The results show that nucleobases and their pre- cursors are rapidly destroyed in most interstel- lar environments and in the solar system, if they

Summary

Many organic molecules have been detected in interstellar environments and even more are found in the solar system, for example in comets and meteorites. Some of these molecules are also found in living organisms on Earth, suggesting that there might be a connection between star- dust and the origin of life on Earth. Before life could emerge on Earth, the building blocks for life had to be present. It is likely that both in- digenous and external sources have contributed to the organic inventory on the early Earth.

In chapter 2, 3, and 4 of this thesis, the photostability of several organic compounds was analysed in simulated interstellar environments.

The results show that nucleobases and their pre- cursors are rapidly destroyed in most interstel- lar environments and in the solar system, if they

reside in the gas phase. These compounds are relatively stable in dark interstellar clouds, that experience a low UV radiation field. Small N- heterocyclic molecules may be formed in the cir- cumstellar envelopes of carbon-rich stars, but have not yet been detected by astronomical ob- servations. N-heterocycles with multiple nitro- gen atoms in the ring are unlikely to be formed and are increasingly unstable against UV radi- ation. In combination with theoretical calcula- tions, our results show that it is not likely that nucleobases and their precursors are present in significant abundances in interstellar and circum- stellar regions.

Stars form by gravitational collapse of the

clumpy cores of dense interstellar clouds. After

formation of the protostar, the remaining dust

and gas accretes to form larger bodies and even- tually planets. Organic material that was present in the presolar nebula is lost during terrestrial planet formation, because of the heat generated by the planet formation process. But organics can survive in the outer solar system and may be incorporated in small solar system bodies. These objects can hit the young planets, thereby de- livering their organic content. Meteorites, frag- ments of comets and asteroids that reach the sur- face of the Earth, have been analyzed and found to contain a large variety of organic molecules, in- cluding amino acids and small amounts of nucle- obases. Our results support the hypothesis that those molecules are formed in the presence of water on the parent bodies of meteorites, where they are protected from UV radiation.

Mars is annually supplied with 2.4×10

kg of carbon by meteorite impact. Despite this sup- ply, the Viking landers did not detect any or- ganic molecules in the martian surface above a detection limit of a few ppb. The discrepancy is usually attributed to oxidizing reactions on the surface and in the subsurface soil. In chapter 5 of this thesis the stability of amino acids in Mars soil analogues is investigated. The results show that the stability is dependent on soil mineral- ogy. While destruction of amino acids in Ata- cama desert soil and Orgueil meteorite samples is rapid, deposits from the Salten Skov region show negligible loss of amino acids after exposure to a simulated Mars environment. We conclude that

clay-type minerals can protect organic molecules against destruction. Our results provide support for future life detection missions to Mars and a proper landing site selection.

Data returned from recent missions to Mars, show that Mars probably had liquid water on its surface in the past. If organic compounds and a suitable energy source were available, life may have emerged on Mars. At a later stage, when water disappeared from Mars, the progres- sive desiccation led to an increase in salt con- centration in the remaining bodies of water. Po- tential martian organisms would have to adapt to these high-salt conditions in order to sur- vive. On Earth, viable halophilic microorgan- isms have been found in inclusions in 290–206 Myr old salt crystals. We have tested the survival of halophilic archaeon Natronorubrum sp. strain HG-1 under conditions similar to those found on Mars. The results in chapter 6 show that Na- tronorubrum strain HG-1 can be desiccated and rehydrated with a negligible effect on its growth, but that these organisms did not survive expo- sure to martian UV. Natronorubrum cells were also incapacitated by high temperature (70

C), while low temperature (−20

C) severely affected the growth. Cells did not survive desiccation in combination with Atacama desert soil. From these results we conclude that Natronorubrum strain HG-1 is not likely to survive in a martian environment.

142 | Summary