Noam R. Izenberg

Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument

Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L’Eau, les Glaces et l’Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars’s history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability.

Impact craters and Venus resurfacing history

Venusian impact crater size-frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size-frequency distribution is close to the atmosphere-free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of ∼ 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end-member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr−1. However, the CRM and ERM are idealized end-member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency ω of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a 0.1 (74° diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number of regions on the planet.

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.

Surface modification of Venus as inferred from Magellan observations of plains

In Sedna Planitia, clear stratigraphic relations can be discerned among volcanic flow units. Young flows exhibit SAR specific cross section values similar to fresh terrestrial basalt flows, whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows. Total degradation of ∼1 m depth over ∼0.6 b.y. is inferred for the Sedna area from radar signatures, impact crater abundances, and ejecta superposition relations with respect to volcanic flow units. Analyses of parabolic ejecta deposits associated with the crater Stuart imply that the material is typically centimeters in thickness. A relatively small fraction (∼10%) of Venusian impact craters exhibit prominent parabolic ejecta deposits. These craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years. The inferred dispersal rate (<10−3 μm/yr) is too low to produce the degradation of flows at Sedna Planitia, and it is concluded that the dominant flow modification process is in situ weathering. In addition, elevation dependent weathering is inferred in western Ovda Regio, where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations. The inferred rate of generation of high reflection coefficient materials is no more than ∼10−2μm/yr, based on the inability of aeolian activity to cover high-reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high-reflectivity surfaces at high elevations. Surface modification rates on Venus are orders of magnitude lower than on Earth. Venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on Mars but are comparable to the estimated rate of weathering and erosion of Martian bedrock. Low surface modification rates imply that it will be possible to determine regional-scale age variations on Venus based on the degree of preservation of volcanic landforms and microwave signatures.

The MESSENGER mission to Mercury: scientific payload

Abstract The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission will send the first spacecraft to orbit the planet Mercury. A miniaturized set of seven instruments, along with the spacecraft telecommunications system, provide the means of achieving the scientific objectives that motivate the mission. The payload includes a combined wide- and narrow-angle imaging system; γ-ray, neutron, and X-ray spectrometers for remote geochemical sensing; a vector magnetometer; a laser altimeter; a combined ultraviolet-visible and visible-infrared spectrometer to detect atmospheric species and map mineralogical absorption features; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere.

Spectroscopic Observations of Mercury's Surface Reflectance During MESSENGER's First Mercury Flyby

During MESSENGERs first flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer made simultaneous mid-ultraviolet to near-infrared (wavelengths of 200 to 1300 nanometers) reflectance observations of the surface. An ultraviolet absorption (<280 nanometers) suggests that the ferrous oxide (Fe2+) content of silicates in average surface material is low (less than 2 to 3 weight percent). This result is supported by the lack of a detectable 1-micrometer Fe2+ absorption band in high-spatial-resolution spectra of mature surface materials as well as immature crater ejecta, which suggests that the ferrous iron content may be low both on the surface and at depth. Differences in absorption features and slope among the spectra are evidence for variations in composition and regolith maturation of Mercurys surface.

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.

MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planets orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planets surface. Now, after MESSENGER, only 10% of Mercurys surface remains to be imaged up close. Denevi et al. (p. 613) use this near-global data to look at the mechanisms that shaped Mercurys crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercurys surface is dotted with impact craters. Watters et al. (p. 618) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercurys contraction as its interior cooled over time. Mercurys exosphere and magnetosphere were also observed (see the Perspective by Glassmeier). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercurys exosphere, which may shed light on the processes that create and maintain the exosphere. High-resolution observations of Mercury’s exosphere reveal different spatial distributions of magnesium, calcium, and sodium. Mercury is surrounded by a tenuous exosphere that is supplied primarily by the planet’s surface materials and is known to contain sodium, potassium, and calcium. Observations by the Mercury Atmospheric and Surface Composition Spectrometer during MESSENGER’s second Mercury flyby revealed the presence of neutral magnesium in the tail (anti-sunward) region of the exosphere, as well as differing spatial distributions of magnesium, calcium, and sodium atoms in both the tail and the nightside, near-planet exosphere. Analysis of these observations, supplemented by observations during the first Mercury flyby, as well as those by other MESSENGER instruments, suggests that the distinct spatial distributions arise from a combination of differences in source, transfer, and loss processes.

Mercury’s Complex Exosphere: Results from MESSENGER’s Third 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. Mercury’s exosphere is more varied and more intertwined with its magnetospheric environment than previously thought. During MESSENGER’s third flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer detected emission from ionized calcium concentrated 1 to 2 Mercury radii tailward of the planet. This measurement provides evidence for tailward magnetospheric convection of photoions produced inside the magnetosphere. Observations of neutral sodium, calcium, and magnesium above the planet’s north and south poles reveal altitude distributions that are distinct for each species. A two-component sodium distribution and markedly different magnesium distributions above the two poles are direct indications that multiple processes control the distribution of even single species in Mercury’s exosphere.

Mercury's Exosphere: Observations During MESSENGER's First Mercury Flyby

During MESSENGERs first Mercury flyby, the Mercury Atmospheric and Surface Composition Spectrometer measured Mercurys exospheric emissions, including those from the antisunward sodium tail, calcium and sodium close to the planet, and hydrogen at high altitudes on the dayside. Spatial variations indicate that multiple source and loss processes generate and maintain the exosphere. Energetic processes connected to the solar wind and magnetospheric interaction with the planet likely played an important role in determining the distributions of exospheric species during the flyby.