allowed only low temperatures, which rather led to ice-age processes with at best the precipitation of snow and associated glacial phenomena. Volcanic activity or regional asteroid impacts could then have sporadically caused dew processes that melted the ice on the surface and in the subsurface and mobilized flowing water in this way.
The latest findings prove that the numerous dried out river valleys on Mars were formed by at least four different drainage processes: Surface runoff due to precipitation, melting glaciers, subglacial runoff below glacial packs and outflowing groundwater (which in turn may have been formed by melting ground ice). The Nereidum Montes is a "key area" where these hypotheses can be well tested. Due to its morphology, the most likely origin of glacial ice is the melting of glacial ice.
Most impact craters in this area are filled with a material that shows a striking concentric pattern on the surface. Such structures indicate glaciers covered by rock debris, so-called block glaciers. In the valleys between the mountain ranges, these forms can also be recognized as extensive deposits. It is assumed that these landscape forms still hide water ice at greater depths under a layer of debris that prevents sublimation, i.e. the evaporation of the ice at the low gas pressure of the Martian atmosphere.