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Project Progress

Task 1. Detection and Characterization of Extrasolar Planets (Butler, Seager, Weinberger)

Alycia Weinberger joined the staff of the Carnegie Institution and the CIW NAI team in August 2001, having moved from UCLA, where she was affiliated with the NAI lead team at that institution. Sara Seager, now at the Institute for Advanced Study, will commence full time at the Carnegie Institution in August 2002.

In the past year Weinberger has continued her investigations into the physical properties of circumstellar disks. With UCLA colleagues Eric Becklin and Ben Zuckerman, she reported the first spatially resolved mid-infrared spectroscopy of a circumstellar disk (Beta Pictoris). The group found an inner warp in the disk that was previously unknown and that may signal the presence of a planet orbiting within the disk. They also found that small silicate grains, similar to those in our Solar System’s comets, are much more prevalent in the inner part of the disk than farther out. This may be because planets stir up the inner disk, causing the collisions that create small dust grains. Hubble Space Telescope (HST) images of the disk of TW Hydrae by Weinberger’s group show that it is flared, in accordance with theories about young disks. Mid-infrared measurements showed that small grains still exist in the disk, even after 8 Myr of evolution and that the disk should be quite massive to explain its energy output. Weinberger also began a project at Las Campanas Observatory to identify young stars very close to Earth and to search for circumstellar disks around them.

Over the last year, Paul Butler and his collaborators, which include postdoctoral scientist McCarthy, announced about 20 new extrasolar planets, bringing the total known exoplanets to nearly 100, including the lowest mass planet yet found (~30 Earth-masses) and the first planet in a Jupiter-like orbit beyond 5 AU. There are approximately 2,000 nearby Sun-like stars (late F – early M) out to 50 parsecs that are at least 2 Gigayears old or older, of which his group is currently surveying approximately 1,200 on the Lick 3-m, Keck 10-m, and the Anglo-Australian 3.9-m telescopes. With the addition of the Carnegie 6.5-m Magellan telescope at Las Campanas, Chile, he plans to expand this survey to all 2,000 nearby stars over the next year and thus carry out the first reconnaissance of all nearby Sun-like stars for planets. With current measurement precision, Butler and colleagues can detect true Solar System analogs, Jupiter and Saturn-like planets orbiting beyond 5 AU. Over the next decade, by discovering many such systems, the long-term goal is to place the Solar System in the broader context of planetary systems for the first time.

Postdoctoral scientists Haghighipour and Rivera have begun a study of the dynamics of bodies ranging from test particles to planetary objects in multi-planet extrasolar planetary systems. With orbital parameters provided by a fitting routine for radial velocities of extrasolar planets, they are investigating the dynamics of systems such as GJ 876, Upsilon Andromedae, and 47 Ursae Majoris using new data from recent observations. Goals include determining their regions of stability and possible islands of instability and their correspondences to mean motion resonances.

Task 2. Formation of Gas-Giant Planets (Boss, Wetherill)

The ice giant planets, Uranus and Neptune, represent a major unsolved problem in the theory of the origin of the Solar System. Boss, Wetherill, and postdoctoral scientist Haghighipour have proposed a radically new scenario for forming the ice giant planets. A gravitationally unstable protoplanetary disk forms four giant gaseous protoplanets within about 1000 years, while dust grains coagulate and add sediment to their centers, forming solid cores. The disk gas beyond Jupiter’s orbit is removed in about a million years by extreme and far-ultraviolet (EUV and FUV) radiation from a nearby massive star, exposing the outermost protoplanets to EUV/FUV radiation, photoevaporating most of their gas in another million years, and leading to 15-Earth-mass cores with thin gas envelopes, i.e., ice giant planets. The innermost gaseous protoplanet is protected from photoevaporative losses by the gravitational potential well of the Sun, becoming Jupiter, while the fourth protoplanet, orbiting around 10 AU, loses most but not all of its gaseous envelope and becomes Saturn. This scenario implies that regions like the Orion Nebula Cluster, where the HST has imaged protoplanetary disks being photoevaporated, as hypothesized in this scenario, may actually be hospitable to the formation of planetary systems similar to our own. Because most stars form in regions like Orion, solar systems similar to our own could be much more prevalent than would otherwise be the case. This scenario also implies that icy dust grains and comets were subjected to a lengthy period of intense ultraviolet (UV) irradiation, leading to photochemical production of surficial organic molecules, such as polycyclic aromatic hydrocarbons and amino acids, perhaps giving a head start for the formation of prebiotic molecules. Comets would build up a layer of organic compounds, similar to the tholins observed on many outer Solar System bodies, which would act as a sunblock coating and protect them from the UV radiation.

In a related project, Haghighipour has begun numerical and analytical studies of the dynamics of small solids in a gaseous disk of non-uniform density. Because of possible connections to the rapid formation of giant planets via the disk instability model, a focus of the work has been the time of migration of solids ranging from micron-sized dust particles to 10-m objects in the vicinity of local density enhancements in a gravitationally unstable disk. If the effects of drag by the gas and the gravitational attraction of the disk are included, it is possible for small solids to migrate rapidly toward the location of spiral arms or clumps of a gravitationally unstable disk where giant planets may form.

Task 3. Evolution of Planetary Water (Solomon)

One of the long-term goals of this NAI project is to assess the likelihood, timing, and physical and chemical environments of hydrothermal systems on Solar System objects other than Earth, particularly Mars and the icy satellites of Jupiter, but also including asteroids and comets. The efforts by Solomon on this project during the past year have focused on Mars, primarily because of the influx of important new data from the Mars Global Surveyor and more recently the Mars Odyssey missions. In particular, he has led a group attempting to synthesize the principal geological events during the earliest history (the Noachian epoch) of Mars, with the goal of understanding the physical and chemical processes linking those events. Of astrobiological relevance, there is abundant evidence for pervasive water-surface interactions on Mars during the Noachian epoch, including large areas displaying extensive erosion and widespread Noachian resurfacing of the northern hemisphere inferred from the high density of partially buried impact structures formed within Utopia basin fill. Early climate was strongly influenced by early atmospheric loss via impact ejection, solar wind sputtering, and formation of carbonates, but the relative importance and timing of these mechanisms and even the differences in climate between the Noachian and later periods are not presently known. Recent upward revisions in the probable water content of Martian magmas suggest that early to middle Noachian construction of Tharsis released substantial quantities of magmatic volatiles to the atmosphere, with possible influences on climate. Open issues currently under study include characteristics of the Noachian cryosphere and hydrosphere (constrained by new estimates for heat flow in the Noachian), the influence of early global differentiation and later magmatism on mantle water content and dynamics (constrained by new estimates of Noachian volcanic flux), and the role of water in modifying upper crustal composition, magnetization, and modes of deformation.

Postdoctoral scientist Dombard has been applying a model for unstable lithospheric contraction to Europa to examine potential long-wavelength folding as a constraint on heat flow through the outer icy shell. A minimum heat flow of 75 mW/m2 is implied, yielding a thermally conductive layer about 8 km thick. In parallel work, Dombard has been modeling relaxation of topography on Ganymede, Callisto, and Europa. On Europa, relaxation has been proposed to be a critical component of the evolutionary sequence for lithospheric folds, but results to date suggest that time scales for this relaxation may be significantly longer than originally estimated. In separate work, Dombard explored the possibility that some of the lineations observed on the near-Earth asteroid Eros are due to thermal stresses associated with a secular change in average surface temperature accompanying orbit transfer. He suggested that study of the stratigraphy of these lineations may provide direct observations of the history of orbit change and, if so, may illuminate the chaotic orbital dynamics of near-Earth asteroids and the threat they pose to terrestrial biota.