Research Interests

The spin state of a planet depends on external torques as well as on the distribution of mass and dissipation mechanisms within the interior. Detailed measurements of the spin dynamics can therefore provide a wealth of information about planetary interior structure and about a variety of important geophysical processes.

Our group has implemented a new Earth-based radar technique promulgated by Igor Holin. The technique provides instantaneous spin state measurements with fractional precision of 1 part in 100,000. The first set of observations reveals that Mercury has a molten core. The state of the core at Mercury has fundamental implications for the composition, internal structure, thermal evolution, and magnetic field of the planet. Click on the image for details about Mercury research.

Spin state measurements can provide important insights into the properties and geophysics of Venus. Atmospheric tides and angular momentum exchange strongly affect the spin state of the solid planet, such that long-term monitoring may provide crucial constraints on the atmospheric dynamics and climate of Venus. Initial measurements indicate a change in the spin rate since the Magellan era (Margot et al., in preparation). Additional observations are planned to measure secular and seasonal variations in the rotation rate. Venus has a large wobble which could be excited by mantle convection, volcanic or seismic activity, resurfacing, or atmospheric changes. Efforts are also underway to measure the spin pole orientation, which may ultimately reveal the precession rate of the spin axis, a direct constraint on the moment of inertia of Venus (i.e. the distribution of mass inside the planet).

This work is performed in collaboration with S.J. Peale, R.F. Jurgens, M.A. Slade, and I.V. Holin.

We study binary asteroids located in the main belt of asteroids between Mars and Jupiter. We use powerful adaptive optics systems on large telescopes to search for asteroid companions and to characterize their orbital parameters (e.g. the Keck II telescope on Mauna Kea, Hawaii and the Very Large Telescope on Cerro Paranal, Chile).

The figure to the left shows the discovery image of a satellite orbiting the 180-km diameter asteroid 22 Kalliope, the first companion detected around an M-class asteroid. The M asteroids are thought to be metal rich, but the density that can be derived from the system's orbital motion rules out a metallic composition. We have also discovered a satellite around 87 Sylvia, one of the 10 largest asteroids. At the time of discovery, Sylvia and its moon were 417 million kilometers away from Earth (about 1100 times the distance to the Moon), and the separation between the two was 1200 km (equivalent to the distance between Houston and El Paso).

Many minor planets orbit beyond the orbit of Neptune in a region aptly called the trans-Neptunian region. Those bodies are asteroids with a larger ice-rock fraction than those found closer to the sun. They are thought to be the precursor of short-period comets. Binary systems exist in that population as well, and Pluto-Charon is the most famous example. I am using the Hubble Space Telescope to characterize trans-Neptunian binaries in detail.

The trans-Neptunian binaries are thought to be primordial, and their abundance and properties constrain the environment in the earliest stages of solar system history, a very important boundary condition for theories of solar system formation. Observations of trans-Neptunian binary systems provide the first measurements of the density and mechanical properties of those distant ice-rock bodies.

This work is performed in collaboration with Mike Brown, Chad Trujillo, and Reem Sari.

High resolution images and shape models of asteroids are important to understand the physical properties and interior structure of asteroids. I have been interested in using Earth-based radar instruments, such as those of Arecibo and Goldstone, to provide images of asteroids during their close approaches to Earth. I am pursuing various ideas to improve the imagery and three-dimensional characterization of such objects, including chirp-based and interferometry techniques.

The first attempt to actually measure the topography of an asteroid was performed during the close approach of Asteroid 6489 Golevka in June 1999. The Arecibo radar was used to illuminate the asteroid, and a fast sampling system was set up to record the echoes at the NASA tracking station in Madrid, Spain, while collaborator Mike Nolan recorded the echoes at Arecibo. Combining signals from Arecibo and Madrid would in principle provide high resolution three-dimensional images of the asteroid. Although the asteroid was successfully detected at Madrid and Arecibo, there were a number of telescope problems which prevented the interferometric analysis. We are anxious to attempt this exciting experiment again on the next good opportunity.

Some of my research projects require the ability to record signals at a very high speed. Designing and building a low-cost, portable fast sampler was therefore one of my priorities when I arrived at Arecibo. These systems have been installed at Arecibo and the Green Bank Telescope. Four units have been replicated at JPL for installation at the NASA tracking stations in Goldstone, California.

The sampling system has been used to provide some of the highest resolution images of asteroids ever obtained. It has also been used to provide the first radar detection of a solar system object at NASA's tracking station in Madrid and the first radar image with NRAO's VLBA antenna in St-Croix. Other observations included OH masers and pulsars, many near-Earth asteroids, the rings of Saturn, and the Earth's ionosphere.