A great variety of fundamental
questions about comets remain to be answered. For example, observations
of many sources, ranging from comets to galaxies have shown the presence
of mid-infrared bands, which appear to be due to the carbon compounds known
generically as PAHs, although this has been a rather controversial subject.
In the case of comets we can study these lines with better resolution and
far better signal to noise than in more distant objects. The possible existence
of complex organic compounds in comets has profound implications for the
formation and later evolution of life on the Earth (and possibly on other
planets of the Solar System). There is increasing speculation that the
emergence of life on Earth may have been greatly aided by the "seeding"
of the first oceans with complex organic compounds from cometary impacts.
It has even been suggested that, given the believed composition of the
primitive Earth's atmosphere, the emergence of life would have been impossible
without aid from impact events. Observations of the observed bands in comets
with higher resolution and greater sensitivity would help to resolve this
issue, allowing an identification of the compounds and transitions that
are involved.
Trans-Neptunian Objects and bare cometary nuclei
Bare cometary nuclei and the recently discovered Trans-Neptunian Objects (which are believed to be Kuiper Belt comets) are very red, which is believed to resultfrom a carbonaceous-rich surface layer. Observations of both types of object in the mid-infrared are of interest because of the possibility of identifying PAHs and because the mid-infrared range allows the thermal emission from the bare cometary nucleus to be measured. Such measurements permit accurate diameters and albedos to be determined. Likewise, thermal infrared observations of asteroids are also of interest for determining their sizes and albedos; several asteroids have been used as calibrators for the ISO satellite in the mid-infrared. Polarimetry in the thermal infrared offers the possibility of studying the physics of cometary dust in more detail and, when extended to bare nuclei, is diagnostic of the surface properties of the nucleus. The current view is that a cometary nucleus is not a single, discrete body, but rather an aggregate of many blocks of material of sizes from tens (hundreds?) of meters, down to fine dust, loosely bound together by a matrix; if this is so we would expect the surface to be highly irregular on all scales.
Polarimetry would also allow the nature and characteristics of the dust coating on the surface of the nucleus to be determined for the first time, well before comet-encounter probes such as ROSETTA make landings on cometary nuclei. An instrument like CanariCam would allow a detailed study to be made of the possible presence of organic compounds in cometary nuclei because it would permit not just the brightest objects to be observed, but would extend observations to the majority of members of the highly-evolved Jupiter family of short-period comets. These represent the most likely bodies to interact with the Earth.
The greatest gains in solar system astronomy may be made though in the observation of the TNOs, mainly because these objects, which hold the key to many questions about the formation and evolution of the Solar System, will only be accessible in the thermal infrared to the most sensitive instruments on the largest telescopes. For these observations though it is necessary to observe as close to the blackbody peak of the TNOs as possible, which implies observing at 25 microns and beyond. At shorter wavelengths, the drop-off of the blackbody curve makes even detection of objects extremely challenging. At present, it seems likely that only the GTC + CanariCam will be capable of observing TNOs in the mid-infrared. Observations at 25 microns, in combination with visible and near infrared observations, will allow the albedo of the objects to be calculated and thus their diameters. The diameters of TNOs are a fundamental parameter for models of the formation of the solar system, as they allow the accretional history of the outer solar system to be studied. This information has important applications to the theoretical modelling of extrasolar planetary systems and protoplanetary disks.
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