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I am a Senior Research Scientist at Southwest Research Institute studying terrestrial planet formation and the geology of small airless bodies. Present research focuses on new modeling techniques for terrestrial planet formation and granular dynamics related to NASA's OSIRIS-REx Asteroid Sample Return mission. In the past I have studied the formation mechanisms of binary asteroids in the Solar System, the history of Saturn's bizarre moon Iapetus, the four small satelites of Pluto and various topics related to small body dynamics and been involved in a couple observing projects (Eulalia/Polana family members, the Deep Impact mission and Irregular Jovian Satellites).

Terrestrial Planet Formation

We have searched for a solution to the "Mars Problem." This problem is based on modern computer simulations of terrestrial planet formation typically fail to create correct-size Mars analogs. We have approached this in a couple ways. First, we investigated the possible role that sweeping resonances from "early" planet migration could play (planetesimal-scattering migration immediately following the dissipation of the proto-solar gas-disk). If these sweeping resonances could remove material down to around 1.0-1.5 AU it could essentially starve the proto-Mars of material, keeping its size small. However, as found in Walsh & Morbidelli 2011, this was found to be too slow of a process, and also one which skew the asteroid belt too dramatically to fit today's constraints. Thus we tried something more drastic.

The "Grand Tack" puts forth a much more dramatic scenario. Here we model the effects on the inner Solar System caused by the inward, then outward, migration of Jupiter. If Jupiter "tacks" at 1.5 AU, the inner disk is truncated and Mars can form with the right mass. Also, Jupiter has a dramatic effect on the asteroid belt as it first empties and then refills the asteroid belt region by scattering asteroids. In the end it allows the asteroid belt to be partially filled with planetesimals originating from beyond the birthplace of the giant planets, which also has implications for the delivery of water to the still-forming Earth. [Nature] See also O'Brien et al. 2014 and Jacobson et al. 2014 for updates on further works.


As Co-Investigator and lead scientist of the Regolith Development Working Group my work in support of the OSIRIS-REx asteroid sample return space mission is focused on understanding the surface properties and behavior of the asteroid to support the selection of the sample site and to assess the liklihood of a successful sampling attempt. After months of surveying asteroid (101955 Bennu), our working group will build a map of the asteroid ranking the locations where our analysis of the measured properties of the asteroid are a good match of our sampling capabilities. This effort, the Sampleability Map, is a key part of the sample site selection process.


Iapetus is a very strange moon. This is the furthest regular moon of Saturn, and is strange for a number of reasons. First, its spin rate is very slow, and synchronous with its orbit, at 79.3 days. This is strange because its orbit is so large tides from Saturn are very weak and unlikely to de-spin Iapetus alone. Second, its shape not in hydrostatic equilibrium. Essentially it has the shape of a body spinning at 16 hours rather than a shape fit to its current rotation. Finally, Iapetus has an equatorial ridge which is up to 10-15 km tall and around 50 km wide (see picture on right).

At SwRI, working with Hal Levison, Amy Barr and Luke Dones, we are trying to undertand the possible evolution of Iapetus if it had had its own satellite in the past. The possible effects of a satellite would be to help de-spin Iapetus through tides, help account for the equatorial ridge through in-falling material from the disk out of which it had formed, and also might be responsible for some of the large impact basins on its surface.

Binary Asteroid Formation

Our models of tidal disruption have shown that it can only account for a small fraction of the observed NEA binary asteroids. With this result, we have focused our efforts on examining the possibility that the thermal YORP-effect may be capable of creating binary asteroids. This effect has recently been measured to change the spin period of an asteroid, and it has been suggested could actually spin-up an asteroid until it "bursts" or simply loses some mass off its surface.

Our work focused on testing the results of slowly spinning-up rubble pile asteroids constructed with different kinds of internal structure, where some behave like piles of rocks on Earth, and others behave very similar to fluids. The results were very nice, with the non-fluid rubble piles producing binary asteroids with properties very similar to those we observe among small asteroids. This is an ideal explanation for the formation mechanism of these observed binaries, and understanding their formation tells us a lot about their physical properties. First, the asteroids bulk structure is quite similar to well-understood granular mechanics. Second, the minor collisions of the material ejected off the asteroid's surface must be quite dissipative, in order to accrete a satellite in orbit. Lastly, the flow of material on the surface of the asteroid during the process may have observable effects on the surface regolith -- with implications for sample return missions. This work was published in Nature.

Granular Mechanics

As part of our effort to understand the re-shaping and disruption of asteroids we have undertaken some basic experiements regarding the physics of granular material. We model entire asteroids as collections of large grains bound only by their own gravity, but using the same tools we can study smaller-scale problems like the movement of their surface material. The first test we are undertaking are matching rotating drum and low-g avalanche experiments (right, a movie if you click on the image).

Tidal Disruption

Recent progress from N-body simulations has produced orbital and physical properties of binary asteroids formed via tidal disruption. These simulations, over 110,000 to date, run on the beowulf cluster at the Department of Astronomy. The properties studied from the simulations are;

  • Binary orbital properties; semi-major axis, eccentricity, inclination,
  • Binary physical properties; shapes, spin-axes, spin-states, size ratios
  • Binary production as a function of encounter parameters and progenitor state.

  • Of specific interest are the properties best constrained by current observations, separation and size ratio of NEA binaries (for a very complete listing of asteroidal binaries, see the Johnston archives). The simulated distribution of separation, when weighted by discvoery liklihood, matches the shape of the observed distribution quite well.

    Preliminary results were presented at DPS in Louisville (#32.20), and more recent results were published in Icarus in 2006, and was presented in a talk at ACM 2005 and a poster at DPS in Cambridge.

    The final part of this project included the creation of a steady-state model of the NEA binary population as produced by tidal disruptions. The results showed that despite the healthy production of binaries by this mechanism, they were evenb more effectively separated during subsequent planetary encounters.

    Small Main Belt Asteroids

    Starting in the fall of 2004 I began an observing program through the UMD-KPNO consortium. The program entails lightcurve observations of small (D < 5km) Main Belt asteroids, in order to constrain the distribution of spin and shape characteristics of this group of bodies. Also of interest is the binary percentage within this subset of the MBAs, which may shed light on possible formation scenarios for all binary asteroids.

    Deep Impact

    On July 4th the impactor from the Deep Impact mission struck comet 9P/Tempel 1. As part of this mission I observed the impact from the San Pedro Martir observatory near Ensenada Mexico. The observations made were optical photometry using the 1.5m telescope in the broadband colors BVRI. As well optical spectroscopy was done on the 2m telescope between 400-600 nm.

    Saturn's F Ring

    Previously I have simulated collisions on planetesimals in strong tidal fields, modeling particle interaction and collision in Saturn's F ring. This study was presented as a poster at the 2003 DPS meeting (abstract #526).

    Side Projects

    To accomodate the great need for computing power in the Astronomy Department, I and two fellow graduate students, Kayhan Gultekin and Zoe Leinhardt, created a network desktop computers to provide free computing time. This network, VAMPIRE, is a series of desktop computers in the department running a job managing program called CONDOR. This network identifies unused computers and runs jobs on them harnassing on average 14 hours of computing time per machine. The network has grown to almost 20 machines.


    My second year project at Maryland consisted of a detailed model of the Shoemaker-Levy 9 breakup at Jupiter in 1993. More to come on this, it is a project still very much underway.


    At the University of Notre Dame I completed a project with Prof. Terry Rettig studying the photometric properties of the irregular jovian satellites. This paper enforced the suspicion that the two main groups (prograde and retrograde) are likely of differing origin. This study has renewed interest as the total number of irregular satellites in Jupiter's system has grown enormously in recent years, with many groups of moons appearing.

    The summer of 1999 I was student in the Northern Arizona REU (Research Experience for Undergraduates). I did photometry on irregular Uranian satellites (Caliban and Sycorax), as well as a distant inactive comet (P/Neujmin 1). My advisor that summer was Prof. Steve Tegler .