Evidence from the neutron detector on Mars Odyssey suggested the possible existence of subsurface water on Mars (Water on Mars, August 2002 Earth Pages News).  I reluctantly succumbed to all the hype about what is implied by that, the more so when reports came in of dendritic drainages revealed by high-resolution elevation data (Case for Martian rainfall strengthens in October 2003 issue of EPN).  In planetary exploration, including remote sensing of the Earth’s surface features, progressive improvement in resolution generally reveals novelty.  The Mars Orbiter Camera, deployed by the Mars Global Surveyor mission has a resolution from 15 down to 2 metres.  For the Earth, you can get 15 m images freely from the ASTER programme, but to match the 2 m images would be very costly.  Given a broadband or better connection you can download the lot for Mars (http://pds-imaging.jpl.nasa.gov/atlas/).  It is this resource that scientists from Brown and Boston Universities in the USA and the Kharkov National University of the Ukraine have used to reveal the latest paradigm buster from the Red Planet (Head, J.W. et al. 2003.  Recent ice ages on Mars.  Nature, v. 426, p. 797-802).

James Head and his colleagues focused on the smooth terrains, or mantles, which drape over older deposits above 30º latitude on both Martian hemispheres, especially where water had been indicated by the Mars Odyssey neutron detector.  They were looking for signs of what on Earth would be regarded as periglacial features, formed by the growth and melting of subsurface ice.  They found lots, including signs of flowing ice-bound debris, but they do not show them in the Article, which deals with the implications of their findings.  An important conclusion is that at least some of the mantle may have formed by what could be described as very dirty snow – a mixture of ice and wind blown dust.  Judging the age of the deposits directly depends on the standard stratigraphic method for all planets other than the Earth and Moon, their relationship to signs of impacts.  There are very few fresh craters in the mantle, but many that have been “blurred” by it.  Head et al. suggest that the mantle dates to at most 10 Ma.  They resort to modelling climate shifts on Mars from its orbital and rotational history. Its rotational axis undergoes the greatest obliquity shifts of any planet, from about 15 to 35º over a 124,000-year cycle (unlike Earth’s tilt, which slowly rocks through a range of only 4 degrees thanks to the stabilising tuggings of our large Moon).  At high obliquity, the polar caps probably evaporate. loading the atmosphere with water vapour, so unlike the Earth it is global warming that induces low-latitude ice accumulation.  It is this modelling that encouraged the authors to suggest an ice age between 2 million and 400 thousand years ago.