Tuesday, January 13, 2015

Astrobiology : Mars & Isotopic Analysis

Two stable isotopes of carbon are found, carbon-12 (12C) and carbon-13 (13C). Carbon-12 is the vast majority, at 98.89% abundance. Individual samples of carbon from different sources can vary strongly from the general abundance ratio, depending on where they came from.

The reactions of biology tend to prefer the lighter form of carbon, so over time biological sources of carbon tend to become relatively enriched in carbon-12 vs. carbon-13. A consequence of this is that carbon samples from a living source are distinct from carbon samples from a geological source.

We've examined the carbon ratios from many sources here on Earth. We've now also examined the carbon ratios from a few sources on Mars. A study was published in the December 2014 edition of Meteoritics & Planetary Science that discussed the examination of carbon ratios in a recently recovered Martian meteorite called Tissint.

There are significant differences in the carbon pattern between Earth and Mars. I put together the above figure from one in references #1 and #2, to better compare the known carbon ratio differences by sample type.

The carbon ratios found in carbonates on Mars suggest they are of inorganic origin. Those from freshwater or marine sources on Earth have ratios consistent with their known biological origin.

The red data-points are from organic material found in Tissint. They compare very well to the blue data-points from coal here on earth. On both planets, atmospheric carbon is relatively reduced in carbon-12 compared to the organic material. This is pretty good evidence that the organics in the meteorite came from some living thing, just as the organics in coal did.

The carbon could still have come from contamination here on Earth, after the meteorite landed. To test for this, the researchers examined the isotopic ratios of the hydrogen in the sample. Mars is enriched in deuterium relative to Earth and this same enrichment was found in the examined organic material.

This research happens to be the best direct evidence so far for the existence of life on Mars (at some point in the past). This still isn't entirely conclusive physical evidence. For that we will have to go to Mars and either find an unambiguous macro-fossil, or culture some micro-organism from one of the brine-water seeps we've been looking at from space.

I've been convinced that there was in the past (and is most likely today) living organisms on Mars, from theoretical considerations about how life works, and how it must have spread in the early solar system. However, I wasn't really expecting to see any convincing physical evidence any time soon.

  1. Tissent Carbon ratios: onlinelibrary.wiley.com/doi/10.1111/maps.12389/full
  2. Earth Carbon ratio: www4.nau.edu/meteorite/meteorite/book-glossaryc.html 
  3. Mars Carbon ratio: www.astrowatch.net/2014/12/scientists-find-new-evidence-for-life.html
  4. Organics on Mars: www.sciencedaily.com/releases/2014/12/141216144137.htm