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. Image: A. Leger et al./Icarus |
In science fiction novels, evil overlords and hostile aliens often threaten to vaporize the Earth. At the beginning of The Hitchhikers Guide to the Galaxy,
the officiously bureaucratic aliens called Vogons, authors of the
third-worst poetry in the universe, actually follow through on the
threat, destroying the Earth to make way for a hyperspatial express
route.
“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.”
And
in fact Fegley, PhD, 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, have vaporized the
Earth—if only by simulation, that is mathematically and inside a
computer.
They
weren’t just practicing their evil overlord skills. By baking model
Earths, they are trying to figure out what astronomers should see when
they look at the atmospheres of super-Earths in a bid to learn the
planets’ compositions.
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 which orbit close to their stars—within
rock-melting distance.
Their NSF- and NASA-funded research, described in the August 10 issue of The Astrophysical Journal,
show 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.
Motivated by degeneracy
Under
favorable circumstances planet hunting techniques allow astronomers not
just to find exoplanets but also to measure their average density.
The
average density together with theoretical models lets the astronomers
figure out the bulk chemical composition of gas giants, 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.
This is an outcome scientists, who would prefer one answer per question, call degeneracy.
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
determine the composition of the planet’s atmosphere, which allows them
to distinguish about 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,” Fegley says. “Right
now, there are eight transiting exoplanets 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,” Fegley says.
Two model Earths
Even
though the planets are called super-Earths, Fegley says, the term is a
reference to their mass and makes no claim about their composition, much
less their habitability. But, he says, you start with what you know.
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, 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.
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.
Raining acid and rock
The
super-Earths the team used as references are thought to have surface
temperatures ranging from about 270 to 1700 C, which
is about 520 to 3,090 F. The Earth, in contrast, has a global
average surface temperature of about 15 C (59 F) and the
oven in your kitchen goes up to about 450 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
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.
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 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.
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.
Asked
whether his team ever cranked the temperature high enough to vaporize
the entire Earth, not just the crust and the mantle, Fegley admits that
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,” he says. “But we didn’t put
that into the paper because the exoplanets the astronomers are finding
are only partially vaporized,” he says.