4,238 research outputs found
Titan's transport-driven methane cycle
The strength of Titan's methane cycle, as measured by precipitation and
evaporation, is key to interpreting fluvial erosion and other indicators of the
surface-atmosphere exchange of liquids. But the mechanisms behind the
occurrence of large cloud outbursts and precipitation on Titan have been
disputed. A gobal- and annual-mean estimate of surface fluxes indicated only 1%
of the insolation, or 0.04 W/m, is exchanged as sensible and/or
latent fluxes. Since these fluxes are responsible for driving atmospheric
convection, it has been argued that moist convection should be quite rare and
precipitation even rarer, even if evaporation globally dominates the
surface-atmosphere energy exchange. In contrast, climate simulations that allow
atmospheric motion indicate a robust methane cycle with substantial cloud
formation and/or precipitation. We argue the top-of-atmosphere radiative
imbalance -- a readily observable quantity -- is diagnostic of horizontal heat
transport by Titan's atmosphere, and thus constrains the strength of the
methane cycle. Simple calculations show the top-of-atmosphere radiative
imbalance is 0.5-1 W/m in Titan's equatorial region, which implies
2-3 MW of latitudinal heat transport by the atmosphere. Our simulation of
Titan's climate suggests this transport may occur primarily as latent heat,
with net evaporation at the equator and net accumulation at higher latitudes.
Thus the methane cycle could be 10-20 times previous estimates. Opposing
seasonal transport at solstices, compensation by sensible heat transport, and
focusing of precipitation by large-scale dynamics could further enhance the
local, instantaneous strength of Titan's methane cycle by a factor of several.Comment: submitted to ApJ Letter
Elastic ice shells of synchronous moons: Implications for cracks on Europa and non-synchronous rotation of Titan
A number of synchronous moons are thought to harbor water oceans beneath
their outer ice shells. A subsurface ocean frictionally decouples the shell
from the interior. This has led to proposals that a weak tidal or atmospheric
torque might cause the shell to rotate differentially with respect to the
synchronously rotating interior. As a result of centrifugal and tidal forces,
the ocean would assume an ellipsoidal shape with its long axis aligned toward
the parent planet. Any displacement of the shell away from its equilibrium
position would induce strains thereby increasing its elastic energy and giving
rise to an elastic restoring torque. We compare the elastic torque with the
tidal torque acting on Europa and the atmospheric torque acting on Titan. For
Europa, the tidal torque is far too weak to produce stresses that could
fracture the ice shell, thus refuting a widely advocated idea. Instead, we
suggest that cracks arise from time-dependent stresses due to non-hydrostatic
gravity anomalies from tidally driven, episodic convection in the interior. Two
years of Cassini RADAR observations of Titan's surface are interpreted as
implying an angular displacement of ~0.24 degrees relative to synchroneity.
Compatibility of the amplitude and phase of the observed non-synchronous
rotation with estimates of the atmospheric torque requires that Titan's shell
be decoupled from its interior. We find that the elastic torque balances the
atmospheric torque at an angular displacement <0.05 degrees, thus coupling the
shell to the interior. Moreover, if Titan's surface were spinning faster than
synchronous, the tidal torque tending to restore synchronous rotation would
certainly be larger than the atmospheric torque. There must either be a problem
with the interpretation of the radar observations, or with our understanding of
Titan's atmosphere and/or interior.Comment: Icarus, in pres
Coupling convectively driven atmospheric circulation to surface rotation: Evidence for active methane weather in the observed spin rate drift of Titan
A large drift in the rotation rate of Titan observed by Cassini provided the
first evidence of a subsurface ocean isolating the massive core from the icy
crust. Seasonal exchange of angular momentum between the surface and atmosphere
accounts for the magnitude of the effect, but observations lag the expected
signal by a few years. We argue that this time lag is due to the presence of an
active methane weather cycle in the atmosphere. An analytic model of the
seasonal cycle of atmospheric angular momentum is developed and compared with
time-dependent simulations of Titan's atmosphere with and without methane
thermodynamics. The disappearance of clouds at the summer pole suggests the
drift rate has already switched direction, signaling the change in season from
solstice to equinox.Comment: 6 pages, 4 figures, published in Ap
Axisymmetric constraints on cross-equatorial Hadley cell extent
We consider the relevance of known constraints from each of Hide's theorem,
the angular momentum conserving (AMC) model, and the equal-area model on the
extent of cross-equatorial Hadley cells. These theories respectively posit that
a Hadley circulation must span: all latitudes where the radiative convective
equilibrium (RCE) absolute angular momentum () satisfies
or or where the RCE absolute
vorticity () satisfies ; all latitudes
where the RCE zonal wind exceeds the AMC zonal wind; and over a range such that
depth-averaged potential temperature is continuous and that energy is
conserved. The AMC model requires knowledge of the ascent latitude
, which need not equal the RCE forcing maximum latitude
. Whatever the value of , we
demonstrate that an AMC cell must extend at least as far into the winter
hemisphere as the summer hemisphere. The equal-area model predicts
, always placing it poleward of . As
is moved poleward (at a given thermal Rossby number), the
equal-area predicted Hadley circulation becomes implausibly large, while both
and become increasingly displaced
poleward of the minimal cell extent based on Hide's theorem (i.e. of
supercritical forcing). In an idealized dry general circulation model,
cross-equatorial Hadley cells are generated, some spanning nearly pole-to-pole.
All homogenize angular momentum imperfectly, are roughly symmetric in extent
about the equator, and appear in extent controlled by the span of supercritical
forcing.Comment: 18 pages, 9 figures, publishe
Total syntheses of conformationally-locked difluorinated pentopyranose analogues and a pentopyranosyl phosphate mimetic
Trifluoroethanol has been elaborated, via a telescoped sequence involving a metalated difluoroenol, a difluoroallylic alcohol, [2,3]-Wittig rearrangement, and ultimately an RCM reaction and requiring minimal intermediate purification, to a number of cyclooctenone intermediates. Epoxidation of these intermediates followed by transannular ring opening or dihydroxylation, then transannular hemiacetalization delivers novel bicyclic analogues of pentopyranoses, which were elaborated (in one case) to an analogue of a glycosyl phosphate
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