377 research outputs found
Soils of eagle crater and Meridiani Planum at the Opportunity Rover landing site
No full textThe soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.Additional co-authors: MP Golombek, R Greeley, JP Grotzinger, KE Herkenhoff, DJ Jerolmack, JR Johnson, B Jolliff, G Klingelhöfer, AH Knoll, ZA Learner, R Li, MC Malin, SM McLennan, HY McSween, DW Ming, RV Morris, JW Rice Jr, L Richter, R Rieder, D Rodionov, FP Seelos IV, JM Soderblom, SW Squyres, R Sullivan, WA Watters, CM Weitz, MB Wyatt, A Yen, J Zipfe
Geology of the Selk crater region on Titan from Cassini VIMS observations
No full textInternational audienceObservations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located similar to 800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is 90 km; its floor diameter is similar to 60 km. A central pit/peak, 20-30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10-15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21 degrees and 122 degrees east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk "bench." Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk's ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidizedejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest
WHITE DWARF COSMOCHRONOLOGY IN THE SOLAR NEIGHBORHOOD
No full textThe study of the stellar formation history in the solar neighborhood is a powerful technique to recover information about the early stages and evolution of the Milky Way. We present a new method that consists of directly probing the formation history from the nearby stellar remnants. We rely on the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The well characterized initial-final mass relation is employed in order to recover the initial masses (1 less than or similar to M-initial/M-circle dot less than or similar to 8) and total ages for the local degenerate sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, which can be compared to similar studies based on the properties of main- sequence stars in the solar neighborhood. Our method provides precise formation rates for all ages except in very recent times, and the results suggest an enhanced formation rate for the solar neighborhood in the last 5 Gyr compared to the range 5 < Age (Gyr) < 10. Furthermore, the observed total age of similar to 10 Gyr for the oldest white dwarfs in the local sample is consistent with the early seminal studies that have determined the age of the Galactic disk from stellar remnants. The main shortcoming of our study is the small size of the local white dwarf sample. However, the presented technique can be applied to larger samples in the future
Derivation of the Stellar Formation History from White Dwarfs in the Solar Neighborhood
No full textThe study of volume complete samples in the solar neighborhood is a powerful technique to learn about the stellar formation history and the initial mass function. We probe the local formation history from the volume complete sample of white dwarfs within 20 pc, where accurate cooling ages and masses have been determined. The observed initial-final mass relation and theoretical stellar isochrones are used to determine the initial stellar parameters of white dwarfs in the local sample. We correct for moderate biases that are necessary to transform our results to a global stellar formation rate, and compare to similar studies based on the properties of main-sequence and turnoff stars. Our results suggest an enhanced formation rate in the last 5 Gyr compared to the range 5 < Age (Gyr) < 10 for stars that are presently in the solar neighborhood
HUBBLE SPACE TELESCOPE MEASURES OF MASS ACCRETION RATES IN THE ORION NEBULA CLUSTER
No full textThe present observational understanding of the evolution of the mass accretion rates ([dot over M][subscript acc]) in pre-main-sequence stars is limited by the lack of accurate measurements of [dot over M][subscript acc] over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of [dot over M][subscript acc] for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M [subscript *] ~ 2 M [subscript ☉]. We derive [dot over M][subscript acc] from both the U-band excess and the Hα luminosity (L [subscript Hα]), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L [subscript acc]) and L [subscript Hα], which is L [subscript acc]/L [subscript ☉] = (1.31 ± 0.03) · L [subscript Hα]/L [subscript ☉] + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between [dot over M][subscript acc] and the parameters of the central star such as mass and age. We clearly find [dot over M][subscript acc] to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of [dot over M][subscript acc] with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log ([dot over M][subscript acc]/M [subscript ☉] yr) = (–5.12 ± 0.86) – (0.46 ± 0.13) · log (t/yr) – (5.75 ± 1.47) · log (M [subscript *]/M [subscript ☉]) + (1.17 ± 0.23) · log (t/yr) · log (M [subscript *]/M [subscript ☉]). These results also suggest that the similarity solution model could be revised for sources with M [subscript *] [> over ~] 0.5 M [subscript ☉]. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources
Fluvial channels on Titan: Initial Cassini RADAR observations
No full textCassini radar images show a variety of fluvial channels on Titan’s surface, often several hundreds of kilometers in length. Some
(predominantly at low- and mid-latitude) are radar-bright and braided, resembling desert washes where fines have been removed by
energetic surface liquid flow, presumably from methane rainstorms. Others (predominantly at high latitudes) are radar-dark and
meandering and drain into or connect polar lakes, suggesting slower-moving flow depositing fine-grained sediments. A third type, seen
predominantly at mid- and high latitudes, have radar brightness patterns indicating topographic incision, with valley widths of up to
3 km across and depth of several hundred meters. These observations show that fluvial activity occurs at least occasionally at all latitudes,
not only at the Huygens landing site, and can produce channels much larger in scale than those observed there. The areas in which
channels are prominent so far amount to about 1% of Titan’s surface, of which only a fraction is actually occupied by channels. The
corresponding global sediment volume inferred is not enough to account for the extensive sand seas. Channels observed so far have a
consistent large-scale flow pattern, tending to flow polewards and eastwards
Overview of the Spirit Mars Exploration Rover Mission to Gusev Crater: Landing site to Backstay Rock in the Columbia Hills
No full textSpirit landed on the floor of Gusev Crater and conducted initial operations on soil-covered, rock-strewn cratered plains underlain by olivine-bearing basalts. Plains surface rocks are covered by wind-blown dust and show evidence for surface enrichment of soluble species as vein and void-filling materials and coatings. The surface enrichment is the result of a minor amount of transport and deposition by aqueous processes. Layered granular deposits were discovered in the Columbia Hills, with outcrops that tend to dip conformably with the topography. The granular rocks are interpreted to be volcanic ash and/or impact ejecta deposits that have been modified by aqueous fluids during and/or after emplacement. Soils consist of basaltic deposits that are weakly cohesive, relatively poorly sorted, and covered by a veneer of wind-blown dust. The soils have been homogenized by wind transport over at least the several kilometer length scale traversed by the rover. Mobilization of soluble species has occurred within at least two soil deposits examined. The presence of monolayers of coarse sand on wind-blown bedforms, together with even spacing of granule-sized surface clasts, suggests that some of the soil surfaces encountered by Spirit have not been modified by wind for some time. On the other hand, dust deposits on the surface and rover deck have changed during the course of the mission. Detection of dust devils, monitoring of the dust opacity and lower boundary layer, and coordinated experiments with orbiters provided new insights into atmosphere-surface dynamics.Additional co-authors: T. Economou, J. Farmer, W. H. Farrand, W. Folkner, M. Golombek, S. Gorevan, J. A. Grant, R. Greeley, J. Grotzinger, E. Guinness, B. C. Hahn, L. Haskin, K. E. Herkenhoff, J. A. Hurowitz, S. Hviid, J. R. Johnson, G. Klingelhöfer, A. H. Knoll, G. Landis, C. Leff, M. Lemmon, R. Li, M. B. Madsen, M. C. Malin, S. M. McLennan, H. Y. McSween, D. W. Ming, J. Moersch, R. V. Morris, T. Parker, J. W. Rice Jr., L. Richter, R. Rieder, D. S. Rodionov, M. Sims, M. Smith, P. Smith, L. A. Soderblom, R. Sullivan, S. D. Thompson, N. J. Tosca, A. Wang, H. Wnke, J. Ward, T. Wdowiak, M. Wolff, A. Ye
Europa Deep Dive Two
No full textThe focus of the Europa Deep Dive 2: Composition workshop is to assess existing laboratory data and identify gaps where laboratory work is still needed; assess existing data on the Europa surface and exosphere; plan how to realize the needs for new data; encourage dialogue among laboratory, observational, and data-analysis communities; and foster collaborations among complementary laboratories.Institutional Support Lunar and Planetary Institute, Universities Space Research Association ; Conveners Louise M. Prockter, USRA/Lunar and Planetary Institute, Jonathan Kay, USRA/Lunar and Planetary Institute ; Science Organizing Committee Murthy S. Gudipati, NASA Jet Propulsion Laboratory, Reggie L. Hudson, NASA Goddard Space Flight Center, Lynnae C. Quick, Smithsonian Institution/Air and Space Museum, Abigal M. Rymer, Johns Hopkins University/Applied Physics Laboratory, Jason M. Soderblom, Massachusetts Institute of Technology.PARTIAL CONTENTS: Towards a Better Understanding of Composition: Status Update on Laboratory Studies of Europa Environment Analogs / B. L. Henderson -- Laboratory Spectral Measurements Needed to Better Model Europa's Surface Composition / C. A. Hibbitts -- Impurities in Europa's Ice Shell: Salty Tree-Rings? / S. M. Howell and E. J. Leonard -- Diving Deep into Europa's Reaction Chemistry / R. L. Hudson -- Plasma and Thermal Processing of Europa's Surface / R. E. Johnson, A. Oza, C. Schmidt, and F. Leblanc -- Investigating the Surface Composition of Europa with the Europa Clipper / R. L. Klima, J. Soderblom, M. Gudipati, and Europa Clipper Composition Working Group -- New Constraints on the Abundances of Sulfur and Chlorine at Europa / M. A. McGrath, W. B. Sparks, and J. R. Spencer -- Melt Production, Eruption and Degassing from Europa's Interior: Effects on Composition over Time / M. Melwani Daswani and S. D. Vance -- Investigating Detectability of Organics on Europa's Surface / I. Mishra and J. Lunine -- Rifting, Convection, and the Elevation of Bands on Europa / L. G. J. Montesi, S. M. Howell, and R. T. Pappalardo
- …
