The cover of this week's Nature features a picture of a largely white Mars and the headline "Mars in an ice age: The red planet before the present interglacial". Inside there's an article by Jim Head, Jack Mustard, Mikhil Kreslavsky, Ralph Milliken and David Marchant which argues that, on Mars as on Earth, orbital changes drive climate changes. The key to the article is the identification over large parts of the surface of "a succession of metres-thick, latitude-dependent surface deposits that are young, ice-rich when formed, and whose deposition and removal is driven by climate change that is induced by spin-axis tilting". This is a deliberately interpretation-free way of describing what appear to be dirty snowcaps currently rotting away over a large part of the surface, left overs from an ice age that ended about 350,000 years ago.
The article is an impressive piece of original geological research which is also, in some ways, a synthesis of emerging conventional wisdom. There will be modifications and disputes over the details of the picture the article draws. Some people think that changes in seasonality, as well as obliquity, will turn out to play a part in the timing of iceages; there will be debates about whether there was real snow at any time, or just moist air that froze itself into existing dusts layers. But in general it looks like ice ages are carving out a lasting place in our mental picture of the planet.
This is all a wonderful example of the power of high-resolution imaging. Before Malin's MOC camera got to Mars on Mars Global Surveyor in 1997, there was thought to be little if anything on the planet that had been formed in the past 10 million years. This is because the features now being interpreted as evidence of recent ice age features were simply too small to see for what they were in the lower-resolution Viking images. (I'd guess some of them can be seen for what they are in those images now; the educated eye sees more). Icy features were clearly visible -- but the evidence that they weren't very, very old was not.
In some ways, clearly, the putative ice ages make Mars seem more earthlike. But these ice ages are very different from the earth's. They are dominated by the behaviour of dust. They have no biological amplifier -- but nevertheless involve significant greenhouse gas changes. There are no oceans involved. The poles get warmed up, not cooled down, as the ice spreads towards the equator. The ice ages are a great illustration of the Necker Cube effect as applied to understanding Mars -- one of those phenomena that can flicker between looking strangely familiar and bizarrely alien.
One implication of this research -- and the one the authors choose to emphasise by concluding with it -- is that there are doubtless records of the ice ages in the shallow subsurface over large parts of the planet that would be accessible to landers. This makes NASA's 2007 Phoenix mission to Vastitas Borealis sound all the more exciting.
A question that the authors choose not to address is whether the movement of ice and water vapour around the planet ever leads to the production of liquid water. The general idea is that liquid water is not necessary. During the ice ages water ice sublimes into water vapour at the poles and condenses on dust particles (perhaps dust in the air, perhaps dust on the ground) in the mid-latitudes. It waits there until the mid-latitudes start to warm, at which point it sublimes away again to eventually refreeze on to the now cooler polar caps. But though this all makes sense as a two-phase system, are there no conditions in which this water might not transiently melt to liquid that would evaporate a little later, rather than subliming straight away from ice to vapour?
That possibility is one of the things that I wrote about in the cover story of the current (January 2004) issue of National Geographic, which is largely about the Martian ice-age theories. The story's key take-home message is in the following paras (here presented in the form they were in in one of my drafts, because I don't seem to have the final form on my machine)
"The idea that the Martian surface is changing right now is opening up a new way of thinking about the planet. From Percival Lowell, at the turn of the 19th century onwards, the story of Mars has been seen as a long planetary diminuendo -- a slow sinking from the vibrant to the moribund. Lowell thought Mars was covered with canals because it was drying out and that the canals were an ancient civilization's last-ditch -- last megaditch -- attempt to stave off the planet's decline into waterless and lifeless senility. From then until now people have argued that planetary activity-volcanism and crustal movement is driven by internal heat, that a smaller world like Mars will necessarily cool down much quicker than a bigger planet like our own, and that we have therefore missed its glory days.
"True as far as it goes: Mars is not the hot-hearted volcano-building place it once was. But if the planet is no longer active in the way the Earth is, it is still exquisitely reactive, responding to little orbital pulls and shoves with planet-girdling sheets of dirty ice or snow that bring striking changes to its surface. Its capacity for change is no longer buried in the dim and distant past. Instead this change is almost contemporary, sometimes happening on the same sort of 10,000- to 100,000-year timescales as the Earth's ice ages, if not quicker. Mars is no longer a world of endless decline, but one of rhythmic regeneration. And that regeneration may include occasional splashes of liquid water, which makes the near-surface of Mars a more likely abode for life than it has seemed since the beginning of the space age."