180 research outputs found
Directional properties of p-wave velocities and acoustic anisotropies in different structural domains of the Northern Barbados Ridge accretionary complex
Ocean Drilling Program (ODP) Leg 156 revisited the northern Barbados Ridge, where the previous Deep Sea Drilling Program Leg 78A and ODP Leg 110 studied the frontal part of this accretionary prism. Drilling and logging-while-drilling at Sites 947, 948, and 949 successfully identified major thrust faults and the décollement, which was the target of several downhole experiments. Two of the eight holes drilled were equipped with borehole observatories that will monitor temperature, pressure, and fluid flow over the next years. Coring at Hole 948C recovered 180 m of sediment, centered around the décollement, which was positively identified based on structural information. The aim of this study is the evaluation of the possible correlation of preferred orientation of acoustic properties and the direction of maximum compressive strain in the frontal part of the accretionary prism. For this purpose, shipboard P-wave velocities from Holes 948C and 949B were reoriented. This information was then used to compare the directional properties of accreted and subducted sediments. In Hole 948C, lowest transverse velocities (Tmin ) were observed to be consistently oriented perpendicular to the maximum horizontal compressive stress, believed to be parallel to the convergence vector. In the underthrust domain of Hole 948C, several preferred orientations for Tmin were detected, but no correlation with the geotectonic reference frame could be identified. Acoustic anisotropy does not show a comparable pattern in Hole 948C. It is concluded that the observed directional dependence of P-wave velocity in the accreted sediment domain in Hole 948B is the result of moderate to steeply inclined bedding, although this conclusion can not adequately be tested due to the lack of corrected structural data
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
Low Temperature Magnetic Properties of Siderite and Magnetite in Marine Sediments
Low temperature magnetic techniques provide useful tools to detect the presence of magnetite and pyrrhotite in sediments through identification of their low temperature transitions, to determine the amount of ultrafine-grained (superparamagnetic) material in sediments, and can potentially detect the presence of certain types of magnetotactic bacteria. Application of these types of experiments to nannofossil chalks from beneath the Barbados accretionary prism led to some unusual results, which are attributed to the presence of siderite. Thermal demagnetization of low-temperature remanence after cooling in zero field and in a 2.5 T field both displayed large remanence losses from 20 K to 40 K. Below 40 K, the magnetization of the chalks was much higher in the field-cooled experiments than in the zero-field-cooled experiments. Low temperature hysteresis experiments, made after cooling in a 2.5 T field, displayed offsets in magnetization parallel to the direction of the initial applied field, when measured below 40 K. The offset loops can be due to either an exchange anisotropy between siderite and magnetite phases in the sediments, a defect moment in the siderites, or a canted moment in the siderites. Apparent similarity between the low-temperature thermal demagnetization results from these siderite-bearing sediments, pure siderite, and pure rhodochrosite samples and the well-known 34 K transition in pyrrhotite should lead to caution in identification of pyrrhotite in marine sediments based on low-temperature remanence studies alone
Modelling and Simulation of Cratering and Ejecta Production During High Velocity Impacts
Anelastic strain recovery reveals extension across SW Japan subduction zone
Sediment dominated convergent margins typically
record substantial horizontal shortening often associated
with great earthquakes. The convergent margin south of
Japan is arguably one of the most extensively investigated
margins and previous studies have documented extensive
evidence for accretion and horizontal shortening. Here, we
show results from anelastic strains recovered from three
partially lithified sediment samples (40~ porosities)
across the southwest Japan accretionary prism and
propose that the margin is dominated by horizontal
extension rather than compression. The anelastic strain
results are also consistent with stress directions interpreted
from two independent techniques - bore hole breakout
orientations and core-scale fault data. We interpret this
unexpected result to reflect geologically recent underthrusting
of a thick sediment package and concomitant
weakening of the decollement
The Role of Ejecta in the Small Crater Populations on the Mid-Sized Saturnian Satellites
We find evidence that crater ejecta play an important role in the small
crater populations on the Saturnian satellites, and more broadly, on cratered
surfaces throughout the Solar System. We measure crater populations in Cassini
images of Enceladus, Rhea, and Mimas, focusing on image data with scales less
than 500 m/pixel. We use recent updates to crater scaling laws and their
constants to estimate the amount of mass ejected in three different velocity
ranges: (i) greater than escape velocity, (ii) less than escape velocity and
faster than the minimum velocity required to make a secondary crater (v_min),
and (iii) velocities less than v_min. Although the vast majority of mass on
each satellite is ejected at speeds less than v_min, our calculations
demonstrate that the differences in mass available in the other two categories
should lead to observable differences in the small crater populations; the
predictions are borne out by the measurements we have made to date. Rhea,
Tethys, and Dione have sufficient surface gravities to retain ejecta moving
fast enough to make secondary crater populations. The smaller satellites, such
as Enceladus but especially Mimas, are expected to have little or no
traditional secondary populations because their escape velocities are near the
threshold velocity necessary to make a secondary crater. Our work clarifies why
the Galilean satellites have extensive secondary crater populations relative to
the Saturnian satellites. The presence, extent, and sizes of sesquinary craters
(craters formed by ejecta that escape into temporary orbits around Saturn
before re-impacting the surface) is not yet well understood. Finally, our work
provides further evidence for a "shallow" size-frequency distribution (slope
index of ~2 for a differential power-law) for comets a few km diameter and
smaller. [slightly abbreviated]Comment: Submitted to Icarus. 77 double-spaced pages, including 25 figures and
5 table
P/2010A2 LINEAR - I: An impact in the Asteroid Main Belt
Comet P/2010A2 LINEAR is a good candidate for membership with the Main Belt
Comet family. It was observed with several telescopes (ESO NTT, La Silla;
Gemini North, Mauna Kea; UH 2.2m, Mauna Kea) from 14 Jan. until 19 Feb. 2010 in
order to characterize and monitor it and its very unusual dust tail, which
appears almost fully detached from the nucleus; the head of the tail includes
two narrow arcs forming a cross. The immediate surroundings of the nucleus were
found dust-free, which allowed an estimate of the nucleus radius of 80-90m. A
model of the thermal evolution indicates that such a small nucleus could not
maintain any ice content for more than a few million years on its current
orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up
and electrostatic dust levitations were also rejected, leaving an impact with a
smaller body as the favoured hypothesis, and ruling out the cometary nature of
the object.
The impact is further supported by the analysis of the tail structure.
Finston-Probstein dynamical dust modelling indicates the tail was produced by a
single burst of dust emission. More advanced models, independently indicate
that this burst populated a hollow cone with a half-opening angle alpha~40degr
and with an ejection velocity v_max ~ 0.2m/s, where the small dust grains fill
the observed tail, while the arcs are foreshortened sections of the burst cone.
The dust grains in the tail are measured to have radii between a=1-20mm, with a
differential size distribution proportional to a^(-3.44 +/- 0.08). The dust
contained in the tail is estimated to at least 8x10^8kg, which would form a
sphere of 40m radius. Analysing these results in the framework of crater
physics, we conclude that a gravity-controlled crater would have grown up to
~100m radius, i.e. comparable to the size of the body. The non-disruption of
the body suggest this was an oblique impact.Comment: 15 pages, 11 figures, in pres
Observing the variation of asteroid thermal inertia with heliocentric distance
Thermal inertia is a useful property to characterise a planetary surface since it can be used as a qualitative measure of the regolith grain size. It is expected to vary with heliocentric distance because of its dependence on temperature. However, no previous investigation has conclusively observed a change in thermal inertia for any given planetary body. We have addressed this by using NEOWISE data and the Advanced Thermophysical Model to study the thermophysical properties of the near-Earth asteroids (1036) Ganymed, (1580) Betulia, and (276049) 2002 CE26 as they moved around their highly eccentric orbits. We confirm that the thermal inertia values of Ganymed and 2002 CE26 do vary with heliocentric distance, although the degree of variation observed depends on the spectral emissivity assumed in the thermophysical modelling. We also confirm that the thermal inertia of Betulia did not change for three different observations obtained at the same heliocentric distance. Depending on the spectral emissivity, the variations for Ganymed and 2002 CE26 are potentially more extreme than that implied by theoretical models of heat transfer within asteroidal regoliths, which might be explained by asteroids having thermal properties that also vary with depth. Accounting for this variation reduces a previously observed trend of decreasing asteroid thermal inertia with increasing size, and suggests that the surfaces of small and large asteroids could be much more similar than previously thought. Furthermore, this variation can affect Yarkovsky orbital drift predictions by a few tens of per cent
Causality guided machine learning model on wetland CH4 emissions across global wetlands
Wetland CH4 emissions are among the most uncertain components of the global CH4 budget. The complex nature of wetland CH4 processes makes it challenging to identify causal relationships for improving our understanding and predictability of CH4 emissions. In this study, we used the flux measurements of CH4 from eddy covariance towers (30 sites from 4 wetlands types: bog, fen, marsh, and wet tundra) to construct a causality-constrained machine learning (ML) framework to explain the regulative factors and to capture CH4 emissions at sub -seasonal scale. We found that soil temperature is the dominant factor for CH4 emissions in all studied wetland types. Ecosystem respiration (CO2) and gross primary productivity exert controls at bog, fen, and marsh sites with lagged responses of days to weeks. Integrating these asynchronous environmental and biological causal relationships in predictive models significantly improved model performance. More importantly, modeled CH4 emissions differed by up to a factor of 4 under a +1C warming scenario when causality constraints were considered. These results highlight the significant role of causality in modeling wetland CH(4 )emissions especially under future warming conditions, while traditional data-driven ML models may reproduce observations for the wrong reasons. Our proposed causality-guided model could benefit predictive modeling, large-scale upscaling, data gap-filling, and surrogate modeling of wetland CH4 emissions within earth system land models.Peer reviewe
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