The MESSENGER Venus Flybys: Opportunities for Synergy with Venus Express

The trajectory of the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, launched by NASA on 3 August 2004 and destined to be the first probe to orbit Mercury, includes two flybys of Venus during the period that the ESA Venus Express mission is operational in Venus orbit. MESSENGER's first Venus flyby occurred on 24 October 2006, at a closest approach distance of 3140 km, but no scientific observations were made because Venus was at superior conjunction and no direct communication with the MESSENGER spacecraft (or with Venus Express) was possible for an extended period. All MESSENGER instruments, however, will be trained on Venus during the spacecraft's second flyby on 6 June 2007, when closest approach will be at 300 km altitude over 12°S, 107°E, in the uplands of Ovda Regio. The Mercury Dual Imaging System will image the night side in near-infrared bands, and color and higher-resolution monochrome mosaics will be made of both the approaching and departing hemispheres. The Ultraviolet and Visible Spectrometer on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument will make profiles of atmospheric species on the day and night sides as well as observations of the exospheric tail on departure. The Visible and Infrared Spectrograph on MASCS will observe the planet near closest approach to sense cloud chemical properties and near-infrared returns from the lower atmosphere and surface. The laser altimeter will serve as a passive 1064-nm radiometer and will measure the range to one or more cloud decks for several minutes near closest approach. The combined observations of Venus Express and MESSENGER will permit simultaneous and complementary observations of particular value for characterization of the particle and field environment at Venus. MESSENGER's Energetic Particle and Plasma Spectrometer (EPPS) will observe charged particle acceleration at the Venus bow shock and elsewhere. The Magnetometer will provide measurements of the interplanetary magnetic field (IMF), bow shock signatures, and pickup ion waves as a reference for EPPS and Venus Express observations. The encounter will enable two-point measurements of IMF penetration into the Venus ionosphere, primary plasma boundaries, and the near-tail region. During the MESSENGER flyby the instruments on Venus Express will be operated to maximize the synergy between the two spacecraft for this unique opportunit

MASCS surface units from cluster analysis

First work on the retrieval of Mercury surface units from cluster analysis of MASCS/MESSENGER instrument data

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

During MESSENGER’s 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 Mercury’s surface

Physical characteristics of Hayabusa target Asteroid 25143 Itokawa

In March 2001, the Hayabusa spacecraft target, Asteroid 25143 Itokawa, made its final close approach to Earth prior to the spacecraft's launch. We carried out an extensive observing campaign from January to September 2001 to better characterize this near-Earth asteroid. Global physical properties of the surface of Itokawa were characterized by analyzing its photometric properties and behavior. Results included here capitalize on analysis of broadband photometric observations taken with a number of telescopes, instruments, and observers. We employed a Hapke model to estimate the surface roughness, single particle scattering albedo, single particle scattering characteristics, phase integral, and geometric and bond albedo. We find that this asteroid has a higher geometric albedo than average main belt S-class asteroids; this is consistent with results from other observers. The broadband colors of Itokawa further support evidence that this is an atypical S-class asteroid. Broadband colors show spectral characteristics more typically found on large-diameter main-belt asteroids believed to be space-weathered, suggesting the surface of this small diameter, near-Earth asteroid could likewise be space-weathered. © 2004 Elsevier Inc. All rights reserved

Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than similar to 400nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits