William Jon Merline

Flood Volcanism in the Northern High Latitudes of Mercury Revealed by MESSENGER

MESSENGER observations of Mercury’s high northern latitudes reveal a contiguous area of volcanic smooth plains covering more than ~6% of the surface that were emplaced in a flood lava mode, consistent with average crustal compositions broadly similar to terrestrial komatiites. MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 × 13 × 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of ‘ejecta blocks’. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a ‘pond’. Properties of the ‘ponds’ are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

Mercury Cratering Record Viewed from MESSENGER's First Flyby

Morphologies and size-frequency distributions of impact craters on Mercury imaged during MESSENGERs first flyby elucidate the planets geological history. Plains interior to the Caloris basin displaying color and albedo contrasts have comparable crater densities and therefore similar ages. Smooth plains exterior to Caloris exhibit a crater density ∼40% less than on interior plains and are thus volcanic and not Caloris impact ejecta. The size distribution of smooth-plains craters matches that of lunar craters postdating the Late Heavy Bombardment, implying that the plains formed no earlier than 3.8 billion years ago (Ga). At diameters less than or equal to 8 to 10 kilometers, secondary impact craters on Mercury are more abundant than primaries; this transition diameter is much larger than that on the Moon or Mars. A low density of craters on the peak-ring basin Raditladi implies that it may be younger than 1 Ga.

Discovery of two new satellites of Pluto

Plutos first known satellite, Charon, was discovered in 1978. It has a diameter (∼1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects ≲150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1s diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ∼38 days (P1) and ∼25 days (P2).

Discovery of a moon orbiting the asteroid 45 Eugenia

Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13 km, and an orbital period of about 4.7 days with a separation of 1,190 km from Eugenia. Using a previously determined diameter for Eugenia, we estimate that its bulk density is about 1.2 g cm-3, which is similar to that of the C-type asteroid Mathilde. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

Secondary craters on Europa and implications for cratered surfaces

For several decades, most planetary researchers have regarded the impact crater populations on solid-surfaced planets and smaller bodies as predominantly reflecting the direct (‘primary’) impacts of asteroids and comets. Estimates of the relative and absolute ages of geological units on these objects have been based on this assumption. Here we present an analysis of the comparatively sparse crater population on Jupiters icy moon Europa and suggest that this assumption is incorrect for small craters. We find that ‘secondaries’ (craters formed by material ejected from large primary impact craters) comprise about 95 per cent of the small craters (diameters less than 1 km) on Europa. We therefore conclude that large primary impacts into a solid surface (for example, ice or rock) produce far more secondaries than previously believed, implying that the small crater populations on the Moon, Mars and other large bodies must be dominated by secondaries. Moreover, our results indicate that there have been few small comets (less than 100 m diameter) passing through the jovian system in recent times, consistent with dynamical simulations.

Near-Infrared Mapping and Physical Properties of the Dwarf-Planet Ceres

Aims. We study the physical characteristics (shape, dimensions, spin axis direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on high-angular resolution near-infrared observations. Methods. We analyze adaptive optics J/H/K imaging observations of Ceres performed at Keck II Observatory in September 2002 with an equivalent spatial resolution of∼50 km. The spectral behavior of the main geological features present on Ceres is compared with laboratory samples. Results. Ceres’ shape can be described by an oblate spheroid ( a = b = 479.7± 2.3 km, c = 444.4± 2.1 km) with EQJ2000.0 spin vector coordinatesα0 = 288 ◦ ± 5 ◦ andδ0 = +66 ◦ ± 5 ◦ . Ceres sidereal period is measured to be 9.074 10 +0.000 10 −0.000 14 h. We image surface features with diameters in the 50-180 km range and an albedo contrast of∼6% with respect to the average Ceres albedo. The spectral behavior of the brightest regions on Ceres is consistent wit h phyllosilicates and carbonate compounds. Darker isolated regions could be related to the presence of frost.

Evidence for young volcanism on Mercury from the third MESSENGER flyby.

MESSENGERs Third Set of Messages MESSENGER, the spacecraft en route to insertion into orbit about Mercury in March 2011, completed its third flyby of the planet on 29 September 2009. Prockter et al. (p. 668, published online 15 July) present imaging data acquired during this flyby, showing that volcanism on Mercury has extended to much more recent times than previously assumed. The temporal extent of volcanic activity and, in particular, the timing of most recent activity had been missing ingredients in the understanding of Mercurys global thermal evolution. Slavin et al. (p. 665, published online 15 July) report on magnetic field measurements made during the 29 September flyby, when Mercurys magnetosphere underwent extremely strong coupling with the solar wind. The planets tail magnetic field increased and then decreased by factors of 2 to 3.5 during periods lasting 2 to 3 minutes. These observations suggest that magnetic open flux loads the magnetosphere, which is subsequently unloaded by substorms—magnetic disturbances during which energy is rapidly released in the magnetotail. At Earth, changes in tail magnetic field intensity during the loading/unloading cycle are much smaller and occur on much longer time scales. Vervack et al. (p. 672, published online 15 July) used the Mercury Atmospheric and Surface Composition Spectrometer onboard MESSENGER to make measurements of Mercurys neutral and ion exospheres. Differences in the altitude profiles of magnesium, calcium, and sodium over the north and south poles of Mercury indicate that multiple processes are at play to create and maintain the exosphere. Volcanism and associated deformation on Mercury may have lasted well into the last half of the history of the solar system. During its first two flybys of Mercury, the MESSENGER spacecraft acquired images confirming that pervasive volcanism occurred early in the planet’s history. MESSENGER’s third Mercury flyby revealed a 290-kilometer-diameter peak-ring impact basin, among the youngest basins yet seen, having an inner floor filled with spectrally distinct smooth plains. These plains are sparsely cratered, postdate the formation of the basin, apparently formed from material that once flowed across the surface, and are therefore interpreted to be volcanic in origin. An irregular depression surrounded by a halo of bright deposits northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on Mercury. Volcanism on the planet thus spanned a considerable duration, perhaps extending well into the second half of solar system history.

A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt

The two newly discovered satellites of Pluto (P1 and P2) have masses that are small compared to both Pluto and Charon—that is, between 5 × 10-4 and 1 × 10-5 of Plutos mass, and between 5 × 10-3 and 1 × 10-4 of Charons mass. This discovery, combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2s proximity to Pluto and Charon, the fact that P1 and P2 are on near-circular orbits in the same plane as Plutos large satellite Charon, along with their apparent locations in or near high-order mean-motion resonances, all probably result from their being constructed from collisional ejecta that originated from the Pluto–Charon formation event. We also argue that dust–ice rings of variable optical depths form sporadically in the Pluto system, and that rich satellite systems may be found—perhaps frequently—around other large Kuiper belt objects.

Learning to Detect Small Impact Craters

Machine learning techniques have shown considerable promise for visual inspection tasks such as locating human faces in cluttered scenes. In this paper, we examine the utility of such techniques for the scientifically-important problem of detecting and cataloging impact craters in planetary images gathered by spacecraft. Various supervised learning algorithms, including ensemble methods (bagging and AdaBoost with feed-forward neural networks as base learners), support vector machines (SVM), and continuously-scalable template models (CSTM), are employed to derive crater detectors from ground-truthed images. The resulting detectors are evaluated on a challenging set of Viking Orbiter images of Mars containing roughly one thousand craters. The SVM approach with normalized image patches provides detection and localization performance closest to that of human labelers and is shown to be substantially superior to boundary-based approaches such as the Hough transform.