695 research outputs found
Asteroid amphitrite: Surface composition and prospects for the possible Galileo flyby
Studies of the trajectory of the Galileo mission to Jupiter recently revealed that the spacecraft can pass close to one of the largest asteroids (#29 Amphitrite). NASA has therefore altered the mission plan of the Galileo spacecraft to include a possible close flyby of Amphitrite in early December 1986, if the condition of the spacecraft allows. If this option is actually implemented, Amphitrite will become the only asteroid for which any high-spatial resolution images and reflection spectra will be available. To evaluate the value of this data and place Amphitrite in the context of the more than 600 asteroids for which some compositional information exists. Existing data was reexamined, new telescopic spectra of Amphitrite were obtained, and simulated Galileo data sets were constructed
Local magneto-shear instability in Newtonian gravity
The magneto-rotational instability (MRI) - which is due to an interplay
between a sheared background and the magnetic field - is commonly considered a
key ingredient for developing and sustaining turbulence in the outer envelope
of binary neutron star merger remnants. To assess whether (or not) the
instability is active and resolved, criteria originally derived in the
accretion disk literature - thus exploiting the symmetries of such systems -
are often used. In this paper we discuss the magneto-shear instability as a
truly local phenomenon, relaxing common symmetry assumptions on the background
on top of which the instability grows. This makes the discussion well-suited
for highly dynamical environments such as binary mergers. We find that -
although this is somewhat hidden in the usual derivation of the MRI dispersion
relation - the instability crucially depends on the assumed symmetries.
Relaxing the symmetry assumptions on the background we find that the role of
the magnetic field is significantly diminished, as it affects the modes' growth
but does not drive it. This suggests that we should not expect the standard
instability criteria to provide a faithful indication/diagnostic of what "is
actually going on" in mergers. We conclude by making contact with a suitable
filtering operation, as this is key to separating background and fluctuations
in highly dynamical systems.Comment: 15 pages, 1 figur
Estimating Lunar Pyroclastic Deposit Depth from Imaging Radar Data: Applications to Lunar Resource Assessment
Lunar pyroclastic deposits represent one of the primary anticipated sources of raw materials for future human settlements. These deposits are fine-grained volcanic debris layers produced by explosive volcanism contemporaneous with the early stage of mare infilling. There are several large regional pyroclastic units on the Moon (for example, the Aristarchus Plateau, Rima Bode, and Sulpicius Gallus formations), and numerous localized examples, which often occur as dark-halo deposits around endogenic craters (such as in the floor of Alphonsus Crater). Several regional pyroclastic deposits were studied with spectral reflectance techniques: the Aristarchus Plateau materials were found to be a relatively homogeneous blanket of iron-rich glasses. One such deposit was sampled at the Apollo 17 landing site, and was found to have ferrous oxide and titanium dioxide contents of 12 percent and 5 percent, respectively. While the areal extent of these deposits is relatively well defined from orbital photographs, their depths have been constrained only by a few studies of partially filled impact craters and by imaging radar data. A model for radar backscatter from mantled units applicable to both 70-cm and 12.6-cm wavelength radar data is presented. Depth estimates from such radar observations may be useful in planning future utilization of lunar pyroclastic deposits
Space station impact experiments
Four processes serve to illustrate potential areas of study and their implications for general problems in planetary science. First, accretional processes reflect the success of collisional aggregation over collisional destruction during the early history of the solar system. Second, both catastrophic and less severe effects of impacts on planetary bodies survivng from the time of the early solar system may be expressed by asteroid/planetary spin rates, spin orientations, asteroid size distributions, and perhaps the origin of the Moon. Third, the surfaces of planetary bodies directly record the effects of impacts in the form of craters; these records have wide-ranging implications. Fourth, regoliths evolution of asteroidal surfaces is a consequence of cumulative impacts, but the absence of a significant gravity term may profoundly affect the retention of shocked fractions and agglutinate build-up, thereby biasing the correct interpretations of spectral reflectance data. An impact facility on the Space Station would provide the controlled conditions necessary to explore such processes either through direct simulation of conditions or indirect simulation of certain parameters
Calculating the mass fraction of primordial black holes
We reinspect the calculation for the mass fraction of primordial black holes (PBHs) which are formed from primordial perturbations, finding that performing the calculation using the comoving curvature perturbation c in the standard way vastly overestimates the number of PBHs, by many orders of magnitude. This is because PBHs form shortly after horizon entry, meaning modes significantly larger than the PBH are unobservable and should not affect whether a PBH forms or not - this important effect is not taken into account by smoothing the distribution in the standard fashion. We discuss alternative methods and argue that the density contrast, Δ, should be used instead as super-horizon modes are damped by a factor k2. We make a comparison between using a Press-Schechter approach and peaks theory, finding that the two are in close agreement in the region of interest. We also investigate the effect of varying the spectral index, and the running of the spectral index, on the abundance of primordial black holes
Signatures of non-gaussianity in the isocurvature modes of primordial black hole dark matter
Primordial black holes (PBHs) are black holes which may have formed very
early on during the radiation dominated era in the early universe. We present
here a method by which the large scale perturbations in the density of
primordial black holes may be used to place tight constraints on
non-gaussianity if PBHs account for dark matter (DM). The presence of
local-type non-gaussianity is known to have a significant effect on the
abundance of primordial black holes, and modal coupling from the observed CMB
scale modes can significantly alter the number density of PBHs that form within
different regions of the universe, which appear as DM isocurvature modes. Using
the recent \emph{Planck} constraints on isocurvature perturbations, we show
that PBHs are excluded as DM candidates for even very small local-type
non-gaussianity, and remarkably the constraint on
is almost as strong. Even small non-gaussianity is excluded if DM is
composed of PBHs. If local non-Gaussianity is ever detected on CMB scales, the
constraints on the fraction of the universe collapsing into PBHs (which are
massive enough to have not yet evaporated) will become much tighter.Comment: 23 pages, 11 figures. V2: minor corrections and changes, matches
published versio
Dynamical non-axisymmetric instabilities in rotating relativistic stars
We present new results on dynamical instabilities in rapidly rotating
neutron-stars. In particular, using numerical simulations in full General
Relativity, we analyse the effects that the stellar compactness has on the
threshold for the onset of the dynamical bar-mode instability, as well as on
the appearance of other dynamical instabilities. By using an extrapolation
technique developed and tested in our previous study [1], we explicitly
determine the threshold for a wide range of compactnesses using four sequences
of models of constant baryonic mass comprising a total of 59 stellar models.
Our calculation of the threshold is in good agreement with the Newtonian
prediction and improves the previous post-Newtonian estimates. In addition, we
find that for stars with sufficiently large mass and compactness, the m=3
deformation is the fastest growing one. For all of the models considered, the
non-axisymmetric instability is suppressed on a dynamical timescale with an m=1
deformation dominating the final stages of the instability. These results,
together with those presented in [1], suggest that an m=1 deformation
represents a general and late-time feature of non-axisymmetric dynamical
instabilities both in full General Relativity and in Newtonian gravity.Comment: To appear on CQG, NFNR special issue. 16 pages, 5 color figures,
movies from http://www.fis.unipr.it/numrel/BarMode/ResearchBarMode.htm
A Spectral Survey of the Crisium Basin Region of the Moon
The Crisium basin region harbors a number of interesting features, including geochemical and radar anomalies, light plains units and possible hidden mare deposits (cryptomaria). This report presents preliminary results of a telescopic near-infrared spectral study concerning a variety of surface units in the Crisium region. Observations were made of Mare Crisium, light plains deposits north of Taruntius crater, and the terra associated with the Crisium basin
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