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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 )


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 help ob­serv­ers 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 af­fect the light given off.

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,” 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 that 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 1,700 de­grees C, or about 520 to 3,090 de­grees F. The team de­ter­mined what would be gas­e­ous at such tem­per­a­tures.

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.”


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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.”