Planetary surfaces

Abstract:

Observations of Neptune were made in September 2009 with the Gemini-North Telescope in Hawaii, using the NIFS instrument in the H-band covering the wavelength range 1.477-1.803 μm. Observations were acquired in adaptive optics mode and have a spatial resolution of approximately 0.15-0.25″. The observations were analysed with a multiple-scattering retrieval algorithm to determine the opacity of clouds at different levels in Neptune's atmosphere. We find that the observed spectra at all locations are very well fit with a model that has two thin cloud layers, one at a pressure level of ∼2. bar all over the planet and an upper cloud whose pressure level varies from 0.02 to 0.08. bar in the bright mid-latitude region at 20-40°S to as deep as 0.2. bar near the equator. The opacity of the upper cloud is found to vary greatly with position, but the opacity of the lower cloud deck appears remarkably uniform, except for localised bright spots near 60°S and a possible slight clearing near the equator. A limb-darkening analysis of the observations suggests that the single-scattering albedo of the upper cloud particles varies from ∼0.4 in regions of low overall albedo to close to 1.0 in bright regions, while the lower cloud is consistent with particles that have a single-scattering albedo of ∼0.75 at this wavelength, similar to the value determined for the main cloud deck in Uranus' atmosphere. The Henyey-Greenstein scattering particle asymmetry of particles in the upper cloud deck are found to be in the range g∼ 0.6-0.7 (i.e. reasonably strongly forward scattering).Numerous bright clouds are seen near Neptune's south pole at a range of pressure levels and at latitudes between 60 and 70°S. Discrete clouds were seen at the pressure level of the main cloud deck (∼2. bar) at 60°S on three of the six nights observed. Assuming they are the same feature we estimate the rotation rate at this latitude and pressure to be 13.2 ± 0.1. h. However, the observations are not entirely consistent with a single non-evolving cloud feature, which suggests that the cloud opacity or albedo may vary very rapidly at this level at a rate not seen in any other giant-planet atmosphere. © 2011 Elsevier Inc.

Abstract:

Recent advancements in visible to near infrared orbital measurements of the lunar surface have allowed the character and extent of the primary anorthositic crust to be studied at unprecedented spatial and spectral resolutions. Here we assess the lunar primary anorthositic crust in global context using a spectral parameter tool for Moon Mineralogy Mapper data to identify and map Fe‐bearing crystalline plagioclase based on its diagnostic 1.25 µm absorption band. This allows plagioclase‐dominated rocks, specifically anorthosites, to be unambiguously identified as well as distinguished from lithologies with minor to trace amounts of mafic minerals. Low spatial resolution global mosaics and high spatial resolution individual data strips covering more than 650 targeted craters were analyzed to identify and map the mineralogy of spectrally pure regions as small as ~400 m in size. Spectrally, pure plagioclase is identified in approximately 450 targets located across the lunar surface. Diviner thermal infrared (TIR) data are analyzed for 37 of these nearly monomineralic regions in order to understand the compositional variability of plagioclase (An#) in these areas. The average An# for each spectrally pure region is estimated using new laboratory measurements of a well‐characterized anorthite (An96) sample. Diviner TIR results suggest that the plagioclase composition across the lunar highlands is relatively uniform, high in calcium content, and consistent with plagioclase compositions found in the ferroan anorthosites (An94–98). Our results confirm that spectrally pure anorthosite is widely distributed across the lunar surface, and most exposures of the ancient anorthositic crust are concentrated in regions of thicker crust surrounding impact basins on the lunar nearside and farside. In addition, the scale of the impact basins and the global nature and distribution of pure plagioclase requires a coherent zone of anorthosite of similar composition in the lunar crust supporting its formation from a single differentiation event like a magma ocean. Our identifications of pure anorthosite combined with the GRAIL crustal thickness model suggest that pure anorthosite is currently observed at a range of crustal thickness values between 9 and 63 km and that the primary anorthositic crust must have been at least 30 km thick.

Abstract:

Initial images of Venus's south pole by the Venus Express mission have shown the presence of a bright, highly variable vortex, similar to that at the planet's north pole. Using high-resolution infrared measurements of polar winds from the Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, we show the vortex to have a constantly varying internal structure, with a center of rotation displaced from the geographic south pole by ∼3 degrees of latitude and that drifts around the pole with a period of 5 to 10 Earth days. This is indicative of a nonsymmetric and varying precession of the polar atmospheric circulation with respect to the planetary axis.