Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation.
Recent discoveries have shown that the very largest Kuiper Belt objects-Eris, 2005 FY9, and Sedna-are coated in methane and may contain other volatile ices as well. New detailed observations show that even within this class of volatile-rich bodies, unexpected differences exist in their surface compositions. 2005 FY9, a body approximately 60% the size of Pluto, with a reflectance spectrum similarly dominated by methane, has a surface depleted in molecular nitrogen by at least an order of magnitude with respect to Pluto. We find that the existence of this new class of volatile-rich objects, the lack of volatiles on most Kuiper Belt objects, and even the otherwise peculiar surface of 2005 FY9 can be explained as a consequence of atmospheric escape of volatile compounds. While previous studies of the surface compositions of objects in the Kuiper Belt have found no explainable patterns, atmospheric escape appears to provide a first-order explanation of the range of surface spectra seen on bodies in the outer solar system.
Supplementary InformationOur GCM is based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory (http://fms.gfdl.noaa.gov). It is similar to standard models for Earth's atmosphere, but with Titan's radius, planetary rotation rate, material properties, and seasonally varying insolation, and with a methane cycle instead of a water cycle. The GCM has similarities to the models in refs. 16, 17, but unlike those, it is 3D and eddy-resolving and has a different representation of radiative transfer and surface processes.Resolution. The GCM solves the hydrostatic primitive equations in vorticity-divergence form, using the spectral transform method in the horizontal and finite differences in the vertical 31 . The horizontal spectral resolution is T21 (corresponding to about 5 • × 5 • resolution of the transform grid). The vertical coordinate is σ = p/p s (pressure p normalized by surface pressure p s ); it is discretized with 18 unequally spaced levels 32 . The top of the model is at σ = 0; the uppermost full level has a mean pressure of 15 mbar (altitude ∼90 km). The time-stepping scheme is a semiimplicit leapfrog scheme (timestep 1600 s).
The spectrum of the bright Kuiper Belt object 2005 FY9 from 0.34 to 2.5 m is dominated by the red coloring of many outer solar system objects in the optical wavelength regime and by absorption due to methane in the near-infrared. The solid methane absorption lines are significantly broader on 2005 FY9 than on any other solar system body, indicating long optical path lengths through the methane. These long path lengths can be parameterized as a methane grain size of approximately 1 cm in a Hapke reflectance model. In addition to large-grained methane, the infrared spectrum also indicates the clear presence of ethane, an expected product of UV photolysis of methane. No evidence for N 2 or CO, both known to be present on Pluto, is found. We suggest that the large differences between the spectrum of 2005 FY9 and that of Pluto and 2003 UB313 is due to a depletion of nitrogen on the surface of 2005 FY9 that leads to large methane grains, abundant sites for ethane formation through UV photolysis, and highly irradiated tholin-like material.
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