How organic molecules on comets survive hitting planets

The question ‘how did life begin on Earth?’ is one of the most important ones we, as a species, are trying to answer. One idea is that the organic molecules that form the building blocks of life were delivered to the early Earth by crashing comets. But comets travel fast, some of them exceeding speeds of 100,000 kilometres per hour, which is nearly 50 kilometres a second. So how could organic molecules on board survive such high speed impacts? A new study from Cambridge University suggests that some of them might have been slowed down on their inbound journey. The paper’s lead author is Richard Anslow...

Richard - The upper limit for these molecules to survive will change molecule by molecule. One potentially important molecule we thought about in this work is hydrogen cyanide because this is a relatively simple molecule and has a strong bond between the carbon and nitrogen atoms. For this molecule, previous work has suggested that impacts below 15 kilometres a second are ideal to have a large amount of hydrogen cyanide surviving the impact.

Will - So 15 kilometres a second is roughly speaking about 54,000 kilometres an hour. If we've got comets going about a hundred thousand kilometres an hour or even more, that seems to be quite a lot of them that are going to burn up their organic compounds on impact. So what kind of mechanisms have you found that might be able to slow these down?

Richard - One idea we had was that if we have a number of planets in between the distant, icy rich regions of the planetary system where we have lots of comets and the habitable zone where we can have liquid water and maybe support life, each of these planets has a chance to interact with the comet and slow it down. And if we have the comets on similar orbits to the planets, this helps us break this distance and have these lower velocity impacts.

Will - So if you'll forgive such a crude metaphor, then the idea might be if you've seen anyone fire a bullet through a load of watermelons, they go through the first few, but by the time they've gone through five or six, they've slowed down to the point where they bounce off a melon and don't really damage the bullet anymore.

Richard - Yes, exactly. And the thing we were really trying to do is work out how similar an orbit we can have between the comet and the planet. And so the closer together the planets are, the smaller the kick each planet has to provide to pass this comet along and this is how we can get some of these low velocity impacts.

Will - Does the actual solar system and the star involved affect this as well?

Richard - Yes, absolutely. So as we were saying before, we need to get the comets from these icy rich regions in the outer planetary system into the habitable zone. And these two locations are set by the temperature of the star and that is set by the mass of the star. And so if we change the mass of the star, we change where the planets are and where the comets are. And this changes their typical impact velocities. And so we see that, around stars like our sun and higher mass stars, we can have much lower velocity impacts than around small red dwarf or M dwarf stars, which are found very commonly across the galaxy.

Will - What about the actual size of the organic compounds themselves? Are any of them more or less prone to survival?

Richard - Yes, definitely. So, we see on comets and some asteroids, some very complicated molecules like amino acids, even though these are much easier to break down and will therefore break down at much lower temperatures compared to smaller molecules such as hydrogen cyanide, which is another potentially important molecule in the origins of life.

Will - You've brought me here and I have to ask, then, how well, if you take all of this into account, the proximity of the planets and the size of the star, how well does Earth fare if you plug the numbers in?

Richard - Yeah, so the earth does quite well with our simulations and we can have a number of these low velocity impacts. A lot of our simulations did work with sort of idealised systems to understand the main trends. So it's difficult to say exactly for any particular system, but it definitely looks like some of these low velocity impacts would've been possible. And so we would've been able to deliver some of these molecules in those types of impacts.

Will - So as a final speculation then, if you wanted to stick your neck out, do you think there's anything to the idea that life started from organic compounds delivered by comets?

Richard - I think that it's definitely possible. And so looking forward to upcoming observations with the new telescopes. If we start to see any signatures of life on planets and we can link these to these trends we've talked about here, then maybe we can start to say that coverage delivery might be important in these first stages of life.