The debate about the origin of life on Earth is having a bit of a moment at our Tuesday meetings. In September, member Mark Rumsby gave an excellent overview of the conditions needed for life to start, develop and be sustained. He also included ideas about how likely it was that we would ever contact or be contacted by other intelligent life-forms if they existed (“not very”, was his conclusion, though that didn’t stop some in the audience thinking we should still be looking). Our October meeting complemented this as it was a presentation by Ben Peerless of Sheffield University entitled “Astrochemistry: the start of life”.
Ben started with some simple ideas about elements in the Periodic Table, atomic structure and how different elements can be built up by nuclear fusion. Nothing too difficult so far. Things got a lot more complicated when he began on his main theme which was the chemistry of the interstellar medium. This is very different from the chemistry we experience on Earth as the temperatures are very low meaning there is insufficient energy to initiate a reaction and the medium is very sparsely populated with compounds and ions. What would be unstable on Earth can be much more stable in the interstellar medium just because there isn’t very much for it to interact with. Such interactions as do happen, though , are very significant, enabling more complex compounds and ions to build up.
One common ion in interstellar medium is the trihydrogen cation H3+ (please excuse the lack of subscripts and superscripts). The starting point for this is the abundant H2 molecules and single H atoms. An H2 molecule can have an electron stripped from it by a passing cosmic ray. The H2+ so produced can then join with an H atom to give H3+. This has three bonds joining the hydrogens into an equilateral triangle, but just two electrons to make those three bonds – weird, seriously weird! Even Ben thought that!
In dark nebulae,the H3+ ions are stable, unlike on Earth, and will eventually encounter and bond with other atoms, molecules or ions, giving a cascade of molecule formation. I liken the H3+ ion to a Lego brick which can attach itself to existing structures enlarging them. Some examples are given in this reference. Other key ions performing a similar role are C+ and O+ though these are more significant in the astrochemistry of diffuse clouds.
You might be interested to also follow this link which includes a list of all the molecules and ions discovered in the interstellar medium. There are about 200 of them, of which about half have just two or three atoms joined together. The list includes some very familiar names : water, of course, ethanol, urea, benzene and acetone. They are not all carbon-based ; sodium chloride is listed.
The real point of interest, certainly for the debate which followed Ben’s presentation, was the possible detection of glycine. Glycine is the simplest of the amino acids which are the building blocks of protein and are thus thought of as precursors of life. Various searches through Google suggest that glycine was first detected in the interstellar medium in the early 2000s and yet it seems that its presence there is still to be confirmed. I haven’t been able to find anything very recent about it, though Ben left me with the impression that its presence was now a certainty. (Glycine has been identified elsewhere, most notably recently by the Rosetta probe on a comet. The real question is where did that compound originate before it was incorporated into the comet?!)
So were the building blocks of life fashioned in the interstellar medium? If so, how did they then get to Earth? Or to put it another way, were the molecules we need delivered somehow from space almost as a “self-assembly kit”? or where they formed here … somehow!
A lively debate ensued !
Talk given by Ben Peerless of Sheffield University
Post written by Katherine Rusbridge
Oct 2016