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Register a new userThe Gas Composition and Deep Cloud Structure of Jupiters Great Red Spot
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We have obtained high-resolution spectra of Jupiters Great Red Spot (GRS) between 4.6 and 5.4 microns using telescopes on Mauna Kea in order to derive gas abundances and to constrain its cloud structure between 0.5 and 5~bars. We used line profiles of deuterated methane CH3D at 4.66 microns to infer the presence of an opaque cloud at 5+/-1 bar. From thermochemical models this is almost certainly a water cloud. We also used the strength of Fraunhofer lines in the GRS to obtain the ratio of reflected sunlight to thermal emission. The level of the reflecting layer was constrained to be at 570+/-30 mbar based on fitting strong ammonia lines at 5.32 microns. We identify this layer as an ammonia cloud based on the temperature where gaseous ammonia condenses. We found evidence for a strongly absorbing, but not totally opaque, cloud layer at pressures deeper than 1.3 bar by combining Cassini/CIRS spectra of the GRS at 7.18 microns with ground-based spectra at 5 microns. This is consistent with the predicted level of an NH4SH cloud. We also constrained the vertical profile of water and ammonia. The GRS spectrum is matched by a saturated water profile above an opaque water cloud at 5~bars. The pressure of the water cloud constrains Jupiters O/H ratio to be at least 1.1 times solar. The ammonia mole fraction is 200+/-50ppm for pressures between 0.7 and 5 bar. Its abundance is 40 ppm at the estimated pressure of the reflecting layer. We obtained 0.8+/-0.2 ppm for PH3, a factor of 2 higher than in the warm collar surrounding the GRS. We detected all 5 naturally occurring isotopes of germanium in GeH4 in the Great Red Spot. We obtained an average value of 0.35+/-0.05 ppb for GeH4. Finally, we measured 0.8+/-0.2 ppb for CO in the deep atmosphere.
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The day and nightside temperatures of hot Jupiters are diagnostic of heat transport processes in their atmospheres. Recent observations have shown that the nightsides of hot Jupiters are a nearly constant 1100 K for a wide range of equilibrium temperatures (T$_{eq}$), lower than those predicted by 3D global circulation models. Here we investigate the impact of nightside clouds on the observed nightside temperatures of hot Jupiters using an aerosol microphysics model. We find that silicates dominate the cloud composition, forming an optically thick cloud deck on the nightsides of all hot Jupiters with T$_{eq}$ $leq$ 2100 K. The observed nightside temperature is thus controlled by the optical depth profile of the silicate cloud with respect to the temperature-pressure profile. As nightside temperatures increase with T$_{eq}$, the silicate cloud is pushed upwards, forcing observations to probe cooler altitudes. The cloud vertical extent remains fairly constant due to competing impacts of increasing vertical mixing strength with T$_{eq}$ and higher rates of sedimentation at higher altitudes. These effects, combined with the intrinsically subtle increase of the nightside temperature with T$_{eq}$ due to decreasing radiative timescale at higher instellation levels lead to low, constant nightside photospheric temperatures consistent with observations. Our results suggest a drastic reduction in the day-night temperature contrast when nightside clouds dissipate, with the nightside emission spectra transitioning from featureless to feature-rich. We also predict that cloud absorption features in the nightside emission spectra of hot Jupiters should reach $geq$100 ppm, potentially observable with the James Webb Space Telescope.
Technique: We present a method to determine the pressure at which significant cloud opacity is present between 2 and 6 bars on Jupiter. We use: a) the strength of a Fraunhofer absorption line in a zone to determine the ratio of reflected sunlight to thermal emission, and b) pressure-broadened line profiles of deuterated methane (CH3D) at 4.66 microns to determine the location of clouds. We use radiative transfer models to constrain the altitude region of both the solar and thermal components of Jupiters 5-micron spectrum. Results: For nearly all latitudes on Jupiter the thermal component is large enough to constrain the deep cloud structure even when upper clouds are present. We find that Hot Spots, belts, and high latitudes have broader line profiles than do zones. Radiative transfer models show that Hot Spots in the North and South Equatorial Belts (NEB, SEB) typically do not have opaque clouds at pressures greater than 2 bars. The South Tropical Zone (STZ) at 32 degrees S has an opaque cloud top between 4 and 5 bars. From thermochemical models this must be a water cloud. We measured the variation of the equivalent width of CH3D with latitude for comparison with Jupiters belt-zone structure. We also constrained the vertical profile of water in an SEB Hot Spot and in the STZ. The Hot Spot is very dry for P<4.5 bars and then follows the water profile observed by the Galileo Probe. The STZ has a saturated water profile above its cloud top between 4 and 5 bars.
We consider the origin of compact, short-period, Jupiter-mass planets. We propose that their diverse structure is caused by giant impacts of embryos and super-Earths or mergers with other gas giants during the formation and evolution of these hot Jupiters. Through a series of numerical simulations, we show that typical head-on collisions generally lead to total coalescence of impinging gas giants. Although extremely energetic collisions can disintegrate the envelope of gas giants, these events seldom occur. During oblique and moderately energetic collisions, the merger products retain higher fraction of the colliders cores than their envelopes. They can also deposit considerable amount of spin angular momentum to the gas giants and desynchronize their spins from their orbital mean motion. We find that the oblateness of gas giants can be used to infer the impact history. Subsequent dissipation of stellar tide inside the planets envelope can lead to runaway inflation and potentially a substantial loss of gas through Roche-lobe overflow. The impact of super-Earths on parabolic orbits can also enlarge gas giant planets envelope and elevates their tidal dissipation rate over $sim $ 100 Myr time scale. Since giant impacts occur stochastically with a range of impactor sizes and energies, their diverse outcomes may account for the dispersion in the mass-radius relationship of hot Jupiters.
Jupiters atmosphere is enriched in C, N, S, P, Ar, Kr and Xe with respect to solar abundances by a factor of ~3. Gas Giant envelopes are mainly enriched through the dissolution of solids in the atmosphere, and this constant enrichment factor is puzzling since several of the above elements are not expected to have been in the solid phase in Jupiters feeding zone; most seriously, Ar and the main carrier of N, N2, only condense at the very low temperatures, 21-26 K, associated with the outer solar nebula. We propose that a plausible solution to the enigma of Jupiters uniform enrichment pattern is that Jupiters core formed exterior to the N2 and Ar snowlines, beyond 30 au, resulting in a Solar composition core in all volatiles heavier than Ne. During envelope accretion and planetesimal bombardment, some of the core mixed in with the envelope causing the observed enrichment pattern. We show that this scenario naturally produces the observed atmosphere composition, even with substantial pollution from N-poor pebble and planetesimal accretion in Jupiters final feeding zone. We note that giant core formation at large nebular radii is consistent with recent models of gas giant core formation through pebble accretion, which requires the core to form exterior to Jupiters current location to counter rapid inward migration during the core and envelope formation process. If this scenario is common, gas giant core formation may account for many of the gaps observed in protoplanetary disks between 10s and 100 au.
We used 0.85 - 5.1 micron 2006 observations by Cassinis Visual and Infrared Mapping Spectrometer (VIMS) to constrain the unusual vertical structure and compositions of cloud layers in Saturns south polar region, the site of a powerful vortex circulation, shadow-casting cloud bands, and spectral evidence of ammonia ice clouds without the lightning usually associated with such features. We modeled spectral observations with a 4-layer model that includes (1) a stratospheric haze, (2) a top tropospheric layer of non-absorbing (possibly diphosphine) particles near 300 mbar, with a fraction of an optical depth (much less than found elsewhere on Saturn), (3) a moderately thicker layer (1 - 2 optical depths) of ammonia ice particles near 900 mbar, and (4) extending from 5 bars up to 2-4 bars, an assumed optically thick layer where NH4SH and H20 are likely condensables. What makes the 3-micron absorption of ammonia ice unexpectedly apparent in these polar clouds, is not penetrating convection, but instead the relatively low optical depth of the top tropospheric cloud layer, perhaps because of polar downwelling and/or lower photochemical production rates. We did not find any evidence for optically thick eyewalls that were previously thought to be responsible for the observed shadows. Instead, we found evidence for small step-wise decreases in optical depth of the stratospheric haze near 87.9 deg S and in the putative diphosphine layer near 88.9 deg S, which are the likely causes of shadows and bright features we call antishadows. We found changes as a function of latitude in the phosphine vertical profile and in the arsine mixing ratio that support the existence of downwelling within 2 deg of the pole.
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