"Long before it's in the papers"
August 03, 2012

RETURN TO THE WORLD SCIENCE HOME PAGE

Earth vaporized in simulation

March 30, 2005
Courtesy of Wash­ing­ton Uni­vers­ity in St. Lou­is
and World Science staff

In sci­ence fic­tion, al­iens of­ten threat­en to va­por­ize the Earth. At the be­gin­ning of The Hitch­hik­ers’ Guide to the Gal­axy, of­fi­ciously bu­reau­crat­ic al­iens called Vo­gons ac­tu­ally fol­low through, zap­ping the plan­et to make way for a high­way.

But va­por­iz­ing Earth might also have sci­en­tif­ic ben­e­fits—at least if done safely with­in the con­fines of a com­put­er sim­ula­t­ion, some re­search­ers say. 

Scientistshave 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. (Im­age cour­te­sy A. Leg­er et al./I­carus )

In science fiction, evil overlords and hostile aliens often threaten to vaporize the Earth. At the beginning of The Hitchhikers Guide to the Galaxy, officiously bureaucratic aliens called Vogons actually follow through, destroying the planet to make way for a hyperspatial express route. But vaporizing the Earth might have scientific benefits—at least if done safely within the confines of a computer simulation, some researchers say. This exercise could help astronomers figure out whether distant, rocky planets are Earth-like in their makeup. Some of these worlds really are partially vaporized, and that may create bizarre effects such as pebble rains, scientists claim. “We scientists are not content just to talk about vaporizing the Earth,” said Bruce Fegley, a planetary scientist at Washington University in St. Louis, tongue firmly in cheek. “We want to understand exactly what it would be like if it happened.” Fegley and colleagues Katharina Lodders, also a scientist at Washington University, and Laura Schaefer, a graduate student at Harvard University, carried out the virtual vaporization. By baking model Earths, they’re trying to figure out what astronomers should see when they look at the atmospheres of alien “super-Earths” in other solar systems. This could help them understand the planets’ compositions. Super-earths are planets outside our solar system that are heavier than Earth but lighter than Neptune, and consist mainly of rock. Because of the techniques used to find them, most known super-Earths orbit close to their stars—within rock-melting distance. The research, described in the Aug. 10 issue of The Astrophysical Journal, finds that Earth-like planets as hot as these 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. The team is collaborating with a research group led by Mark Marley at the NASA Ames Research Center to define how these gas abundances would be detectable in terms of reaching our telescopes. Under favorable circumstances planet hunting techniques let astronomers not just find alien planets but also to measure their average density, or compactness. That, along with theoretical models, lets astronomers figure out the chemical composition of giant gaseous planets. But in the case of rocky planets the possible variety of rocky ingredients can often add up several different ways to the same average density. If a planet passes in front of its star so that astronomers can observe the light from the star filtered by the planet’s atmosphere, they can also determine the makeup of the planet’s atmosphere, which lets them distinguish alternative planetary compositions. “More people are looking at the atmospheres,” Fegley said. “Right now, there are eight transiting exoplanets [planets in other solar systems] where astronomers have done some atmospheric measurements and more will probably be reported in the near future.” “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,” he added. The team ran calculations on two types of “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. The difference is water, Fegley said. Earth’s continental crust is dominated by granite, but you need water to make granite. Without it, you get a “basaltic” crust like Venus. Both are mostly silicon and oxygen, but a basaltic crust is richer in elements such as iron and magnesium. The super-Earths the team used as references are thought to have surface temperatures ranging from about 270 to 1700 degrees Celsius (C), or about 520 to 3,090 degrees F. The team determined what would be gaseous at such 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 said. “Ultimately this puts all the constituents of the rock into the atmosphere.” The continental crust melts at about 940 C (1,720 F), Fegley said, 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. The calculations showed that the atmospheres of both model Earths would be dominated over a wide temperature range by steam and carbon dioxide. The major difference between the models is that the BSE atmosphere is more “reducing,” meaning it contains gases that would combine with oxygen if it were present. At temperatures below about 730 C (1,346 F) the BSE atmosphere, for example, contains methane and ammonia. That’s interesting, Fegley said, because methane and ammonia, when sparked by lighting, combine to form amino acids, key ingredients of life. At temperatures above about 730 C, sulfur dioxide would enter the atmosphere, Fegley said. “Then the exoplanet’s atmosphere would be like Venus’s, but with steam,” Fegley said. 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 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, Fegley said. In only some runs of the simulation did the team crank the temperature high enough to vaporize the whole Earth, just to see what would happen. “You’re left with a big ball of steaming gas that’s knocking you on the head with pebbles and droplets of liquid iron,” Fegley said. “But we didn’t put that into the paper because the exoplanets the astronomers are finding are only partially vaporized.”