Earth model vaporized to learn about atmospheres of hot exoplanets
Washington, August 3 (ANI): Scientists have vaporized the Earth-only by simulation, that is, mathematically and inside a computer - as they try to figure out what astronomers should see when they look at the atmospheres of super-Earths in a bid to learn the planets' compositions.
This is the work of Bruce Fegley, professor of Earth and planetary sciences at Washington University in St. Louis, 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.
Super-Earths are planets outside our solar system (exoplanets) that are more massive than Earth but 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 that orbit close to their stars-within rock-melting distance.
Their NSF- and NASA-funded research showed that 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.
The WUSTL team is collaborating with Dr. Mark Marley's research group at the NASA Ames Research Center to convert the gas abundances they have calculated into synthetic spectra the planet hunters can compare to spectra they measure.
"We modelled 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," Fegley said.
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.
The difference between the two models, said 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.
Fegley is quick to admit the Earth's continental crust is not a perfect analog for lifeless planets because it has been modified by the presence of life over the past four billion years, which both oxidized the crust and also 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 about 520 to 3,090 degrees F. The Earth, in contrast, has a global average surface temperature of about 15 degrees C (59 degrees F) and the oven in your kitchen goes up to about 450 Fahrenheit.
Using thermodynamic equilibrium calculations, the team determined which elements and compounds would be gaseous at these alien temperatures.
Their calculations showed that 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.
That's interesting, Fegley noted, 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 said.
"Then the exoplanet's atmosphere would be like Venus's, but with steam," he added.
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 amusing 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.
Their research has been described in the August 10 issue of The Astrophysical Journal. (ANI)
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