Francesca DeMeo

Current Research

The Distribution of Asteroids: Advances in the discovery and characterization of asteroids over the past decade have revealed an unanticipated underlying structure that points to a dramatic early history of the inner Solar System. The asteroids in the main asteroid belt have been discovered to be more compositionally diverse with size and distance from the Sun than had previously been known. This implies substantial mixing through processes such as planetary migration and the subsequent dynamical processes. This review is published in Nature (DeMeo & Carry 2014, Nature, 505:629-634) .


Ice variability on Charon: Improved technology, especially adaptive optics systems, now enable the clear angular separation of Pluto and Charon from ground-based telescopes. Near- infrared spectral data have revealed Charon’s surface to be rich in crystalline water ice and ammonia hydrates. In this work we search for spectral differ- ences across Charon’s surface with new near-infrared spectral data taken in the K-band (2.0-2.4 μm) with SINFONI on the VLT and NIRI on Gemini North as well as with previously published spectral data. Focusing particularly on the distribution of ammonia hydrate across the surface, we find that the band center and band depth vary weakly across Charon’s surface. The strength of the absorption band of ammonia hydrate is dependent on the state of the ice, concentration in H2O, grain size, temperature and exposure to radiation. The spectral variation suggests that the New Horizons flyby could find the concentration of ammonia hydrate heterogeneously distributed across the surface. This work is in preparation.


Rogue asteroids in the inner belt Very red featureless asteroids (spectroscopic D-types) are expected to have formed in the outer solar system far from the sun. They comprise the majority of asteroids in the Jupiter Trojan population, and are also commonly found in the outer main belt and among Hildas. The first evidence for D-types in the inner and middle parts of the main belt was seen in the Sloan Digital Sky Survey (SDSS). Here we report follow-up observations of SDSS D-type candidates in the near-infrared. Based on follow up observations of 13 SDSS D-type candidates, we find a ~20% positive confirmation rate. Known inner belt D-types range in diameter from roughly 7 to 30 kilometers. Based on these detections we estimate there are ~100 inner belt D-types with diameters between 2.5 and 20km. The total mass of inner belt D-types is 4x10^16kg which represents 0.01% of the mass of the inner belt. The inner belt D-types have albedos slightly higher that typical D-types (0.1 versus 0.06) which raises the question to whether these inner belt bodies represent only a subset of D-types, they have been altered by external factors such as weathering processes, or if they are compositionally distinct from other D-types. Dynamical models have yet to show how D-types originating from the outer solar system could penetrate into the inner reaches of the Main Belt under current scenarios of planet formation and subsequent Yarkovsky drift. (DeMeo et al. 2014, Icarus, 229:392-399) .


Asteroid resurfacing mechanisms: All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1My) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: i) the relatively steady fraction of Q-types with increasing semi-major axis and ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces (DeMeo et al. 2014, Icarus, 227:112–122) .


Previous Research


1. Compositional Studies of Main Belt Asteroids


Asteroid Taxonomy: Taxonomic classification systems of asteroid spectra have been used for decades to group bodies with similar surface properties. Previous systems, however, were defined in the visible wavelength range. The past decade has seen an explosion of research in the near-infrared due to better CCD technology and the usefulness of this wavelength range to identify spectral absorptions indicating key minerals on asteroid surfaces. To incorporate this wealth of new data, I created a taxonomy of asteroids based on a few hundred visible plus near-infrared spectra using Principal Component Analysis (DeMeo et al. 2009, Icarus, 202:160-180). This tool is used extensively by the asteroid community (http://smass.mit.edu/busdemeoclass.html).


Binary asteroid formation: Binary asteroids are unique markers of collisional history in the solar system, although it is extremely difficult to resolve the two components. There are two proposed scenarios for creating binary asteroids including capture of one asteroid around another and fission of on asteroid into two components. Asteroid (379) Huenna, because of the highly eccentric orbit of its secondary, was a rare candidate for the capture scenario. I took one of the first spectral measurements separating two components of a binary asteroid to constrain formation scenarios (DeMeo et al. 2011, Icarus, 212:677-681). Because the two asteroids had similar spectral characteristics both the capture and the fission formation scenarios were valid.


2. Physical characteristics and sources of the near-Earth object population


Composition v. Size for NEOs: It was expected that the most common meteorites should match the composition of the most common near-Earth objects. However, Vernazza et al. (2008, Nature, 454:7206:858-860) found that the most common type of NEO in the few kilometer and larger range did not match the meteorite flux. This suggests that the sampling of large NEOs is different from smaller ones, likely due to forces such as Yarkovsky drift that is more effective at smaller sizes. I have undertaken a survey of the smallest NEOs measured spectroscopically to-date using the 6.5-meter Magellan telescope to determine whether or not sub-km NEOs are a better match to the meteorite record (DeMeo et al., 2011, LPSC abstract, 1608:2055). By sampling the 200-1,000 meter size range I seek to determine how the NEO population transitions compositionally as a function of size. The three potential outcomes include a steady transition, an abrupt change, or no change over the size range I cover. Any outcome will provide constraints on the mechanisms for delivery to near-Earth space particularly as a function of size. This work is in preparation.


The comet contribution to NEOs: The lifespan of near-Earth objects (NEOs) is much shorter than the age of the Solar System, necessitating resupply from other source regions, particularly the main asteroid belt and the Jupiter family comets. One outstanding question has been what is the relative contribution from each source population. To address this, I identified comet candidates among asteroid-like NEOs using three constraints: the Jupiter Tisserand dynamical parameter, the surface albedo, and spectral properties (DeMeo & Binzel 2008, Icarus, 194:436-449) . I conclude that 8 ± 5% of the total asteroid-like NEO population have the requisite orbital properties, physical properties, and dynamical likelihood to have originated as comets from the outer Solar System.


3. Volatiles on Kuiper Belt Objects


Compositions of small KBOs: Kuiper Belt Objects (KBOs) are small bodies residing past Neptune. Studying their surface compositions helps to understand where these bodies originally formed, what compositional and temperature conditions they formed in, and whether or not they were capable of retaining the volatiles that were originally on their surface. During my PhD I was part of a large program observing 45 KBOs spectroscopically and photometrically in the visible and near-infrared using the 8-meter Very Large Telescope in Chile. I interpreted the surface composition of three outer Solar System small bodies, (52872) Okyrhoe, (90482) Orcus, and (73480) 2002 PN34, by modeling spectroscopic measure- ments in the visible and near-infrared wavelength ranges (DeMeo et al. 2010, A&A, 521:A35). The spectra reveal varying amounts of H2O ice among these bodies, as potential surface heterogeneity. Orcus, a special intermediate-size object that lies in a transition region for volatile retention, is further explored in Carry et al. (2011, A&A, 534:A115). In that work we seek to constrain the potential presence of volatiles other than water, such as methane or ammonia. We find no evidence for other volatiles suggesting Orcus was not able to retain volatiles over its lifetime. Additionally, I explore the photometric colors of 23 TNOs, refining absolute magnitude values and detecting surface variation (DeMeo et al. 2009, A&A, 493:283-290).


Constraining the presence of Ethane on Pluto and Triton: Using spectral measurements of Pluto and Triton on the 8-meter VLT, I search for ethane on their surfaces. Pluto and Triton’s surfaces and atmospheres are rich in methane and because ethane is an irradiation by-product of methane it is reasonable to expect it is also present. By using radiative transfer models I constrained the potential presence of ethane on both bodies to less than a few percent of the surface area, though no clear detection was found. (DeMeo et al. 2010, Icarus, 208:412-424)