Vaporising the Earth Helps Hunt for Earth-like Planets

“We scientists are not content just to talk about vaporizing the Earth,” says Bruce Fegley, professor of earth and planetary sciences at Washington University in St. Louis, tongue firmly in cheek. “We want to understand exactly what it would be like if it happened.”

Now, there’s scientific curiosity, which is all well and good, but this latest research — conducted by Fegley and his colleagues Katharina Lodders, PhD, a research professor of earth and planetary sciences who is currently on assignment at the National Science Foundation, and Laura Schaefer, currently a graduate student at Harvard University — also has the added bonus of helping astronomers searching for super-Earths.

Scientists at Washington University have simulated the atmospheres of hot Earth-like planets, such as CoRoT-7b, shown here in an artist’s conception. CoRoT-7b orbits so close to its star that its starward side is an ocean of molten rock. By looking for atmospheres like those generated by the simulations, astronomers should be able to identify Earth-like exoplanets.
A super-Earth is not necessarily as the name would lead you to think, being simply an exoplanet more massive than Earth and less massive than Neptune made up of rock instead of gas. And, given that the current method of detecting super-Earths leads us to find planets orbiting very close to their stars, what we’re finding are planets within rock-melting distance. Subsequently, understanding the properties of a planet that has been mostly vaporised is intensely helpful.

The research shows that Earth-like planets orbiting so close to their stars would have atmospheres composed mostly of steam and carbon dioxide, with smaller levels of other gases that may be helpful in determining the composition of one plant from another.

Two Model Earths

Fegley and his team ran calculations on two Earth simulations; one was an Earth very like our own, and the other was an Earth modeled after an Earth prior to the formation of its continental crust, known as a bulk silicate Earth. The difference between the two models, says Fegley, is water. The Earth’s continental crust is dominated by granite, but you need water to make granite. If you don’t have water, you end up with a basaltic crust like Venus. Both crusts are mostly silicon and oxygen, but a basaltic crust is richer in elements such as iron and magnesium.

One issue to arise from the simulations is the fact that, currently, our own Earth is not necessarily a great analog for hunting lifeless super-Earths. We’ve affected the planet too much, oxidizing the crust which also led to the production of vast reservoirs of reduced carbon.

Raining Rock

“The vapor pressure of the liquid rock increases as you heat it, just as the vapor pressure of water increases as you bring a pot to boil,” Fegley says. “Ultimately this puts all the constituents of the rock into the atmosphere.”

The continental crust melts at about 940 C (1,720 F), Fegley says, and the bulk silicate Earth at roughly 1730 C (3,145 F). There are also gases released from the rock as it heats up and melts.

On both Earths the simulations were dominated by a wide temperature range of steam (from vaporizing water and hydrated minerals) and carbon dioxide (from vaporizing carbonate rocks). The main difference between the two models is that the bulk silicate Earth would contain gases that would oxidize if oxygen were present. At temperatures below about 730 C (1,346 F) the BSE atmosphere, for example, contains methane and ammonia.

That’s interesting, Fegley says, because methane and ammonia, when sparked by lighting, combine to form amino acids, as they did in the classic Miller-Urey experiment on the origin of life.

At temperatures above about 730 C, sulfur dioxide would enter the atmosphere, Fegley says. “Then the exoplanet’s atmosphere would be like Venus’s, but with steam,” Fegley says.

Proving that even scientists have a sense of humour, the team admitted that they had cranked the temperature up on their Earth simulation to the point where the entire planet vaporised.

“You’re left with a big ball of steaming gas that’s knocking you on the head with pebbles and droplets of liquid iron,” he says. “But we didn’t put that into the paper because the exoplanets the astronomers are finding are only partially vaporized,” he says.

Source: Washington University in St. Louis
Image Source: A. LEGER ET AL./ICARUS

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