Ryan S. Park

Dawn at Vesta: Testing the Protoplanetary Paradigm

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta’s south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta’s crust by melting of a chondritic parent body. Vesta’s mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawns results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.

Gravity Field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) Mission

The Holy GRAIL? The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moons gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668, published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moons upper crust while fracturing it extensively. Wieczorek et al. (p. 671, published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675, published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moons history. The Moons gravity field reveals that impacts have homogenized the density of the crust and fractured it extensively. Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

Dawn Arrives at Ceres: Exploration of a Small Volatile-Rich World

On 6 March 2015, Dawn arrived at Ceres to find a dark, desiccated surface punctuated by small, bright areas. Parts of Ceres’ surface are heavily cratered, but the largest expected craters are absent. Ceres appears gravitationally relaxed at only the longest wavelengths, implying a mechanically strong lithosphere with a weaker deep interior. Ceres’ dry exterior displays hydroxylated silicates, including ammoniated clays of endogenous origin. The possibility of abundant volatiles at depth is supported by geomorphologic features such as flat crater floors with pits, lobate flows of materials, and a singular mountain that appears to be an extrusive cryovolcanic dome. On one occasion, Ceres temporarily interacted with the solar wind, producing a bow shock accelerating electrons to energies of tens of kilovolts.

Lunar interior properties from the GRAIL mission

The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 ± 0.40 kg/m3, are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J2 and C22, as well as the Love number k2 describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k2 determinations, with uncertainties of ~1%, differ by 1%; the average value is 0.02416 ± 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C − A)/B and (B − A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 ± 4. The moment of inertia computation combines the GRAIL-determined J2 and C22 with LLR-derived (C − A)/B and (B − A)/C. The normalized mean moment of inertia of the solid Moon is Is/MR2 = 0.392728 ± 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200–380 km, a solid inner core with radius of 0–280 km and mass fraction of 0–1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.

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 2,460–2,900 kilograms per cubic metre (that is, composed of CI and CM chondrites), which yields an outer-shell thickness of 70–190 kilometres. The density of this outer shell is 1,680–1,950 kilograms 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 1,000 kilometres across) ice-rich rocky bodies.

Cratering on Ceres: Implications for its crust and evolution

INTRODUCTION Thermochemical models have predicted that the dwarf planet Ceres has, to some extent, formed a mantle. Moreover, due to viscous relaxation, these models indicate that Ceres should have an icy crust with few or no impact craters. However, the Dawn spacecraft has shown that Ceres has elevation excursions of ~15 km, cliffs, graben, steep-sided mountains, and a heavily cratered surface. RATIONALE We used Dawn’s Framing Camera to study the morphology, size frequency, and spatial distribution of the craters on Ceres. These data allow us to infer the structure and evolution of Ceres’ outer shell. RESULTS A large variety of crater morphologies are present on Ceres, including bowl-shaped craters, polygonal craters, floor-fractured craters, terraces, central peaks, smooth floors, flowlike features, bright spots, secondary craters, and crater chains. The morphology of some impact craters is consistent with water ice in the subsurface. Although this might have favored relaxation, there are also large unrelaxed craters. The transition from bowl-shaped simple craters to modified complex craters occurs at diameters of about 7.5 to 12 km. Craters larger than 300 km are absent, but low-pass filtering of the digital elevation model suggests the existence of two quasi-circular depressions with diameters of ~570 km (125.56°E and 19.60°N) and ~830 km (24.76°W and 0.5°N). Craters are heterogeneously distributed across Ceres’ surface, with more craters in the northern versus the southern hemisphere. The lowest crater densities are associated with large, well-preserved southern hemisphere impact craters such as Urvara and Yalode. Because the low crater density (LCD) terrain extends across a large latitude range in some cases (e.g., Urvara and Yalode: ~18°N and 75°S; Kerwan: ~30°N and 46°S), its spatial distribution is inconsistent with simple relaxation driven by warmer equatorial temperatures. We instead propose that impact-driven resurfacing is the more likely LCD formation process, although we cannot completely rule out an internal (endogenic) origin. We applied two different methodologies to derive absolute model ages from observed crater size-frequency distributions. The lunar-derived model adapts the lunar production and chronology functions to impact conditions on Ceres, taking into account impact velocities, projectile densities, current collision probabilities, and surface gravity. The asteroid-derived model derives a production function by scaling the directly observed object size-frequency distribution from the main asteroid belt (extended to sizes <5 km by a collisional model) to the resulting size-frequency distribution of cerean craters, using similar cerean target parameters as the lunar-derived model. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages of 550 million and 720 million years, depending on which chronology model is applied. CONCLUSION Crater morphology and the simple-to-complex crater transition indicate that Ceres’ outer shell is likely neither pure ice nor pure rock but an ice-rock mixture that allows for limited relaxation. The heterogeneous crater distribution across the surface indicates crustal heterogeneities and a complex geologic evolution of Ceres. There is evidence for at least some geologic activity occurring in Ceres’ recent history. Spatial density of craters larger than 20 km on Ceres. Crater rims are shown as black solid circles. Blue indicates areas with LCDs; yellow and red represent more highly cratered areas. The smallest dashed ellipse denotes the idealized former rim of an extremely degraded impact crater at 48.9°E and 44.9°S, which is barely recognizable in imagery but apparent from the global digital elevation model. Also shown as dashed circles are the outlines of two large putative basins. Unambiguously recognized basins >300 km in diameter are missing, and there are several areas with LCDs associated with large impact craters (e.g., Yalode, Urvara, Kerwan, Ezinu, Vinotonus, Dantu, and two unnamed craters northeast and southeast of Oxo). Areas A and B are topographic rises with central depressions that also show LCDs. Thermochemical models have predicted that Ceres, is to some extent, differentiated and should have an icy crust with few or no impact craters. We present observations by the Dawn spacecraft that reveal a heavily cratered surface, a heterogeneous crater distribution, and an apparent absence of large craters. The morphology of some impact craters is consistent with ice in the subsurface, which might have favored relaxation, yet large unrelaxed craters are also present. Numerous craters exhibit polygonal shapes, terraces, flowlike features, slumping, smooth deposits, and bright spots. Crater morphology and simple-to-complex crater transition diameters indicate that the crust of Ceres is neither purely icy nor rocky. By dating a smooth region associated with the Kerwan crater, we determined absolute model ages (AMAs) of 550 million and 720 million years, depending on the applied chronology model.

High‐resolution lunar gravity fields from the GRAIL Primary and Extended Missions

The resolution and accuracy of the lunar spherical harmonic gravity field have been dramatically improved as a result of the Gravity Recovery and Interior Laboratory (GRAIL) mission. From the Primary Mission, previous harmonic gravity fields resulted in an average n = 420 surface resolution and a Bouguer spectrum to n = 330. The GRAIL Extended Mission improves the resolution due to a lower average 23 km altitude orbit. As a result, new harmonic degree 900 gravity fields (GL0900C and GL0900D) show nearly a factor of 2 improvement with an average surface resolution n = 870 and the Bouguer spectrum extended to n = 550. Since the minimum spacecraft altitude varies spatially between 3 km and 23 km, the surface resolution is variable from near n = 680 for the central farside to near n = 900 for the polar regions. These gravity fields with 0.8 million parameters are by far the highest-degree fields of any planet ever estimated with a fully dynamic least squares technique using spacecraft tracking data.

SURFACE ALBEDO AND SPECTRAL VARIABILITY OF CERES

Previous observations suggested that Ceres has active, but possibly sporadic, water outgassing as well as possibly varying spectral characteristics over a timescale of months. We used all available data of Ceres collected in the past three decades from the ground and the Hubble Space Telescope, as well as the newly acquired images by the Dawn Framing Camera, to search for spectral and albedo variability on Ceres, on both a global scale and in local regions, particularly the bright spots inside the Occator crater, over timescales of a few months to decades. Our analysis has placed an upper limit on the possible temporal albedo variation on Ceres. Sporadic water vapor venting, or any possibly ongoing activity on Ceres, is not significant enough to change the albedo or the area of the bright features in the Occator crater by >15%, or the global albedo by >3% over the various timescales that we searched. Recently reported spectral slope variations can be explained by changing Sun–Ceres–Earth geometry. The active area on Ceres is less than 1 km2, too small to cause global albedo and spectral variations detectable in our data. Impact ejecta due to impacting projectiles of tens of meters in size like those known to cause observable changes to the surface albedo on Asteroid Scheila cannot cause detectable albedo change on Ceres due to its relatively large size and strong gravity. The water vapor activity on Ceres is independent of Ceres’ heliocentric distance, ruling out the possibility of the comet-like sublimation process as a possible mechanism driving the activity.

Jupiter gravity field estimated from the first two Juno orbits

The combination of the Doppler data from the first two Juno science orbits provides an improved estimate of the gravity field of Jupiter, crucial for interior modeling of giant planets. The low-degree spherical harmonic coefficients, especially J4 and J6, are determined with accuracies better than previously published by a factor of 5 or more. In addition, the independent estimates of the Jovian gravity field, obtained by the orbits separately, agree within uncertainties, pointing to a good stability of the solution. The degree 2 sectoral and tesseral coefficients, C2,1, S2,1, C2,2, and S2,2, were determined to be statistically zero as expected for a fluid planet in equilibrium.

The NANOGrav 11 Year Data Set: Pulsar-timing Constraints on the Stochastic Gravitational-wave Background

We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released 11 year data set from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no evidence for a GWB, we place constraints on a population of inspiraling supermassive black hole (SMBH) binaries, a network of decaying cosmic strings, and a primordial GWB. For the first time, we find that the GWB constraints are sensitive to the solar system ephemeris (SSE) model used and that SSE errors can mimic a GWB signal. We developed an approach that bridges systematic SSE differences, producing the first pulsar-timing array (PTA) constraints that are robust against SSE errors. We thus place a 95% upper limit on the GW-strain amplitude of A_(GWB) < 1.45 × 10^(−15) at a frequency of f = 1 yr^(−1) for a fiducial f^(−2/3) power-law spectrum and with interpulsar correlations modeled. This is a factor of ~2 improvement over the NANOGrav nine-year limit calculated using the same procedure. Previous PTA upper limits on the GWB (as well as their astrophysical and cosmological interpretations) will need revision in light of SSE systematic errors. We use our constraints to characterize the combined influence on the GWB of the stellar mass density in galactic cores, the eccentricity of SMBH binaries, and SMBH–galactic-bulge scaling relationships. We constrain the cosmic-string tension using recent simulations, yielding an SSE-marginalized 95% upper limit of Gμ < 5.3 × 10^(−11)—a factor of ~2 better than the published NANOGrav nine-year constraints. Our SSE-marginalized 95% upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation universe) is Ω_(GWB)(f) h^2 < 3.4 × 10^(−10).