5,832 research outputs found
I. Collisional evolution and reddening of asteroid surfaces: The problem of conflicting timescales and the role of size-dependent effects
Space weathering is the generic term used for processes that modify the
optical properties of surfaces of atmosphereless rocky bodies under exposure to
the space environment. The general agreement about the relevance of the effects
of space weathering on the spectral properties of S-complex asteroids fails
when some basic quantitative estimates are attempted. In particular, there is
severe disagreement regarding the typical timescales for significant spectral
reddening to occur, ranging from 1 Myr to 1 Gyr. Generally speaking, the
spectral reddening of an individual object can be considered as the sum of
three terms, one (which is relevant for statistical analyses) depending on the
exposure of the object to space weathering during its lifetime, a second one
due to the original surface composition, and a third one (a "noise" term) due
to the combination of poorly constrained effects (e.g., structure and texture
of the surface). The surface of an asteroid is usually covered by regolith, and
its presence and properties presumably play a critical role in the weathering
processes. In this paper we discuss the role played by collisional evolution in
affecting the spectral properties of asteroids and refreshing the surfaces due
to the formation of ejecta, and the necessity of a simultaneous modeling of
collisions and weathering processes. We introduce a new idea, based on the
possibility of a sort of saturation of the refreshing process whenever a
massive reaccumulation of the impact ejecta takes place. In this case, a
dependence of the overall reddening on the asteroid size should naturally come
out. We show that this conclusion is indeed supported by available main belt
asteroid spectroscopic data.Comment: Accepted by MNRA
Observations of "Fresh" and Weathered Surfaces on Asteroid Pairs and Their Implications on the Rotational-Fission Mechanism
The rotational-fission of a rubble-pile asteroid can result in an "asteroid
pair", two un-bound asteroids sharing similar orbits. This mechanism might
exposes material that previously had never have been exposed to the weathering
conditions of space. Therefore, the surfaces of asteroid pairs offer the
opportunity to observe non-weathered fresh spectra. We report near-IR
spectroscopic observations of 31 asteroids in pairs. We analyze their spectral
slopes, 1 {\mu}m absorption band, taxonomy, and estimate the time elapsed since
their separation. Analyzing the 19 S-complex objects in our sample, we find two
fresh Q-type asteroids that are the first of their kind to be observed in the
main-belt over the full visible and near-IR range. This solidly demonstrates
that Q-type objects are not limited to the NEA population. The pairs in our
sample present a range of fresh and weathered surfaces with no clear evidence
for a correlation with the ages of the pairs. However, our sample includes old
pairs (1 to 2 My) that present low spectral slopes. This illustrates a
timescale of at least ~2 My before an object develops high spectral slope that
is typical for S-type asteroids.
We discuss mechanisms that explain the existence of weathered pairs with
young dynamical ages and find that the "secondary fission" model (Jacobson &
Scheeres 2011) is the most robust with our observations since: 1) the secondary
members in our sample present fresh parameters that tend to be fresher than
their weathered primaries; 2) most of the fresh pairs in our sample have low
size ratios between the secondary and the primary; 3) 33% of the primaries in
our sample are fresh, similar to the prediction set by this model; 4) known
satellites orbit two of the pairs in our sample with low size ratio and fresh
surface; 5) there is no correlation between the weathering state and the
primary shape as predicted by other models.Comment: 19 pages, 17 figures, 4 tables. Accepted to Icaru
Extreme AO Observations of Two Triple Asteroid Systems with SPHERE
We present the discovery of a new satellite of asteroid (130) Elektra -
S/2014 (130) 1 - in differential imaging and in integral field spectroscopy
data over multiple epochs obtained with SPHERE/VLT. This new (second) moonlet
of Elektra is about 2 km across, on an eccentric orbit and about 500 km away
from the primary. For a comparative study, we also observed another triple
asteroid system (93) Minerva. For both systems, component-resolved reflectance
spectra of the satellites and primary were obtained simultaneously. No
significant spectral difference was observed between the satellites and the
primary for either triple system. We find that the moonlets in both systems are
more likely to have been created by sub-disruptive impacts as opposed to having
been captured.Comment: 8 pages, 4 figures, 1 table, accepted to be published in the
Astrophysical Journal Letter
Sapping Features of the Colorado Plateau: a Comparative Planetary Geology Field Guide
This book is an attempt to determine geomorphic criteria to be used to distinguish between channels formed predominantly by sapping and seepage erosion and those formed principally by surface runoff processes. The geologic nature of the Colorado Plateau has resulted in geomorphic features that show similarities to some areas on Mars, especially certain valley networks within thick sandstone formations. Where spring sapping is an effective process, the valleys that develop are unique in terms of their morphology and network pattern
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