Solar system

Abstract:

We present new measurements and modelling of low-resolution transmission spectra of self-broadened ammonia gas, one of the most important absorbers found in the near-infrared spectrum of the planet Jupiter. These new spectral measurements were specifically designed to support measurements of Jupiter's atmosphere made by the Near-Infrared Mapping Spectrometer (NIMS) which was part of the Galileo mission that orbited Jupiter from 1995 to September 2003. To reach approximate jovian conditions in the lab, a new gas spectroscopy facility was developed and used to measure self-broadened ammonia spectra from 0.74 to 5.2 μm, virtually the complete range of the NIMS instrument, for the first time. Spectra were recorded at temperatures varying from 300 to 215 K, pressures from 1000 to 33 mb and using three different path lengths (10.164, 6.164 and 2.164 m). The spectra were then modelled using a series of increasingly complex physically based transmittance functions. © 2008 Elsevier Inc. All rights reserved.

Abstract:

We present zonal and meridional wind measurements at three altitude levels within the cloud layers of Venus from cloud tracking using images taken with the VIRTIS instrument on board Venus Express. At low latitudes, zonal winds in the Southern hemisphere are nearly constant with latitude with westward velocities of 105 ms-1 at cloud-tops (altitude ∼ 66 km) and 60-70 ms-1 at the cloud-base (altitude ∼ 47 km). At high latitudes, zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear (values lower than 15 ms-1), indicating the possibility of a vertically coherent vortex structure. Meridional winds at the cloud-tops are poleward with peak speed of 10 ms-1 at 55° S but below the cloud tops and averaged over the South hemisphere are found to be smaller than 5 ms-1. We also, report the detection at subpolar latitudes of wind variability due to the solar tide. Copyright 2008 by the American Geophysical Union.

Abstract:

We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio 12C/13C in each of three chemical species in Titan's stratosphere: CH4, C2H2 and C2H6. This is the first measurement of 12C/13C in any C2 molecule on Titan, and the first measurement of 12CH4/13CH4 (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of 12C/13C within a given species, and also for differences between the 12C/13C in the three gases. For CH4 alone, we find 12C / 13C = 76.6 ± 2.7 (1-σ), essentially in agreement with the 12CH4/13CH4 measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3 ± 1.0, and also with measured values in H13CN and 13CH3D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C2 species, we find 12C / 13C = 84.8 ± 3.2 in C2H2 and 89.8 ± 7.3 in C2H6, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of 12C / 13C = 80.8 ± 2.0, also compatible with GCMS. Therefore, the evidence is mounting that 12C/13C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88 ± 7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work. © 2008 Elsevier Inc. All rights reserved.