Atmospheres of the Solid Worlds I: Origin and Composition and Magnetospheres
The atmosphere of Titan from Wikipedia
- Gasses present in the solar nebula (mostly hydrogen and helium)
- Volatiles outgassed from planetary interiors.
Oxidation and reduction:
- To a chemist, the term oxidation occurs when an atom loses electrons during a chemical reaction. For example in:
Na + F -> Na+F-
sodium is oxidized by fluorine. Because the electron is being removed from the sodium by the fluorine, we call fluorine the oxidizing agent
- Reduction is the opposite - occurring when an element gains electrons during a chemical reaction. In the example above, fluorine is reduced by sodium. We refer to sodium as the reducing agent.
Major gas components of atmospheres: From the strongest reducing agent to the strongest oxidizing agent.
- H, atomic and molecular (H2)
- Methane (CH4)
- Ammonia (NH3)
- Water, H2O
- Ozone (O3)
The solid worlds with atmospheres: Venus, Earth, Mars, and Titan to scale
- Spectroscopy (can be used remotely)
- Mass spectrometry (requires sample)
- Gas chromatography (requires sample)
The atmospheres of the solid worlds vary significantly in mass. The image at right shows the planets and their surface atmospheric pressures to scale.
- Venus: 96% CO2, 4% N2
- Earth: 78% N2, 21% O2, 1-4% H2O
- Mars: 95% CO2, 3% N2, 2% Ar
- Titan: 98.4% N2, 1.4% CH4 (methane), 0.2% H2
- On Venus, this enters the atmosphere from volcanoes and either just accumulates.
- On Mars, this enters the atmosphere from volcanoes and slowly leaks into space.
- On Earth, it becomes dissolved in water and deposited as chemical or biochemical sedimentary rocks.
- On Earth, water is present as vapor in the atmosphere and as liquid on the surface.
- On Mars, it is mostly frozen in the ice caps or buried as permafrost.
- On Venus, it seems actually to be depleted.
- On Venus, this relatively non-reactive gas sits in the atmosphere.
- On Mars, it has slowly leaked into space.
Nevertheless, some differences remain when we compare volatile inventories:
- Venus really has significantly less water then Earth or Mars. Apparently ultraviolet radiation striking the upper atmosphere dissociates it into O2, that then forms compounds with other elements, and H2 that leaks into space.
- Mars really has less nitrogen than Earth or Venus.
- Earth really has more O2 because of.......
Global distribution of chlorophyll from Wikipedia
6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2
As a consequence:
- O2 entered the atmosphere in increasing quantities.
- Ozone: As O2 accumulated, free O2 in upper atmosphere recombined to form ozone layer (O3). Because ozone is opaque to ultraviolet radiation, it allowed life to colonize surface waters.
- Oceanic acidity: By eating up atmospheric CO2, photosynthesizers caused the acidity of the oceans to diminish, allowing the direct precipitation of carbonate rocks for the first time. Once that was possible, atmospheric CO2 concentrations fell very rapidly. as carbon became locked up in rock.
Planetary atmospheric composition idiosyncrasies:
Venus in ultraviolet from Wikipedia
- Clouds of sulfuric acid? It is thought that the surface has a lot of sulfide minerals. At the high surface temperature (750 K), these sulfides react with the CO2 atmosphere to produce SO2 gas. This then reacts in the atmosphere with a small amount of water vapor to produce H2SO4.
Sulfuric acid has a boiling point of 530 K. Why is Venus' atmosphere clear up to 30 km? Because below this altitude H2SO4 is a gas. (Now breathe deeply. Yikes.)
- Snow or plating? Atmospheric sulfur may react with surface rocks. Strange to say, Venus' mountains seem to be capped with some substance that it highly reflective to radar. Predicted by Brackett et al., 1995, and reported by Schaefer and Fegeley, 2003. Certainly not water ice. Pyrite (iron sulfide FeS2) or other metal sulfides may have condensed and "snowed" onto regions of high elevations.
- The fate of the water: Gas molecules in Venus' upper atmosphere are bombarded by intense solar radiation because:
- Venus is close to the Sun.
- Venus lacks a magnetic field with which to shield its atmosphere from direct exposure to the solar wind.
Mars dust storms
- Condensation flow: Mars' axial inclination is slightly greater than Earth's so Mars experiences seasons for the same reason. What makes this interesting is that during winter, both water and CO2 condense on the ice cap. CO2 is the major component of Mars' atmosphere. The result is a seasonal global flow of CO2 from one pole (the one experiencing spring sublimation of CO2) to the other (experiencing autumn condensation of CO2).
- Modern Mars:
- Has no global magnetic field
- Is close enough to the Sun to have its atmosphere eroded by the solar wind.
- Ancient Mars: We know from ancient (~ during its first 500 million years) evidence of:
- flowing water
- a planetary magnetic field
- The geochemical comparison of Mars surface (observed by rovers) and deep (sent to Earth as mars meteorites) rocks by Tuff et al., 2013 suggests active subduction of rocks that had been exposed to an oxidizing atmosphere during Mars' first 0.5 gy.
- Iron oxidizes readily, even in environments with low oxygen concentration, so finding iron oxide is not surprising. Manganese, however, requires high oxygen concentrations to form oxides. Thus, Lanza et al.'s 2016 report of manganese oxides by the Curiosity rover in Gale Crater come as a big surprise.
Solar wind and magnetospheres
Solar wind: The stream of ionized particles (plasma) flowing outward from the sun. Their flow is controlled by the sun's magnetic field. The Sun has tremendous magnetic activity. Near the sun this is manifested in sunspots, solar flares and related phenomena. Farther away we can compare its magnetic
field to a dipole, like a bar magnet and call it the interplanetary magnetic field (IMF). The particles of the solar wind follow its field lines. What happens when this magnetically guided wind encounters planets?
No atmosphere or magnetic field: In the case of a body like the moon, with no atmosphere or magnetic field, the solar wind simply impacts the dayside surface, and cases a plasma shadow on the night side.
No atmosphere but has magnetic field: What if the planet is airless but has a magnetic field like Mercury? The IMF interacts with Mercury's magnetic field. We describe this in terms of two limits:
- Magnetopause: Inside this boundary, Mercury's magnetic field exerts the predominant force. Outside, the IMF dominates.
- Bow shock: The solar wind slows down abruptly as it nears the magnetopause, producing a shock wave like that of a supersonic aircraft. This is the bow shock. The region between the bow shock and magnetopause is the magnetosheath.
No magnetic field but has atmosphere: What if the planet has no magnetic field but a thick atmosphere, like Venus? The solar wind strikes the upper atmosphere directly, heating the thermosphere and ionizing its gasses. The resulting layer of ionized gasses is the ionosphere. The ionized gasses of the ionosphere can deflect the solar wind. This barrier is the ionopause. As before, the solar wind decelerates approaching the ionopause, yielding a bow shock. Here the region between the ionopause and the bow shock is the magnetosheath.
Remember: Gasses in the thermosphere are effected by interactions with the solar wind. The disassociation of water molecules into hydrogen and oxygen in Venus' thermosphere is thought to account for its low global volatile inventory of water.
Both: Earth has both a substantial atmosphere (with ionosphere) and magnetic field. Because the magnetic field extends farther out, it dominates interaction with the solar wind, yielding a bow shock and magnetopause like Mercury's, only larger. The geometry of the magnetic field is such that charged particles following magnetic field lines strike the upper atmosphere mostly at the poles. These collisions cause ions and atoms to emit light that we see as auroras.
Earth's magnetic field also traps ions in two torus-shaped loops - the Van Allen Radiation Belts:
- The outer belt's ions are mostly derived from the solar wind.
- The inner one from earth's ionosphere.
Key concepts and vocabulary:
- Four solid worlds with substantial atmospheres
- Analytic methods
- Gas chromatography
- Mass spectrometry
- Be familiar with relative atmospheric pressures
- Be familiar with relative atmospheric compositions
- Volatile inventories
- Venus low on H2O
- Mars low on N2
- Earth high in O2
- Earth oxygen from the biosphere
- Ozone (O3)
- Oceanic acidity
- Atmospheric retention
- Planetary idiosyncrasies:
- Sulfuric acid (H2SO4) clouds
- Sulfide "frost" (or is that "plating") at high elevations.
- Removal of water by solar wind
- Condensation flow
- Modern Mars losing atmosphere
- Ancient Mars with oxidizing atmosphere
- Interplanetary magnetic field (IMF)
- Bow shock
- Van Allen Radiation Belt
- Robert Brackett, Bruce Fegeley, Jr., and Raymond Arvidson. 1995. Volatile transport on Venus and implications for surface geochemistry and geology. Journal of Geophysical Research 100(1), 1553-1563.
- Nina L. Lanza, Roger C. Wiens, Raymond E. Arvidson, Benton C. Clark, Woodward W. Fischer, Ralf Gellert, John P. Grotzinger, Joel A. Hurowitz, Scott M. McLennan, Richard V. Morris, Melissa S. Rice, James F. Bell III, Jeffrey A. Berger, Diana L. Blaney, Nathan T. Bridges, Fred Calef III, John L. Campbell, Samuel M. Clegg, Agnes Cousin, Kenneth S. Edgett, Cecile Fabre, Martin R. Fisk, Olivier Forni, Jens Frydenvang, Keian R. Hardy, Craig Hardgrove, Jeffrey R. Johnson, Jeremie Lasue, Stephane Le Mouelic, Michael C. Malin, Nicolas Mangold, Javier Martin-Torres, Sylvestre Maurice, Marie J. McBride, Douglas W. Ming, Horton E. Newsom, Ann M. Ollila, Violaine Sautter, Susanne Schröder, Lucy M. Thompson, Allan H. Treiman, Scott VanBommel, David T. Vaniman, Maria-Paz Zorzano. 2016. Oxidation of manganese in an ancient aquifer, Kimberley formation, Gale crater, Mars. Geophysical Research Letters preprint June 2016.