The impacts of bits of asteroids and comets with Mars have revealed it. The channels which were cut when water trickled out the resulting craters are clearly visible in the current Mars imagery.

Now not only do these impacts occur all over the surface of Mars, revealing the water content in the different formations in different areas, but also due to Mars relative lack of atmosphere, even small impactors survive to stirke, thus revealing the water content in each region at the different depths.

Since this is so, the problem then becomes calibrating the results of the gamma ray and neutron sensors to reflect the actual condition on Mars.

In closing this note, given this natural exccavation, it is a wonder to hear the watery-Mars "scientists" seeking funding for the development of burrowing probes... perhaps the key word here is "funding". Roving robots capable of navigating up channels to examine their "stratigraphy" would be a far better use of funds.

ep