Drilling Mars: Curiosity calculations

Martian life

 

New calculations show how deep into Martian surface rocks a probe might need to drill to realistically extract organic molecules due to "life" that have not degraded in the harsh world of Mars. Apparently, signs of life may be mere centimetres deep.

When NASA's Curiosity Rover from the Mars Science Laboratory touches down on the Red Planet on 6 August 2012, it will not have to dig too deep to look for the molecular signature for life on Mars, according to new calculations. Curiosity's mission is primarily to search for past or present life, to study the Martian climate, geology and to collect data to assist in the planning of a putative trip to Mars by humans.

Alexander Pavlov of the NASA Goddard Space Flight Center in Greenbelt, Maryland, explains that research by he and his colleagues has determined the optimal depth and location for probing the Martian terrain for organic molecules similar to those we know to comprise life on Earth, such as sugars and amino acids. Earlier studies have very much suggested that amino acids and other "biochemicals" can exist in space and on other worlds. However, tests also show that in certain harsh environments exposed to interminable bombardment by radiation, such delicate molecules do not survive long. "Cosmic rays can get about 2 metres into the solid rock regardless of the rock's composition. That's why cosmic rays are so important for the search of organic molecules," Pavlov told SpectroscopyNOW.

Pavlov and his colleagues have worked out what amount of material is needed to cover and protect so that such compounds might survive for many years and lie waiting to be discovered by Curiosity's instrumentation. The data suggest that the prospect for discovering such compounds, should they be present in the first place, is better than previous estimated suggested. "It is  important to be clear that 2 cm rock depth is a good protection against Solar Cosmic rays but not Galactic Cosmic rays (GCRs). GCRs would still destroy organic molecules at 5 cm or even at a metre depth. But the rate of destruction slows down significantly and there is a good chance that small organic molecules will survive even billions years of exposure," Pavlov told us.

Of course, the presence of amino acids and other biological molecules will not prove that life ever existed on Mars. Such simple molecules have been detected in interstellar space and could reach the Martian surface on board meteorites. Lesser carbon-containing molecules known to be present on Earth because of the activities of living things, such as methane and carbon dioxide, may also have a simpler explanation for their presence in volcanic activity.

50 million grays

The researchers explain that their work suggests that the chances of finding such molecules within a depth of 2 centimetres, is essentially zero. There would be too little overlying soil to protect delicate organic molecules. They calculate that the top layer absorbs a total of 500 million grays of cosmic radiation over the course of one billion years, which would destroy all organic matter. However, dig just a little deeper, to within 5 to 10 centimetres, which is within the drilling capability of Curiosity and the radiation exposure falls off tenfold, to 50 million grays. Although that is an extremely damaging radiation rate, very simple organic molecules, such as formaldehyde, might resist degradation. In a young impact crater, the team suggests that even more complex molecules might survive.

The team reports details of their study in the July issue of Geophysical Research Letters. "Right now the challenge is that past Martian landers haven't seen any organic material whatsoever," Pavlov concedes. "We know that organic molecules have to be there but we cannot find any of them in the soil." Past Martian rovers collected only loose soil from the Martian surface for analysis. The odds of finding organic matter a 1.5 metres are very high, but that is too deep for Curiosity's drill, the team is thus relived that 5 centimetres may be just deep enough for molecular survival and at the same time be accessible to Curiosity's probes. The team also points out that some regions of the planet's surface may not be exposed to such high levels of radiation pinpointing such areas would increase the chances of finding carbon-containing material.

Fresh craters

"When you have a chance to drill, don't waste it on perfectly preserved (landscapes)," Pavlov said. "You want to go to fresh craters because there's probably a better chance to detect complex organic molecules. Let Nature work for you." Curiosity will land in Gale crater, the landing site of the Spirit rover in 2004. Whether or not there are newer craters in this 3.5-billion-year-old crater is not yet known for sure. Pavlov hopes that his team's findings will at least help guide NASA on where to drill once the rover has landed and influence where future generations of rover landers will touch down.