Cometary atmospheres. Observations of comets with the LSA/MMA will directly aid our understanding of the nature and origin of comets and complement the planned space probes (in particular Rosetta) which will be able to sample only a few comets. Millimeter and submillimeter spectroscopy have by now discovered more than 20 molecules in comets, and have shown that the composition of cometary ices is highly analogous to interstellar material, indicating that comets are relics of protosolar chemistry. The large collecting area of the array will permit us to search for less abundant molecules, radicals, and new ions in bright comets coming from the Oort cloud and to investigate the composition of periodic comets born in the Kuiper belt. It will be possible to detect CO and other species in many distant comets and to study the evolution of their outgassing as they approach the Sun. CO could also be detected in giant comet-like objects such as Chiron. Such studies will provide clues to the sublimation processes and physical properties of cometary ices. The LSA/MMA will be also a key instrument for investigating isotopic ratios (such as D/H) in several species and many comets. Important information will be obtained on the origin of comets and the formation of the Solar System.
The LSA/MMA will be able to map molecules and dust continuum emission in the coma of comets, as in the maps of comet Hale-Bopp, shown in Fig. 2.7, but with much greater sensitivity and resolution. Note that with beams of < 0.1'' pointed at a comet at a distance of 1 AU, a new array will have a linear resolution of 70 km ! Because cometary activity varies on short time scales, the new array will be ideal for providing fast images. Such images will allow us to study structures in the inner coma, such as fans and rotating jets, and to investigate to what extent dust and chemistry in the coma are involved in the production of molecules. Maps of the distribution of rotational temperatures of different molecular species will help us study the thermodynamics, excitation processes, and physical conditions in the coma of comets.
Thermal emission of small Solar System bodies. The LSA/MMA will enable us to make sensitive studies of the thermal emission of small Solar System bodies. Asteroids and cometary nuclei of small sizes, and even distant objects such as Centaurs and trans-Neptunian objects, will be detectable in the mm and sub-mm continuum. Together with observations in the mid-infrared, centimeter, and visible bands, mm/sub-mm observations with the LSA/MMA will allow to probe the temperature of these objects at various depths and to measure their albedo and size. Imaging thermal emission from the planetary satellites, the Pluto/Charon system and the largest asteroids will provide clues to their thermal properties and the degree of heterogeneity of their surface.
Atmospheres of the Planets and their Satellites. A new large array could map CO and HDO in Mars and Venus and give data on wind, temperature, CO and water distribution, and atmospheric dynamics on spatial scales less than 100 km, which is comparable to regional weather scales. The analysis of meteorological and climatic variations in the atmosphere of Mars with the LSA/MMA will be a worthwhile complement to future space missions. Moreover, the large collecting area of the array will permit us to search for molecular trace species likely to be present in these planets, such as sulfur-bearing compounds in Venus and organic species in Mars. Wide bandwidth capabilities will allow to probe the deep atmosphere of Venus. The LSA/MMA will be the first mm interferometer that will be able to map planetary atmospheres on short timescales.
Mapping HCN and CO, and searching for other nitriles (HCN, CH3CN) on Neptune would provide information on whether the origin of such molecules is internal or external. A long-term mapping of CO, CS and HCN in the atmosphere of Jupiter will allow us to follow the evolution of the spatial distribution of the large quantity of such molecules that were recently deposited at Southern latitudes during the impacts of comet Shoemaker-Levy. During very dry conditions at the high-altitude site proposed for the LSA/MMA, the mapping of H2O and HDO on the four Giant Planets would provide clues as to the origin of water (interplanetary dust or ring/satellite material). A backend as broad as 8 GHz would allow us to detect and map tropospheric species such as PH3 in the Giant Planets.
In addition, the LSA/MMA will permit us to probe with high resolution and sensitivity the atmospheres of Pluto and the satellites of the giant planets. An array with beams < 0.1 arcsec could detect SO2and SO in the plumes of the volcanoes on Jupiter's moon Io and enable the discovery of other trace constituents. Profound advances in the general understanding of the composition and thermal structure of the plumes may be expected from such observations. Mapping the millimeter lines of CO, HCN, HC3N, and CH3CN in the stratosphere of Titan with high spectral resolution will provide the vertical and latitudinal distributions of these constituents, enabling better constraints on the photochemistry that occurs in Titan's atmosphere and its response to seasonal effects. Other nitriles, like HC5N and C2H3CN, might be detected, allowing considerable advances in our knowledge of Titan's organic chemistry. Such future ground-based mm/sub-mm observations of Titan will complement in many aspects the on-the-spot measurements in a time-limited period by the Cassini/Huygens spacecraft that arrives at Saturn in 2004. The LSA/MMA will have sufficient sensitivity to detect and map CO (and perhaps other species, such as HCN) in the tenuous atmospheres of Pluto and Triton. This will provide hints on the nature of the interaction between their icy surfaces and their atmospheres. Finally, there is another object with a very tenuous atmosphere which could possibly be studied with the LSA/MMA -- the Moon. If, as is currently suspected, there are icy craters present near the poles, and if water is outgassed from those places, the LSA/MMA will be able to detect it.
