6,622 research outputs found
Mercurian impact ejecta: Meterorites and mantle
We have examined the fate of impact ejecta liberated from the surface of
Mercury due to impacts by comets or asteroids, in order to study (1) meteorite
transfer to Earth, and (2) re-accumulation of an expelled mantle in
giant-impact scenarios seeking to explain Mercury's large core. In the context
of meteorite transfer, we note that Mercury's impact ejecta leave the planet's
surface much faster (on average) than other planet's in the Solar System
because it is the only planet where impact speeds routinely range from 5-20
times the planet's escape speed. Thus, a large fraction of mercurian ejecta may
reach heliocentric orbit with speeds sufficiently high for Earth-crossing
orbits to exist immediately after impact, resulting in larger fractions of the
ejecta reaching Earth as meteorites. We calculate the delivery rate to Earth on
a time scale of 30 Myr and show that several percent of the high-speed ejecta
reach Earth (a factor of -3 less than typical launches from Mars); this is one
to two orders of magnitude more efficient than previous estimates. Similar
quantities of material reach Venus.
These calculations also yield measurements of the re-accretion time scale of
material ejected from Mercury in a putative giant impact (assuming gravity is
dominant). For mercurian ejecta escaping the gravitational reach of the planet
with excess speeds equal to Mercury's escape speed, about one third of ejecta
re-accretes in as little as 2 Myr. Thus collisional stripping of a silicate
proto-mercurian mantle can only work effectively if the liberated mantle
material remains in small enough particles that radiation forces can drag them
into the Sun on time scale of a few million years, or Mercury would simply
re-accrete the material.Comment: 14 pages. Submitted to Meteoritics and Planetary Scienc
Pencil-Beam Surveys for Faint Trans-Neptunian Objects
We have conducted pencil-beam searches for outer solar system objects to a
limiting magnitude of R ~ 26. Five new trans-neptunian objects were detected in
these searches. Our combined data set provides an estimate of ~90
trans-neptunian objects per square degree brighter than ~ 25.9. This estimate
is a factor of 3 above the expected number of objects based on an extrapolation
of previous surveys with brighter limits, and appears consistent with the
hypothesis of a single power-law luminosity function for the entire
trans-neptunian region. Maximum likelihood fits to all self-consistent
published surveys with published efficiency functions predicts a cumulative sky
density Sigma(<R) obeying log10(Sigma) = 0.76(R-23.4) objects per square degree
brighter than a given magnitude R.Comment: Accepted by AJ, 18 pages, including 6 figure
A Derivation of the Luminosity Function of the Kuiper Belt from a Broken Power-Law Size Distribution
We have derived a model of the Kuiper belt luminosity function exhibited by a
broken power-law size distribution. This model allows direct comparison of the
observed luminosity function to the underlying size distribution. We discuss
the importance of the radial distribution model in determining the break
diameter. We determine a best-fit break-diameter of the Kuiper belt
size-distribution of 30<Db<90 km via a maximum-likelihood fit of our model to
the observed luminosity function. We also confirm that the observed luminosity
function for m(R) ~ 21-28 is consistent with a broken power-law size
distribution, and exhibits a break at m(R)=26.0+0.7-1.8.Comment: Accepted by Icarus 25 pages, 2 tables, 2 figure
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