163 research outputs found
Mars Encounters cause fresh surfaces on some near-Earth asteroids
All airless bodies are subject to the space environment, and spectral
differences between asteroids and meteorites suggest many asteroids become
weathered on very short (<1My) timescales. The spectra of some asteroids,
particularly Q-types, indicate surfaces that appear young and fresh, implying
they have been recently been exposed. Previous work found that Earth encounters
were the dominant freshening mechanism and could be responsible for all
near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type
sample of by a factor of three. We present the orbital distributions of 64
Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for
refreshing their surfaces. Our sample reveals two important results: i) the
relatively steady fraction of Q-types with increasing semi-major axis and ii)
the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection
Distances (MOID) that do not have orbit solutions that cross Earth. Both of
these are evidence that Earth-crossing is not the only scenario by which NEO
Q-types are freshened. The high Earth-MOID asteroids represent 10% of the
Q-type population and all are in Amor orbits. While surface refreshing could
also be caused by Main Belt collisions or mass shedding from YORP spinup, all
high Earth-MOID Q-types have the possibility of encounters with Mars indicating
Mars could be responsible for a significant fraction of NEOs with fresh
surfaces.Comment: Accepted for publication in Icarus -- 14 pages, 8 figures, 1 table, 2
appendice
Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry
The nature and origin of the asteroids orbiting in near-Earth space,
including those on a potentially hazardous trajectory, is of both scientific
interest and practical importance. We aim here at determining the taxonomy of a
large sample of near-Earth (NEA) and Mars-crosser (MC) asteroids and analyze
the distribution of these classes with orbit. We use this distribution to
identify their source regions and to study the strength of planetary encounters
to refresh asteroid surfaces. We measure the photometry of these asteroids over
four filters at visible wavelengths on images taken by the SDSS. These colors
are used to classify the asteroids into a taxonomy consistent with the widely
used Bus-DeMeo taxonomy based on spectroscopy. We report here on the taxonomic
classification of 206 NEAs and 776 MCs determined from SDSS photometry,
representing an increase of 40% and 663% of known taxonomy classifications in
these populations. Using the source region mapper by Greenstreet et al. (2012),
we compare the taxonomic distribution among NEAs and main-belt asteroids of
similar diameters. Both distributions agree at the few percent level for the
inner part of the Main Belt and we confirm this region as a main source of
near-Earth objects. The effect of planetary encounters on asteroid surfaces are
also studied by developing a simple model of forces acting on a surface grain
during planetary encounter, which provides the minimum distance at which a
close approach should occur to trigger resurfacing events. By integrating
numerically the orbit of the 519 S-type and 46 Q-type asteroids back in time
and monitoring their encounter distance with planets, we seek to understand the
conditions for resurfacing events. The population of Q-type is found to present
statistically more encounters with Venus and the Earth than S-types, although
both types present the same amount of encounters with Mars.Comment: Accepted for publication in Icarus. 45 pages, 11 figures, 4 tables, 2
tables in appendix (supplementary material
Categorization of asteroids in the near-infrared
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2007.Includes bibliographical references (leaves 40-42).This work presents the DeMeo taxonomy, an asteroid taxonomy with 24 classes based on Principal Component Analysis of spectral data over the visible and near-infrared wavelengths, specifically the 0.45 to 2.45 micron range. Principal Component Analysis was used by both Tholen (1984) and Bus (1999) to create taxonomies on visible data. There is no pre-existing taxonomic system for the entire suite of asteroid characteristics because only in the current decade has spectral data collection become available in the near-infrared for asteroids down to relatively faint (V= 17) limiting magnitudes. (Rayner et al. 2003) With a larger data range, which includes important absorption features at one and two microns suggesting the presence of minerals, there is a need for an extended system to encompass this range of information. In this work we explain the process of creating the taxonomy, the method for finding an object's taxonomic class under this system, and present spectral types for the 365 objects that were used to create the system.by Francesca E. DeMeo.S.M
Unexpected D-type Interlopers in the Inner Main Belt
Very red featureless asteroids (spectroscopic D-types) are expected to have
formed in the outer solar system far from the sun. They comprise the majority
of asteroids in the Jupiter Trojan population, and are also commonly found in
the outer main belt and among Hildas. The first evidence for D-types in the
inner and middle parts of the main belt was seen in the Sloan Digital Sky
Survey (SDSS). Here we report follow-up observations of SDSS D-type candidates
in the near-infrared. Based on follow up observations of 13 SDSS D-type
candidates, we find a ~20% positive confirmation rate. Known inner belt D-types
range in diameter from roughly 7 to 30 kilometers. Based on these detections we
estimate there are ~100 inner belt D-types with diameters between 2.5 and 20km.
The lower and upper limits for total mass of inner belt D-types is 2x
kg to 2x kg which represents 0.01% to 0.1% of the mass of the inner
belt. The inner belt D-types have albedos at or above the upper end typical for
D-types which raises the question as to whether these inner belt bodies
represent only a subset of D-types, they have been altered by external factors
such as weathering processes, or if they are compositionally distinct from
other D-types. All D-types and candidates have diameters less than 30km, yet
there is no obvious parent body in the inner belt. Dynamical models have yet to
show how D-types originating from the outer solar system could penetrate into
the inner reaches of the Main Belt under current scenarios of planet formation
and subsequent Yarkovsky drift.Comment: 16 pages, 3 figures, 4 tables -- accepted for publication in Icaru
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
The Compositional Structure of the Asteroid Belt
The past decade has brought major improvements in large-scale asteroid
discovery and characterization with over half a million known asteroids and
over 100,000 with some measurement of physical characterization. This explosion
of data has allowed us to create a new global picture of the Main Asteroid
Belt. Put in context with meteorite measurements and dynamical models, a new
and more complete picture of Solar System evolution has emerged. The question
has changed from "What was the original compositional gradient of the Asteroid
Belt?" to "What was the original compositional gradient of small bodies across
the entire Solar System?" No longer is the leading theory that two belts of
planetesimals are primordial, but instead those belts were formed and sculpted
through evolutionary processes after Solar System formation. This article
reviews the advancements on the fronts of asteroid compositional
characterization, meteorite measurements, and dynamical theories in the context
of the heliocentric distribution of asteroid compositions seen in the Main Belt
today. This chapter also reviews the major outstanding questions relating to
asteroid compositions and distributions and summarizes the progress and current
state of understanding of these questions to form the big picture of the
formation and evolution of asteroids in the Main Belt. Finally, we briefly
review the relevance of asteroids and their compositions in their greater
context within our Solar System and beyond.Comment: Accepted chapter in Asteroids IV in the Space Science Series to be
published Fall 201
Physical characterization and origin of binary near-Earth asteroid (175706) 1996 FG3
The near-Earth asteroid (NEA) (175706) 1996 FG3 is a particularly interesting
spacecraft target: a binary asteroid with a low-DeltaV heliocentric orbit. The
orbit of its satellite has provided valuable information about its mass density
while its albedo and colors suggest it is primitive or part of the C-complex
taxonomic grouping. We extend the physical characterization of this object with
new observations of its emission at mid-Infrared (IR) wavelengths and with
near-IR reflection spectroscopy. We derive an area-equivalent system diameter
of 1.90 \pm 0.28 km (corresponding to approximate component diameters of 1.83
km and 0.51 km, respectively) and a geometric albedo of 0.039 \pm 0.012.
1996 FG3 was previously classified as a C-type asteroid, though the combined
0.4--2.5 micron spectrum with thermal correction indicates classification as
B-type; both are consistent with the low measured albedo. Dynamical studies
show that 1996 FG3 has most probably originated in the inner main asteroid
belt. Recent work has suggested the inner Main Belt (142) Polana family as the
possible origin of another low-DeltaV B-type NEA, (101955) 1999 RQ36. A similar
origin for 1996 FG3 would require delivery by the overlapping Jupiter 7:2 and
Mars 5:9 mean motion resonances rather than the nu-6 resonance, and we find
this to be a low probability, but possible, origin.Comment: Published in Ap
A spectral comparison of (379) Huenna and its satellite
We present near-infrared spectral measurements of Themis family asteroid
(379) Huenna (D~98 km) and its 6 km satellite using SpeX on the NASA IRTF. The
companion was farther than 1.5" from the primary at the time of observations
and was approximately 5 magnitudes dimmer. We describe a method for separating
and extracting the signal of a companion asteroid when the signal is not
entirely resolved from the primary. The spectrum of (379) Huenna has a broad,
shallow feature near 1 {\mu}m and a low slope, characteristic of C-type
asteroids. The secondary's spectrum is consistent with the taxonomic
classification of C-complex or X-complex. The quality of the data was not
sufficient to identify any subtle feature in the secondary's spectrum.Comment: 6 pages, 4 figures, 2 tables - Accepted for publication in Icaru
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