Rock Atmosphere
The following passage is excerpted from an article entitled “Cloudy with a chance of pebble showers” (© 2009 by Diana Lutz).
- So accustomed are we to the
- sunshine, rain, fog and snow
- of our home planet that we
- find it next to impossible to
- imagine other forms of
- precipitation. To be sure, Dr.
- Seuss came up with a green
- gluey substance called oobleck
- that fell from the skies, but it
- had to be conjured up by
- wizards. Not so the atmosphere
- of COROT-7b, an exoplanet
- discovered last February by the
- COROT space telescope.
- According to models,
- COROT-7b’s atmosphere is
- made up of the ingredients of
- rocks and when “a front
- moves in,” pebbles condense
- out of the air and rain into
- lakes of molten lava below.
- In August 2009 a consortium
- of European observatories
- reported the discovery of
- COROT-7c, a second planet
- orbiting COROT-7. Using the
- data from both planets, they
- were able to calculate that
- COROT-7b has an average
- density about the same as
- Earth’s. This means it is
- almost certainly a rocky
- planet made up of silicate
- rocks like those in Earth’s
- crust. Not that anyone would
- call it Earth. The planet and
- its star are separated by only
- 1.6 million miles, 23 times
- less than the distance between
- the parboiled planet Mercury
- and our Sun. Because the
- planet is so close to the star,
- it is gravitationally locked to
- it in the same way the Moon
- is locked to Earth. One side
- of the planet always faces its
- star. This star-facing side
- has a temperature of about
- 4220 degrees Fahrenheit.
- That’s hot enough to vaporize
- rocks. The global average
- temperature of Earth’s
- surface, in contrast, is only
- about 59 degrees. The side in
- perpetual shadow, on the
- other hand, is positively
- chilly at -369 degrees
- Fahrenheit.
- Perhaps because they were
- cooked off, COROT-7b’s
- atmosphere has none of the
- volatile elements or
- compounds that make up
- Earth’s atmosphere, such
- as water, nitrogen and carbon
- dioxide. “The only atmosphere
- this object has is produced
- from vapor arising from hot
- molten silicates in a lava lake
- or lava ocean,” Fegley says.
- What might that atmosphere
- be like? To find out Fegley,
- along with research assistant
- Laura Schaefer, have used
- thermochemical equilibrium
- calculations to model
- COROT-7b’s atmosphere.
- The calculations, which reveal
- which mineral assemblages are
- stable under different
- conditions, were carried out
- with MAGMA, a computer
- program Fegley developed in
- 1986. Fegley and Schaefer
- modified the MAGMA program
- in 2004 in order to study
- high-temperature volcanism
- on Io, Jupiter’s innermost
- satellite. Because the scientists
- didn’t know the exact
- composition of the planet,
- they ran the program with
- four different starting
- compositions. “We got
- essentially the same result
- in all four cases,” says Fegley.
- “Sodium, potassium, silicon
- monoxide and then oxygen
- make up most of the
- atmosphere.” But there are
- also smaller amounts of the
- other elements found in
- silicate rock, such as iron.
- Why is there oxygen on a
- dead planet, when it didn’t
- show up in Earth’s atmosphere
- until 2.4 billion years ago,
- when plants started to produce
- it? Oxygen is the most abundant
- element in rock, so when you
- vaporize rock, you end up
- producing a lot of oxygen.
- The peculiar atmosphere has
- its own singular weather.
- “As you go higher the
- atmosphere gets cooler and
- eventually you get saturated
- with different types of ‘rock’
- the way you get saturated
- with water in the atmosphere
- of Earth,” explains Fegley.
- “But instead of a water cloud
- forming and then raining
- water droplets, you get a ‘rock
- cloud’ forming and it starts
- raining out little pebbles of
- different types of rock.” Even
- more strangely, the kind of
- rock condensing out of the
- cloud depends on the
- altitude. The atmosphere
- works the same way as
- fractionating columns, the
- tall knobby columns that
- make petrochemical plants
- recognizable. In a
- fractionating column, crude
- oil is boiled and its
- components condense out,
- with the heaviest one at the
- bottom, and the lightest
- rising to the top.
- Instead of condensing out
- hydrocarbons such as
- petroleum and gasoline, the
- exoplanet’s atmosphere
- condenses out minerals. In
- both cases the fractions fall
- out in order of boiling point.
- Elemental sodium and
- potassium, which have very
- low boiling points in
- comparison with rocks, do
- not rain out but would
- instead stay in the
- atmosphere, where they
- would form high gas
- clouds buffeted by the
- stellar wind from COROT-7.