66 research outputs found
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
The Cosmic Shoreline
Volatile escape is the classic existential problem of planetary atmospheres. The problem has gained new currency now that we can study the cumulative effects of escape from extrasolar planets. Escape itself is likely to be a rapid process, relatively unlikely to be caught in the act, but the cumulative effects of escape in particular, the distinction between planets with and without atmospheres should show up in the statistics of the new planets. The new planets make a moving target. It can be difficult to keep up, and every day the paper boy brings more. Of course most of these will be giant planets loosely resembling Saturn or Neptune albeit hotter and nearer their stars, as big hot fast-orbiting exoplanets are the least exceedingly difficult to discover. But they are still planets, all in all, and although twenty years ago experts could prove on general principles that they did not exist, we have come round rather quickly, and they should be welcome now at LPSC. Here we will discuss the empirical division between planets with and without atmospheres. For most exoplanets the question of whether a planet has or has not an atmosphere is a fuzzy inference based on the planet's bulk density. A probably safe presumption is that a low density planet is one with abundant volatiles, in the general mold of Saturn or Neptune. On the other hand a high density low mass planet could be volatile-poor, in the general mold of Earth or Mercury. We will focus on planets, mostly seen in transit, for which both radius and mass are measured, as these are the planets with measured densities. More could be said: a lot of subtle recent work has been devoted to determining the composition of planets from equations of state or directly observing atmospheres in transit, but we will not go there. What interests us here is that, from the first, the transiting extrasolar planets appear to have fit into a pattern already seen in our own Solar System, as shown in Fig. 1. We first noticed this in 2004 when there were just two transiting exoplanets to consider. The trend was well-defined by late 2007. Figure 1 shows how matters stood in Dec 2012 with approx.240 exoplanets. The figure shows that the boundary between planets with and without active volatiles - the cosmic shoreline, as it were - is both well-defined and follows a power law
Photolytic Hazes in the Atmosphere of 51 Eri b
We use a 1D model to address photochemistry and possible haze formation in
the irradiated warm Jupiter, 51 Eridani b. The intended focus was to be carbon,
but sulfur photochemistry turns out to be important. The case for organic
photochemical hazes is intriguing but falls short of being compelling. If
organic hazes form, they are likeliest to do so if vertical mixing in 51 Eri b
is weaker than in Jupiter, and they would be found below the altitudes where
methane and water are photolyzed. The more novel result is that photochemistry
turns HS into elemental sulfur, here treated as S. In the cooler
models, S is predicted to condense in optically thick clouds of solid
sulfur particles, whilst in the warmer models S remains a vapor along with
several other sulfur allotropes that are both visually striking and potentially
observable. For 51 Eri b, the division between models with and without
condensed sulfur is at an effective temperature of 700 K, which is within error
its actual effective temperature; the local temperature where sulfur condenses
is between 280 and 320 K. The sulfur photochemistry we have discussed is quite
general and ought to be found in a wide variety of worlds over a broad
temperature range, both colder and hotter than the 650-750 K range studied
here, and we show that products of sulfur photochemistry will be nearly as
abundant on planets where the UV irradiation is orders of magnitude weaker than
it is on 51 Eri b.Comment: 24 pages including 11 figures and a tabl
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
Transient Climate Effects of Large Impacts on Titan
Titan's thick atmosphere and volatile-rich surface cause it to respond to big impacts in a somewhat Earth-like manner. Here we construct a simple globally-averaged model that tracks the flow of energy through the environment in the weeks, years, and millenia after a big comet strikes Titan. The model Titan is endowed with 1.4 bars of N2 and 0.07 bars of CH4, methane lakes, a water ice crust, and enough methane underground to saturate the regolith to the surface. We find that a nominal Menrva impact is big enough to raise the surface temperature by approx. 80 K and to double the amount of methane in the atmosphere. The extra methane drizzles out of the atmosphere over hundreds of years. An upper-limit Menrva is just big enough to raise the surface to water's melting point. The putative Hotei impact (a possible 800-1200 km diameter basin, Soderblom et al., 2009) is big enough to raise the surface temperature to 350-400 K. Water rain must fall and global meltwaters might range between 50 m to more than a kilometer deep, depending on the details. Global meltwater oceans do not last more than a few decades or centuries at most, but are interesting to consider given Titan's organic wealth. Significant near-surface clathrate formation is possible as Titan cools but faces major kinetic barriers
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