Exobiology and Future Mars Missions

Nitrogen is an essential element for the evolution of life as we know it, because it is found in avariety of biologically important molecules. Therefore, N is an important element to study from anexobiological perspective. In particular, “fixed nitrogen” is the biologically useful form of nitrogen.Fixed nitrogen is generally defined as “3, NH;, NO,, or N that is chemically bound to eitherinorganic or organic molecules, and releasable by hydrolysis to NH3 or NH:. On Earth, the vastmajority of nitrogen exists as N2 in the atmosphere, and not in the fixed form. On early Marsthe same situation probably existed. The partial pressure of N2 on early Mars is thought to havebeen 18 mb, significantly less than that of early Earth. Dinitrogen can be fixed abiotically byseveral mechanisms. These mechanisms include thermal shock from meteoritic infall and lightning,which could produce NOat a rate of 1015 molecules J-l in a primitive Martian atmosphere(1 bar total pressure; COz:N2=33), as well as the interaction of light and sand containing Ti02which produces NHJ that would be rapidly destroyed by photolysis and reaction with OH radicals.These mechanisms could have been operative on primitive Mars. Any NO produced under theseconditions reacts with H forming HNO. In the Martian ocean HNO would react to form N20,,N30; , and their conjugate acids. These species would decay leaving a nitrogen pool in the oceansof NO, and NO,. In addition, there may have been organic nitrogen compounds available in theenvironment. These compounds then would have formed sediment, which may have been buriedby volcanic lava to a depth at which gaseous products would be produced and released to theatmosphere. If this burial process did not occur, or was insufficient to produce enough heat andpressure, then any nitrogen that was fixed would remain buried on Mars, and not recycled. Wouldthe relatively low abundance of nitrogen, compared to primitive Earth, have an impact on theorigin and early evolution of a living system on Mars? Data gathered in our laboratory suggestthat the low abundance of nitrogen alone may not significantly deter the origin and early evolutionof a nitrogen utilizing organism. However, the conditions on current Mars with respect to nitrogenI Iare quite different, and organisms may not be able to utilize all of the available nitrogen.