Cassini-VIMS observations of particles in Enceladus' plume and the E ring

Details Cassini-VIMS observations of particles in Enceladus' plume and the E ring Cassini-VIMS observations of particles in Enceladus' plume and the E ring

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..760h&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

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Scientific paper

One of the most surprising discoveries from the Cassini mission is that Saturn's moon Enceladus vents material from its interior, launching gas and particulate matter high above its surface to supply water molecules to the inner magnetosphere and ice-rich grains to the E ring. While data from various instruments onboard Cassini have already provided important information about such parameters as the heat flux from the surface and the distributions of dust and gas within the plume, there is still considerable debate regarding several aspects of this phenomenon. In particular, there are several different models for how solid material is produced and accelerated within the moon, some involve boiling liquids [1] and others disassociating clathrates [2]. We report here on an investigation of the near infrared spectra of the plume between 0.85 and 5.1microns obtained by the Visual and Infrared Mapping Spectrometer (VIMS) instrument [3]. These data provide information on the velocity distribution of particles launched from the surface, which should constrain possible models for the acceleration of solids within the moon [4]. The relevant plume spectral data were obtained at high phase angles (roughly 160 ), where diffraction by small particles is the dominant process responsible for scattering light from the sun into the instrument. In fact, provided the particle size distribution is sufficiently steep and the particles' optical constants do not vary too much with wavelength (which should be true outside of the strong water-ice absorption band at three microns), particles of a given size s will most efficiently scatter light with a wavelength proportional to s (the constant of proportionality being determined primarily by the phase angle of the observation). Thus there is a relatively direct mapping between the shape of the spectrum and the shape of the particle size distribution, which allows us to compute approximate model spectra for a fairly comprehensive suite of particle size distributions. By comparing the observed VIMS spectra to these model predictions, we are able to derive constraints on the detailed shape of the particle size distribution in the plume between 1 and 5 microns. VIMS has sufficient spatial resolution during these observations to obtain spectra of the plume at about six different altitudes between 50 km and 300 km above Enceladus' surface. We can therefore obtain profiles of particle densities versus height for a range of different particle sizes. Only particles launched with sufficient speed at the surface can reach a given altitude, so these profiles can then be converted into estimates of the velocity distributions of particles launched from the surface. These calculations indicate that the velocity distribution is significantly steeper for threemicron- sized particles than for one-micron-sized particles. In fact, few particles larger than a few microns across have sufficient velocity at the surface to escape Enceladus' gravity, and so most fall back to the surface. This is consistent with the relative rarity of such particles in the E ring that has been inferred from ground-based remote sensing [5] and Cassini in-situ [6],[7] data (high-phase E ring spectra obtained by VIMS also confirm this). These findings should place important constraints on how particles are accelerated beneath Enceladus' surface and help unravel the processes responsible for generating the plume. The work was supported by NASA via the Cassini Project and a grant from the Cassini Data Analysis Program.

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