CASSINI/VIMS-V at Jupiter: Radiometric calibration test and data results

Abstract

During the Cassini–Huygens flyby of Jupiter in December 2000, VIMS-V acquired multispectral data cubes of Jupiter's atmosphere. The visual and infrared imaging spectrometer-visual channel (VIMS-V) is one of the principal contributions of Italian Space Agency (ASI) to the Cassini–Huygens mission to Saturn. VIMS-V is an imaging spectrometer operating in the wavelength range 300– 1050 nm , with a (nominal) spectral resolution of 7.3 nm , and a (nominal) spatial resolution of 500 μrad . VIMS-V is boresighted with the VIMS-IR channel operating in the wavelength range 0.8– 5.1 μm . During the early phases of the Cassini mission, the spacecraft encountered Venus (June 23, 1999), followed shortly thereafter by a flyby of the Earth. During the Earth flyby the Moon (August 17, 1999) was observed. Following the Earth–Moon flyby, the spacecraft encountered Jupiter (closest approach on December 31, 2000), and during the roughly 6 months prior to Jupiter closest approach a series of observations were made of most of the objects in the Jovian system. We have determined the instrumental transfer function of VIMS-V using the Moon and Venus day side data. This transfer function was then used to remove instrumental effects from the Jupiter data and to convert raw instrumental response numbers to spectral radiance from the target. It was thus possible to study the spectral variability of Jupiter's atmosphere across its disk using data from both the visual (V) and infrared (IR) channels of VIMS. In this paper we discuss the main results obtained by the V channel. We have analyzed the principal spectral features of Jupiter atmosphere, and in particular, the spatial variation of methane and ammonia absorption bands over the Jovian disk. Using the instrument's spatial mapping capabilities we have investigated the nature of the absorption band in the spectrum of Jupiter's atmosphere at 929 nm that is consistent with the presence of ammonia or water vapor. After comet Shoemaker–Levy 9 impacted Jupiter, water vapor was considered the most likely cause of the 929 nm absorption feature, but our data indicate that ammonia is the source of this band. Other analyses were performed using standard techniques such as forming band ratios and removal of the continuum. Our analyses confirm previous ground or satellite based observations. We were also able to verify the instrument radiometric calibration, using observations conducted during the close encounters with Venus and the Moon.