Atmospheric escape

Atmospheric escape is the loss of planetary atmospheric gases to outer space.

Contents

Thermal escape mechanisms

One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies continuously as they collide with one another, gaining and losing kinetic energy. The variation in kinetic energy among the molecules is described by the Maxwell distribution. The kinetic energy and mass of a molecule determine its velocity by E_{\mathit{kin}}=\frac{1}{2}mv^2.

Individual molecules in the high tail of the distribution may reach escape velocity, at a level in the atmosphere where the mean free path is comparable to the scale height, and leave the atmosphere.

The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature, and the less likely it is that any of them reach escape velocity.

This is why hydrogen escapes from a given atmosphere more easily than carbon dioxide. Also, if the planet has a higher mass, the escape velocity is greater, and fewer particles will escape. This is why the gas giant planets have significant amounts of hydrogen and helium, which escape on Earth. Distance from the star also plays a part; a close planet has a hotter atmosphere, with a faster range of velocities, and more chance of escape. A distant body has a cooler atmosphere, with a slower range of velocities, and less chance of escape. This helps Titan, which is small compared to Earth but further from the Sun, keep its atmosphere.

However, while it has not been observed, it is theorized that an atmosphere with a high enough pressure and temperature can undergo a 'blow-off'. In this situation molecules basically just flow off into space. Here it is possible to lose heavier molecules that would not normally be lost.

Significance of solar winds

The relative importance of each loss process is a function of planet mass, atmosphere composition, and distance from a star. A common erroneous belief is that the primary non-thermal escape mechanism is atmospheric stripping by a solar wind in the absence of a magnetosphere. Excess kinetic energy from solar winds can impart sufficient energy into atmospheric particles to reach escape velocity, causing atmospheric escape. The solar wind, composed of ions, is deflected by magnetic fields because the charged particles within the wind flow along magnetic field lines. The presence of a magnetic field thus deflects solar winds, preventing atmospheric loss to solar winds. On Earth, for instance, the interaction between the solar wind and magnetic field deflects the solar wind around the planet, with near total deflection around 10 Earth radii away.[1] This region of deflection is called a bow shock.

Depending on planet size and atmospheric composition, however, a lack of magnetic field does not determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitudes denser than Earth’s.[2] Recent models indicate that stripping by solar wind accounts for less than 1/3 of total non-thermal loss processes.[2]

While Venus and Mars have no magnetosphere to protect the atmosphere from solar winds, photoionizing radiation (sunlight) and the interaction of the solar wind with the atmosphere of the planets causes ionization of the uppermost part of the atmosphere. This ionized region of atmosphere, in turn, induces magnetic moments that deflect solar winds much like a magnetic field, limiting solar wind effects to the uppermost altitudes of atmosphere, roughly 1.2-1.5 planetary radii away from the planet, or an order of magnitude closer to the surface than Earth's magnetic field creates. Past this region, also called a bow shock, the solar wind is slowed to subsonic velocities.[1] Nearer to the surface, solar wind dynamic pressure balances with pressure from the ionosphere, at a region called the ionopause. This interaction typically prevents solar wind stripping from being the dominant loss process of the atmosphere.

Comparison of non-thermal loss processes based on planet and particle mass

Dominant non-thermal loss processes differ based on the planetary body in discussion. The varying relative significance of each process is based on planetary mass, atmospheric composition, and distance from the sun. The dominant nonthermal loss processes for Venus and Mars, two terrestrial planets without magnetic fields, are dissimilar. The dominant nonthermal loss process on Mars is pick-up from solar winds, because the atmosphere is not dense enough to shield itself from the winds during peak solar activity.[2] Venus is somewhat shielded from solar winds by merit of a denser atmosphere, and solar pick-up is not the dominant nonthermal loss process on Venus. Smaller bodies without magnetic fields are more likely to suffer from solar winds, because the planet is too small to hold sufficient atmosphere to stop solar winds.

The dominant loss process for Venus is loss through electric force field acceleration. Because electrons are more mobile than other particles, they are more likely to escape from the top of the ionosphere of Venus.[2] As a result, a minor net positive charge can develop. The net positive charge, in turn, creates an electric field that can accelerate other positive charges out of the system. Through this, H+ ions are accelerated beyond escape velocity, causing atmospheric escape through this process. Other important loss processes on Venus are photochemical reactions, driven by proximity to the Sun. Photochemical reactions rely on splitting the molecules into constituent atoms, often with a significant portion of kinetic energy maintained in the less massive particle. This particle is of sufficiently low mass and high kinetic energy to escape from Venus. Oxygen, relative to hydrogen, is not of sufficiently low mass to escape through this mechanism on Venus.

Phenomena of non-thermal loss processes on moons with atmospheres

Several moons within our solar system have atmospheres and are subject to atmospheric loss processes. They typically have no magnetic fields of their own, but orbit planets with powerful magnetic fields. Many of these moons lie within the magnetic fields generated by the planets and are less likely to undergo sputtering and pick-up. The shape of the bow shock, however, allows for some moons, such as Titan, to pass through the bow shock when their orbits takes them between the sun and their primary. Titan spends roughly half of its transit time outside of the bow-shock and being subjected to unimpeded solar winds. The kinetic energy gained from pick-up and sputtering associated with the solar winds increases thermal escape throughout the transit of Titan, causing neutral hydrogen to escape from the moon.[3] The escaped hydrogen maintains an orbit following in the wake of Titan, creating a neutral hydrogen torus around Saturn. Io, in its transit around Jupiter, encounters a plasma cloud.[4] Interaction with the plasma cloud induces sputtering, kicking off sodium particles. The interaction produces a stationary banana-shaped charged sodium cloud along a part of the orbit of Io.

Impact erosion

The impact of a large meteoroid can lead to the loss of atmosphere. If a collision is energetic enough, it is possible for ejecta, including atmospheric molecules, to reach escape velocity. Just one impact such as the Chicxulub event does not lead to a significant loss, but the terrestrial planets went through enough impacts when they were forming for this to matter.

Sequestration

This is a loss, not an escape; it is when molecules solidify out of the atmosphere onto the surface. This happens on Earth, when water vapor forms glacial ice or when carbon dioxide is sequestered in sediments. The dry ice caps on Mars are also an example of this process.

One mechanism for sequestration is chemical; for example, most of the carbon dioxide of the Earth's original atmosphere has been chemically sequestered into carbonate rock. Very likely a similar process has occurred on Mars. Oxygen can be sequestered by oxidation of rocks; for example, by increasing the oxidation states of ferric rocks from Fe2+ to Fe3+. Gases can also be sequestered by adsorption, where fine particles in the regolith capture gas which adheres to the surface of particles.

Dominant atmospheric escape and loss processes on Earth

Earth is too large to lose particles efficiently through Jeans Escape. The exosphere is the high altitude region where atmospheric density is sparse and Jeans Escape occurs. Jeans escape calculations assuming an exosphere temperature of 1,800 degrees show that to deplete O+ ions by a factor of e (2.78...) would take nearly a billion years. 1,800 degrees is higher than the actual observed exosphere temperature; at the actual average exosphere temperature, depletion of O+ ions would not occur even over a trillion years. Furthermore, most oxygen on Earth is bound as O2, which is too massive to escape Earth by Jeans Escape.

Earth’s magnetic field protects it from solar winds and prevents escape of ions, except along open field lines at the magnetic poles. The gravitational attraction of Earth’s mass prevents other non-thermal loss processes from appreciably depleting the atmosphere. Yet Earth’s atmosphere is two orders of magnitude less dense than that of Venus at the surface. Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere. H2O vapor is sequestered as liquid H2O in oceans, greatly decreasing the atmospheric density. With liquid water running over the surface of Earth, CO2 can be drawn down from the atmosphere and sequestered in sedimentary rocks. Some estimates indicate that nearly all carbon on Earth is contained in sedimentary rocks, with the atmospheric portion being approximately 1/250,000 of Earth’s CO2 reservoir. If both of the reservoirs were released to the atmosphere, Earth’s atmosphere would be denser than even Venus’s atmosphere. Therefore, the dominant “loss” mechanism of Earth’s atmosphere is not escape to space, but sequestration.

Sources

References

  1. ^ a b Shizgal, 1996
  2. ^ a b c d Lammer, 2006
  3. ^ Lammer, 1998
  4. ^ Wilson, 2002