ALICE: Ultraviolet Imaging Spectrometer
ALICE, an Ultraviolet Imaging Spectrometer, will characterize the composition of the nucleus and coma and the nucleus/coma coupling of comet 67 P/Churyumov-Gerasimenko. This will be accomplished through the observation of spectral features in the extreme and far ultraviolet spectral regions ranging from 70 to 205 nm.
ALICE will make measurements of noble gas abundances in the coma, the atomic budget in the coma, and major ion abundances in the tail and in the region where solar wind particles interact with the ionosphere of the comet. ALICE will determine the production rates, variability, and structure of H2O and CO, and CO2 gas surrounding the nucleus and the far-ultraviolet properties of solid grains in the coma.
ALICE will also map the cometary nucleus in the far ultraviolet and study the Rosetta asteroid flyby targets while en route to Churyumov-Gerasimenko. It has already studied Mars, during the Mars flyby on 25 February 2007.
The scientific objectives of the ALICE investigation are to characterize the composition of the nucleus and coma, and the coma/nucleus of comet 67 P/Churyumov-Gerasimenko. This will be accomplished through the observation of spectral features in the 70 - 205 nm extreme and far ultraviolet spectral region.
Ultraviolet spectroscopy is a powerful tool for studying astrophysical objects, and has been applied with dramatic success to the study of comets. ALICE will provide unprecedented improvements in sensitivity and spatial resolution over previous cometary ultraviolet observations. For example, ALICE will move the sensitivity threshold from the ~1 Rayleigh level achievable with the Hubble Space Telescope to the milliRayleigh level. In addition, ALICE will (by virtue of its location at the comet) move the spatial exploration of nucleus ultraviolet surface properties from the present-day state-of-the-art (no data available on any comet) to complete nuclear maps at Nyquist-sampled resolutions of a few hundred meters. Stars occulted by the absorbing coma will also be observed and used to map the water molecule spatial distribution, giving hints as to the location of the production regions on the nuclear surface.
Through its remote-sensing nature, ALICE will be able to:
- Obtain compositional and morphological information on the comet prior to the rendezvous, thereby providing planning observations for in situ instruments prior to entering orbit about the comet
- Map the spatial distribution of key species in the coma, and small coma dust grains, as a function of time as the comet responds to the changing solar radiation field during its approach to the Sun
- Obtain compositional and production rate measurements of nuclear jets and other inner coma features even when the Orbiter is not in the vicinity of these structures
- Obtain certain ion abundance measurements around perihelion in order to connect nucleus activity to changes in tail morphology and structure, and coupling to the solar wind
The primary scientific themes of the ALICE investigation are the following:
- Determine the rare gas content of the nucleus to provide information on the temperature of formation and the thermal history of the comet since its formation. Argon and Neon will be primary targets of the ALICE investigations
- Determine the production rates and spatial distributions of the key parent molecule species, H2O, CO and CO2, thereby allowing the nucleus/coma coupling to be directly observed and measured on many time-scales in order to study the chemical heterogeneity of the nucleus and its coupling to the coma
- Obtain an unambiguous budget of the cosmogonically most important atoms (Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur through the detection of their emissions far from the nucleus. This is required to understand their production processes and to derive the elemental composition of the volatile fraction of the nucleus. Coupled to the measurement of the major molecule abundances of the nucleus, this will give us the total contribution of the secondary parent species to the composition of the nucleus
- Study the onset of nuclear activity and nucleus output variations related to changing solar aspect and nuclear rotation with unprecedented sensitivity
Additional scientific themes ALICE will address include the following:
- Spectral mapping of the complete nucleus at far-ultraviolet wavelengths to characterize the distribution of ultraviolet absorbers on the surface, in particular icy patches and organics
- Photometric properties and ice/rock ratio of small grains in the coma as an aid to understanding the size distribution of cometary grains and how they vary in time. Also, studying the grain coma to establish the relative contributions of the nucleus and coma grains to the observed gases
- Mapping the time variability of O+, N+, and possibly S+ and C+ emissions in the coma and ion tail in order to connect nuclear activity to changes in tail morphology and structure, and tail interaction/coupling to the solar wind
Light enters the ALICE telescope through a 40 × 40 mm entrance aperture and is collected and focused by an off-axis paraboloidal primary mirror onto the approximately 0.1° × 6° spectrograph entrance slit. After passing through the entrance slit, the light falls onto the toroidal holographic grating of a Rowland Circle style imaging spectrograph, where it is dispersed onto a microchannel plate detector. The 2-D (1024 × 32 pixel) format MCP detector uses dual, side-by-side, solar-blind photocathodes of potassium bromide (KBr) and cesium iodide (CsI). The predicted spectral resolving power (λ/Δλ) of ALICE is in the range of 105 - 330 for an extended source that fills the instantaneous field-of-view defined by the size of the entrance slit.