Solar Wind Sputtering
Solar Wind Sputtering is a process where the solar wind slowly strips away the atmosphere of a planet without a strong magnetic field. It is thought to have removed about 1/3 of Mars' early atmosphere. See Atmospheric Loss for a full description of what has happened to Mars' originally thick atmosphere.
Contents
Overview
Atmospheric Loss of Light Gases
Atoms and molecules with low mass move faster at a given temperature. On worlds with lower gravity, gases with low masses such as hydrogen, helium, neon, may be moving fast enough that those moving the fastest exceed the escape velocity of the planet and thus are lost to space.
Thus, hot planets will lose gases faster than cold ones and planets with low surface gravity will lose gases faster than ones with higher gravity.
Mars has a surface gravity that it should be able to hang on to heavier molecules such as oxygen (O2), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), and heavier gases.
However, Mars originally had a thick atmosphere of 3 or 4 bar, but now is a near vacuum (0.6 bar). What caused the atmosphere to be stripped away? (Note: some scientists think that Mars could not have such a thick early atmosphere, but it is hard to explain how Mars could have had running water ~4 to 5 billion years ago without it.)
Gases Absorbed Into the Crust
See Atmospheric Loss for more details, but it is clear that most of Mars' early atmosphere has been absorbed into the crust, but this does not account for all of the missing atmosphere.
Solar Wind Sputtering=
When high energy electromagnetic radiation (such as X-rays or Ultraviolet Radiation hits the top of the Martian atmosphere, it can ionize gases. High energy impact of particles such as cosmic rays or Solar Radiation can also cause ionization.
Ionized gases are strongly effected by electric charges and magnetic fields. They can get caught up by the magnetic fields of the solar wind, and be pulled away from the planet, despite the planet's gravity being strong enough to hold on to them. It is thought that Mars has lost about 1/3 of its early atmosphere via this method. Satellites have detected this loss around Mars, and tho slow, it is significant over hundred of millions of years.
Sputtering should allow gases to escape from heavy planets, but Venus (also with no magnetic field) has a solar wind more than 4.4 times that of Mars and has not lost much atmosphere to solar wind sputtering. This is far less than predicted so our models need to be adjusted.
