Evaporating Rocks and Planetary Formation: An Interview with Edward Young

On Earth, no one thinks of molten lava as being able to evaporate into the atmosphere. But in the emptiness of space, such evaporation may have played an important role in the formation of Earth and other rocky bodies in our solar system.

 In a new paper [1] in Nature, Remco Hin at the University of Bristol, UK, and colleagues, examined magnesium isotopes to show that when materials in the early solar system collided and melted together into small bodies called planetesimals, evaporation occurred. This evaporation may be responsible for creating the current distribution of elements and isotopes that scientists observe in rocky bodies in the solar system. In a paper [2] published concurrently in Nature, Ashley Norris and Bernard Wood at the University of Oxford, UK, measured the vaporization of different elements at high temperatures in a controlled lab setting and came to the same conclusion. If these processes occurred as described, then they may explain the longstanding question of why the chemistry of Earth and other rocky planets differs from the primitive meteorites, called chondrites, that likely aggregated to form them. 

Co-Chair of the DCO’s Deep Energy community, Edward Young, a professor of geology at the University of California Los Angeles, USA, wrote a News and Views commentary [3] to accompany these papers. In his own research at UCLA, Young uses isotope profiles to understand the processes that formed the solar system.

Young talked with DCO science writer Patricia Waldron over Skype about how the new papers revive an old idea about planetary formation and the controversial role of evaporation in creating rocky planets as we know them today.

Let’s begin at the beginning. Can you tell me a little bit about what we think we know about how Earth and other rocky planets formed?

It really is like a snowball. Planets form by the sequential accretion of larger and larger bodies, so starting from dust, to boulders, to asteroid-sized objects, to moon-sized objects, and upward. Starting with little grains, they accrete and make a kind of snowball. And because of gravity, these “snowballs” are moving through the protoplanetary disk and things are piling on. And then these snowballs start hitting each other and sticking and then they make bigger and bigger boulders.

These two papers use different approaches to argue that the elements and isotopes on Earth and other rocky bodies differ from the chondrite meteorites that may have been the raw planetary material because during accretion, collisions caused melting and vaporization of certain elements. What do you think of their individual approaches?

What we’re really talking about is the evaporation of lava in space. The Hin et al. group thinks that they see isotopic evidence for this. They weren’t the first people to suggest it, but they’re the first people to suggest it for magnesium isotopes. And they built a rather elaborate, comprehensive story to explain the isotopes of magnesium, silicon, and iron, all by this process.

The Norris and Wood paper evaporated some rock in the lab. They found that when they evaporate certain elements that never were well explained before, called chalcophiles, which means they love copper, they get a good match between what we see in Earth and these evaporation experiments. The paper is cool because we just haven’t done enough experiments on evaporation. It’s nice to get data that are rooted in experiments rather just in calculations.

So these two groups are reviving an old idea at the same time, with new data, so that made a bit of a splash.

Do you have any reservations about these two studies?

What I worry about is that these two papers are inconsistent. The Hin et al. group requires that after a collision, the vapor and the liquid were in equilibrium, meaning that they have to sit there for a long time to equilibrate. In the Norris and Wood paper, material is just evaporating continuously, and those are two different processes in many ways. The details are yet to be worked out.

Are there constraints on the size of the bodies where this evaporation occurs?

Evaporation is more likely to occur on a regular basis between little planetesimals hitting each other. When you’re the mass of Earth, molecules don’t achieve escape velocity, due to gravity, so you have to be the size of Pluto or smaller for things to escape.

So, this process could have had an impact on Earth’s chemistry if the little planetesimals underwent evaporation and then accreted to become Earth?

Yes. It’s an inherited signal from the evaporation caused by collisions of the “snowballs.” And that makes it kind of cool, because that can tell us about the nature of these bodies and how fast they were colliding and so forth and so on.

How does this work fit into the larger field?

There has been a lot of work in the last decade assigning differences in elemental concentrations and isotopes between different rocks in the solar system to the formation of metal cores. When you accrete these planetesimals, eventually they get so hot that they begin to melt on the inside. The iron sinks to the core with all the elements that like iron. So there’s been this sort of tug of war: is it volatility or core formation that has been the dominant process? What you’re seeing is a swinging of the pendulum from “everything is core formation,” back to “volatility is important.”

Does this work have any applications for deep carbon science?

The elements that we’re talking about are what a geochemist would call moderately volatile, but they’re not what most chemists would call “the volatiles,” like water, carbon dioxide, and methane. If we’re seeing evidence for elements like magnesium and silicon evaporating away, then you can imagine that the water and carbon dioxide evaporated away too. That means, after you accrete the Earth, you have to bring the volatiles back, by asteroids and comets, for example. How much carbon did the Earth actually accrete with? We don’t know because we don’t know the budget for the deep Earth. That’s what the DCO is trying to do.

What should be the next step for this work?

More experiments. We’re proposing to boil more rocks and see exactly what happens in the laboratory. 

Ultimately, how do you think this work will be applied?

It will be applied to understand how Earth got its volatiles. If these planetesimals were experiencing that much melting and evaporation, then we really do have to bring our volatiles in later, after Earth's initial formation. People like me have been more on the side of the volatiles coming from the volcanoes belching out from the interior of Earth. But if these planetesimals are boiling all the time, then it’s score one for adding water late. So I think that’s how it’s going to be applied – trying to understand how Earth got its volatiles.


A chondrite meteorite that fell on Alberta, Canada on June 9, 1952. Credit: Captmondo, via Wikimedia Commons

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