Mars - Making a case for life

Was doing Scopus searches for relevant articles in my field and saw the following article (entitled: The microbial case for Mars and its implication for human expeditions to Mars) and thought: Research Blogging! With a title as cool as that, it deserves to be blogged on!

Gerda Horneck starts the Introduction by making a case for life on Mars. There are several factors which lean positively towards this being the case.

1. The physical and chemical surfaces of the early Earth and Mars were similar.
2. Early Mars (3.5 Ga ago) was wet and had a more temperate climate.
3. Microbial life existed on Earth by 3.5 to 3.8 Ga ago.

Water, Water Everywhere
According to Horneck: As well as carbon based chemistry and an adequate energy source, water in liquid phase has been considered as one of the prerequisites for habitability. To which, is outlined the case for water on Mars.

1. Erosional patterns on Mars.
2. Presence of hematite, which is only formed in the presence of water.
3. Presence of sodium chloride which also is only formed when water is present.
4. The global mineralogical data (clay minerals) of Mars also points towards liquid water.

Given Mars present, extremely cold, atmosphere, water is unable to persist in a liquid state on the surface. Therefore, Horneck states: The search for putative extant Martian life must therefore concentrate on the subsurface biological oases where liquid water still exists under the current conditions.

Don't the current conditions suck?
Well, for most life, the answer to that question is: Yep. However, there is always the exceptions to the rule. Section 3 lays the groundwork for the possibility of life on Mars by citing the many instances of life on Earth in very "inhospitable" places.

Arid environments
Though not cited directly by name, the organism Deinococcus radiodurans is one of my favorite examples of survival in one environment, providing protection in another. D. radiodurans has been shown to be quite resistant to extremely high levels of radiation, able to effectively stitch its DNA back together after receiving doses of radiation which would kill most everything else. And it's not that D. radiodurans grew up next to Chernobyl. This bacterium is often isolated in very arid environments, and such areas where dessication is a common threat, methods to repair such damage (which is very similar to radiation damage) were necessarily evolved. Horneck also speaks of cryptoendolithic microbial ecosystems in deserts which ... give an example how life has withdrawn into protected zones. They colonize sandstones a few millimeters below the surface, forming layers of algae and cyanobacteria as primary producers as well as fungi and bacteria as consumers, thereby producing their own microhabitate in an otherwise hostile arid environment.

Cold environments
Psychrophiles, one of my favorite groups of organisms. These organisms don't just live, they thrive in places where the warmest it gets is 0 degrees centigrade. Some have been shown to grow in Antarctica at temperatures below -20 C (-4 F). Nevermind the fact that long term storage of bacteria usually involves freezing at -80 C or in liquid nitrogen (-346 F).

Salty environments
Halophiles (salt loving) organisms thrive in areas where salt is abundant. Brine, salt lakes and inside rock salts. They have evolved to regulate osmotic flow to survive.

Intense UV radiation
Here Horneck states that during the Archean (3.8 to 2.5 billion years ago), UV irradiation has been calculated to be about 1000 times greater than it is today (we have photosynthesis and our ozone-ified atmosphere to thank for the protection). Yet, during this period, life still found a way to survive. Not only that, by UV radiation is a mutagen and could have promoted biological evolution. What it certainly did do, was force bacteria to evolve DNA repair mechanisms.

Intense ionizing radiation
Here Horneck cites the story of D. radiodurans (Dr), and says that while Dr can tolerate radiation does of 3 x 10^3 Gy, the dosages on Mars are 30,000 to 40,000 lower. Horneck also states: Recently, hyperthermophilic archaea have been described that exert a similar high radiation resistance as D. radiodurans. Even the more radiation sensitive spores of Bacillus subtilis or vegetative cells of Escherichia coli would be able to survive radiation exposure under Martian conditions for extended periods of time.

So, as we can see ... there are many conditions on Earth where we wouldn't expect life, but it's there anyways. Can the same be the case for Mars? Could there perhaps be areas on Mars similar to these inhospitable areas on Earth which teem with life?


So, where?
Where are these, as Horneck calls them: putative Martian oases?

There is a general consensus that the present surface of Mars is not likely to be a habitable place. Putative habitable regions may exist in the sub-surface of Mars, where liquid water might be present, temperature fluctuations are low and the harmful UV radiation is attenuated.

Perhaps given such cruddy environmental conditions the microorganisms are currently "hiding out", as Horneck states:

On Earth, microorganisms could survive for hundred thousand years or even longer in ice or permafrost. Bacterial spores, which are reported to survive interim hostile conditions over millions of years, are another example of potential survivors over extreme intervals on Mars.

Implications for human expeditions to Mars
Why do it? Horneck says that to search for life, it's really a task best suited for humans. Factors such as critical thinking, and real-time repairs play a role in any successful attempt at examining the planet. Of course, this comes with downsides as well, and Horneck focuses on three: the risk of contamination of astronauts by Martian microbes, the risk of contamination of Earth by Martian microbes on a return trip, and the risk of contaminating Mars with Earth microorganisms. It appears that NASA already has #3 in check, or is working on it. The issue is, as Horneck argues:

Strict requirements to keep Mars clean can only be met with robotic missions to Mars. The scenario changes when humans are involved in the mission. Since humans carry vast amounts of microbes required to sustain important body functions, Mars will become inevitably contaminated with terrestrial microorganisms as soon humans arrive on its surface. Although the surface of Mars seems to be very hostile to microbial life, it cannot be excluded that some terrestrial microorganisms accidentally imported may find protective ecological niches where they could survive or even metabolize, grow and eventually propagate.

Conclusions
I think Horneck summarizes the issue quite nicely in the closing paragraphs:

Finally the question arises, whether the increasing robotic exploration of Mars and the eventual human exploration and settlement of that planet is likely to cause an environmental impact to scientifically important sites, regions of natural beauty and historically important regions, in the form of contamination with spacecraft parts and microbiota. Already the presence of crashed robots on Mars begs important questions on the type of wilderness ethic one may apply to Mars and how this ethic is emboided within practical environmental policy.

Good points, all of them. I would argue that if we eventually "colonize" Mars, we're going to have to accept a certain level of contamination of Mars with terrestrial organisms. I'd say that it gives us a perfect opportunity to "do it right" with this second chance, which should minimize the extent of this contamination, but it's probably unavoidable.

Overall though, this was a very interesting read. If you have access to the journal, it's definitely worth a download (PDF, 10 pages).

References
G HORNECK (2008). The microbial case for Mars and its implication for human expeditions to Mars Acta Astronautica, 63 (7-10), 1015-1024 DOI: 10.1016/j.actaastro.2007.12.002