872 research outputs found
Earth's Early Reduced Atmospheres
Earths early atmosphere evolved in response to cosmic impacts, photochemistry generated by the copious extreme ultraviolet radiation emitted by the young Sun, hydrogen escape, mineral redox buffering, the evolution of iron in the mantle, and the character of crustal weathering and subduction. No single chemical composition, surface pressure, or temperature was typical. After the biggest impacts the temperature likely rose above 3000 K and the atmosphere held rock vapor. As the atmosphere cooled its constituents rained out: first the silicates, then the geochemical volatiles (such as salt), and last the water. Depending on the size of the impact, cooling took thousands to millions of years. Between impacts the atmosphere was thick with gases like CO2, H2, and CH4 that do not condense at Earth but provide greenhouse warming that could easily have maintained surface temperatures in excess of 500 K. At some point the early atmosphere must have been conducive to the evolution of organic chemistry and the origin of life. It was therefore --- at times --- dominated by reduced gases like hydrogen and methane. Hydrogen-methane atmospheres are easily formed thermochemically after major impacts if H2O and CO2 were present on Earth in quantities consistent with their inventories on Earth today the key is high pressure. Hydrogen-methane atmospheres slowly break down over millions of years by hydrogen escape and photochemistry. At first methane decomposes into tars and nitriles that precipitate, but later as the atmosphere grows more oxidized from hydrogen escape the methane oxidizes to CO and CO2, with a small fraction going to HCN. Impacts can also produce NH3 directly from N2 and H2O, which is expected to rain out with the H2O. As impacts grew infrequent the climate cooled in response to the fixing of CO2 as carbonate and carbonate subduction. Eventually the CO2 was mostly removed to the mantle and the surface mostly frozen
Fractionation of terrestrial neon by hydrodynamic hydrogen escape from ancient steam atmospheres
Atmospheric neon is isotopically heavier than mantle neon. By contrast, nonradiogenic mantle Ar, Kr, and Xe are not known to differ from the atmosphere. These observations are most easily explained by selective neon loss to space; however, neon is much too massive to escape from the modern atmosphere. Steam atmospheres are a likely, if intermittent, feature of the accreting Earth. They occur because, on average, the energy liberated during accretion places Earth above the runaway greenhouse threshold, so that liquid water is not stable at the surface. It is found that steam atmospheres should have lasted some ten to fifty million years. Hydrogen escape would have been vigorous, but abundant heavy constituents would have been retained. There is no lack of plausible candidates; CO2, N2, or CO could all suffice. Neon can escape because it is less massive than any of the likely pollutants. Neon fractionation would have been a natural byproduct. Assuming that the initial Ne-20/Ne-22 ratio was solar, it was found that it would have taken some ten million years to effect the observed neon fractionation in a 30 bar steam atmosphere fouled with 10 bars of CO. Thicker atmospheres would have taken longer; less CO, shorter. This mechanism for fractionating neon has about the right level of efficiency. Because the lighter isotope escapes much more readily, total neon loss is pretty minimal; less than half of the initial neon endowment escapes
Exchange of ejecta between Telesto and Calypso: Tadpoles, horseshoes, and passing orbits
We have numerically integrated the orbits of ejecta from Telesto and Calypso,
the two small Trojan companions of Saturn's major satellite Tethys. Ejecta were
launched with speeds comparable to or exceeding their parent's escape velocity,
consistent with impacts into regolith surfaces. We find that the fates of
ejecta fall into several distinct categories, depending on both the speed and
direction of launch.
The slowest ejecta follow sub-orbital trajectories and re-impact their source
moon in less than one day. Slightly faster debris barely escape their parent's
Hill sphere and are confined to tadpole orbits, librating about Tethys'
triangular Lagrange points L4 (leading, near Telesto) or L5 (trailing, near
Calypso) with nearly the same orbital semi-major axis as Tethys, Telesto, and
Calypso. These ejecta too eventually re-impact their source moon, but with a
median lifetime of a few dozen years. Those which re-impact within the first
ten years or so have lifetimes near integer multiples of 348.6 days (half the
tadpole period).
Still faster debris with azimuthal velocity components >~ 10 m/s enter
horseshoe orbits which enclose both L4 and L5 as well as L3, but which avoid
Tethys and its Hill sphere. These ejecta impact either Telesto or Calypso at
comparable rates, with median lifetimes of several thousand years. However,
they cannot reach Tethys itself; only the fastest ejecta, with azimuthal
velocities >~ 40 m/s, achieve "passing orbits" which are able to encounter
Tethys. Tethys accretes most of these ejecta within several years, but some 1 %
of them are scattered either inward to hit Enceladus or outward to strike
Dione, over timescales on the order of a few hundred years
Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
We address disequilibrum abundances of some simple molecules in the
atmospheres of solar composition brown dwarfs and self-luminous extrasolar
giant planets using a kinetics-based 1D atmospheric chemistry model. Our
approach is to use the full kinetics model to survey the parameter space with
effective temperatures between 500 K and 1100 K. In all of these worlds
equilibrium chemistry favors CH4 over CO in the parts of the atmosphere that
can be seen from Earth, but in most disequilibrium favors CO. The small surface
gravity of a planet strongly discriminates against CH4 when compared to an
otherwise comparable brown dwarf. If vertical mixing is like Jupiter's, the
transition from methane to CO occurs at 500 K in a planet. Sluggish vertical
mixing can raise this to 600 K; but clouds or more vigorous vertical mixing
could lower this to 400 K. The comparable thresholds in brown dwarfs are
K. Ammonia is also sensitive to gravity, but unlike CH4/CO, the
NH3/N2 ratio is insensitive to mixing, which makes NH3 a potential proxy for
gravity. HCN may become interesting in high gravity brown dwarfs with very
strong vertical mixing. Detailed analysis of the CO-CH4 reaction network
reveals that the bottleneck to CO hydrogenation goes through methanol, in
partial agreement with previous work. Simple, easy to use quenching relations
are derived by fitting to the complete chemistry of the full ensemble of
models. These relations are valid for determining CO, CH4, NH3, HCN, and CO2
abundances in the range of self-luminous worlds we have studied but may not
apply if atmospheres are strongly heated at high altitudes by processes not
considered here (e.g., wave breaking).Comment: Astrophysical Journal, in press. Clarity improvements throughout and
one new figure. 17 figures, 20 page
Impact production of NO and reduced species
It has recently been suggested that a reported spike in seawater (87)Sr/(86)Sr at the K-T boundary is the signature of an impact-generated acid deluge. However, the amount of acid required is implausibly large. Some about 3 x 10 to the 15th power moles of Sr must be weathered from silicates to produce the inferred Sr spike. The amount of acid required is at least 100 and probably 1000 times greater. Production of 3 x 10 to the 18th power moles of NO is clearly untenable. The atmosphere presently contains only 1.4 x 10 to the 20th power moles of N-sub 2 and 3.8 x 10 to the 19th power moles of O sub 2 If the entire atmosphere were shocked to 2000 K and cooled within a second, the total NO produced would be about 3 x 10 to the 18th power moles. This is obviously unrealistic. A (still to short) cooling time of 10th to the 3rd power sec reduces NO production by an order of magnitude. In passing, we note that if the entire atmosphere had in fact been shocked to 2000 K, acid rain would have been the least of a dinosaur's problems. Acid rain as a mechanism poses poses other difficulties. Recently deposited carbonates would have been most susceptable to acid attack. The researchers' preferred explanation is simply increased continental erosion following ecological trauma, coupled with enchanced levels of CO-sub 2
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