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.
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.
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. Note that then, the Sun was only 70% as warm as it is now, so running water on an early Mars is doubly hard to explain.)
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.
Detailed Discussion
Paleomagnetism
When molten rock cools, magnetic particles inside it (such as iron, nickel, or cobalt) align with the planet's magnetic field, and as the rock freezes, these particle are locked in place. This gives the rock a weak permanent magnetic field. Martian rocks exhibit this, so we know that early Mars did have a magnetic field. By looking at areas with such 'paleomagnetism' scientists can determine the strength and direction of the magnetic field around the rocks at the time they solidified.
It has been found that this very weak paleomagnetic field provides enough of a magnetic field to protect the atmosphere above some regions on Mars. So some areas above large, ancient lava flows suffer less gas loss to sputtering.
These Martian crustal fields cover ~2/3 of the Martian surface, but the strongest are in the Southern highlands, particularly in the Sirenum and Terra Cimmerian regions. Above these regions, the magnetic field is about 1,000 nanoTeslas, where as the induced magnetic field at the top of the atmosphere is typically 30nT. (The Earth's magnetic field is about 30,000 nT in comparison.)
Originally it was thought that these regions had only local influence, but we are finding that even these extremely weak fields are enough to effect the global dynamics. They are strongest in the Southern Hemisphere, which is the side of Mars which points at the Sun when the planet is closest to the Sun. Thus, it gets the most protection from these paleomagnetic fields, when it is feeling the strongest effects from the solar wind.
The topology of these magnetic fields reacts in complex ways with the Interplanetary Magnetic Field, which may shield the planet, or direct streams of particles into the atmosphere. They also increase the turbulence and complexity of the magnetic fields lines flowing around the planet. Generally, when they are facing the sun, they push the Bow Shock (see below) further from the planet.
In places where the magnetic field lines are vertical, they promote the solar wind hitting the atmosphere, and allow easier mixing of the upper and lower levels of the ionosphere.
It was surprising how strongly these weak magnetic fields effect the solar wind.
Isotope Ratios
Many gases are made of atoms with more than one Isotope. Since these differ in mass, lighter isotopes are less dense and tend to rise to the top of the Martian atmosphere. These are more subject to sputtering, and thus, lighter isotopes tend to be lost more quickly than heavier ones. By measuring the ratios between light and heavier isotopes, scientists have estimated that about 1/3 of the Martian atmosphere has been lost to space.
Direct Measurements by Orbiting Spacecraft
Three spacecraft have collected most of the data on this subject, namely: Phobos 2, Mars Global Surveyor, and Mars Express. Other space craft have provided some data. The Pioneer Venus Orbiter, and the Venus Express have looked at Venus' solar wind interactions and given data that can be contrasted with Martian conditions.
They have found that Mars (and Venus) has no large magnetosphere, but interactions with the solar wind on the upper atmosphere of the planets create an induced magnetic field. (The upper atmosphere becomes ionized, which can then carry electric currents.) Such currents are inducted by the magnetic field, which create their own magnetic field in opposition to the Interplanetary Magnetic Field. These induced fields are much weaker than Earth's magnetic field, but still help to protect the atmosphere. To create this induced field, the magnetic field must vary. The Interplanetary Magnetic Field (IMF) does in fact vary a great deal, so such protective fields are induced, but at random intervals, the IMF is steady enough that the induced fields collapse regionally, so the solar wind CAN entrain gas particles and lift them away from the planet.
The interactions of the solar wind this Mars' upper atmosphere are complex, varying with time, region, and altitude. Numerous models have been created and while no single model can describe the whole of this system, they are very successful in describing subsets of the environment.
Bow Shock
The bow shock is where the Interplanetary Magnetic Field (IMF) hits this induced magnetic field and is directed around Mars. It marks where the flow of the solar wind goes from supersonic to subsonic flow. It lies closest to Mars under the sub-solar point, about 1.6 Mars radii, from the planet.
Similar to Venus, the shock distance and the flow of plasma around the planet is close relative to the size of the planet. (Earth's powerful magnetic field keeps this flow much further from us.) Models have so far done a poor job of modelling the details of the flow around the planet; the flow around Mars (and Venus) show various turbulence and asymmetries, which the models do not capture.
As the particles of the solar wind hit the bow shock they are suddenly slowed and heat up. Thus they are considerably hotter than particles further out from the planet. As the solar wind picks up atmospheric particles it is further slowed, and extends partly around the planet into the night side. The details of the region where the solar wind dominates and where the planetary ions dominate have proved hard to model. Scientists are unsure if there is a single boundary, or several closely spaced ones, and instrument readings are inconsistent in time.
Flow of Ions Around and Away From Mars
Some ions are directed towards the planet and impact the exosphere heating it. Others become magnetically entrained with the solar wind and are directed away from the planet. Note, that if an ion missing an electron, then absorbs one, it decouples from the magnetic field and is strongly effected by gravity. If it is not moving too fast, it will fall back to Mars.
The solar side of Mars' atmosphere is ionized by solar radiation. However, the night side has very few ions. This effects the dynamics of the plasma flow as it moves into the Martian night side, cooling it and decoupling it from the Martian atmosphere. The magnetic field of this plasma flow is very weak on the Martian night side, being about 10 nanoTeslas. (Tho this is still stronger than the Interplanetary Magnetic Field (IMF) far from Mars.)
The induced magnetic lines (in the atmosphere) and the IMF interact with each other in very complex, turbulent ways. But they can locally connect, which results in a great deal of magnetic energy being transferred to the plasma, heating it. This typically happens at the terminator, or at the near night side of the planet.
The magnetic fields entrained in the plasma flow around Mars, and move away from the planet into deep space behind it. This flow induces an 'induced magnetic tail' behind Mars, which is very weak as there are few ions in this region.
Solar Storms
Observations of young stars suggest that Coronal Mass Ejections (CME) or solar storms were more common, and more powerful with our young sun. However, early in Mars' history, it had a magnetic field protecting it. It is debated if the early solar storms would have had an outsized effect on the rate of Martian atmospheric loss, or if the magnetic field lasted long enough to cancel this effect.
Modern solar storms are expected to increase the rate of atmospheric loss, but the rate of the increase has not been well measured.
The sun produces very few x-rays. Most of the ionization of the atmosphere is caused by UV rays. However in a large CME, there is a significant increase in UV and x-rays, which means the upper atmosphere is more strongly ionized.
While a CME (on the side of the sun facing Mars) will always cause more ionization, the flood of solar particles usually will miss Mars. In the case when the energetic solar particles hit the planet, there is far more energy available to pick up Martian ions. (And some of these high energy solar particles will collide with the ionosphere, ionizing it more, and heating it.)
It is estimated that a CME hitting Mars will increase the rate of atmospheric loss by 2 to 100 times (depending on the CME intensity) for the couple hour duration of the solar storm.
