Planetary Science

My primary scientific interest is in the search for life and habitable environments on other worlds. Projects to date under this broad theme include:

MARS

For my dissertation, I am working with Steve Squyres on high-resolution studies of the martian surface. This work relies heavily on images returned by the HiRISE camera on board NASA's Mars Reconnaissance Orbiter (MRO). HiRISE is the most powerful telescope ever sent to another planet, and provides views of the martian surface at a typical resolution of 30 cm (1 foot) per pixel.

Initially, much of our work focused on layered rock (probably sedimentary) outcrops surrounding Mawrth Vallis, an ancient valley that may have been carved by floodwaters. The layered rocks in this region have been previously found to contain clay minerals, which usually form through prolonged interaction between rocks and liquid water. With HiRISE and CRISM, another instrument on MRO, we have been studying the relationships between rock layers of different chemical composition. These may preserve a record of changing environmental conditions during a period of martian history in which water appears to have been more abundant than it is today, and which may have been conducive to the origin of life. Mawrth Vallis and other areas of Mars in which clay minerals have been detected are prime candidate landing sites for future missions to the surface of the Red Planet.

More recently, we have begun using HiRISE and CRISM to search for additional exposures of clays and hydrated sulfate salts across a much broader span of the Martian surface. Initial results of this survey are briefly summarized here.


ICY MOONS

For my senior thesis at Princeton, advised by Ed Turner, I conducted telescopic observations of icy moons in the outer solar system. We focused on Jupiter's moon Europa, which is believed to have a liquid water ocean buried beneath its icy surface, and on Saturn's moon Enceladus, which has recently been found (by the Cassini spacecraft) to have jets of water vapor and ice bursting forth from cracks in its surface, possibly from a subsurface body of liquid water. Many have proposed that such buried seas, while utterly starved of sunlight, could still support life, analogous to hydrothermal vent environments in the deep oceans of Earth.

Our observations were designed to look for gases--including water--in the tenuous atmospheres of these and other icy moons, which could have been outgassed from depth. We made our observations with the Echelle Spectrograph on the 3.5-meter telescope at the Apache Point Observatory in New Mexico. These ground-based observations cannot provide the spatial resolution of spacecraft missions to the outer Solar system, but they do give much higher spectral resolution, and allow for long-term monitoring of interesting features. Analysis of the resulting data is ongoing.

In summer 2006, I participated in the NASA Academy program at the Goddard Space Flight Center. With 19 other space science and engineering students from across the US and the world, I designed a potential future robotic mission to Enceladus, to follow up on the exciting discoveries made by Cassini. Our mission concept would send two spacecraft to the Saturn system: one would orbit Enceladus and ultimately land on its surface, while the other would remain in orbit around Saturn and provide telecommunications relay. We presented our design to NASA's Outer Planets Assessment Group as input into the ongoing process of selecting the next robotic mission to the outer solar system.


PLANETESIMAL DEBRIS DISKS

Over 200 planets orbiting other stars have been discovered in the past decade. Most of these are gas giants like Jupiter and Saturn, but discoveries of numerous smaller, rocky planets like Earth and Mars are expected and eagerly anticipated from upcoming space missions such as NASA's Kepler. Some of these planets could have conditions suitable for life.

Before any extrasolar planets had been identified, evidence for planetary systems was found in the form of dusty disks circling other stars. The dust in these disks is interpreted as debris from collisions between planetesimals--small rocky bodies similar to asteroids or comets--occurring while larger planets formed. With Michael Liu at the University of Hawaii, I searched for new dust disks around K and M dwarfs, the smallest stars other than brown dwarfs. These stars are faint and have low masses, making both dust disks and planets difficult to detect around them. Yet they are the most common types of stars in the universe, and are very long-lived, so understanding the planetary systems they may support is of great interest in the search for other habitable worlds. We identified several new candidate dust disks around these stars.



Structure of the Universe

I am fascinated by cosmology and the large-scale structure of the universe, and have worked on several projects in this theme, mostly during my time as an undergraduate.

SUPERCLUSTERS

For the most part, galaxies throughout the universe are grouped into clusters, and these clusters are in turn grouped into "superclusters", or clusters of clusters, which may be hundreds of millions of light-years across! Because they are so large, superclusters are difficult to study observationally, as their many tens to hundreds of constituent galaxies are spread out over vast regions of space. Furthermore, only a small fraction of the mass in superclusters is luminous; the rest is invisible dark matter.

To study the true structure and spatial extent of superclusters, I (with Neta Bahcall) used the results of a simulation of dark matter evolution throughout the history of the universe. We found, among other things, that the superclusters formed by this simulation typically have a triaxial shape (imagine a squashed football), and that many have flattened over time to become nearly two-dimensional structures at present. These results can be compared to actual observations from ongoing galaxy surveys as a check on our current understanding of cosmology and structure formation.


PHOTOMETRIC REDSHIFTS

To map large-scale structure using actual observations, each galaxy's position must be known in all three dimensions, including its distance from the observer on Earth. The most common method for calculating a galaxy's distance uses its redshift, i.e. the Doppler shift of its emitted light that occurs due to the continuous expansion of the universe. This redshift is most commonly and easily measured from a spectrum of the galaxy, but can also be estimated from the galaxy's color in visible images (more distant galaxies will appear redder, on average).

One ongoing sky survey with great potential for revealing the large-scale structure is the Sloan Digital Sky Survey (SDSS). The SDSS has collected spectra of roughly one million galaxies, but has imaged 50-100 million galaxies in color. Thus the number of galaxies available for structure studies can be greatly increased by measuring the redshift photometrically, i.e. using only color images.

With Jim Gunn, I developed a new method for estimating photometric redshifts of galaxies. Whereas many other methods to date have used only galaxy colors, our method also incorporates the galaxy's apparent brightness and information about its internal structure, which leads to improved precision in our estimate of its distance.


SUBMILLIMETER INSTRUMENTATION

While at the NASA Academy, I worked with Ed Wollack and Dave Chuss to develop more efficient detectors for millimeter wavelengths of light (between infrared and radio). These wavelengths are ideal for studying the cosmic microwave background radiation, the remnant light from the Big Bang that has provided much of our most fundamental cosmological knowledge. Submillimeter wavelengths are also well suited for studying cold dust in galaxies and in planetesimal debris disks in our own galaxy.

My work involved electromagnetic modeling of multi-layer millimeter-wave detectors, to determine what combinations of materials would maximize the light absorbed by these detectors. I also participated in lab tests of several prototype detectors.



Other Astrophysics

My first major scientific project--with Bohdan Paczynski and Laurent Eyer--was a study of variable stars in the stellar "bulge" at the center of our galaxy. We focused on a particular class of red giant variable stars that had been little studied previously, and demonstrated that they are concentrated along the galactic bar of stars that passes through the galaxy's center.


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