According to our best models, Mars' axial tilt varies from its current 25 degrees to as little as 15 degrees and as much as 80 degrees. With an 80-degree tilt, it seems pretty obvious that the rotational poles will lose *all* of their residual ice caps and that polar conditions could exist almost all the way down to the equator.

So, for a significant fraction of the geologic history of Mars, areas that are not now subject to a polar climate were covered with dense caps of water and dry ice.

So... since we know (if our models are correct) that Mars *must* have huge areas that were once polar, what kinds of terrain alteration ought we be looking for as remnant indicators? If current polar geology is any guide, I'd think you would see surfaces covered over with the layered deposits that form as dust forms films between successive seasonal ice buildups.

What areas of Mars seem to be covered with such deposits? Do we have the kind of imaging we need to detect this kind of thing?

I will point out that Meridiani sure looks like it could be a candidate area...

-the other Doug

The polygonal terrain should set up rock lanes and brain coral patterns that should remain visible as sediments get exhumed.

While the obliquity cycle will affect the ice caps I didn't think that any of the modelling indicated ice forming in the equatorial regions? The Niles and Michalski hypothesis to explain the berries required a complete shift in the axis of rotation but there does not seem to be any irrefutable evidence that this occurred. It would be a more comfortable hypothesis if there were another Meridiani like area at the antipodes but there does not seem to be any similar region. If Meridiani had been a polar cap wouldn't we see the remnants of outflow channels created as the cap melted, similar to those in the basal layer of the current north cap?

Are we even sure that the possible buried ice deposits found by MARSIS are indeed ice and not just low density deposits? Even if ice they could be remnants of a wetter past.

The problem is that even if there were low-latitude icecaps in the past, they would almost certainly have been "cold-based", i. e. frozen to the ground. Such icecaps leave very little trace of their existence, certainly none of the "classical" signs like striae, drumlins, eskers etc. There might be a some glacial erratics, left behind when the icecaps melted.

The best chance of finding traces would probably be in volcanic areas. Subglacial volcanoes have a number of peculiarities and geothermic energy in such areas might even raise subglacial temperatures to the pressure melting point in which case "classic" glacial landforms might occur.

Some of these might be relevant:

3461 - Dickson, J. L., J. W. Head, and D. R. Marchant (2008), Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases, Geology, 36, 411-414, doi: 10.1130/G24382A.1.

3459 - Fastook, J. L., J. W. Head, D. R. Marchant, and F. Forget (2008), Tropical mountain glaciers on Mars: Altitude-dependence of ice accumulation, accumulation conditions, formation times, glacier dynamics, and implications for planetary spin-axis/orbital history, Icarus, 198, 305-317, doi:10.1016/j.icarus.2008.08.008.

3411 - Levy, J. S., J. W. Head, and D. R. Marchant (2007), Lineated valley fill and lobate debris apron stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacial modification of the dichotomy boundary, J. Geophys. Res., 112, E08004, doi: 10.1029/2006JE002852.

3292 - Morgan, G. A., J. W. Head, and D. R. Marchant (2009), Lineated valley fill (LVF) and lobate debris aprons (LDA) in the Deuteronilus Mensae northern dichotomy boundary region, Mars: Constraints on the extent, age and episodicity of Amazonian glacial events, Icarus, 202, 22-38, doi:10.1016/j.icarus.2009.02.017.

3281 - Shean, D. E., J. W. Head, J. L. Fastook, and D. R. Marchant (2007), Recent glaciation at high elevations on Arsia Mons, Mars: Implications for the formation and evolution of large tropical mountain glaciers, J. Geophys. Res., 112, E03004, doi: 10.1029/2006JE002761.

3250 - Forget, F., R. M. Haberle, F. Montmessin, B. Levrard, J. W. Head (2006), Formation of Glaciers on Mars by Atmospheric Precipitation at High Obliquity, SCIENCE, January 20, 2006 Vol. 311, www.sciencemag.org, pp. 368-371

3240 - Head, J. W., A. L. Nahm, D. R. Marchant, and G. Neukum (2006), Modification of the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation, Geophys. Res. Lett., 33, L08S03, doi: 10.1029/2005GL024360.

There are probably scads more on the Brown publications page. Try searching for "glaci" or "obliq".

Just wanted to clarify here: the Niles and Michalski hypothesis suggests that Meridiani's sediments (and sulfates) could be formed through massive ice deposits. In order to explain how there are massive ice deposits at the equator we suggest a couple of ideas: 1) obliquity variations and 2) polar wander. Also it does not explain the berries - it rather calls on the same diagenesis explanation forwarded by Squyres. It could also be consistent with impact reworking as described by Knauth and Burt.

I personally favor obliquity variations because I think this process occurred all across the equatorial regions -- Valles Marineris, Aram Chaos, etc. which explains why we seem the same gray hematite in all of those places. This is hard to explain with polar wander because you would need the pole to shift all across the planet before reaching it's final resting place.

However, in defense of polar wander we do see antipodal H-enrichments near Meridiani and Amazonis Planitia. There are also MASSIVE outflow channels near Amazonis Planitia which show up in the MOLA data and not so much in the visible imaging data. There are also finely layered sediments - the Medusa Fossae Formation.