One of the key observations that determines which volatiles we will find in the regolith of Mars and what state we will find them in is the amount of heat flowing out from the warm interior. Like all Planets, internal radiogenic heat production leaks out by convection and conduction, giving rise to a thermal gradient with depth. If that thermal gradient is high, then liquids will be found at shallow depths, but if it is low, then they will be frozen until much deeper.

Evidence on heat flow is hard to find. On Earth we measure the temperature in deep mines or boreholes or insert sensitive probes into the seabed to measure the temperature at depth. We have not yet been able to do this for Mars but we do have some indirect measurements. For a start, we can gues, based on the volume to surface area of Mars compared to earth that it has cooled about twice as fast, and therefore has about half the heat flow and temperature gradient, but we can improve on this guess:-

1) MOLA lithosphere thickness

One outcome of the MGS mission has been an analysis of the MOLA topography and the gravity field of Mars, as measured from spacecraft orbit perturbations. This has been used by the MOLA team to calculate how rigid the lithosphere is on Mars, or more precisely how rigid it was when various topographic loads were imposed on it.  A thick rigid lithosphere can support a large load without sagging while a thin lithosphere will buckle at short wavelength. By comparing the ratio of topography to gravity at various wavelengths, lithospheric thickness can be measured. The Zuber et al. Science paper includes a neat figure(fig 4)  showing the evolution of lithospheric thickness for different aged constructs. This is reproduced here:

Older terrains are near the base of the figure, and younger terrains near the top. Note the steady growth with time of the apparent lithospheric thickness from ~20 km near the end of the Noachian some 3.8 billion years ago to 250 km or more at the time of Mons Olympus construction, about 0.5 to 1 billion years ago. Extrapolating that to the present day, we might expect a lithospheric thickness of ~300 km.

Now on Earth, the continental lithosphere is about 120 km thick and since the base of the lithosphere is where the rocks get warm enough to soften and begin to move under slow plastic creep. This is essentially a fixed temperature of ~ 1300 K (1000 degrees C). From this, we can calculate the average temperature gradient of the planet. For Earth, we have ~300K at surface, 1300K at 120 km, so the thermal gradient is around 8 K/km. For Mars, we have nearer to 200K at surface, and 300 km lithosphere which gives less than 4 K/km.
 
 

2) MOLA surface data

MOLA images of "young" terrains on Mars such as the Northern plains, Hellas basin, volcanic constructs etc. would expect to be fairly smooth. However, MOLA data reveals a pattern of "wrinkles" everywhere on these terrains:

Isidis basin - beautiful and striking images of ridge textures - note the "rosette" marking the buried central rings of Isidis Basin.
Frame width 1,700 km at mid height (cylindrical projection) MOLA data gridded by Adrian Lark, displayed by Nick Hoffman

Frame width 1,700 km at mid height (cylindrical projection) MOLA data gridded by Adrian Lark, displayed by Nick Hoffman

This is the region around Dao Valles which shows clearly the imprint of wrinkles across the formerly smooth floor of Hellas basin and on "young" volcanic constructs at upper right. Within Hellas Basin the wrinkles pick out a buried pattern of overlapping impact craters - the old floor of Hellas that was buried by the younger sediment fill. On the volcanoes, a pattern of radial and circumferential wrinkles results.

The heavily cratered terrains do not at first sight appeasr to show similar textures, but careful inspection will begin to reveal a pattern of ridges and lines cutting through and between the craters - evidence that similar wrinkles exist here but are camouflaged by the large impact craters.

What are these ridges? The simplest explanation is that they are contractional thrust faults. The spacing of the wrinkles is ~50 to 100 km, and each wrinkle is a few hundred metres in height. This implies that in the time since the smooth plains deposits were formed, Mars surface has contracted by 1% or so. This can be interpreted as cooling of the lithosphere, as it thickens, in agreement with the above MOLA/gravity calculation.

Combined with the evidence that the volcanic activity on Mars has been progressively dropping off, we find a concensus of evidence that Mars today is cooling down, and has been doing so for at least 1 billion years.

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au