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Why is the Solar System the way it is? Many of the properties of planet Earth, including its size, its orbit, its temperature, and its composition are the direct result of processes that occurred 4.5 billion years ago in a collapsing, rotating cloud of interstellar gas and dust. Planetary astronomers at IfA are trying to discover what these processes were.
As the interstellar cloud shrank to form the solar nebula, its gas density grew, allowing dust grains in the cloud to coalesce into planetesimals that in turn grew to form planets. The planetesimals near the Sun were made of rock and metal. Those farther from the Sun, where the temperature was cold enough and the gas was thin, were made of ice particles and other frozen molecules along with dust grains and trapped gases.
Billions of these primitive objects still exist beyond the orbit of Neptune, though they are very hard to find and study. These small icy objects may tell us as much about the origin of the solar system as do the major planets. The deep freeze of the outer reaches of the solar system has preserved compounds from the interstellar cloud that produced the Sun and the planets. It is a rich place to look for clues about our origins.
Pluto and its moon Charon are the largest and most famous of these icy objects. David Tholen has made careful studies of the orbits of these two bodies. He has used both the telescopes on Mauna Kea and the Hubble Space Telescope to try to discover what they are made of and why Charon's orbit has a nonzero eccentricity. He also participates in the planning for NASA's spacecraft mission to Pluto.
David Jewitt studies objects beyond Neptune. Using highly sensitive large-format CCD cameras on the UH 2.2-meter and CFHT 3.6-m telescopes, he and past and present graduate students look for faint objects that move between one exposure and the next. They have found over 300 objects so far. Some of these, the "Plutinos," move in orbits that resemble that of Pluto. Others move in slow orbits in the "Kuiper Belt," a region of the outer solar system about twice the size of Neptune's orbit. Jewitt and his colleagues estimate that there are at least 70,000 objects in the Kuiper Belt that have diameters greater than 100 km. The colors of their surfaces range from gray to red. The reason for these variations is unclear, but they could be due to collisions among these objects. The Kuiper Belt objects are so faint that it is difficult to secure useful spectra even with the Keck telescopes, but plans are underway to rendezvous with one or more of them during NASA's mission to Pluto.
Recently, Jewitt and JCMT postdoc Hervé Aussel used the JCMT and the UH 2.2-m telescope to simultaneously measure thermal radiation and reflected light from KBO (2000) Varuna. From these data they were able to determine that the object is very much darker than Pluto, and is only about one third of its diameter.
Comets are icy objects from the outer solar system. They move in orbits that bring them close to the Sun. As the Sun heats them, gases and dust grains evaporating off their surfaces produce large comae. Karen Meech has been monitoring more than fifty comets as they pass through the inner solar system and disappear beyond the range of telescope view. The growth and fading of their tails as the comets' temperatures change reveal the kinds of gases they contain, while the oscillations of their brightness reveal how they spin as they orbit the Sun. Meech has found differences in the physical properties of the comets with different orbital characteristics. Since different dynamic families of comets originated and aged in different regions of the solar system, Meech's research will help us understand how and where different materials condensed in the early solar nebula.
Karen Meech is also an investigator in the NASA's Deep Impact mission to excavate a crater in the comet P/Tempel 1 in July 2005. The spacecraft will fire a half-ton copper impactor to the nucleus of the comet to excavate a crater > 20m deep and >100m in diameter. By watching how the crater is formed, and analyzing the debris that are thrown upwards by the collision, the Deep Impact team hopes to learn what lies below the skin of a comet, and sample pristine material that dates from the origin of the solar system.
The appearance of bright comets such as Hyakutake in 1996 and Hale-Bopp in 1997 gives astronomers the chance to analyze comets much more thoroughly than ever before. Several IfA astronomers have carried out extensive programs of imagery and spectroscopy at both infrared and visible wavelengths. David Jewitt has pioneered the use of submillimeter spectroscopy to study molecules and dust grains in comets. His observations with the JCMT have revealed enormous quantities of carbon monoxide in comets that are too far from the Sun, and too cold, for water ice to sublimate. Carbon monoxide sublimes at a much lower temperature than water ice, which explains why some comets become bright even while they are beyond Jupiter. Dr Meech has several collaborators in her comet group. NATO Postdoctoral Fellow Jana Pittichova, studies visible and near-infrared images of comets to try to understand how dust is ejected from their nuclei. Postdoctoral Fellow Yan Fernandez studies how comet nuclei rotate with a particular focus on extinct comets that have evolved to resemble asteroids. Graduate student Gerbs Bauer is writing a dissertation on the Centaur objects, which are believed to be transition objects between KBOs and short-period comets
The asteroids are the other leftovers from the formation of the solar system. Most are between Mars and Jupiter, but some have orbits that cross that of Earth. Of these, about two thousand are big enough (more than a kilometer in diameter) to pose a serious threat to our planet if they were to enter our atmosphere. Several new such objects are discovered each year. David Tholen measures the sizes and compositions of these Earth-crossing asteroids to find out which ones could pose an actual threat to Earth. He is also a team member for the orbiter camera in the Muses-C project; this Japanese spacecraft will be launched towards an asteroid in 2002, and bring back a sample of its surface material to the Earth.
SIRTF Fellow Yan Fernandez, working with David Jewitt and Scott Sheppard, is studying the albedos of small solar system objects, including the near Earth objects (NEOs). These objects are responsible for impacts on the Earth ranging in scale from negligible to devastating (e.g. the KT impactor which wiped out the dinosaurs). The NEOs comprise a mixture of escaped main-belt asteroids and dead comets in uncertain proportions. Fernandez hopes to measure the fraction of NEOs which might be dead comets by measuring the albedos. Cometary nuclei are coated in very dark, carbon rich materials that have a distinctively low albedo whereas asteroids are, on the whole, more reflective. In an initial study, 90% of the NEOs having dynamical similarities to comets also had the very low albedos typical of comets, confirming that dead comets persist amongst the NEOs. NEOs are one of the major science targets of the Pan STARRS all-sky telescope under development at IfA
Graduate student Scott Sheppard and David Jewitt are studying the small outer satellites of Jupiter using wide field CCD surveys conducted at the UH 2.2 m, CFHT and SUBARU telescopes. With their colleagues, they have discovered 41 new irregular satellites of Jupiter, more than quadrupling the number previously known. With these discoveries, Jewitt and Sheppard hope to learn more about the origin of the irregular satellites. Already they have found that the satellites belong to five distinct dynamical groups, each likely to have been produced by collisional disruption of a precursor object. The capture of the satellites by Jupiter probably occurred in the very early solar system, perhaps in the first million years after the collapse from the interstellar cloud. Understanding the Jovian satellites may throw new light on the properties and processes of the very young solar system.
Using both ground-based telescopes and NASA space probes, Toby Owen searches for clues to the origin of the planets and their atmospheres in the isotope ratios of elements such as oxygen, carbon, and sulfur. Recently, he has championed the idea that comets deliver volatiles to the Earth and other inner planets. The oceans, for example, may have arrived as cometary ice during the early phases of Earth history. This model is based on abundance and isotope ratio measurements of chemically inert gases in the atmospheres of Mars and Earth, interpreted with the help of laboratory studies performed in Israel by Akiva Bar-Nun. With telescopes on Mauna Kea, Owen plans to gather more evidence to test this model by obtaining new high-resolution spectroscopy of Mars and by studying molecules containing deuterium in comets. Owen's participation in the NASA Galileo mission to Jupiter, and the upcoming Cassini mission to Saturn and its giant satellite Titan should provide exciting new data with which to examine the origin of these bodies. He is also participating in the Japanese Planet B mission to Mars in 1998, which will allow testing of various possible escape processes that have controlled the evolution of the Martian atmosphere.
The University of Hawaii Adaptive Optics group, led by François Roddierand Claude Roddier has been using the Canada-France-Hawaii Telescope to study Solar System phenomena with resolutions that sometimes exceed that of the Hubble Space Telescope. Using the Hokupa'a wavefront curvature system they have mapped the moons and rings of Saturn, Uranus and Neptune, and watched the motions of clouds in Neptune's atmosphere. The picture on the right, obtained in collaboration with graduate student Christophe Dumas, shows a 2.26 µm image of Jupiter's moon Io, which is barely 1 arcsec across as seen from the Earth. The bright spots are erupting volcanos.