Wednesday, January 31, 2007
Astrobiology: Dig deeper to find Martian life
Astrobiology: A UCL-led study has found that probes designed to find life on Mars do not drill deep enough to find the living cells that scientists believe may exist well below the surface.
Although current drills may find essential tell-tale signs that life once existed on Mars, cellular life could not survive the radiation levels for long enough any closer to the surface of Mars than a few metres deep - beyond the reach of even state-of-the-art drills."
The study, published in the journal 'Geophysical Research Letters', maps out the cosmic radiation levels at various depths, taking into account different surface conditions on Mars, and shows that the best place to look for living cells is within the ice at Elysium (see "ESA's Mars Express sees signs of a 'frozen sea'" below), the location of the newly discovered frozen sea on Mars.
Lead author Mr Lewis Dartnell, (UCL Centre for Mathematics and Physics in the Life Sciences and Experimental Biology - CoMPLEX), said: "Finding hints that life once existed - proteins, DNA fragments or fossils - would be a major discovery in itself, but the Holy Grail for astrobiologists is finding a living cell that we can warm up, feed nutrients and reawaken for studying."
"It just isn't plausible that dormant life is still surviving in the near-subsurface of Mars - within the first couple of metres below the surface - in the face of the ionizing radiation field. Finding life on Mars depends on liquid water surfacing on Mars, but the last time liquid water was widespread on Mars was billions of years ago. Even the hardiest cells we know of could not possibly survive the cosmic radiation levels near the surface of Mars for that long."
Survival times near the surface reach only a few million years. This means that the chance of finding life with the current probes is slim. Scientists will need to dig deeper and target very specific, hard-to-reach areas such as recent craters or areas where water has recently surfaced.
Dr Andrew Coates (UCL Space and Climate Physics) said: "This study is trying to understand the radiation environment on Mars and its effect on past and present life. This is the first study to take a thorough look at how radiation behaves in the atmosphere and below the surface and it's very relevant to planned missions. The best chance we have of finding life is looking in either the sea at Elysium or fresh craters."
The team found that the best places to look for living cells on Mars would be within the ice at Elysium because the frozen sea is relatively recent - it is believed to have surfaced in the last five million years - and so has been exposed to radiation for a relatively short amount of time.
The team developed a radiation dose model to study the radiation environment for possible life on Mars. Unlike Earth, Mars is not protected by a global magnetic field or thick atmosphere and for billions of years it has been laid bare to radiation from space. The team quantified how solar and galactic radiation is modified as it goes through the thin Martian atmosphere to the surface and underground.
Taking the known radiation resistance of terrestrial cells combined with the annual radiation doses on Mars, the team calculated the survival time of dormant populations of cells. Some strains are radiation-resistant and are able to survive the effects because, when active, they successfully repair the DNA breaks caused by ionising radiation. However, when cells are dormant, such as when frozen as in the subsurface of Mars, they are preserved but unable to repair the damage, which accumulates to the point where the cell becomes permanently inactivated.
Mr Dartnell said: "With this model of the subsurface radiation environment on Mars and its effects on the survival of dormant cells we have been able to accurately determine the drilling depth required for any hope of recovering living cells. We have found that this suspected frozen sea in Elysium represents one of the most exciting targets for landing a probe, as the long-term survival of cells here is better than underground in icy rock. This could be crucial for the scientists and engineers planning future Mars missions to find life."
Source: University College London, 30 January 2007
Based on the paper:
Dartnell, L. R., L. Desorgher, J. M. Ward, and A. J. Coates (2007), Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology, Geophys. Res. Lett., 34, L02207, doi:10.1029/2006GL027494
The damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. Here we model the propagation of solar energetic protons and galactic cosmic ray particles through the Martian atmosphere and three different surface scenarios: dry regolith, water ice, and regolith with layered permafrost. Particle energy spectra and absorbed radiation dose are determined for the surface and at regular depths underground, allowing the calculation of microbial survival times. Bacteria or spores held dormant by freezing conditions cannot metabolise and become inactivated by accumulating radiation damage. We find that at 2 m depth, the reach of the ExoMars drill, a population of radioresistant cells would need to have reanimated within the last 450,000 years to still be viable. Recovery of viable cells cryopreserved within the putative Cerberus pack-ice requires a drill depth of at least 7.5 m.
"ESA's Mars Express sees signs of a 'frozen sea'" - A February 2005 Press Release from the European Space Agency:
The above image, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, shows what appears to be a dust-covered frozen sea near the Martian equator.
It shows a flat plain, part of the Elysium Planitia, that is covered with irregular blocky shapes. They look just like the rafts of fragmented sea ice that lie off the coast of Antarctica on Earth. This scene, taken during orbit 32, is a few tens of kilometres across, and is centred on latitude 5 degrees North and longitude 150 degrees East.
The water that formed the sea appears to have originated beneath the surface of Mars, and to have come out through a series of fractures known as the Cerberus Fossae, from where it flowed in a catastrophic flood.
It collected in a vast area about 800 kilometres long and 900 kilometres wide with a depth of about 45 metres. As the water started to freeze, floating pack ice broke up into rafts. These became later covered in ash and dust from volcanic eruptions in the region.
Ice is unstable at the surface of Mars because of the low atmospheric pressure, and sublimates away (changes straight from ice to vapour without passing through the liquid state) into the atmosphere, but some of the ice rafts appear to have been protected by layers of volcanic dust. While the entire sea froze solid, the unprotected ice between the rafts sublimated to leave 'ice plateaus' surrounded by bare rock.
The sparse cratering of this region shows that it cannot have formed more than about five million years ago, meaning this is a relatively young feature.
The question remains as to whether the frozen body of water is still there, or whether the visible floes are just the remains of the sublimation process. Two observations suggest that the ice is still there: first, the submerged craters are too shallow, indicating most of the ice is still in the craters; and second, the surface is too horizontal - if the ice had been lost, there would be a greater height variation.
These findings were presented on 21 February at ESA's Mars Express Science Conference at ESTEC in Noordwijk, the Netherlands, where about 250 scientists from all over the world are discussing the first year of scientific results from Mars Express. The complete scientific paper by Dr J. Murray et al. describing the frozen sea results will be published by the journal Nature in March 2005. 
The colour images were processed using the HRSC nadir (vertical view) and three colour channels. The perspective views were calculated from the digital terrain model derived from the stereo channels.
Image Caption: Map showing location of the 'frozen sea' in context
 Nature 434, 352-356 (17 March 2005) | doi: 10.1038/nature03379
Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator
John B. Murray et al.
It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5 degrees north latitude and 150 degrees east longitude in southern Elysium. The frozen lake measures about 800 times 900 km in lateral extent and may be up to 45 metres deep - similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 plus or minus 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.
See the Sunday, December 10, 2006 post "NASA Images Suggest Water Still Flows in Brief Spurts on Mars"
Video: "Mars Discoveries - Liquid Water and Impact Craters" (03.19)
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