Vaporizing the Earth for science

Posted by Trent Nouveau

Evil galactic overlords and hostile aliens are often threatening to vaporize the Earth in science fiction novels and films.

Indeed, in The Hitchhikers Guide to the Galaxy, the officiously bureaucratic aliens known as the Vogons actually follow through on their nefarious threats - destroying planet Earth simply to make way for a hyperspatial express route (intergalactic highway).

Apparently, there are some scientists who are not content with just talking about vaporizing the Earth, but rather, want to understand exactly what it would be like if such an event actually occurred.

That is why Professor Bruce Fegley of Washington University, Katharina Lodders of the National Science Foundation and Laura Schaefer of Harvard University have vaporized the Earth - if only by simulation, that is mathematically and inside a computer.

However, the team wasn’t just practicing its evil overlord skills. No, the researchers were attempting to determine what astronomers would see when analyzing the atmospheres of super-Earths in a bid to learn the planets' compositions.

To be sure, super-earths are planets outside our solar system (exoplanets) that are more massive than Earth, yet less massive than Neptune and made of rock instead of gas. Because of the techniques used to find them, most of the detected super-Earths are those which orbit close to their stars - within rock-melting distance.

Earth-like planets as hot as these exoplanets would have atmospheres composed mostly of steam and carbon dioxide, with smaller amounts of other gases that could be used to distinguish one planetary composition from another. Under favorable conditions, planet hunting techniques allow astronomers not just to locate exoplanets but also measure their average density.

Together with theoretical models, this allows astronomers to calculate the bulk chemical composition of gas giants. However, in the case of rocky planets the possible variety of rocky ingredients often adds up in several different ways to the same average density. This is an outcome scientists, who would prefer one answer per question, refer to as degeneracy.

Alternatively, if a planet passes in front of its star astronomers can observe the light from the star filtered by the planet's atmosphere and determine the composition of the planet's atmosphere - allowing them to distinguish alternative bulk planetary compositions.

"It's not crazy that astronomers can do this and more people are looking at the atmospheres of these transiting exoplanets. Right now, there are eight transiting exoplanets where astronomers have done some atmospheric measurements and more will probably be reported in the near future,” Fegley explains. 

"We modeled the atmospheres of hot super-Earths because that's what astronomers are finding and we wanted to predict what they should be looking for when they look at the atmospheres to decipher the nature of the planet."

Even though the planets are called super-Earths, the term is a reference to their mass and does not refer to their composition or habitability.

"The team ran calculations on two types of pseudo-Earths, one with a composition like that of the Earth's continental crust and the other, called the BSE (bulk silicate Earth), with a composition like the Earth's before the continental crust formed, which is the composition of the silicate portion of the primitive Earth before the crust formed," says Fegley.

"The difference between the two models? 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."

Fegley is quick to acknowledge the Earth's continental crust is not a perfect analog for lifeless planets as it has been modified by the presence of life over the past four billion years - which both oxidized the crust and led to production of vast reservoirs of reduced carbon, for example in the form of coal, natural gas, and oil.

The super-Earths the team used as references are thought to have surface temperatures ranging from about 270 to 1700 degrees Celsius (C), which is approximately 520 to 3,090 degrees F. The Earth, in contrast, has a global average surface temperature of about 15 degrees C (59 degrees F).

Using thermodynamic equilibrium calculations, the team determined which elements and compounds would be gaseous at these alien temperatures.

"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 notes. "Ultimately this puts all the constituents of the rock into the atmosphere."

The continental crust melts at about 940 C (1,720 F) 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. Their calculations indicated the atmospheres of both model Earths would be dominated over a wide temperature range by steam (from vaporizing water and hydrated minerals) and carbon dioxide (from vaporizing carbonate rocks).

The major difference between the models is that the BSE atmosphere is more reducing, meaning that it contains 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. This is of interest, the scientists say, 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. "Then the exoplanet's atmosphere would be like Venus's, but with steam," Fegley confirms.

The gas most characteristic of hot rocks, however, is silicon monoxide, which would be found in the atmospheres of both types of planets at temperatures of 1,430 C (2,600 F) or higher.

This leads to the possibility that as frontal systems moved through this exotic atmosphere, the silicon monoxide and other rock-forming elements might condense and rain out as pebbles. Asked whether his team ever cranked the temperature high enough to vaporize the entire Earth, not just the crust and the mantle, Fegley admits they did.

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