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121.
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123.
A.W.R. Bevan R. Hutchison A.J. Easton G.P. Durant C.M. Farrow 《Meteoritics & planetary science》1985,20(3):491-501
High Possil and Strathmore are the first (1804) and last (1917) meteorite falls, respectively, of the three recorded in Scotland. Olivine compositions and total Fe contents in High Possil (Fa25.2; 21.35 wt %) and Strathmore (Fa25.3; 20.6 wt %) confirm their classification as L-group chondrites (Mason, 1963), and the presence of abundant plagioclase feldspar shows that both chondrites belong to petrologic type 6. Both chondrites display thermal and mechanical alteration attributable to moderate shock-loading appropriate to facies c (High Possil) and c-d (Strathmore) (Dodd and Jarosewich, 1979). Incipient shock-melting of metal and troilite in both chondrites is the first described from Lc chondrites, and differences in the responses of metallic and silicate minerals to shock-loading are discussed. 相似文献
124.
We present the results of extensive thermal-infrared observations of the C-type near-Earth Asteroid (1580) Betulia obtained in June 2002 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Betulia is a highly unusual object for which earlier radiometric observations, interpreted on the basis of simple thermal models, indicated a surface of high thermal inertia. A high thermal inertia implies a lack of thermally insulating regolith. Radiometric observations of other asteroids of comparable size indicate that regolith is present in nearly all cases. Knowledge of the surface thermal properties of small near-Earth asteroids is crucial for meaningful calculations of the Yarkovsky effect, which is invoked to explain the delivery of collisional fragments from the main belt into near-Earth orbits, and apparently has a significant influence on the orbital evolution of potentially hazardous near-Earth asteroids. Furthermore, apart from being an indicator of the presence of thermally insulating regolith on the surface of an asteroid, the thermal inertia determines the magnitude of the diurnal temperature variation and is therefore of great importance in the design of instrumentation for lander missions to small asteroids. In the case of Betulia our database is sufficiently broad to allow the use of more sophisticated thermal models than were available for earlier radiometric observations. The measured fluxes have been fitted with thermal-model emission continua to determine the asteroid's size and geometric albedo, pv. Fits obtained with a new thermophysical model imply an effective diameter of 4.57±0.46 km and an albedo of 0.077±0.015 and indicate a moderate surface thermal inertia of around 180 J m−2 s−0.5 K−1. It is difficult to reconcile our results with earlier work, which indicate a larger diameter for Betulia and a high-thermal-inertia surface of bare rock. 相似文献
125.
126.
The Mangala Valles system is an ∼
∼900 km fluvially carved channel system located southwest of the Tharsis rise and is unique among the martian outflow channels
in that it heads at a linear fracture within the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse
channels. Mangala Valles is confined within a broad, north–south trending depression, and begins as a single valley measuring
up to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands toward the northern
lowlands. Approximately 600 km downstream, this single valley branches into multiple channels, which ultimately lose their
expression at the dichotomy boundary. Previous investigations of Mangala Vallis suggested that many of the units mapped interior
to the valley were depositional, related to flooding, and that a minimum of two distinct periods of flooding separated by
tens to hundreds of millions of years were required to explain the observed geology. We use infrared and visible images from
the THermal EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter (MOLA), to investigate
the nature of the units mapped within Mangala Vallis. We find that the geomorphology of the units, as well as their topographic
and geographic distribution, are consistent with most of them originating from a single assemblage of volcanic flow deposits,
once continuous with volcanic flows to the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south
trending depression into which Mangala Vallis formed prior to any fluvial activity. Later flooding scoured and eroded this
volcanic assemblage north of the Mangala source graben, resulting in the present distribution of the units within Mangala
Vallis. Additionally, our observations suggest that a single period of catastrophic flooding, rather than multiple periods
separated by tens to hundreds of millions of years, is consistent with and can plausibly explain the interior geology of Mangala
Vallis. Further, we present a new scenario for the source and delivery of water to the Mangala source graben that models flow
of groundwater through a sub-cryosphere aquifer and up a fracture that cracks the cryosphere and taps this aquifer. The results
of our model indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have required
less than several days to fill up prior to any spill-over of water to the north. Through estimates of the volume of material
missing from Mangala (13,000–20,000 km3), and calculation of mean discharge rates through the channel system (∼
∼5 × 106 m3 s−1), we estimate that the total duration of fluvial activity through the Mangala Valles was 1–3 months. 相似文献
127.
128.
CO maps of the Bok globule B335 are presented and used to derive its density profile, mass distribution, and rotational velocity structure. It is found that the cloud is in nearly hydrostatic equilibrium with a density profile that varies roughly as r?1 in the core and r?3 in the envelope. The observed rotation is unimportant in the force balance at the present stage of evolution. 相似文献
129.
Analyses of sea level and current-meter data using digital filters and a variety of statistical methods show a variety of phenomena related to non-local coastal forcing and local tidal forcing in the northern reach of San Francisco Bay, a partially mixed estuary. Low-frequency variations in sea level are dominated by non-local variations in coastal sea level and also show a smaller influence from tidally induced fortnightly sea level variations. Low-frequency currents demonstrate a gravitational circulation which is modified by changes in tidal-current speed over the spring-neap tidal cycle. Transients in gravitational circulation induce internal oscillations with periods of two to four days. 相似文献
130.
R. W. Komm 《Solar physics》1995,157(1-2):45-50
I analyze a quiet-Sun magnetogram with an orthogonal wavelet transform, which allows me to define an entropy measure. The entropy measure of the magnetogram as a function of spatial scale obeys a scaling law, which leads to a fractal dimension ofD
f
= 1.7. Furthermore, the entropy scaling law is directly related to the intermittency of magnetic features, which increases for decreasing spatial scales, as expected for a turbulent signal. In this context, the scaling law parameter can be interpreted as a fractional reduction in volume from one step of the turbulent cascade to the next.Operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. 相似文献