Other
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..516y&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Other
Scientific paper
Ground-based or near-Earth (e.g., HST) stellar occultations of every atmosphere in our solar system has been observed: Venus, Mars, Jupiter, Saturn, Titan, Uranus, Neptune, Triton, and Pluto [1]. These observations probe the atmospheres at roughly 0.1 to 100 microbar. I will talk about three aspects of stellar occultations: one-dimensional vertical profiles of the atmosphere, two- or three-dimensional atmospheric states, and the time evolution of atmosphere. In all three, I will draw on recent observations, with an emphasis on Pluto. Occultations are particularly important for the study of Pluto's atmosphere, which is impossible to study with imaging, and extremely difficult to study with spectroscopy. It was discovered by stellar occultation in 1988 [2]. No subsequent Pluto occultations were observed until two events in 2002 [3]. Pluto is now crossing the galactic plane, and there have been several additional occultations observed since 2006. These include a high signal-to-noise observation from the Anglo Australian Observatory in 2006 [4] (Fig 1), densely spaced visible and infrared observations of Pluto's upper atmosphere from telescopes in the US and Mexico in March, 2007 [5] (Fig. 2), and a dualwavelength central flash observation from Mt. John in July, 2007 [6] (Fig 3). The flux from a star occulted by an atmosphere diminishes primarily due to the increase in refraction with depth in the atmosphere, defocusing the starlight, although absorption and tangential focusing can also contribute. Because the atmospheric density, to first order, follows an exponential, it is feasible to derive a characteristic pressure and temperature from isothermal fits to even low-quality occultation light curves. Higher quality light curves allow fits with more flexible models, or light curve inversions that derive temperatures limited by the resolution of the data. These allow the derivation of one-dimensional profiles of temperature and pressure vs. altitude, which are critical for understanding the energy balance in upper atmospheres, interpreting thermal emission, and studying dynamics. These vertical profiles include small-scale fluctuations generally associated with gravity waves. While the 1988 Pluto occultation light curve was remarkably smooth, more recent Pluto occultations show spikes indicative of these small-scale dynamics. When an atmospheric occultation is observed from several sites, or when a single site is near enough to the shadow center to observe solar flux refracted from multiple locations in the occulting atmosphere, it is possible to study the two- or three-dimensional structure of an atmosphere. The simplest example is the oblateness of the atmosphere derived from the shape of an isobar [7], but more complex analyses are also possible. A comparison of the temperatures at different latitudes or local times of day can shed light on the relative importance of radiative equilibrium and dynamics to the energetics of an atmosphere, as has been done for the 2006 June 12 occultation by Pluto [4]. Closely spaced sites can be used to derive the two-dimensional shape and aspect ratio of temperature and density fluctuations [5,8], aiding the identification of the generating sources of these fluctuations. Occultation observations over a long time span are used to study the long-term evolution of an atmosphere. Both Triton and Pluto have shown large changes in their pressures since the late 1980's that is almost certainly related to changes in the temperature of their surface ices. The temperatures in the Uranian upper atmosphere increased before the previous solstice, and reverted to cooler temperatures a decade later, perhaps indicative of adiabatic cooling [9]. Recent improvements in astrometric catalogs, occultation-capable cameras (with low read noise, high readout rates, and little or no dead time), and easy access to accurate timing can greatly improve the quality, spatial sampling, and frequency of occultations by planetary atmospheres.
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