Julie C. Castillo-Rogez

Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres

The typically dark surface of the dwarf planet Ceres is punctuated by areas of much higher albedo, most prominently in the Occator crater. These small bright areas have been tentatively interpreted as containing a large amount of hydrated magnesium sulfate, in contrast to the average surface, which is a mixture of low-albedo materials and magnesium phyllosilicates, ammoniated phyllosilicates and carbonates. Here we report high spatial and spectral resolution near-infrared observations of the bright areas in the Occator crater on Ceres. Spectra of these bright areas are consistent with a large amount of sodium carbonate, constituting the most concentrated known extraterrestrial occurrence of carbonate on kilometre-wide scales in the Solar System. The carbonates are mixed with a dark component and small amounts of phyllosilicates, as well as ammonium carbonate or ammonium chloride. Some of these compounds have also been detected in the plume of Saturn’s sixth-largest moon Enceladus. The compounds are endogenous and we propose that they are the solid residue of crystallization of brines and entrained altered solids that reached the surface from below. The heat source may have been transient (triggered by impact heating). Alternatively, internal temperatures may be above the eutectic temperature of subsurface brines, in which case fluids may exist at depth on Ceres today.

A partially differentiated interior for (1) Ceres deduced from its gravity field and shape

Remote observations of the asteroid (1) Ceres from ground- and space-based telescopes have provided its approximate density and shape, leading to a range of models for the interior of Ceres, from homogeneous to fully differentiated. A previously missing parameter that can place a strong constraint on the interior of Ceres is its moment of inertia, which requires the measurement of its gravitational variation together with either precession rate or a validated assumption of hydrostatic equilibrium. However, Earth-based remote observations cannot measure gravity variations and the magnitude of the precession rate is too small to be detected. Here we report gravity and shape measurements of Ceres obtained from the Dawn spacecraft, showing that it is in hydrostatic equilibrium with its inferred normalized mean moment of inertia of 0.37. These data show that Ceres is a partially differentiated body, with a rocky core overlaid by a volatile-rich shell, as predicted in some studies. Furthermore, we show that the gravity signal is strongly suppressed compared to that predicted by the topographic variation. This indicates that Ceres is isostatically compensated, such that topographic highs are supported by displacement of a denser interior. In contrast to the asteroid (4) Vesta, this strong compensation points to the presence of a lower-viscosity layer at depth, probably reflecting a thermal rather than compositional gradient. To further investigate the interior structure, we assume a two-layer model for the interior of Ceres with a core density of 2,460–2,900u2009kilograms per cubic metre (that is, composed of CI and CM chondrites), which yields an outer-shell thickness of 70–190u2009kilometres. The density of this outer shell is 1,680–1,950u2009kilograms per cubic metre, indicating a mixture of volatiles and denser materials such as silicates and salts. Although the gravity and shape data confirm that the interior of Ceres evolved thermally, its partially differentiated interior indicates an evolution more complex than has been envisioned for mid-sized (less than 1,000u2009kilometres across) ice-rich rocky bodies.

Constraining Ceres’ interior from its rotational motion

Context. Ceres is the most massive body of the asteroid belt and contains about 25 wt.% (weight percent) of water. Understanding its thermal evolution and assessing its current state are major goals of the Dawn Mission. Constraints on internal structure can be inferred from various observations. Especially, detailed knowledge of the rotational motion can help constrain the mass distribution inside the body, which in turn can lead to information on its geophysical history. Aims. We investigate the signature of the interior on the rotational motion of Ceres and discuss possible future measurements performed by the spacecraft Dawn that will help to constrain Ceres internal structure. Methods. We compute the polar motion, precession-nutation, and length-of-day variations. We estimate the amplitudes of the rigid and non-rigid response for these various motions for models of Ceres interior constrained by recent shape data and surface properties. Results. As a general result, the amplitudes of oscillations in the rotation appear to be small, and their determination from spaceborne techniques will be challenging. For example, the amplitudes of the semi-annual and annual nutations are around ~364 and ~140 milli-arcseconds, and they show little variation within the parametric space of interior models envisioned for Ceres. This, combined with the very long-period of the precession motion, requires very precise measurements. We also estimate the timescale for Ceres orientation to relax to a generalized Cassini State, and we find that the tidal dissipation within that object was probably too small to drive any significant damping of its obliquity since formation. However, combining the shape and gravity observations by Dawn offers the prospect to identify departures of non-hydrostaticity at the global and regional scale, which will be instrumental in constraining Ceres past and current thermal state. We also discuss the existence of a possible Chandler mode in the rotational motion of Ceres, whose potential excitation by endogenic and/or exogenic processes may help detect the presence of liquid reservoirs within the asteroid.

Rotational motion of Phobos

Context. Phobos is in synchronous spin-orbit resonance around Mars, like our Moon around the Earth. As a consequence, the rotational period of Phobos is equal in average to its orbital period. The variations of its rotational motion are described by oscillations, called physical librations, which yield information of its interior structure. The largest libration of Phobos rotational motion was first detected in 1981 and the determination of this libration has recently been improved using Mars EXpress observations. Aims. The objective of this paper is to present the spectrum of Phobos’ librations by using recent orbital ephemerides and geophysical knowledge of this Martian satellite. The analysis of the librational spectrum highlights the relationship between dynamical and geophysical properties of the body, but is also useful for cartographic and geodetic purposes for future space missions dedicated to Phobos. Methods. We developed a numerical model of Phobos’ rotation that includes the point-mass Mars acting on the dynamical shape of Phobos, expanded to the third degree, and the e ect of Mars’ oblateness. The forced librations spectrum is extracted through a frequency analysis. Results. We find that the libration in longitude presents a quadratic term that coincides with the secular acceleration of Phobos falling onto Mars. The primary libration in longitude has a period equal to the anomalistic mean motion, whereas the primary libration in latitude has a period equal to the draconic mean motion (node to node). Both librations have amplitudes of about one degree leading to a surface displacement of about 200 m. These two components dominate the libration spectrum by a factor one hundred. Phobos’ third degree gravity harmonics and Mars’ oblateness a ect the librations amplitude at 10 4 degree. This is small but detectable from long-term tracking of a lander. The determination of the librational spectrum would bring strong constraints on the principal torques acting on the Martian moon, as well as on the possible presence of lateral variations in density predicted by certain geophysical models of the Stickney crater formation. We also investigate the obliquity variations of Phobos and find that their amplitudes are larger than the mean value of the obliquity. Conclusions. Phobos exhibits a rich and varied set of librational oscillations. The main librations and the librations close to the proper frequencies are the most sensitive to the interior structure. On the other hand, the superimposed e ect of large amplitude oscillations is likely to make the determination of the mean obliquity challenging.

COMPOSITIONS AND ORIGINS OF OUTER PLANET SYSTEMS: INSIGHTS FROM THE ROCHE CRITICAL DENSITY

We consider the Roche critical density (ρRoche), the minimum density of an orbiting object that, at a given distance from its planet, is able to hold itself together by self-gravity. It is directly related to the more familiar Roche limit, the distance from a planet at which a strengthless orbiting object of given density is pulled apart by tides. The presence of a substantial ring requires that transient clumps have an internal density less than ρRoche. Conversely, in the presence of abundant material for accretion, an orbiting object with density greater than ρRoche will grow. Comparing the ρRoche values at which the Saturn and Uranus systems transition rapidly from disruption-dominated (rings) to accretion-dominated (moons), we infer that the material composing Uranus rings is likely more rocky, as well as less porous, than that composing Saturns rings. From the high values of ρRoche at the innermost ring moons of Jupiter and Neptune, we infer that those moons may be composed of denser material than expected, or more likely that they are interlopers that formed farther from their planets and have since migrated inward, now being held together by internal material strength. Finally, the Portia group of eight closely packed Uranian moons has an overall surface density similar to that of Saturns A ring. Thus, it can be seen as an accretion-dominated ring system, of similar character to the standard ring systems except that its material has a characteristic density greater than the local ρRoche.

Mars-Moons Exploration, Reconnaissance, and Landed Investigation (MERLIN)

MERLIN, the Mars-Moons Exploration, Reconnaissance and Landed Investigation, is a concept for the first mission to land on the Martian moon Phobos and the first U.S. mission to conduct an in situ investigation of a D-type body typical of the outer solar system. Understanding Phobos and Deimos provides key information for understanding the history and evolution of our solar system and drives MERLINs combined orbital and landed mission design. MERLIN would perform 9 months of orbital reconnaissance of Phobos and Deimos, characterizing their geology and a landing site on Phobos. Once landed, MERLIN would perform 90 days of complementary measurements of chemical and mineralogic composition. Phobos size and mass provide a low-risk landing environment for a small-body lander. Controlled descent is so slow that the landing can be rehearsed and even repeated, yet gravity is high enough that surface operations do not require anchoring. Imaging of Phobos from past missions demonstrates the existence of regions suitable for landing and provides knowledge for planning the orbital and landed investigations. MERLINs dual orbital and landed data would deliver seminal science directly traceable to NASAs Strategic Goals and Objectives, NASAs Science Plan, and Decadal Survey goals, while simultaneously closing strategic knowledge gaps (SKGs) to prepare for future human exploration. MERLINs landed compositional measurements would unravel the origin of Mars moons, addressing the goal to understand how solar system objects formed and evolved. MERLIN would determine the inventory of prebiotic materials on Phobos, addressing the goal focused on the distribution of volatiles and organics across the solar system, and the origin and requirements of life. MERLINs high-resolution images during low flyovers would investigate processes that affect the local regolith and provide geologic context for landed measurements. MERLIN would characterize the geology, surface regolith, and internal structure of Mars moons, addressing the goal to understand processes that shape planetary bodies, and how those processes operate and interact. MERLINs combined remote and landed investigations would deliver pioneering data about Phobos, characterizing an object on the flexible path for human exploration.

Observing Near-Earth Objects with the James Webb Space Telescope

The James Webb Space Telescope (JWST) has the potential to enhance our understanding of near-Earth objects (NEOs). We present results of investigations into the observability of NEOs given the nominal observing requirements of JWST on elongation (85°–135°) and non-sidereal rates (<30 mas s−1). We find that approximately 75% of NEOs can be observed in a given year. However, observers will need to wait for appropriate observing windows. We find that JWST can easily execute photometric observations of meter-sized NEOs that will enhance our understanding of the small NEO population.

Strontium iodide gamma ray spectrometers for planetary science(Conference Presentation)

Gamma rays produced passively by cosmic ray interactions and by the decay of radioelements convey information about the elemental makeup of planetary surfaces and atmospheres. Orbital missions mapped the composition of the Moon, Mars, Mercury, Vesta, and now Ceres. Active neutron interrogation will enable and/or enhance in situ measurements (rovers, landers, and sondes). Elemental measurements support planetary science objectives as well as resource utilization and planetary defense initiatives. Strontium iodide, an ultra-bright scintillator with low nonproportionality, offers significantly better energy resolution than most previously flown scintillators, enabling improved accuracy for identification and quantification of key elements. Lanthanum bromide achieves similar resolution; however, radiolanthanum emissions obscure planetary gamma rays from radioelements K, Th, and U. The response of silicon-based optical sensors optimally overlaps the emission spectrum of strontium iodide, enabling the development of compact, low-power sensors required for space applications, including burgeoning microsatellite programs. While crystals of the size needed for planetary measurements (>100 cm3) are on the way, pulse-shape corrections to account for variations in absorption/re-emission of light are needed to achieve maximum resolution. Additional challenges for implementation of large-volume detectors include optimization of light collection using silicon-based sensors and assessment of radiation damage effects and energetic-particle induced backgrounds. Using laboratory experiments, archived planetary data, and modeling, we evaluate the performance of strontium iodide for future missions to small bodies (asteroids and comets) and surfaces of the Moon and Venus. We report progress on instrument design and preliminary assessment of radiation damage effects in comparison to technology with flight heritage.

Origin and Evolution of Dwarf Planet Ceres from Dawn

RAYMOND, Carol A.1, RUSSELL, C.T.2, AMMANNITO, E.3, BUCZKOWSKI, D.L.4, CASTILLO-ROGEZ, J.C.1, COMBE, J-P.5, DE SANCTIS, M.C.6, JAUMANN, R.7, MCCORD, T.B.5, MCSWEEN, H.Y.8, NATHUES, A.9, PRETTYMAN, T.H.10 and SCHENK, P.M.11, (1)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, (2)Earth, Planetary and Space Sciences/IGPP, University of California, Los Angeles, 603 Charles Young Drive, 3845, Los Angeles, CA 90095, (3)EPSS-IGPP, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90025, (4)Space Departrment, Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, (5)Bear Fight Institute, P.O. Box 667, 22 Fiddlers Rd, Winthrop, WA 98862, (6)INAF, Instituto di Astrofisica Spaziale e Fisica Cosmica, Rome, Italy, (7)German Aerospace Center (DLR) Berlin, Institute of Planetary Research, Rutherfordstrasse 2, Berlin, D-12489, Germany, (8)Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, University of Tennessee, Knoxville, TN 37996-1410, (9)Max-Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, (10)Planetary Science Institute, Los Ranchos de Albuquerque, NM 87107, (11)Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, carol.a.raymond@jpl.nasa.gov