Planetary atmosphere observation analysis

Abstract:

Observations of Uranus were made in September 2009 with the Gemini-North telescope in Hawaii, using both the NIFS and NIRI instruments. Observations were acquired in Adaptive Optics mode and have a spatial resolution of approximately 0.1″ NIRI images were recorded with three spectral filters to constrain the overall appearance of the planet: J, H-continuum and CH4(long), and long slit spectroscopy measurements were also made (1.49-1.79μm) with the entrance slit aligned on Uranus' central meridian. To acquire spectra from other points on the planet, the NIFS instrument was used and its 3″×3″ field of view stepped across Uranus' disc. These observations were combined to yield complete images of Uranus at 2040 wavelengths between 1.476 and 1.803μm. The observed spectra along Uranus central meridian were analysed with the NEMESIS retrieval tool and used to infer the vertical/latitudinal variation in cloud optical depth. We find that the 2009 Gemini data perfectly complement our observations/conclusions from UKIRT/UIST observations made in 2006-2008 and show that the north polar zone at 45°N has continued to steadily brighten while that at 45°S has continued to fade. The improved spatial resolution of the Gemini observations compared with the non-AO UKIRT/UIST data removes some of the earlier ambiguities with our previous analyses and shows that the opacity of clouds deeper than the 2-bar level does indeed diminish towards the poles and also reveals a darkening of the deeper cloud deck near the equator, perhaps coinciding with a region of subduction. We find that the clouds at 45°N,S lie at slightly lower pressures than the clouds at more equatorial latitudes, which suggests that they might possibly be composed of a different condensate, presumably CH4 ice, rather than H2S or NH3 ice, which is assumed for the deeper cloud. In addition, analysis of the centre-to-limb curves of both the Gemini/NIFS and earlier UKIRT/UIST IFU observations shows that the main cloud deck has a well-defined top, and also allows us to better constrain the particle scattering properties. Overall, Uranus appeared to be less convectively active in 2009 than in the previous 3years, which suggests that now the northern spring equinox (which occurred in 2007) is passed the atmosphere is settling back into the quiescent state seen by Voyager 2 in 1986. However, a number of discrete clouds were still observed, with one at 15°N found to lie near the 500 mb level, while another at 30°N, was seen to be much higher at near the 200 mb level. Such high clouds are assumed to be composed of CH4 ice. © 2011 Elsevier Inc.

Abstract:

Asteroid surface material is expected to become photoelectrically charged, and is likely to be transported through electrostatic levitation. Understanding any movement of the surface material is relevant to proposed space missions to return samples to Earth for detailed isotopic analysis. Motivated by preparations for the Marco Polo sample return mission, we present electrostatic modelling for a real asteroid, Itokawa, for which detailed shape information is available, and verify that charging effects are likely to be significant at the terminator and at the edges of shadow regions for the Marco Polo baseline asteroid, 1999JU3. We also describe the Asteroid Charge Experiment electric field instrumentation intended for Marco Polo. Finally, we find that the differing asteroid and spacecraft potentials on landing could perturb sample collection for the short landing time of 20min that is currently planned.

Abstract:

The retrieval of surface composition from IR measurements of airless bodies requires a model capable of computing the significant thermal gradients present in the top few hundred microns of the regolith. In this study we introduce a model which reproduces most of the features found in controlled experiments made in the simulated lunar environment emission chamber (SLEEC). Although the model presented here is forced by a lower boundary held at a fixed temperature, we conclude that a similar algorithm driven by solar illumination may be used as a forward model to retrieve composition, particle size and effective thermal conductivity from IR measurements of airless bodies. Copyright 2011 by the American Geophysical Union.

Abstract:

Nightside infrared limb spectra of the Venus upper atmosphere, obtained by Venus Express VIRTIS, show strong scattering of thermal radiation. This scattering of upward-going radiation into the line-of-sight is dominant below 82.5. km even at a wavelength of 5 μm, which is indicative of relatively large particles. We show that 1 μm-sized sulfuric acid particles (also known as mode 2 particles) provide a good fit to the VIRTIS limb data at high altitudes. We retrieve vertical profiles of the mode 2 number density between 75 and 90. km at two latitude ranges: 20-30°N and 47-50°N. Between 20 and 30°N, scattering by mode 2 particles is the main source of radiance for altitudes between 80 and 85. km. Above altitudes of 85. km smaller particles can also be used to fit the spectra. Between 47 and 50°N mode 2 number densities are generally lower than between 20 and 30°N and the profiles show more variability. This is consistent with the 47-50° latitude region being at the boundary between the low latitudes and high latitudes, with the latter showing lower cloud tops and higher ultraviolet brightness (Titov, D.V., Taylor, F.W., Svedhem, H., Ignatiev, N.I., Markiewicz, W.J., Piccioni, G., Drossart, P. [2008]. Nature 456, 620-623). © 2010 Elsevier Inc.