Paulo Fernando Penteado

Evidence for a Polar Ethane Cloud on Titan

Spectra from Cassinis Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51° to 68° north and all longitudes observed (10° to 190° west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moons north pole. Preferential condensation of ethane, perhaps as ice, at Titans poles during the winters may partially explain the lack of liquid ethane oceans on Titans surface at middle and lower latitudes.

Possible tropical lakes on Titan from observations of dark terrain

Titan has clouds, rain and lakes—like Earth—but composed of methane rather than water. Unlike Earth, most of the condensable methane (the equivalent of 5 m depth globally averaged) lies in the atmosphere. Liquid detected on the surface (about 2 m deep) has been found by radar images only poleward of 50° latitude, while dune fields pervade the tropics. General circulation models explain this dichotomy, predicting that methane efficiently migrates to the poles from these lower latitudes. Here we report an analysis of near-infrared spectral images of the region between 20° N and 20° S latitude. The data reveal that the lowest fluxes in seven wavelength bands that probe Titans surface occur in an oval region of about 60 × 40 km2, which has been observed repeatedly since 2004. Radiative transfer analyses demonstrate that the resulting spectrum is consistent with a black surface, indicative of liquid methane on the surface. Enduring low-latitude lakes are best explained as supplied by subterranean sources (within the last 10,000 years), which may be responsible for Titan’s methane, the continual photochemical depletion of which furnishes Titans organic chemistry.

Characterization of clouds in Titan's tropical atmosphere

Images of Titans clouds, possible over the past 10 years, indicate primarily discrete convective methane clouds near the south and north poles and an immense stratiform cloud, likely composed of ethane, around the north pole. Here we present spectral images from Cassinis Visual Mapping Infrared Spectrometer that reveal the increasing presence of clouds in Titans tropical atmosphere. Radiative transfer analyses indicate similarities between summer polar and tropical methane clouds. Like their southern counterparts, tropical clouds consist of particles exceeding 5 μm. They display discrete structures suggestive of convective cumuli. They prevail at a specific latitude band between 8°-20° S, indicative of a circulation origin and the beginning of a circulation turnover. Yet, unlike the high latitude clouds that often reach 45 km altitude, these discrete tropical clouds, so far, remain capped to altitudes below 26 km. Such low convective clouds are consistent with the highly stable atmospheric conditions measured at the Huygens landing site. Their characteristics suggest that Titans tropical atmosphere has a dry climate unlike the south polar atmosphere, and despite the numerous washes that carve the tropical landscape.

Measurements of CH3D and CH4 in titan from infrared spectroscopy

We measured the CH4 column abundance in Titans atmosphere through an analysis of Titans monodeuterated methane (CH3D) spectral features. CH3D is several orders of magnitude less abundant in Titans atmosphere than CH4. Thus, unlike CH4, the strong and well-studied CH3D 3ν2 lines are not saturated and provide a sensitive measure of its column abundance. We recorded the CH3D 3ν2 lines at 1.55 μm at NASAs Infrared Telescope Facility (IRTF) equipped with the Cryogenic Echelle Spectrograph. We derive a total integrated column abundance of 2.1 ± 0.1 m amagat for CH3D. We also measured stratospheric emission lines of both CH3D and CH4 at 8.6 μm at higher resolution than previously possible to better constrain the CH3D/CH4 ratio. These observations, recorded at the IRTF using the Texas Echelon Cross Echelle Spectrograph, were analyzed with radiative transfer calculations. We determine a CH3D/CH4 ratio of (50 ± 10) × 10-5. Taken together, our measurements of the CH3D column abundance and the CH3D/CH4 ratio indicate a total CH4 column abundance of 4.2 km amagat, close to the column abundance of an atmosphere with 100% saturation in the entire troposphere.

OBSERVATIONS OF TITAN'S MESOSPHERE

We have recorded spectra of Titans ν4 band of CH4 at a higher resolving power (R = 70,000) than prior measurements to better constrain the thermal structure of Titans atmosphere from 100 to 600 km altitude. Radiative transfer analyses of the spectra indicate a temperature profile below 300 km that is consistent with past measurements. The high resolving power of our observations provides the first infrared measurement of Titans thermal structure between 300 and 600 km. We detect the presence of a mesosphere, with a drop of temperature above 380 km altitude of at least 15 K, consistent with radiative cooling of the atmosphere by emission from hydrocarbons.