This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Why is there water on Earth? We asked this question because one might naively expect water to be too volatile to be retained by the hot early Earth. Rather, water should have been confined to the outer solar system, beyond the so-called snow line, where the distance from the sun is such that water can freeze. In the solar system, that line runs between the orbits of Mars and Jupiter.

Lindy Elkins-Tanton’s research has focused on how rocky planets form and evolve. She has a particular interest in where the Earth’s water came from and is a major proponent of one of the competing theories that attempt to answer this question. She’s a professor in the School of Earth and Space Exploration at Arizona State University. Lindy Elkins-Tanton, welcome to Geology Bites.

Lindy Elkins-Tanton: Thank you so much, Oliver. It’s a pleasure to be here. Thanks for inviting me.

Oliver Strimpel: Did I summarize the problem of the origin of the Earth’s water correctly?

Lindy Elkins-Tanton: Yeah, this is just how people think about it, with the snow line. The old view of water in the planets, in the rocky planets, and water in the Earth, is that the planets formed from a planetary nebula that consisted of gas and dust orbiting around our young sun while the sun was forming. And high temperatures and the solar wind from the active young sun evaporated the volatiles from the inner parts of that planetary disk and blew them outwards past this so-called snow line. And so the inner terrestrial planets like the Earth formed dry, with just rock and metal.

But there’s a new view. First of all, we’ve learned that that planetary nebula only lasted a few million years, and then it dispersed. We can see it in exoplanets, and we can measure it in certain ways in our own solar system. So the planets had to have formed very rapidly, because after that there’s no more material available to form a planet. So even though volatiles were relatively depleted from the inner solar system by those very processes of solar wind and high temperature, minerals still formed with water trapped within them, in crystal voids or as part of the crystal lattices themselves. And fast-forming planetary cores of metal and rock in the outer solar system grew large enough to gravitationally gather their volatile envelopes before the planetary nebula dispersed, whereas the littler planets in the inner solar system never got big enough to gather that water. So they look very different. But it turns out the materials that made those cores are not very different, because a lot of material migrated regularly both inward and outward while the planets were forming. And so that idea that the inner ones were just dry and the outer ones were wet is really almost the product of what the solar system looks like today, and not the actual process by which things formed.

Oliver Strimpel: Two-thirds of the Earth’s surface is covered by oceans. It certainly looks as if there’s a lot of water on Earth, but actually, what proportion of the Earth is water, and is it mainly in the oceans?

Lindy Elkins-Tanton: We are such surface creatures. We really feel that all the water is here and we’re standing in it. But the interior of the Earth still contains quite a lot of water. And again, it’s mostly water that’s either just an oxygen and a hydrogen, a hydroxyl ion instead of a full water molecule, and in some cases a full water molecule, trapped in crystal voids or part of crystal lattices. So they almost seem like they’re part of the solid Earth. But inside the Earth, the estimates range from two to ten times as much water as we have here on the surface. So there’s actually more water inside the Earth than there is outside.

Oliver Strimpel: Okay, can you summarize the theories that have been put forward to explain the origin of Earth’s water?

Lindy Elkins-Tanton: There’s a range. Of course, there’s always a range. It’s not just one thing or exactly another thing, the way these theories go. But the simplest way to describe it is that one end of the idea is that Earth’s water came from cometary impacts: that the Earth formed really dry, and then it was later wet impactors like comets that brought water to the Earth. And then the other end-member concept is that although comets have definitely hit the Earth in the past, water was delivered to the Earth in the very rocky building blocks of the Earth itself, found inside those rocky material crystals. And so the Earth actually formed with water from its original rocky material.

Oliver Strimpel: So what observations can we make to help distinguish among the different theories?

Lindy Elkins-Tanton: One thing is detailed analysis of meteorites and asteroids and comets by audacious space missions that have measured particles of comets, and also observations from the Earth. We now know a lot more about what comets are made of, even on the isotopic level. So what we can do is compare the isotopes of certain elements as they’re found on the Earth versus the ones that are found in comets, or in meteorites that are parts of asteroids, or in asteroids themselves. And when you look at deuterium to hydrogen, those two hydrogen isotopes, or nitrogen-14 to nitrogen-15, those nitrogen isotopes, and also noble gas compositions, it’s been shown that cometary water contributed 1% or less of the Earth’s water. Some people say perhaps up to 15%, but really a very low amount. So water really can’t have come from comets. It had to have come from the water that’s bound up in rocky asteroidal material like meteorites.

Oliver Strimpel: When you talked about the isotopic measurements, you talked about nitrogen. Does that actually bear directly on where the hydrogen and oxygen in the water came from as well?

Lindy Elkins-Tanton: Yeah. We think of comets mainly as ice balls, balls of ice, and we think of them as water ice, but they’re about half ice. And that ice is not just water ice, but it’s also things like ammonia ice that contains a lot of nitrogen. And the other half of comet materials, broadly speaking, are organic materials, so things that are made of carbon and nitrogen and hydrogen, and then other elements as well. And so when you add the water from comets, you actually are forced to also add those other ices and the organic material as well. And so you can compare the whole volatile inventory of comets to what we find on our Earth.

Oliver Strimpel: So if we can say that the other volatiles have the isotopic signature of comets, then we can infer that the water must have come along for the ride.

Lindy Elkins-Tanton: Yeah, that’s right. Or conversely, if none of the other measurements fit, then cometary water is not the main source.

Oliver Strimpel: Perhaps you’ve already indicated your preference, but which theory do you favor and why?

Lindy Elkins-Tanton: I came at this not so much to solve the problem of Earth’s water, but instead looking at the processes by which rocky planets form from giant impacts of material in early solar systems. And those giant impacts, as they crash into the planet, deliver enough kinetic energy that they melt the target body as it’s growing. And the target body goes through this phase called a magma ocean. And so my interest was: how can we use our quite extensive knowledge of how magma solidifies and how volatiles interact with magma to predict and understand how rocky planets form from that starting point of a magma ocean?

It had been thought that magma oceans were necessarily fully dry, that the water was driven off by heat. And again, I think that some of our scientific intuition comes from our simple experiences at one atmosphere of pressure and Earth gravity and the kind of conditions we normally live in, where if you, for example, put your clothes in a hot dryer, it drives off the water. And that’s true, but the thing that we forget is that water is actually incredibly sticky, and you never drive it all off. So is the amount of water that is retained enough to do what we need to make a rocky planet? My work showed that if you build a rocky planet with rocky material that contains just 100 parts per million of water bound up in crystals, just the way we find it in meteorites and in asteroids (many of them are in fact wetter than 100 parts per million), and you process that through a magma ocean, a lot of water is released into an early steam atmosphere that’s free to later cool and collapse into an ocean, but a lot is actually retained inside in the magma, because the magma won’t give up all the water. So it turns out that starting with just 100 parts per million of water produces a nice wet planet, one with water inside, similar to what the Earth has today, and with a deep water ocean. In fact, the problem becomes getting rid of the surface water, not making more.

And so what that work indicated, and what really confirmed my opinions about this whole field, is that any rocky planet born anywhere in our universe out of material that has just 100 parts per million of water will be born wet. And therefore, if it’s in a habitable zone, it will be born habitable. And so when you think about it that way, requiring comets to strike the surface later, which is a stochastic process that you can’t count on, sounds a little bit like special pleading when you know that the planet’s going to be wet from the beginning. The problem is not delivering water to a planet; the problem is getting rid of it. These planets are born with so much water.

Oliver Strimpel: Did the water in the oceans then have to be extracted from the magma ocean, or do we think it just came out of the steam atmosphere?

Lindy Elkins-Tanton: That steam atmosphere is a bit vulnerable. A hot steam atmosphere is expansive. It rises up from the planet’s surface farther than a cold atmosphere like what we have today. And the early young sun was very active, and so a lot of that steam could have been lost to the solar wind, but not all of it, before it cooled. In fact, this is one of the ideas about the difference between Venus and the Earth. Just that distance from the sun could have meant that Venus’s atmosphere was stripped, but the Earth retained more of its atmosphere to allow it to collapse into an ocean. And so I think that the physics and chemistry would indicate that some steam atmospheres may be stripped, and then the planet needs to start over with volcanism releasing water from the inside, and others will collapse and make something of an ocean before they’re stripped.

Oliver Strimpel: Is that the main mechanism then for getting the water from the inside to the outside then, via volcanism?

Lindy Elkins-Tanton: That’s right. That’s part of why people care so much about plate tectonics when we think about habitability. That’s the way that we understand that carbon is controlled on our planet and water is refreshed into the atmosphere.

Oliver Strimpel: If the problem then becomes how to get rid of the excess water, so to speak, what do we think happened to make it disappear?

Lindy Elkins-Tanton: I think that the primary mechanism would be stripping of that inflated hot steam atmosphere by the young sun. But another way to remove water from the surface is through ongoing impacts. For example, on the Earth, our last really giant impact had to have been the one that formed our moon. But impacts didn’t just end then. In fact, the moon-forming impact, or impacts of that scale, would create a trail of debris in the Earth’s orbit, which the Earth and the moon would necessarily sweep up again over time. And so those impacts, plus other objects flung in from the outer solar system or out from the inner solar system during those early chaotic times before the planets completely cleared their orbits, all of those impacts would have removed some amount of atmosphere and water from the surface. Although it’s also been shown that that removal is not as efficient as our surface-bound brains might think. They remove some, but not everything.

Oliver Strimpel: As you already mentioned, water plays a critical role in plate tectonics, as the presence of even minute quantities of water in a rock has a very big effect on its strength and melting temperature. This in turn affects then what happens to plate boundaries, especially at subduction zones, where water is released by dehydration reactions in the downgoing plate. And as you just mentioned, that’s the source of the volcanism that we now see above subduction zones in volcanic arcs. Can we envisage how the Earth might have evolved if it had much less water? And you mentioned Venus. Is that a case in point?

Lindy Elkins-Tanton: That’s certainly one of the predominant views in the scientific community, that Venus is just dry on the inside. Also, as you point out, even a very small amount of water, again within the crystals, not wandering around as a free fluid, reduces the viscosity tremendously. And the thing to think about here is that the interior of the Earth is moving centimeters to meters per year, so very, very slowly, and it moves through the deformation of solid crystals. And those solid crystals are the same as you might pick up in a piece of rock on the surface, but under heat and pressure they can flow. And you can imagine this makes some kind of intuitive sense: if in their little crystal voids are bits of water, or in their crystal lattices are water, those are generally weaker crystals than ones without, and so they flow more easily. So just little bits of water inside the solid mineral crystals actually allow the inside of the Earth to flow more easily, particularly right up underneath the tectonic plates, and that’s where there’s an area of particularly low viscosity that allows that plate tectonic movement, which we don’t think is happening on Venus, although that’s debated. It could still be happening on Venus. But the idea is that the interior of Venus in fact is drier and therefore much stiffer and much less willing to flow.

But an interesting question is: if there were plate tectonics on Venus, how would we really know? Because the way that we absolutely confirmed it on the Earth was with seismic networks that were able to locate earthquakes that conform to the shape of a downgoing slab under these volcanic arcs that you referred to. We have no seismic network on Venus. We could hope, maybe someday.

Oliver Strimpel: So just to get back to why Venus is dry and the Earth is not, is it, you think, fundamentally because it’s slightly closer to the sun and therefore it couldn’t retain water in an atmosphere?

Lindy Elkins-Tanton: That is a good, solid hypothesis for why that is. And I think that’s probably where I would go first to look for an answer. But there’s still so much that’s not known about Venus, so much more that we need to learn. And so I wouldn’t venture to say that I’d place money on that bet, but it’s absolutely plausible.

Oliver Strimpel: I said in the introduction that the snow line runs between Mars and Jupiter. So is Mars wet?

Lindy Elkins-Tanton: Mars is a bit wet at the present time and has a bunch of water ice on its surface that migrates back and forth. But there’s evidence that it was much wetter in the past, and so there’s the thought that in the distant past Mars was as wet and habitable as the Earth, and that it lost its water over time and became much, much drier, which is where it is today. The thought for that water loss is mainly just that Mars is much smaller than the Earth, and water is not as well gravitationally bound to it. And especially if Mars lost its magnetic field early on (it has none now), it’s not as protected on Mars as it is on the Earth, and so that water will be lost over time to the solar wind. That’s one set of ideas that hangs together well and might explain what we see on Mars today.

Oliver Strimpel: As we search for exoplanets, there’s a lot of discussion about the habitable zone, defined as the band within which liquid water can exist, because we believe water is essential for life. But could it be that a typical planet in the habitable zone might just be dry, even if any water would have been in liquid form if it was there? Would you think the sort of mechanisms that you describe for Earth would prevail in any plausible exoplanetary system?

Lindy Elkins-Tanton: Well, I hate to generalize to the universe, because we know so little about it. I can only say that if a planet like the Earth was formed around another star from materials similar to what we have in our solar system, then it would also be wet. But the concept of a habitable zone is a very, very simplistic concept, because everything about the habitability of a planet depends upon its chemistry, in fact even more than its distance from its star or how large it is. In a solar system where for some reason there’s a higher percentage of radiogenic elements in the interior, heating up the planet can keep its surface wet longer, and you don’t need to rely on that star distance. The composition of its atmosphere will entirely determine the current surface temperatures of that planet. And if you look at the difference between Venus’s atmosphere and the Earth’s atmosphere, and how different our surface conditions are, knowing we could not discriminate between Venus and the Earth when looking through telescopes, you see the composition is everything. And I would just add that even in planets that are not in the classical habitable zone, there could be liquid water just under the surface of those planets, either brines or waters that are heated by radiogenic heating inside, or in the moons of planets that are not at all in the habitable zone, where tidal heating from their main planet keeps them warm enough to be habitable.

Oliver Strimpel: Have we in fact seen evidence of water on an exoplanet?

Lindy Elkins-Tanton: Yes, traces of water in exoplanetary atmospheres have been found.

Oliver Strimpel: So we know there’s water out there.

Lindy Elkins-Tanton: There’s water out there, and then a whole chain of questions need to be asked about what that water might mean, and whether it indicates any kind of habitable situation that we could understand with our comprehension of life. Unfortunately, it seems that the most likely kind of life elsewhere in the universe is going to be microbial life living under the surface of a planet, which is unfortunately going to be virtually undetectable using today’s technologies.

Oliver Strimpel: If you were to be in charge of formulating missions to try and figure out if and where there might be life, how would you go about it?

Lindy Elkins-Tanton: Well, my favorite thing to do would be to go to Mars and drill into the groundwater system and measure what’s there. That’s where I would go first. But that’s already technology that we don’t have.

Oliver Strimpel: What about under the surface of Europa or Enceladus, under those icy exteriors?

Lindy Elkins-Tanton: Sure. Famously great places that people want to go look for life. Absolutely.

Oliver Strimpel: What are you working on at the moment?

Lindy Elkins-Tanton: I am absolutely focused on the Psyche mission, which is a mission to a metal asteroid. Not a site for searching for life, and no liquid water. I’ve been working on that since 2011. We won our NASA competition in 2017, we launched in 2023, and we’ll get to the asteroid in 2029. And so all of my time is spent leading this very large team of people on this project, making sure that our spacecraft stays happy and healthy on its way to the asteroid so that we can get our science.

Oliver Strimpel: Lindy Elkins-Tanton, thank you very much.

Lindy Elkins-Tanton: Thank you very much indeed for this time, Oliver. I appreciate it.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you’ll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.