125,193 research outputs found
Spectral properties and geology of bright and dark material on dwarf planet Ceres
Variations and spatial distributions of bright and dark material on dwarf
planet Ceres play a key role in understanding the processes that have led to
its present surface composition. We define limits for bright and dark material
in order to distinguish them consistently, based on the reflectance of the
average surface using Dawn Framing Camera data. A systematic classification of
four types of bright material is presented based on their spectral properties,
composition, spatial distribution, and association with specific
geomorphological features. We found obvious correlations of reflectance with
spectral shape (slopes) and age; however, this is not unique throughout the
bright spots. Although impact features show generally more extreme reflectance
variations, several areas can only be understood in terms of inhomogeneous
distribution of composition as inferred from Dawn Visible and Infrared
Spectrometer data. Additional material with anomalous composition and spectral
properties are rare. The identification of the composition and origin of the
dark, particularly the darkest material, remains to be explored. The spectral
properties and the morphology of the dark sites suggest an endogenic origin,
but it is not clear whether they are more or less primitive surficial exposures
or excavated subsurface but localized material. The reflectance, spectral
properties, inferred composition, and geologic context collectively suggest
that the bright and dark material tends to gradually change toward the average
surface over time. This could be because of multiple processes, i.e., impact
gardening/space weathering, and lateral mixing, including thermal and aqueous
alteration, accompanied by changes in composition and physical properties such
as grain size, surface temperature, and porosity (compaction).Comment: Meteoritics and Planetary Science; Dawn at Ceres special issu
Science with a lunar low-frequency array: from the dark ages of the Universe to nearby exoplanets
Low-frequency radio astronomy is limited by severe ionospheric distortions
below 50 MHz and complete reflection of radio waves below 10-30 MHz. Shielding
of man-made interference from long-range radio broadcasts, strong natural radio
emission from the Earth's aurora, and the need for setting up a large
distributed antenna array make the lunar far side a supreme location for a
low-frequency radio array. A number of new scientific drivers for such an
array, such as the study of the dark ages and epoch of reionization,
exoplanets, and ultra-high energy cosmic rays, have emerged and need to be
studied in greater detail. Here we review the scientific potential and
requirements of these and other new scientific drivers and discuss the
constraints for various lunar surface arrays. In particular we describe
observability constraints imposed by the interstellar and interplanetary
medium, calculate the achievable resolution, sensitivity, and confusion limit
of a dipole array using general scaling laws, and apply them to various
scientific questions. Whichever science is deemed most important, pathfinder
arrays are needed to test the feasibility of these experiments in the not too
distant future. Lunar low-frequency arrays are thus a timely option to
consider, offering the potential for significant new insights into a wide range
of today's crucial scientific topics. This would open up one of the last
unexplored frequency domains in the electromagnetic spectrum.Comment: 36 pages, many figures, accepted for publication by New Astronomy
Review
Night Matters—Why the Interdisciplinary Field of “Night Studies” Is Needed
The night has historically been neglected in both disciplinary and interdisciplinary research. To some extent, this is not surprising, given the diurnal bias of human researchers and the difficulty of performing work at night. The night is, however, a critical element of biological, chemical, physical, and social systems on Earth. Moreover, research into social issues such as inequality, demographic changes, and the transition to a sustainable economy will be compromised if the night is not considered. Recent years, however, have seen a surge in research into the night. We argue that “night studies” is on the cusp of coming into its own as an interdisciplinary field, and that when it does, the field will consider questions that disciplinary researchers have not yet thought to ask
Delivery of Dark Material to Vesta via Carbonaceous Chondritic Impacts
NASA's Dawn spacecraft observations of asteroid (4) Vesta reveal a surface
with the highest albedo and color variation of any asteroid we have observed so
far. Terrains rich in low albedo dark material (DM) have been identified using
Dawn Framing Camera (FC) 0.75 {\mu}m filter images in several geologic
settings: associated with impact craters (in the ejecta blanket material and/or
on the crater walls and rims); as flow-like deposits or rays commonly
associated with topographic highs; and as dark spots (likely secondary impacts)
nearby impact craters. This DM could be a relic of ancient volcanic activity or
exogenic in origin. We report that the majority of the spectra of DM are
similar to carbonaceous chondrite meteorites mixed with materials indigenous to
Vesta. Using high-resolution seven color images we compared DM color properties
(albedo, band depth) with laboratory measurements of possible analog materials.
Band depth and albedo of DM are identical to those of carbonaceous chondrite
xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison
CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band
depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance
in DM (1-6 vol%) is consistent with howardite meteorites. We find no evidence
for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of
DM. Our modeling efforts using impact crater scaling laws and numerical models
of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta
during the formation of the ~400 km Veneneia basin by a low-velocity (<2
km/sec) carbonaceous impactor. This discovery is important because it
strengthens the long-held idea that primitive bodies are the source of carbon
and probably volatiles in the early Solar System.Comment: Icarus (Accepted) Pages: 58 Figures: 15 Tables:
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