Sami W. Asmar

The Crust of the Moon as Seen by GRAIL

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 shows that the lunar crust is less dense and more porous than was thought. High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moons highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

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.

The vertical profile of winds on Titan

One of Titans most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titans atmospheric pressure and temperature and evidence for the presence of strong zonal winds. It was realized that the motion of an atmospheric probe could be used to study the winds, which led to the inclusion of the Doppler Wind Experiment on the Huygens probe. Here we report a high resolution vertical profile of Titans winds, with an estimated accuracy of better than 1 m s-1. The zonal winds were prograde during most of the atmospheric descent, providing in situ confirmation of superrotation on Titan. A layer with surprisingly slow wind, where the velocity decreased to near zero, was detected at altitudes between 60 and 100 km. Generally weak winds (∼1 m s-1) were seen in the lowest 5 km of descent.

Gravity Field, Shape, and Moment of Inertia of Titan

Titan Through to the Core Gravity measurements acquired from orbiting spacecraft can provide useful information about the interior of planets and their moons. Iess et al. (p. 1367; see the Perspective by Sohl) used gravity data from four flybys of the Cassini spacecraft past Saturns moon, Titan, to model the moons gravity field and probe its deep interior structure. Their analysis implies that Titan is a partially differentiated body with a core consisting of a mix of ice and rock or hydrated silicates. Analysis of gravity data reveals that Saturn’s moon Titan has a partially differentiated internal structure. Precise radio tracking of the spacecraft Cassini has provided a determination of Titan’s mass and gravity harmonics to degree 3. The quadrupole field is consistent with a hydrostatically relaxed body shaped by tidal and rotational effects. The inferred moment of inertia factor is about 0.34, implying incomplete differentiation, either in the sense of imperfect separation of rock from ice or a core in which a large amount of water remains chemically bound in silicates. The equilibrium figure is a triaxial ellipsoid whose semi-axes a, b, and c differ by 410 meters (a – c) and 103 meters (b – c). The nonhydrostatic geoid height variations (up to 19 meters) are small compared to the observed topographic anomalies of hundreds of meters, suggesting a high degree of compensation appropriate to a body that has warm ice at depth.

The Gravity Field and Interior Structure of Enceladus

Inside Enceladus Saturns moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturns largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep. The saturnian moon is differentiated and likely hosts a regional subsurface sea at its southern pole. The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

The Tides of Titan

Getting to Know Titan Gravity-field measurements provide information on the interior structure of planets and their moons. Iess et al. (p. 457; published online 28 June) analyzed gravity data from six flybys of Saturns moon, Titan, by the Cassini spacecraft between 2006 and 2011. The data suggest that Titans interior is flexible on tidal time scales with the magnitude of the observed tidal deformations being consistent with the existence of a global subsurface water ocean. Gravity measurements by the Cassini spacecraft suggest that Saturn’s moon Titan hosts a subsurface ocean. We have detected in Cassini spacecraft data the signature of the periodic tidal stresses within Titan, driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of Cassini during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k2 = 0.589 ± 0.150 and k2 = 0.637 ± 0.224 (2σ). Such a large response to the tidal field requires that Titan’s interior be deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.

Radio science observations with Mars Global Surveyor: Orbit insertion through one Mars year in mapping orbit

Mars Global Surveyor (MGS) radio science comprises studies of the atmosphere and gravity of the planet. Perturbations of the 3.6-cm λ radio path by the atmosphere during periods of atmospheric occultation provide the vertical temperature-pressure structure T[p(r)] to accuracies at the surface of ΔT ≈ 0.4 K and Δp ≈ 2 Pa, and ∼10 K and ∼0.6 Pa at altitudes of 40–50 km; the error in radius is Δr ≈ 1 m at all levels. Accurate knowledge of the radius permits fixing of the T-p structure to the geopotential and use of the gradient wind equation to calculate components of the wind. Systematic sampling of the atmosphere in combination with the accuracy of the MGS radio system supports studies of the large-scale dynamics of the atmosphere, including seasonal variations of the atmospheric fields and embedded waves such as Kelvin and Rossby waves. Terminator region ionospheric electron density profiles are obtained successfully much of the time but on occasion are undetectable with the MGS system. Two-way radio tracking of MGS with uncertainties in the line-of-sight velocity of several to tens of μm s−1 and less supports solution for spherical harmonic models of the gravity field of order and degree in the range of 75×75, although the degree and order of meaningful terms is limited by the ∼400 km spacecraft altitude to ∼62×62, corresponding to a resolution of a few degrees of arc on the surface. This resolution of gravity is sufficient to support geophysical studies of the planets interior structure and history. Additional radio science investigations include the search for gravitational radiation and observation of very low grazing angle forward scattering by the surface of Mars.

Ancient Igneous Intrusions and Early Expansion of the Moon Revealed by GRAIL Gravity Gradiometry

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 map shows that the crust is cut by extensive magmatic dikes, perhaps implying a period of early expansion. The earliest history of the Moon is poorly preserved in the surface geologic record due to the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory (GRAIL) mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive-density anomalies that are interpreted to be ancient vertical tabular intrusions or dikes formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moons radius by 0.6 to 4.9 kilometers early in lunar history, consistent with predictions of thermal models.

The structure of Venus’ middle atmosphere and ionosphere

The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50–90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day–night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus’ ionosphere.