This latest earthly explorer to be sent to the red planet is set to carry out a host of geological and meteorological measurements, important for understanding the interaction between Mars’ polar regions and the global climate. But the Phoenix mission has also got astrobiologists very excited. The landing zone for Phoenix has been targeted to the martian polar region somewhere between the latitudes 65º and 72º North, which on Earth would correspond to the chilly climes of Northern Alaska. Mars is not blanketed by the greenhouse effect of a thick atmosphere, and the arctic plains here are truly cold — during the winter the air itself freezes out on the ground as a thick frost of solid carbon dioxide (‘dry ice’). But paradoxically, this is exactly the sort of location where astrobiologists think martian life may be still surviving today.
The possibility of the prior, or perhaps even present, existence of life on Mars is wedded to the presence of water, and we suspect from earlier probes that there are substantial amounts of water ice locked up in the martian soil, or regolith, in both the northern and southern polar regions. Nearer the equator the warmth of the sun has long since sublimed away any water near the surface — with such a low air pressure water cannot exist as a liquid and sublimes straight from ice into vapour and escapes from the ground. Such numbingly-cold conditions are also very favourable for the long-term preservation of any microbes — cryopreserved in the permafrost in the same way microbiologists routinely store cells in terrestrial laboratories.
No probe yet has successfully touched-down in these polar regions (a consequence of orbital dynamics makes it very difficult to reach high latitudes), and so Phoenix promises some very original results. Furthermore, Phoenix will be the first lander since the Viking probes of the 1970s to be able to access beneath the martian surface, using a specially designed digger arm and scoop. Another consequence of the thin martian atmosphere is that it lets through high levels of solar ultraviolet light to bathe the ground, and this is believed to have created very chemically-oxidising conditions in the martian soil. Any organic molecules, the building blocks of life as we know it, will be rapidly destroyed in the martian topsoil. Indeed, the Viking landers could find no evidence of organics at all in the soil they scooped up for analysis. This is only to say, however, that no organics were found at these two specific locations on Mars down to the detection-levels of the Viking instruments. Phoenix will be landing far away from either of the Viking landing sites, and so will fall onto completely fresh hunting grounds. And hopefully its digging arm will get deep enough to reach beneath this sterilising layer. This digging arm has been exquisitely cleaned and sterilised to thoroughly remove any traces of Earth’s biota — you wouldn’t want to go all the way to Mars to hunt for signs of alien biochemistry, only to be fooled by gunk falling off your own dirty sleeve…
The northern plains are also of particular astrobiological interest because some researchers claim evidence for an ancient ocean filling this basin during the earliest periods of martian history [see a recent paper on this in last month’s Nature].
If Phoenix finds fine sediments of mud and silt it will lend support to this exciting hypothesis. Alternatively, coarse sediments of sand might indicate the past action of flowing water, especially if the grains are rounded and size-sorted, as would be expected from the action of flowing water. Any evidence of the prior presence of liquid water would be a profound result for astrobiologists. Ancient expanses of surface water, as already inferred at Meridiani Planum by the Mars Exploration Rover Opportunity, might have provided a cosy environment for the emergence of life, which could remain preserved to this day in the icy soil around Phoenix’s landing site. Even more exciting is the possibility that dormant life in the arctic plains may be able to periodically reanimate from its frozen slumber. Unlike Earth, the tilt of the martian rotational axis (which gives the cycling of day and night) is unstable and tumbles back and forth over long time periods. As the arctic regions tip forward toward the sun the additional heating, and possibly thickening of the atmosphere, would produce a warmer surface climate that may allow thawing of the permafrost and a transient burst of activity in martian microbes.
We know of terrestrial cold-loving (‘psychrophilic’) microbes that continue growing at –20ºC, and there is even mounting evidence that some ultra-hardy cells remain metabolically active far below this temperature. Although it is true to say that, unlike Viking, Phoenix will carry no specifically-biological experiments (it will not have the capability to attempt to sample and incubate martian bacteria) it is still of paramount astrobiological importance. The major role of this probe will be in assessing the habitability of the martian subsurface. If Phoenix does dig deep enough to reach the expected underground ice it will constitute the first on-the-spot direct examination of martian water – all of the orbiter and lander discoveries so far have inferred its existence. Once scooped up and delivered to its on-board scientific instruments, Phoenix will also analyse the chemistry and mineralogy of the soil, including determining the composition of life-giving elements such as carbon, nitrogen, and phosphorus, and attempt to sniff-out any organic volatiles. The physical environment of the wet soil will also be looked at, to check that it doesn’t isn’t laced with hazardous oxidants, or too acidic, or alkaline, or salty for known life to tolerate.
This ice-soil boundary beneath the martian surface represents an enticing potential habitat in the solar system, a region where extraterrestrial life may persist even today and be awaiting detection by probes following Phoenix’s lead.
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