Francesca DeMeo

The Distribution of Olivine-Rich Asteroids: Differentiated asteroids are rare in the main asteroid belt despite evidence for ∼100 distinct differentiated bodies in the meteorite record. We have sought to understand why so few main belt asteroids differentiated and where those differentiated bodies or fragments reside. Using the Sloan Digital Sky Survey (SDSS) to search for a needle in a haystack we identify spectral A-type asteroid candidates, olivine-dominated asteroids that may represent mantle material of differentiated bodies. We have performed a near-infrared spectral survey with SpeX on the NASA IRTF and FIRE on the Magellan Telescope. We report results from having doubled the number of known A-type asteroids. We deduce a new estimate for the overall abundance and distribution of this class of olivine-dominated asteroids. We find A-type asteroids account for less than 0.16% of all main-belt objects larger than 2 km and estimate there are a total of ∼600 A-type asteroids above that size. They are found rather evenly distributed throughout the main belt, are even detected at the distance of the Cybele region, and have no statistically significant concentration in any asteroid family. We conclude the most likely implication is the few fragments of olivine-dominated material in the main belt did not form locally, but instead were implanted as collisional fragments of bodies that formed elsewhere. This work is submitted to Icarus.

The MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS): The MITHNEOS Survey systematically measures near-infrared spectra of potentially hazardous (PHA) and mission-accessible NEOs using the instrument SpeX on the NASA IRTF telescope (PI R. P. Binzel 2001-2014, PI F. E. DeMeo 2015 - present). Since inception, this program has measured near-infrared spectra for:

> 700 Near-Earth Objects.

~3% of the known Δv < 7 km/s population, representing ~270 NEOs, 80 of which have Δv < 6 km/s.

~15% of the known PHA population or about 270 objects, 115 of which have Δv < 7km/s.

53 NEOs with Jupiter Tisserand Parameter (Tj) < 3, a metric to determine if a body has a comet-like orbit.

19 asteroids with H > 25, these are small objects with D< ~30 m, assuming an albedo of 0.15.

The impact from this longstanding program includes:

Enhanced understanding of space weathering and refreshing mechanisms (Vernazza et al., 2009 Nature. Binzel et al. 2010, Nature, DeMeo et al. 2014)

Meteorite-NEO links and NEO-main belt source region links (Vernazza et al, 2008, Nature. Thomas, C.A. & Binzel R. P. 2010)

Characterization of extremely small and rapidly rotating NEOs (Polishook et al., 2012)

Development of techniques to determine albedo and spin orientation from thermal emission past 2 microns (Rivkin et al. 2005, Moskovitz et al. 2017)

Contributed to the vis+near-infrared spectral taxonomy used by the community to classify asteroids (DeMeo et al. 2009, 250 citations)

Much, much more on the advancement of NEO science that was largely impacted by this program is detailed in a dedicated Asteroids IV chapter (Binzel et al. 2015)

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 is published in Icarus (DeMeo & Carry 2013, Icarus, 226:723-741) and a review is published in Nature (DeMeo & Carry 2014, Nature, 505:629-634) .

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) .

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 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.

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.

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. (DeMeo et al. 2015, Icarus, 246:213-219)

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)