Astrobiology Revealed #13: Craig Walton
on cosmic dust, glaciers, and the origins of life
by Aubrey Zerkle
For this AR, we chatted with planetary scientist and science fiction author Craig Walton about his paper “Cosmic dust fertilization of glacial prebiotic chemistry on early Earth.” Craig is the NOMIS–ETH Fellow at the Centre for Origin and Prevalence of Life at ETH Zurich and a Junior Research Fellow at Trinity College, University of Cambridge. In this Q&A, Craig discusses how closed environments have become the new hotspots for origins of life research, and why prebiotic chemistry likes it cold. (This interview has been edited for length and clarity.)
How did you become involved in origins of life research?
That’s a fun question! As with most things in science it was definitely not planned! It was at St Andrews, we had a visit from Professor Bob Hazen from the Carnegie Institute. He gave a keynote talk at a conference that was being hosted, and I went to the pub and met with him after his talk, which was about mineral evolution and mentioned the origins of life. He gave me the opportunity to help him out with some of that work, which happened to connect with something I was already thinking about, phosphorus. The idea was to think about which minerals containing phosphorus would’ve been on the Earth when it first formed and available for origin of life chemistry to use. It’s still a problem I haven’t solved, and no one else has solved, which is how do you get the ingredients for life in abundance.
It was basically a mini project over the summer, but one of the people who was involved in it was Matthew Pasek, a professor at University of South Florida, who was a big expert on prebiotic phosphorus, and was impressed. He invited me to a Gordon Research Conference on the origins of life in Galveston, Texas. I was still an undergrad, but it was just an incredible experience that set my path towards working on this, because I got to spend a week rubbing shoulders with really big names in the field. It’s a very small conference, so I got to meet so many of them, and see what was cutting edge, and what people knew. It was like a crash course. By the time I left, I knew [there was] so much that I didn’t know! But it really shaped that I was working on a good area, and I had many ideas for a PhD application. I was lucky enough to get to design my own [PhD] project, and it’s gone from there. I’m still thinking about phosphorus and the origins of life. I like to think I’ve made progress and not just been going around the hamster wheel. It’s still an unsolved problem and a fascinating one!
It’s amazing how many origins stories begin at a pub, or a GRC conference! I think it’s quite unusual to get invited to one of those as an undergrad, so it’s really cool that you got to experience that so early in your career.
Yes, definitely! I feel really lucky.
What aspects of the origin of life were you addressing with your recent paper in Nature Astronomy?
The critical concept in that one is cosmic dust, and whether or not it could have supplied ingredients necessary for the origin of life to the surface of the early Earth. There are several pieces of motivation that go into that. I hinted earlier at the phosphorus problem. Planets aren’t particularly stocked with ingredients life needs, they’re unreactive, or in the wrong form, or not very abundant. So, for CO2, for N2, you have to transform those things and break the nitrogen triple bond to make it available for biochemistry. For phosphorus, there’s quite a lot of it in the crust actually. It’s a minor element, so it’s not totally absent. But it’s largely locked up in minerals that aren’t very soluble, so it’s limiting for life in many environments today. When you model the early Earth, and you think about the sorts of reactions that people can reproduce in the lab, trying to build DNA from scratch, for example, [these reactions] need incredibly high concentrations and reactive forms of nutrient elements. As far as we’re aware, planets don’t typically start with [these reactive forms of nutrients] being prevalent. So you end up trying to think of localized environments that can beat the odds and get yourself on a nice chemical pathway [towards life].
People have long thought about meteorites as patching this gap, because a lot of them are full of nitrogen and phosphorus and sulfur, many of they key elements of life, and in reactive forms. The reason for that is because they haven’t gone through the planet-forming process, they’re sampling the building blocks of planets. They haven’t devolatilized and heated up for core formation, etc. And they fall to Earth now. So basically, you can tap into this chemically-rich reservoir and stick it on the surface [of Earth], and if it hit in a pond, then maybe <bam> you have the origin of life. This idea goes back to the founding of the field, with Alexander Oparin in the early 1900s. He observed that meteorites have what you need [for life], and said “well they’re probably going to be falling to Earth the whole time, maybe that’s all you need [to start it].”
But there are many problems with this idea, which is where my work came in. One key problem is that meteorites fall randomly. So you get a lot [of nutrients] once, but it’s not reliable. If I’m trying to keep you alive, and I deliver you your meals randomly at any point on the surface of the Earth every few million years, it’s a real problem. This is not how life likes to operate, life likes a continuous source [of food], and the origin of life was probably much the same. The other problem is that meteorites arrive destructively. It’s the most famous thing about them, they make big craters, and they get really hot, and they destroy your volatile elements.
But cosmic dust doesn’t have these problems. So, the thought that occurred to me towards the end of the PhD, is that you can deliver exactly the same composition of materials in a much more gentle and reliable way via cosmic dust rather than meteorites. These are just smaller particles, but they arrive consistently and continuously across the whole surface [of Earth]. So, basically what we wanted to address [with this paper] was whether we can use extraterrestrial materials [to kickstart life], but provide a new, more reliable, source.
I love it! Can you briefly describe what cosmic dust is?
Cosmic dust in this context is the smallest size category of extraterrestrial material that falls to Earth. The cutoff is 3 millimeters, above which you’re into micro-meteorites. It is made up of fragments of asteroids and larger objects that have been liberated by collisions. They can come from asteroids, they can come from comets, and they can even come from larger objects, but most of the dust comes from asteroids and comets.
Is there an important compositional distinction between cosmic dust from asteroids versus comets, in terms of what they supply for prebiotic chemistry?
The compositional difference there, which is potentially important for our theory, is that comets originate from the outer solar system. So they formed under cooler conditions in the solar nebula than asteroids that came from in the solar system, and therefore [comets] have a higher percentage in their chemical makeup of volatile elements. Things like carbon, nitrogen, sulfur and phosphorus - all the things that we care about for life. So one of the key questions in this work was what would the sources of dust have been, and would they have delivered enough of the right stuff.
You estimated with your models that the cosmic dust flux to Earth was up to 10,000 times higher in the first 500 million years after the moon-forming impact. Why was it so high then?
That’s a good point, that’s almost critical for the whole paper. Right now, there’s not very much dust coming in. There’s some, it’s the greatest source of material falling to Earth per year, I think it’s 10,000 tons a year. The reason it was much higher back in the day is because you’ve got this big reservoir of material the dust is coming from, in the asteroid belt and the cometary regions, and you’re not replenishing this. The solar system has what mass it has, and collisions liberate material, they spiral it towards the sun, and it’s gone. The asteroid belt used to be much more massive, in terms of the amount of material it had, than it is today. So, the reason you had more [cosmic dust] arriving at Earth back then is because the source had not yet been depleted.
Also, the planetary architecture [of the solar system] was still being defined. During the origins of life, the Earth was a young planet, the solar system was a young place, and the orbits were still settling out. We think the moon-forming collision was related to the Giant Planet Instability, where Jupiter wandered towards the sun and then back out again, scattering everything, totally destabilizing the asteroid belt, and probably sending Theia on its collision course to Earth. Then you had this declining period for several hundred million years afterward where the debris that was generating all that chaos fell to Earth. So that’s why the first 500 million years in particular were so much more active in delivering material to Earth. Your chances of being blown up by a sizeable dinosaur-killer asteroid were also much higher back then. Then it’s been quiescent, much more chilled out, for the remainder of Earth history. So it’s amazing that life got along at all, frankly, at that time!
Right on! When you were looking at how dust might accumulate in specific environments on the early Earth, you assumed that the dust would be concentrated by the same mechanisms back then as it is today. What are the biggest uncertainties in that assumption?
There are many assumptions <laughs>! The biggest one is that the Earth was in any way like it is today. The title of the paper is “Did cosmic dust fertilize glacial prebiotic chemistry.” Glaciers are where cosmic dust deposits accumulate most readily today, but the early Earth may not have had glaciers, so that would immediately weaken our hypothesis. If it’s true the early Earth was really hot, or an ocean world and there was no ice, then the concentration mechanisms that we are invoking, to take the cosmic dust from across a large area and build deposits you can do origin of life chemistry with, just wouldn’t have been there. That’s not clear though, and the nature of the early Earth is very uncertain.
What you need for cosmic dust to accumulate is an arid area, which could be glacial but it could also be deserts. So, if there was any place arid on the early Earth you could do this [concentrate cosmic dust], but it could’ve been cold. I think most people naively think the Hadean Earth was full of volcanoes and boiling hot. That may have been true, but you also have the Faint Young Sun. The sun was a lot fainter at that time, so you’ve got less solar luminosity. And without subduction, volcanism may have been less active. The process of injecting water [into the deep Earth during subduction] lowers the solidus and makes melting happen more readily, and you can potentially degas and have lots of volcanism.
It’s true that the planet has been cooling down over time, but it’s not clear how that translates into CO2 balance in the atmosphere, the planetary thermostat. A lot of the evidence that early Earth was boiling hot compared to today is from oxygen isotopes, which is a proxy you measure to estimate paleotemperatures. It’s been under fire recently, because it turns out that the oxygen isotope composition of the oceans has been changing, which may have thrown off this proxy. If you correct for this [change in ocean oxygen isotopes over time], it might be that the ocean temperatures were basically within error of today going back through most of Earth history.
So, [the Hadean Earth] could’ve been cold. And, if it was then you would’ve had glaciers and you would’ve had cosmic dust deposits. There’s no way around that. There’s not so many assumptions in the sorting and concentration mechanisms beyond that, because other than the [cosmic dust] deposits themselves, these environments don’t have much life. The fact that you’re talking about a prebiotic life regime doesn’t matter so much - the wind is going to behave in the same way, and so will water, and that’s what’s doing the concentrating [of the dust]. There aren’t any trees that are changing transport regimes on a glacier.
How big of an issue is the question of when the continents formed? If you need an arid environment, you presumably need to have a land surface above water.
Absolutely, that’s a really good point! This is another hotly debated thing, which is the whole problem with the origin of life research, where a lot of it just comes down to pure speculation. People can’t agree about emergent land area in the past 4 billion years, and whether or not there was 100% continental crust or 0% compared to today. The reason for that is because the zircon record, which most continental crust volume estimates are derived from, ends at 4 billion [years ago], because the bulk rock record ends at 4 billion. There are a couple of grains beyond that, but they’ve been through so many sedimentary cycles that it’s hard to say what Earth was like [that far back in time]. So, it is a problem.
If there was no land area at all, if it was an ocean world, then it’s a problem for all origin of life hypotheses that don’t involve the ocean. Today there are many more [origin of life] scenarios being favored that happened on land, in closed, small environments where you can concentrate things, than in the oceans where dilution is this constant enemy. I think it’s quite hard to imagine an early Earth that had no land, but to get this sort of arid environment you might want quite a lot of it. It really comes down to who you believe, what you think about the early Earth, and the jury’s out so much on this that I think it’s worth exploring the idea. Especially if it makes testable predictions for the lab, then that’s where this work will go. There’s a scenario here that can be reproduced [in the lab], and we can see if it can give rise to life. We’re never going to know exactly how [the origin of life] happened, all we can come up with is the range of environments where it’s possible.
That’s absolutely true! In your models, you estimated that the highest dust concentrations would’ve built up in these glacial environments called “cryoconite-type sediments.” Can you explain what those are?
Yes, these are really fun! I only came to learn about them because of a good friend of mine from Cambridge and collaborator, Rob Law, who is visiting ETH this week! Cryoconite sediments are the sediments with the highest concentration of cosmic dust that are known on Earth. That’s because you’ve got a very limited supply of terrestrial sediment in the area [of glaciers], so they’re just wind-blown particles from far afield. The percentage [of cosmic dust] that’s raining from space directly onto the ice sheet is high compared to other places on Earth. [The cryoconites] are essentially wind-blow deposits of these grains of sediment that have an albedo difference from the ice around them, so they melt into the ice and form these column-like structures. They melt in, they sink, then more dust ends up being trapped in these dark columns in the ice.
They can make a bunch of different morphologies - sometimes they can be quite big, sometimes they can be very small, sometimes they can freeze over at the top and seal themselves off and get pressurized with weird gases. Whenever I show them in cross-section to the origin of life chemists they get excited, because it’s the closest they’ve ever seen in nature to a cuvette, the lab equipment. It looks exactly the same. They’re also some of the most radioactive sediments on Earth, lots of nuclear fallout has accumulated there. And they’re an oasis for life because they’re full of nutrients, whereas on the rest of the ice sheet there’s almost nothing there.
Are these cryoconites otherwise plausible settings for the origin of life, beyond concentrating nutrients from dust?
I think so. The main question about an environment like that is whether or not being cold is good or bad. When I first started thinking about this my assumption was that cold would be bad somehow, because chemical reactions slow down when things are cold. But that’s not the case, actually. For origins of life chemistry, cold is better. It’s because some of the intermediates and the products you’re trying to accumulate, you will destroy them if it’s too warm. RNA, for example, and many other species, will just go crazy and you end up with a big mess of horrible side products if it’s warm. But if it’s near freezing, then [the reactions] get much more selective. Yeah, it maybe takes a little longer to do your reactions, but you produce the thing you want to produce. So, people do glacial prebiotic chemistry all the time. The real problem with glacial prebiotic chemistry in the past has been, where is your source of ingredients? You’re in ice, so where’s the carbon coming from? And that’s why I quite like the cryoconite - it gets you both [cold temperatures and nutrients].
That’s really cool! Do you think cosmic dust could be a viable planetary fertilizer elsewhere in the universe?
Yeah, definitely! The reason I think that is because, as I mentioned earlier, it’s really easy to produce dust. You can’t produce a planet without producing dust. The planet formation process involves collisions between objects, and they combine and get bigger and bigger. You also produce smaller and smaller fragments at the same time. So anywhere you produce an Earth-like planet, you produce dust. And, if the planet in principle could have life on it, dust that’s falling to the surface will give you a concentrated punch of the same stuff. So, it definitely could be a viable fertilizer for anywhere that has a land area.
I guess the question would be the rate at which [the dust is] delivered, and whether or not the composition is right. And there’s a fundamental question underlying all of this, which is, is it really enough to put dust in a pond? Or is there more to it? Because it could be that the volatiles that survive passage through the atmosphere and end up in the pond and don’t just get immediately leached en route aren’t enough. That is possible. There are big caveats to all of this, in terms of actual chemistry. But there are some experiments planned, so we’ll see!
Do you have experiments in mind, or other plans to follow this up?
My project here at ETH has a bunch of different threads, but this is definitely one of them. The question is can we come up with an experiment that is informative. Having thought about various more selective approaches, I think what I’m going to do is literally take some actual cosmic dust from my collaborator Martin Suttle, probably cometary cosmic dust which is full of the carbon and the nitrogen, and incubate it under UV light, under anoxic conditions, and analyze the residue. And see what happens. At the end of the day, there are certain environmental factors you could play around with, but that is roughly what had to have happened to it. Nobody has tried, by the way. And if that doesn’t give you anything interesting, then that tells you a lot as well. There may be ways to improve the scenario somehow, in terms of the exact pH conditions or whatever, but I think a Miller-Urey type experiment to start with would be wise.
But I do have several plan B’s if that is just a complete no-go. I mean that’d be great, because so much of origin of life science isn’t making testable predictions about the geology. This at least gives you quite a concrete scenario to test. I think maybe by next year I’ll know if this is working or not. The concept of dust still has so many nice advantages, and I think you can generalize it, since many of the closed environments [for the origin of life] people are interested in nowadays suffer from a lack of food. Lots of people are interested in closed lakes, like Mono Lake in California. Some of these closed lakes have lots of life in them, but life is doing lots of recycling, lots of very efficient reutilization of nutrients. Origin of life chemistry did not do that. I think if you can imagine having a whole stockpile of dusty minerals at the bottom [of a lake like that], your chemistry starts to deplete the water, and you dissolve more of your feedstock. There are ways I think in which the general concepts of constant input and geological sources could be generalized too. So, I do think we need a fertilizer that early life could tap into, but if cosmic dust is not the right stuff I won’t be totally dead in the water!