A linear correlation analysis between the Mars Odyssey neutron fluxes and various surface parameters indicates that the annual maximum surface temperature is the most important factor controlling the soil water content in the surficial (upper few tens of centrimetres) layers of the Martian soil. This is likely to be associated with the higher enthalpy of hydration of minerals in comparison with the enthalpy of sublimation of ice, which is presumably almost absent in the surficial layer. While presently the maximum surface temperature occurs near 30° S because of perihelion in late southern spring, the season of perihelion periodically migrates by virtue of precession. Consequently, the maximum surface temperature as well as the driest place on Mars should move from one hemisphere to the other with a period of about 51 000 yr. A significant amount of surficial (adsorbed) water would then be exchanged between the hemispheres and between the soil and other reservoirs, especially the polar caps and the polar layered deposits, and is probably borne out by the stratigraphic structure of these deposits. It is suggested that the water migration driven by the orbital eccentricity and precession may be as important as the obliquity-driven exchange of water, particularly very close to the surface, where ground ice is unstable.
The zones of liquid water stability oscillate somewhat in a north–south direction in the course of the precession cycle, but are most prevalent in parts of the low and mid northern latitudes as well as in the Hellas Basin. The thermal stability of liquid water tends to be high where the near-surface soil water content is low, indicating that the periodic melting of ground ice by solar heating is not a likely source of liquid water on the surface, but some episodic processes should provide water, if any.
An enhanced soil water content near the surface is always accompanied with a reduced peak ultraviolet flux, both reducing the chemical reactivity of the soil. In the present epoch the northern hemisphere may represent astrobiologically more clement environmental conditions.