174 research outputs found
The Electron Proton Helium INstrument as an Example for a Space Weather Radiation Instrument
The near-Earth energetic particle environment has been monitored since the
1970's. With the increasing importance of quantifying the radiation risk for,
e.g. for the human exploration of the Moon and Mars, it is essential to
continue and further improve these measurements. The Electron Proton Helium
INstrument (EPHIN) on-board SOHO continually provides these data sets to the
solar science and space weather communities since 1995. Here, we introduce the
numerous data products developed over the years and present space weather
related applications. Important design features that have led to EPHINs success
as well as lessons learned and possible improvements to the instrument are also
discussed with respect to the next generation of particle detectors
Lithologic Mapping of HED Terrains on Vesta using Dawn Framing Camera Color Data
The surface composition of Vesta, the most massive intact basaltic object in
the asteroid belt, is interesting because it provides us with an insight into
magmatic differentiation of planetesimals that eventually coalesced to form the
terrestrial planets. The distribution of lithologic and compositional units on
the surface of Vesta provides important constraints on its petrologic
evolution, impact history and its relationship with Vestoids and
howardite-eucrite-diogenite (HED) meteorites. Using color parameters (band tilt
and band curvature) originally developed for analyzing lunar data, we have
identified and mapped HED terrains on Vesta in Dawn Framing Camera (FC) color
data. The average color spectrum of Vesta is identical to that of howardite
regions, suggesting an extensive mixing of surface regolith due to impact
gardening over the course of solar system history. Our results confirm the
hemispherical dichotomy (east-west and north-south) in albedo/color/composition
that has been observed by earlier studies. The presence of diogenite-rich
material in the southern hemisphere suggests that it was excavated during the
formation of the Rheasilvia and Veneneia basins. Our lithologic mapping of HED
regions provides direct evidence for magmatic evolution of Vesta with diogenite
units in Rheasilvia forming the lower crust of a differentiated object.Comment: Accepted for Meteoritics and Planetary Science special issue for
Composition of Vesta/Dawn Missio
Simulation and experiment of gas diffusion in a granular bed
The diffusion of gas through porous material is important to understand the
physical processes underlying cometary activity. We study the diffusion of a
rarefied gas (Knudsen regime) through a packed bed of monodisperse spheres via
experiments and numerical modelling, providing an absolute value of the
diffusion coefficient and compare it to published analytical models. The
experiments are designed to be directly comparable to numerical simulations, by
using precision steel beads, simple geometries, and a trade-off of the sample
size between small boundary effects and efficient computation. For direct
comparison, the diffusion coefficient is determined in Direct Simulation Monte
Carlo (DSMC) simulations, yielding a good match with experiments. This model is
further-on used on a microscopic scale, which cannot be studied in experiments,
to determine the mean path of gas molecules and its distribution, and compare
it against an analytical model. Scaling with sample properties (particle size,
porosity) and gas properties (molecular mass, temperature) is consistent with
analytical models. As predicted by these, results are very sensitive on sample
porosity and we find that a tortuosity depending linearly on
the porosity can well reconcile the analytical model with
experiments and simulations. Mean paths of molecules are close to those
described in the literature, but their distribution deviates from the
expectation for small path lengths. The provided diffusion coefficients and
scaling laws are directly applicable to thermophysical models of idealised
cometary material.Comment: accepted by MNRA
Olivine-rich exposures at Bellicia and Arruntia craters on (4) Vesta from Dawn FC
We present an analysis of the olivine-rich exposures at Bellicia and Arruntia
craters using Dawn Framing Camera (FC) color data. Our results confirm the
existence of olivine-rich materials at these localities as described by
Ammannito et al. (2013a) using Visual Infrared Spectrometer (VIR) data.
Analyzing laboratory spectra of various Howardite-Eucrite-Diogenite meteorites,
high-Ca pyroxenes, olivines and olivine-orthopyroxene mixtures, we derive three
FC spectral band parameters that are indicators of olivine-rich materials.
Combining the three band parameters allows us, for the first time, to reliably
identify sites showing modal olivine contents >40%. The olivine-rich exposures
at Bellicia and Arruntia are mapped using higher spatial resolution FC data.
The exposures are located on the slopes of outer/inner crater walls, on the
floor of Arruntia, in the ejecta, as well as in nearby fresh small impact
craters. The spatial extent of the exposures ranges from a few hundred meters
to few kilometers. The olivine-rich exposures are in accordance with both the
magma ocean and the serial magmatism model (e.g., Righter and Drake 1997;
Yamaguchi et al. 1997). However, it remains unsolved why the olivine-rich
materials are mainly concentrated in the northern hemisphere (~36-42{\deg} N,
46-74{\deg} E) and are almost absent in the Rheasilvia basin.Comment: Accepted for publication in Meteoritics and Planetary Scienc
Bilobate comet morphology and internal structure controlled by shear deformation
Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.Additional co-authors: M. A. Barucci, J.-L. Bertaux, I. Bertini, D. Bodewits, G. Cremonese, V. Da Deppo, S. Debei, M. De Cecco, J. Deller, S. Fornasier, M. Fulle, P. J. Gutiérrez, C. Güttler, W.-H. Ip, H. U. Keller, L. M. Lara, F. La Forgia, M. Lazzarin, A. Lucchetti, J. J. López-Moreno, F. Marzari, M. Massironi, S. Mottola, N. Oklay, M. Pajola, L. Penasa, F. Preusker, H. Rickman, F. Scholten, X. Shi, I. Toth, C. Tubiana & J.-B. Vincen
Comparing Dawn, Hubble Space Telescope, and Ground-Based Interpretations of (4) Vesta
Observations of asteroid 4 Vesta by NASA's Dawn spacecraft are interesting
because its surface has the largest range of albedo, color and composition of
any other asteroid visited by spacecraft to date. These hemispherical and
rotational variations in surface brightness and composition have been
attributed to impact processes since Vesta's formation. Prior to Dawn's arrival
at Vesta, its surface properties were the focus of intense telescopic
investigations for nearly a hundred years. Ground-based photometric and
spectroscopic observations first revealed these variations followed later by
those using Hubble Space Telescope. Here we compare interpretations of Vesta's
rotation period, pole, albedo, topographic, color, and compositional properties
from ground-based telescopes and HST with those from Dawn. Rotational spectral
variations observed from ground-based studies are also consistent with those
observed by Dawn. While the interpretation of some of these features was
tenuous from past data, the interpretations were reasonable given the
limitations set by spatial resolution and our knowledge of Vesta and HED
meteorites at that time. Our analysis shows that ground-based and HST
observations are critical for our understanding of small bodies and provide
valuable support for ongoing and future spacecraft missions.Comment: Pages: 51, Figures: 9, Tables:
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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