According to THEMIS Gusev seems to be in an area where there is a higher amount of hydrogen, presumably ice in the ground. The surface of Gusev though seems dry with various kinds of salts and olivine which normally occurs away from water.

So the question is how did the ice get under the ground there without destroying the olivine? There only seem to be a few possibilities. One is the ice was a water table that never reached the surface. The second is there was water there a long time ago and the icy layer is totally buried by aeolian and other deposits that were always dry. The third is there may not have been water on the surface but ice.

IIRC ice on the surface if it doesn't melt doesn't destroy olivine. What if then there was water here at one stage and this became salty for example from leaching out of basalt and other rocks. Then the upper ice, perhaps less salty because of snow, etc started to freeze at a lower temperature than the salty brine below.

Then for a long time there was ice here and large deposits of material accumulated above the ice by aeolian deposition, material that had never been wet so it contained olivine. The lower brine would have frozen and precipitated out some salts and other salts would have frozen in the ice.

This would have been when the atmosphere was thicker. When the atmosphere lost enough pressure (perhaps as the planet cooled and the CO2 started to freeze at the poles) the ice started sublimating leaving some ice in the deep soil. This ice then would have come from the brine seeping into the ground.

This allowed the upper layers containing the olivine to land on the previous salty areas. Some ice remained in the soil where it couldn't be exposed enough to the atmosphere to sublimate. Other ice turned to gas and the two layers, one of salts from the brine and one from the aeolian sediments containing olivine mixed in together.

So the soil came to be a mixture of salts from the previous brine, olivine from the layers above the ice, and with some ice still under the ground.

Any merit in this?