Introduction

Mars is the world that has generated the most interest in life beyond the Earth. There are three reasons why Mars is the prime target for a search for signs of life. First, there is direct evidence that Mars had liquid water on its surface in the past, and there is the possibility that there is liquid water in the subsurface at the present time. Second, Mars has an atmosphere, albeit a thin one, that contains CO2 and N2 . Third, conditions on Mars are cold and dry and thus are favourable for the preservation of evidence of organic remains of life that may have formed under more clement past conditions.

Mars may be cold and dry today but there is compelling evidence that earlier in its history Mars did have liquid water. This evidence comes primarily from the images taken from orbital spacecraft. Figure 12.1 from Malin and Carr (1999) shows an image of a canyon on Mars and represents probably the best evidence for extended and repeated, if not continuous, flows of liquid water on Mars. Water is the common ecological requirement for life on Earth. No organisms are known that can grow or reproduce without liquid water. Thus, the evidence that sometime in its early history Mars had liquid water is the primary motivation for the search for evidence of life (McKay, 1997).

The search for life beyond the Earth is one of the main goals of astrobiology. If life is or was present on Mars it would be important to understand the relationship of Martian life to Earth life. It is possible that Martian life and Earth life are related - part of the same tree of life. This could have resulted either from an exchange of life from one of these worlds to the other via meteorites or by the seeding of both worlds by infalling material carrying life. This later concept, known as panspermia, has been the focus of renewed interest in recent years primarily as an explanation for the origin of life on Earth very soon after the end of the impact bombardment (Davies, 1996).

Planetary Systems and the Origins of Life, eds. Ralph E. Pudritz, Paul G. Higgs, and Jonathon R. Stone. Published by Cambridge University Press. © Cambridge University Press 2007.

Fig. 12.1. Liquid water on another world. Mars Global Surveyor image showing Nanedi vallis in the Xanthe Terra region of Mars. The image covers an area 9.8 km by 18.5 km; the canyon is about 2.5 km wide. This image is the best evidence we have of liquid water anywhere outside the Earth. Photo from NASA/Malin Space Sciences.

Fig. 12.1. Liquid water on another world. Mars Global Surveyor image showing Nanedi vallis in the Xanthe Terra region of Mars. The image covers an area 9.8 km by 18.5 km; the canyon is about 2.5 km wide. This image is the best evidence we have of liquid water anywhere outside the Earth. Photo from NASA/Malin Space Sciences.

It would be more interesting scientifically and philosophically if Martian life were not related to Earth life but represented a second genesis of life (McKay, 1997). This case is interesting scientifically because we would then have, for the first time, an alternative biochemistry to compare with terrestrial biochemistry. In addition, the fact that life arose independently twice in our Solar System would be persuasive evidence that life is common in the Universe.

Thus, the full astrobiological investigation of Mars goes beyond just a search for signs of life. It is also a study of the nature of that life and its genetic relationship to Earth life. To study the nature of Martian life requires that we access biological material on Mars - organisms either living or dead. In this chapter I discuss the implications of this requirement to access Martian biology and suggest approaches and locations on Mars that may be fruitful in this respect.

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