free debate

March 2, 2011

Scientific Unknowns: Life on Mars

The question of how much life there is in the universe is one of the most fundamental questions in astronomy, and possibly all of science. While we are just now on the brink of finding Earth-like planets around other stars, the possibility of life existing on planets very different from our own still remains. While scientists have dismissed the old notions of grand civilizations on Venus and Mars, there is tantalizing evidence of microbial life throughout the Solar System. This is a first in a series of posts I will do over the next few weeks looking at some of the top contenders.

Of all the places we look for life in our solar system, none seems to have the mystique of Mars. The search for life on Mars has deep historical roots. Percival Lowell infamously reported his evidence for a grand civilization on Mars with a grand irrigation network. This and other stories firmly set Mars as the home of extraterrestrials in science fiction. While scientists are no longer searching little green men on the surface, microbes on the red planet have become hot topic of debate. Mars surface is bathed in radiation that would likely prevent anything from living on its surface. Liquid water also can no longer exist on the surface of Mars. These both come from the fact Mars lacks the thick atmosphere and magnetic field we have on the Earth. So planetary scientists are now searching for microbes under the surface of mars. For those scientists there are three main lines of evidence.

1. Recently, there has been a growing consensus that a few billion years ago, Mars had at least a some running water on its surface. The Earth appears to have developed life fairly quickly once the oceans formed. The thinking goes that if Mars had conditions similar to that of the early Earth, it is reasonable to assume that it would have developed life in a similar way. Mars has changed so much over the last few billion years to become the dry and harsh world it is today, that it is not surprising that we don't see the abundance of life we now have on the Earth. This idea gives hope that possibly some microbial life was able to take hold on Mars during the planet's youth and maybe still be surviving under the surface.

2. If life ever existed on Mars, we could maybe find evidence of it the same way we find evidence of past life on the Earth: fossils. It is well accepted that large impacts from meteorites on the surface of Mars can lob chunks of the Red Planet into space. The Sun can then pull those into the more inner part of the solar system where they collide with the Earth. What is controversial is a claim made by a group of scientists in 1996 that one such Martian meteorite contained a fossilized bacteria. If true, this would be sure evidence of at the very least past life on Mars. After 15 years of research however, the scientific community is still far from a consensus.

In science, extraordinary claims require extraordinary evidence. Life on Mars is certainly an extraordinary claim. Unfortunately, the evidence has yet to match. Without going into extreme technical detail, the core issue is whether or not the meteorite in question, Martian meteorite ALH84001, could have plausibly formed without life. Scientists look at a multitude of characteristics, mostly based around the various minerals including their isotopes, distribution, and texture. I think it is safe to say that our current results have been inconclusive. There are scientists with strong points on either side. In the end, the evidence is just not there to make a solid decision.

3. The final hint of life on Mars is methane. This is something I touched on briefly in the past and the basic story is this. The chemistry of Mars atmosphere is such that methane breaks down over time. We have found large quantities of methane in Mars atmosphere, so we know it must be replenished by some ongoing process. At this point all we have is the observation of the methane itself, and speculation about what is causing it. The reason people get excited is because one of the ways methane is produced here on Earth is by bacteria. Scientists, however, are also quick to point out that we know of geologic processes that can produce methane in the quantities we seen on Mars. Again, this line of evidence seems tantalizing, but non-conclusive.

So, what is the final story for Mars? I have to say that, while there is good reason to continue looking for microbes under the surface, we can't say we know they will be there. Either we will see exciting geology or revolutionary biology. Regardless, Mars continues to be a place worth exploring with mysteries lurking just beneath the surface.