Reading 31

Rock Atmosphere

The following passage is excerpted from an article entitled “Cloudy with a chance of pebble showers” (© 2009 by Diana Lutz).

  1. So accustomed are we to the
  2. sunshine, rain, fog and snow
  3. of our home planet that we
  4. find it next to impossible to
  5. imagine other forms of
  6. precipitation. To be sure, Dr.
  7. Seuss came up with a green
  8. gluey substance called oobleck
  9. that fell from the skies, but it
  10. had to be conjured up by
  11. wizards. Not so the atmosphere
  12. of COROT-7b, an exoplanet
  13. discovered last February by the
  14. COROT space telescope.
  15. According to models,
  16. COROT-7b’s atmosphere is
  17. made up of the ingredients of
  18. rocks and when “a front
  19. moves in,” pebbles condense
  20. out of the air and rain into
  21. lakes of molten lava below.
  22. In August 2009 a consortium
  23. of European observatories
  24. reported the discovery of
  25. COROT-7c, a second planet
  26. orbiting COROT-7. Using the
  27. data from both planets, they
  28. were able to calculate that
  29. COROT-7b has an average
  30. density about the same as
  31. Earth’s. This means it is
  32. almost certainly a rocky
  33. planet made up of silicate
  34. rocks like those in Earth’s
  35. crust. Not that anyone would
  36. call it Earth. The planet and
  37. its star are separated by only
  38. 1.6 million miles, 23 times
  39. less than the distance between
  40. the parboiled planet Mercury
  41. and our Sun. Because the
  42. planet is so close to the star,
  43. it is gravitationally locked to
  44. it in the same way the Moon
  45. is locked to Earth. One side
  46. of the planet always faces its
  47. star. This star-facing side
  48. has a temperature of about
  49. 4220 degrees Fahrenheit.
  50. That’s hot enough to vaporize
  51. rocks. The global average
  52. temperature of Earth’s
  53. surface, in contrast, is only
  54. about 59 degrees. The side in
  55. perpetual shadow, on the
  56. other hand, is positively
  57. chilly at -369 degrees
  58. Fahrenheit.
  59. Perhaps because they were
  60. cooked off, COROT-7b’s
  61. atmosphere has none of the
  62. volatile elements or
  63. compounds that make up
  64. Earth’s atmosphere, such
  65. as water, nitrogen and carbon
  66. dioxide. “The only atmosphere
  67. this object has is produced
  68. from vapor arising from hot
  69. molten silicates in a lava lake
  70. or lava ocean,” Fegley says.
  71. What might that atmosphere
  72. be like? To find out Fegley,
  73. along with research assistant
  74. Laura Schaefer, have used
  75. thermochemical equilibrium
  76. calculations to model
  77. COROT-7b’s atmosphere.
  78. The calculations, which reveal
  79. which mineral assemblages are
  80. stable under different
  81. conditions, were carried out
  82. with MAGMA, a computer
  83. program Fegley developed in
  84. 1986. Fegley and Schaefer
  85. modified the MAGMA program
  86. in 2004 in order to study
  87. high-temperature volcanism
  88. on Io, Jupiter’s innermost
  89. satellite. Because the scientists
  90. didn’t know the exact
  91. composition of the planet,
  92. they ran the program with
  93. four different starting
  94. compositions. “We got
  95. essentially the same result
  96. in all four cases,” says Fegley.
  97. “Sodium, potassium, silicon
  98. monoxide and then oxygen
  99. make up most of the
  100. atmosphere.” But there are
  101. also smaller amounts of the
  102. other elements found in
  103. silicate rock, such as iron.
  104. Why is there oxygen on a
  105. dead planet, when it didn’t
  106. show up in Earth’s atmosphere
  107. until 2.4 billion years ago,
  108. when plants started to produce
  109. it? Oxygen is the most abundant
  110. element in rock, so when you
  111. vaporize rock, you end up
  112. producing a lot of oxygen.
  113. The peculiar atmosphere has
  114. its own singular weather.
  115. “As you go higher the
  116. atmosphere gets cooler and
  117. eventually you get saturated
  118. with different types of ‘rock’
  119. the way you get saturated
  120. with water in the atmosphere
  121. of Earth,” explains Fegley.
  122. “But instead of a water cloud
  123. forming and then raining
  124. water droplets, you get a ‘rock
  125. cloud’ forming and it starts
  126. raining out little pebbles of
  127. different types of rock.” Even
  128. more strangely, the kind of
  129. rock condensing out of the
  130. cloud depends on the
  131. altitude. The atmosphere
  132. works the same way as
  133. fractionating columns, the
  134. tall knobby columns that
  135. make petrochemical plants
  136. recognizable. In a
  137. fractionating column, crude
  138. oil is boiled and its
  139. components condense out,
  140. with the heaviest one at the
  141. bottom, and the lightest
  142. rising to the top.
  143. Instead of condensing out
  144. hydrocarbons such as
  145. petroleum and gasoline, the
  146. exoplanet’s atmosphere
  147. condenses out minerals. In
  148. both cases the fractions fall
  149. out in order of boiling point.
  150. Elemental sodium and
  151. potassium, which have very
  152. low boiling points in
  153. comparison with rocks, do
  154. not rain out but would
  155. instead stay in the
  156. atmosphere, where they
  157. would form high gas
  158. clouds buffeted by the
  159. stellar wind from COROT-7.