Jean-Luc Margot

Gravity Field and Internal Structure of Mercury from MESSENGER

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s outer solid shell is denser than expected, suggesting a deep reservoir of high-density material, possibly iron-sulfide. Radio tracking of the MESSENGER spacecraft has provided a model of Mercury’s gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury’s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia C/MR2 = 0.353 ± 0.017, where M and R are Mercury’s mass and radius, and a ratio of the moment of inertia of Mercury’s solid outer shell to that of the planet of Cm/C = 0.452 ± 0.035. A model for Mercury’s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core.

Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ∼1.5-kilometer-diameter primary (Alpha) is an unconsolidated gravitational aggregate with a spin period ∼2.8 hours, bulk density ∼2 grams per cubic centimeter, porosity ∼50%, and an oblate shape dominated by an equatorial ridge at the objects potential-energy minimum. The ∼0.5-kilometer secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius ∼2.5 kilometers and period ∼17.4 hours, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.

Topography of the Northern Hemisphere of Mercury from MESSENGER Laser Altimetry

Mercury Inside and Out The MESSENGER spacecraft orbiting Mercury has been in a ∼12-hour eccentric, near-polar orbit since 18 March 2011 (see the Perspective by McKinnon). Smith et al. (p. 214, published online 21 March) present the most recent determination of Mercurys gravity field, based on radio tracking of the MESSENGER spacecraft between 18 March and 23 August 2011. The results point to an interior structure that differs from those of the other terrestrial planets: the density of the planets solid outer shell suggests the existence of a deep reservoir of high-density material, possibly an Fe-S layer. Zuber et al. (p. 217, published online 21 March) used data obtained by the MESSENGER laser altimeter through to 24 October 2011 to build a topographic map of Mercurys northern hemisphere. The map shows less variation in elevation, compared with Mars or the Moon, and its features add to the body of evidence that Mercury has sustained geophysical activity for much of its history. Mercury’s topography indicates sustained geophysical activity for most of the planet’s geological history. Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury’s topography occurred after the earliest phases of the planet’s geological history.

ARCHITECTURE OF PLANETARY SYSTEMS BASED ON KEPLER DATA: NUMBER OF PLANETS AND COPLANARITY

We investigated the underlying architecture of planetary s ystems by deriving the distribution of planet multiplicity (number of planets) and the distribution of orbital inclinations based on the sample of planet candidates discovered by the Kepler mission. The scope of our study included solar-like stars an d planets with orbital periods less than 200 days and with radii between 1.5 and 30 Earth radii, and was based on Kepler planet candidates detected during Quarters 1 through 6. We created models of planetary systems with different distributions of planet multiplicity and inclinations, simulated observat ions of these systems by Kepler, and compared the properties of the transits of detectable objects to actual Kepler planet detections. Specifically, we compared with both the Kepler sample’s transit numbers and normalized transit duration r atios in order to determine each model’s goodness-of-fit. We did not include any constra ints from radial velocity surveys. Based on our best-fit models, 75-80% of planetary systems have 1 or 2 plane ts with orbital periods less than 200 days. In addition, over 85% of planets have orbital inclinations les s than 3 degrees (relative to a common reference plane). This high degree of coplanarity is comparable to tha t seen in our Solar System. These results have implications for planet formation and evolution theories. Low inclinations are consistent with planets forming in a protoplanetary disk, without significant and lasting pe rturbations from other bodies capable of increasing inclinations. Subject headings:methods: statistical ‐ planetary systems ‐ planets and sate llites: general ‐ planets and satellites: detection

Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ 36), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate da=dt ¼ð � 19:0 � 0:1 Þ� 10

Mercury's moment of inertia from spin and gravity data

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00L09, doi:10.1029/2012JE004161, 2012 Mercury’s moment of inertia from spin and gravity data Jean-Luc Margot, 1,2 Stanton J. Peale, 3 Sean C. Solomon, 4,5 Steven A. Hauck II, 6 Frank D. Ghigo, 7 Raymond F. Jurgens, 8 Marie Yseboodt, 9 Jon D. Giorgini, 8 Sebastiano Padovan, 1 and Donald B. Campbell 10 Received 15 June 2012; revised 31 August 2012; accepted 5 September 2012; published 27 October 2012. [ 1 ] Earth-based radar observations of the spin state of Mercury at 35 epochs between 2002 and 2012 reveal that its spin axis is tilted by (2.04 AE 0.08) arc min with respect to the orbit normal. The direction of the tilt suggests that Mercury is in or near a Cassini state. Observed rotation rate variations clearly exhibit an 88-day libration pattern which is due to solar gravitational torques acting on the asymmetrically shaped planet. The amplitude of the forced libration, (38.5 AE 1.6) arc sec, corresponds to a longitudinal displacement of

No evidence for thick deposits of ice at the lunar south pole

450 m at the equator. Combining these measurements of the spin properties with second-degree gravitational harmonics (Smith et al., 2012) provides an estimate of the polar moment of inertia of Mercury C/MR 2 = 0.346 AE 0.014, where M and R are Mercury’s mass and radius. The fraction of the moment that corresponds to the outer librating shell, which can be used to estimate the size of the core, is C m /C = 0.431 AE 0.025. Citation: Margot, J.-L., S. J. Peale, S. C. Solomon, S. A. Hauck II, F. D. Ghigo, R. F. Jurgens, M. Yseboodt, J. D. Giorgini, S. Padovan, and D. B. Campbell (2012), Mercury’s moment of inertia from spin and gravity data, J. Geophys. Res., 117, E00L09, doi:10.1029/2012JE004161. 1. Introduction [ 2 ] Bulk mass density r = M/V is the primary indicator of the interior composition of a planetary body of mass M and volume V. To quantify the structure of the interior, the most useful quantity is the polar moment of inertia Z r ð x; y; z Þ x 2 þ y 2 dV : C ¼ V In this volume integral expressed in a cartesian coordinate system with principal axes {x, y, z}, the local density is multiplied by the square of the distance to the axis of rotation, which is assumed to be aligned with the z axis. Moments of Department of Earth and Space Sciences, University of California, Los Angeles, California, USA. Department of Physics and Astronomy, University of California, Los Angeles, California, USA. Department of Physics, University of California, Santa Barbara, California, USA. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D. C., USA. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA. Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA. National Radio Astronomy Laboratory, Green Bank, West Virginia, USA. Jet Propulsion Laboratory, Pasadena, California, USA. Royal Observatory of Belgium, Uccle, Belgium. Department of Astronomy, Cornell University, Ithaca, New York, USA. Corresponding author: J.-L. Margot, Department of Earth and Space Sciences, University of California, 595 Charles Young Dr. E., Los Angeles, CA 90095, USA. ( jlm@ess.ucla.edu) ©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2012JE004161 inertia computed about the equatorial axes x and y are denoted by A and B, with A < B < C. The moment of inertia (MoI) of a sphere of uniform density and radius R is 0.4 MR 2 . Earth’s polar MoI value is 0.3307 MR 2 [Yoder, 1995], indicating a concentration of denser material toward the center, which is recognized on the basis of seismological and geochemical evidence to be a primarily iron-nickel core extending

Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4

55% of the planetary radius. The value for Mars is 0.3644 MR 2 , suggesting a core radius of

Focused 70-cm Wavelength Radar Mapping of the Moon

50% of the planetary radius [Konopliv et al., 2011]. The value for Venus has never been measured. Here we describe our determina- tion of the MoI of Mercury and that of its outer rigid shell (C m ), both of which can be used to constrain models of the interior [Hauck et al., 2007; Riner et al., 2008; Rivoldini et al., 2009]. [ 3 ] Both the Earth and Mars polar MoI values were secured by combining measurements of the precession of the spin axis due to external torques (Sun and/or Moon), which depends on [C A (A + B)/2]/C, and of the second-degree harmonic coefficient of the gravity field C 20 = A [C A (A + B)/2]/(MR 2 ). Although this technique is not applicable at Mercury, Peale [1976] proposed an ingenious procedure to estimate the MoI of Mercury and that of its core based on only four quantities. The two quantities related to the gravity field, C 20 and C 22 = (B A A)/(4MR 2 ), have been determined to better than 1% precision by tracking of the MESSENGER spacecraft [Smith et al., 2012]. The two quantities related to the spin state are the obliquity q (tilt of the spin axis with respect to the orbit normal) and amplitude of forced libration in longitude g (small oscillation in the orientation of the long axis of Mercury relative to uniform spin). They have been measured by Earth-based radar observations at 18 epochs between 2002 and 2006. These data provided strong obser- vational evidence that the core of Mercury is molten, and that E00L09 1 of 11

The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36

Shackleton crater at the Moon’s south pole has been suggested as a possible site of concentrated deposits of water ice, on the basis of modelling of bi-static radar polarization properties and interpretations of earlier Earth-based radar images. This suggestion, and parallel assumptions about other topographic cold traps, is a significant element in planning for future lunar landings. Hydrogen enhancements have been identified in the polar regions, but these data do not identify the host species or its local distribution. The earlier Earth-based radar data lack the resolution and coverage for detailed studies of the relationship between radar scattering properties, cold traps in permanently shadowed areas, and local terrain features such as the walls and ejecta of small craters. Here we present new 20-m resolution, 13-cm-wavelength radar images that show no evidence for concentrated deposits of water ice in Shackleton crater or elsewhere at the south pole. The polarization properties normally associated with reflections from icy surfaces in the Solar System were found at all the observed latitudes and are strongly correlated with the rock-strewn walls and ejecta of young craters, including the inner wall of Shackleton. There is no correlation between the polarization properties and the degree of solar illumination. If the hydrogen enhancement observed by the Lunar Prospector orbiter indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith.