Caleb I. Fassett

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.

Global Distribution of Large Lunar Craters: Implications for Resurfacing and Impactor Populations

Lunar Reconnaissance The Lunar Reconnaissance Orbiter reached lunar orbit on 23 June 2009. Global data acquired since then now tell us about the impact history of the Moon and the igneous processes that shaped it. Using the Lunar Orbiter Laser Altimeter, Head et al. (p. 1504; see the cover) provide a new catalog of large lunar craters. In the lunar highlands, large-impact craters have obliterated preexisting craters of similar size, implying that crater counts in this region cannot be used effectively to determine the age of the underlying terrain. Crater counts based on the global data set indicate that the nature of the Moons impactor population has changed over time. Greenhagen et al. (p. 1507) and Glotch et al. (p. 1510) analyzed data from the Diviner Lunar Radiometer Experiment, which measures emitted thermal radiation and reflected solar radiation at infrared wavelengths. The silicate mineralogy revealed suggests the existence of more complex igneous processes on the Moon than previously assumed. An analysis of high-resolution global topography data advances our understanding of the impact history of the Moon. By using high-resolution altimetric measurements of the Moon, we produced a catalog of all impact craters ≥20 kilometers in diameter on the lunar surface and analyzed their distribution and population characteristics. The most-densely cratered portion of the highlands reached a state of saturation equilibrium. Large impact events, such as Orientale Basin, locally modified the prebasin crater population to ~2 basin radii from the basin center. Basins such as Imbrium, Orientale, and Nectaris, which are important stratigraphic markers in lunar history, are temporally distinguishable on the basis of crater statistics. The characteristics of pre- and postmare crater populations support the hypothesis that there were two populations of impactors in early solar system history and that the transition occurred near the time of the Orientale Basin event.

Global resurfacing of Mercury 4.0–4.1 billion years ago by heavy bombardment and volcanism

The most heavily cratered terrains on Mercury have been estimated to be about 4 billion years (Gyr) old, but this was based on images of only about 45 per cent of the surface; even older regions could have existed in the unobserved portion. These terrains have a lower density of craters less than 100 km in diameter than does the Moon, an observation attributed to preferential resurfacing on Mercury. Here we report global crater statistics of Mercury’s most heavily cratered terrains on the entire surface. Applying a recent model for early lunar crater chronology and an updated dynamical extrapolation to Mercury, we find that the oldest surfaces were emplaced just after the start of the Late Heavy Bombardment (LHB) about 4.0–4.1 Gyr ago. Mercury’s global record of large impact basins, which has hitherto not been dated, yields a similar surface age. This agreement implies that resurfacing was global and was due to volcanism, as previously suggested. This activity ended during the tail of the LHB, within about 300–400 million years after the emplacement of the oldest terrains on Mercury. These findings suggest that persistent volcanism could have been aided by the surge of basin-scale impacts during this bombardment.

Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity

Rimless shallow depressions on Mercury may still be forming by outgassing, volcanism, sublimation, or space weathering. High-resolution images of Mercury’s surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury’s interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system’s innermost planet.

Unique chronostratigraphic marker in depositional fan stratigraphy on Mars: Evidence for ca. 1.25 Ma gully activity and surficial meltwater origin

The origin of gullies on Mars is controversial (e.g., catastrophic groundwater release, debris flows, dry granular flows, or meltwater from surface ice and snow) and their ages are difficult to determine due to their small size. We describe a gully depositional fan that contains a unique chronostratigraphic marker (secondary crater clusters) between episodes of gully activity during fan development. This marker can be traced to its source, a 7-km-diameter rayed crater that we have dated as ca. 1.25 Ma. This age links gully activity to the emplacement of dust-ice mantling deposits interpreted to represent recent ice ages on Mars. This association, together with multiple episodes of depositional fan formation, favors an origin for these gullies from top-down melting of snow and ice during multiple favorable spin-axis and orbital variations. This melting mechanism is consistent with the occurrence of gullies in unique steep-sloped, poleward-facing insolation microenvironments that favor the melting of small amounts of surficial snow and ice.

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.

Constraints on the volatile distribution within Shackleton crater at the lunar south pole

Shackleton crater is nearly coincident with the Moon’s south pole. Its interior receives almost no direct sunlight and is a perennial cold trap, making Shackleton a promising candidate location in which to seek sequestered volatiles. However, previous orbital and Earth-based radar mapping and orbital optical imaging have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than the surrounding terrain and the interiors of nearby craters, but not as bright as the interior walls. The combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material. The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility.

Crater degradation on the lunar maria: Topographic diffusion and the rate of erosion on the Moon

Landscape evolution on the Moon is dominated by impact cratering in the post-maria period. In this study, we mapped 800 m to 5 km diameter craters on >30% of the lunar maria and extracted their topographic profiles from digital terrain models created using the Kaguya Terrain Camera. We then characterized the degradation of these craters using a topographic diffusion model. Because craters have a well-understood initial morphometry, these data provide insight into erosion on the Moon and the topographic diffusivity of the lunar surface as a function of time. The average diffusivity we calculate over the past 3 Ga is ~5.5 m2/Myr. With this diffusivity, after 3 Ga, a 1 km diameter crater is reduced to approximately ~52% of its initial depth and a 300 m diameter crater is reduced to only ~7% of its initial depth, and craters smaller than ~200–300 m are degraded beyond recognition. Our results also allow estimation of the age of individual craters on the basis of their degradation state, provide a constraint on the age of mare units, and enable modeling of how lunar terrain evolves as a function of its topography.

Thickness of proximal ejecta from the Orientale Basin from Lunar Orbiter Laser Altimeter (LOLA) data: Implications for multi‐ring basin formation

[1] Quantifying the ejecta distribution around large lunar basins is important to understanding the origin of basin rings, the volume of the transient cavity, the depth of sampling, and the nature of the basin formation processes. We have used newly obtained altimetry data of the Moon from the Lunar Orbiter Laser Altimeter (LOLA) instrument to estimate the thickness of ejecta in the region surrounding the Orientale impact basin, the youngest and best preserved large basin on the Moon. Our measurements yield ejecta thicknesses of ∼2900 m near the Cordillera Mountains, the topographic rim of Orientale, decaying to ∼ 1k m in thickness at a range of 215 km. These measurements imply a volume of ejecta in the region from the Cordillera ring to a radial range of one basin diameter of ∼2.9 × 10 6 km 3 and permit the derivation of an ejecta‐thickness decay model, which can be compared with estimates for the volume of excavation and the size of the transient cavity. These data are consistent with the Outer Rook Mountains as the approximate location of the transient cavity’s rim crest and suggest a volume of ∼4.8 × 10 6 km 3 for the total amount

Amazonian‐aged fluvial valley systems in a climatic microenvironment on Mars: Melting of ice deposits on the interior of Lyot Crater

Valley networks, regional drainage patterns suggesting liquid water stability at the surface, are confined to early in the history of Mars (the Noachian/Hesperian boundary and before), prior to a major climate transition to the hyperarid cold conditions of the Amazonian. Several later fluvial valley systems have been documented in specific Hesperian and Early Amazonian environments, and are thought to have formed due to local conditions. Here we describe fluvial valley systems within Lyot crater that have the youngest well-constrained age reported to date (Middle or Late Amazonian) for systems of this size (tens of km). These valleys are linked to melting of near-surface ice-rich units, extend up to ∼50 km in length, follow topographic gradients, and deposit fans. The interior of Lyot crater is an optimal micro-environment, since its low elevation leads to high surface pressure, and temperature conditions at its location in the northern mid-latitudes are sufficient for melting during periods of high-obliquity. This micro-environment in Lyot apparently allowed melting of surface ice and the formation of the youngest fluvial valley systems of this scale yet observed on Mars.