Extraterrestrial life

Researchers: Organic chemicals essential for life found in Martian meteorite; Rob Sheldon responds

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Readers will note that Timothy Standish responded earlier, to put the find in context. Now, it’s our physics color commentator Rob Sheldon’s turn. He looks at the wider issues around nitrogen:

My NASA colleague with the electron microscope having an X-ray k-edge spectrometer, Richard Hoover, has been analyzing samples of old organic material (mammoth hair, egyptian mummy hair, etc) and always detects nitrogen in the materials, basically amino acids. As far as he can tell, the half-life of nitrogen in amino acids in old organic matter is over 100,000 years old, approaching 1,000,000 years old.

However, when he looks at meteorites with fossilized cyanobacteria or fossilized diatoms, he finds no nitrogen. Mind you, the Xray method has a lower threshold of something like 0.5% IIRC, so it isn’t as sensitive as a mass spectrometer. But fresh hair, or fresh diatoms have something like 15% nitrogen in them.

So where did all the nitrogen go?

Nitrogen is a funny element. When it triple bonds with itself, it forms nitrogen gas, which is what makes up 71% of our atmosphere. And that N=N is a strong bond and very, very stable. So as chemical bonds rupture in old organic material, the nitrogen atom finds another nitrogen and escapes as a gas. As you are no doubt aware, the chemical nitroglycerine is unstable because when you rearrange the nitrogen atoms you get N2, H20 and a lot of energy. Ditto for NH4NO3, ammonium nitrate, which rearranges to form N2, H20, H20 and a leftover O when used in blasting slurry. (In my youth I once tried and failed to set off 25 lbs of it, which is why I am a living physicist and not a dead chemist.)

The point is that we don’t expect to find nitrate and ammonia in the soil of Mars, not unless some nitrogen fixing bacteria put it there recently, because over time it will all come out of the soil as N2 gas. Claiming that the process goes the other way, from N2 in atmosphere to nitrates in soils, goes backwards, from high entropy to low entropy. Whoever mainntains that needs to bone up on physical chemistry. Finding nitrogen in this meteorite is quite unusual. Richard has looked at 100’s of samples of 100’s of meteorites and never found any nitrogen, at least, not at the 0.5% level.

In addition to geochronology, one can also talk about chemical-chronology based on the speed at which these reorganizing reactions take place, such as racemization, denitrogenation, etc. That’s why finding left-handed amino acids on the Moon, comets, meteorites (and an unpublished paper on Triton), has me convinced that life is more widespread than anyone suspected.

Rob Sheldon is the author of The Long Ascent, Vol. 1 and The Long Ascent, Vol. 2.

See also: Researchers: Organic chemicals essential for life found in Martian meteorite; Tim Standish responds. Standish: If the nitrogen cycle isn’t established within a certain time, nitrogen will be removed from the atmosphere and the surface will become rich in nitrate (bad) or, in a reducing atmosphere, ammonia (really bad). The bottom line is that there are speculations that probably get around this, but it is one more needle that has to be threaded for chemical evolution to produce the first life, or a problem for the first life to quickly take care of.

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