The Ashima/MIT Mars GCM and argon in the martian atmosphere     

Yuan Lian  Mark I. Richardson  Claire E. Newman  Christopher Lee  Anthony D. Toigo  Michael A. Mischna  Jean-Michel Campin 《Icarus》2012,218(2):1043-1070

We investigate the ability of modern general circulation models (GCMs) to simulate transport in the martian atmosphere using measurements of argon as a proxy for the transport processes. Argon provides the simplest measure of transport as it is a noble gas with no sinks or sources on seasonal timescales. Variations in argon result solely from ‘freeze distillation’, as the atmosphere condenses at the winter poles, and from atmospheric transport. Comparison of all previously published models when rescaled to a common definition of the argon enhancement factor (EF) suggest that models generally do a poor job in predicting the peak enhancement in southern winter over the winter pole – the time when the capability of the model transport approaches are most severely tested. Despite observed peak EF values of ～6, previously published model predictions peaked at EF values of only 2–3. We introduce a new GCM that provides a better treatment of mass conservation within the dynamical core, includes more sophisticated tracer transport approaches, and utilizes a cube–sphere grid structure thus avoiding the grid-point convergence problem at the pole that exists for most current Mars GCMs. We describe this model – the Ashima Research/Massachusetts Institute of Technology Mars General Circulation Model (Ashima/MIT Mars GCM) and use it to demonstrate the significant sensitivity of peak EF to the choices of transport approach for both tracers and heat. We obtain a peak EF of 4.75 which, while over 50% higher than any prior model, remains well short of the observed value. We show that the polar EF value in winter is primarily determined by the competition between two processes: (1) mean meridional import of lower-latitude air not enriched in argon and (2) the leakage of enriched argon out of the polar column by eddies in the lowest atmospheric levels. We suggest possibilities for improving GCM representation of the CO₂ cycle and the general circulation that may further improve the simulation of the argon cycle. We conclude that current GCMs may be insufficient for detailed simulation of transport-sensitive problems like the water cycle and potentially also the dust cycle.  相似文献   

Experimental determination of photostability and fluorescence‐based detection of PAHs on the Martian surface     

Lewis R. DARTNELL  Manish R. PATEL  Michael C. STORRIE‐LOMBARDI  John M. WARD  Jan‐Peter MULLER 《Meteoritics & planetary science》2012,47(5):806-819

Abstract– Even in the absence of any biosphere on Mars, organic molecules, including polycyclic aromatic hydrocarbons (PAHs), are expected on its surface due to delivery by comets and meteorites of extraterrestrial organics synthesized by astrochemistry, or perhaps in situ synthesis in ancient prebiotic chemistry. Any organic compounds exposed to the unfiltered solar ultraviolet spectrum or oxidizing surface conditions would have been readily destroyed, but discoverable caches of Martian organics may remain shielded in the subsurface or within surface rocks. We have studied the stability of three representative polycyclic aromatic hydrocarbons (PAHs) in a Mars chamber, emulating the ultraviolet spectrum of unfiltered sunlight under temperature and pressure conditions of the Martian surface. Fluorescence spectroscopy is used as a sensitive indicator of remaining PAH concentration for laboratory quantification of molecular degradation rates once exposed on the Martian surface. Fluorescence‐based instrumentation has also been proposed as an effective surveying method for prebiotic organics on the Martian surface. We find the representative PAHs, anthracene, pyrene, and perylene, to have persistence half‐lives once exposed on the Martian surface of between 25 and 60 h of noontime summer UV irradiation, as measured by fluorescence at their peak excitation wavelength. This equates to between 4 and 9.6 sols when the diurnal cycle of UV light intensity on the Martian surface is taken into account, giving a substantial window of opportunity for detection of organic fluorescence before photodegradation. This study thus supports the use of fluorescence‐based instrumentation for surveying recently exposed material (such as from cores or drill tailings) for native Martian organic molecules in rover missions.  相似文献   

Constraining albedo,diameter and composition of near-Earth asteroids via near-infrared spectroscopy     

Vishnu Reddy  Michael J. Gaffey  Paul A. Abell  Paul S. Hardersen 《Icarus》2012,219(1):382-392

We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5 μm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed.  相似文献   

Extending the Capture Map Concept to Estimate Discrete and Risk-Based Streamflow Depletion Potential     

Jeremy T. White  Linzy K. Foster  Michael N. Fienen 《Ground water》2021,59(4):571-580

A popular and contemporary use of numerical groundwater models is to estimate the discrete relation between groundwater extraction and surface-water/groundwater exchange. Previously, the concept of a “capture map” has been put forward as a means to effectively summarize this relation for decision-making consumption. While capture maps have enjoyed success in the environmental simulation industry, they are deterministic, ignoring uncertainty in the underlying model. Furthermore, capture maps are not typically calculated in a manner that facilitates analysis of varying combinations of extraction locations and/or reaches. That is, they are typically constructed with focus on a single reach or group of reaches. The former of these limitations is important for conveying risk to decision makers and stakeholders, while the latter is important for decision-making support related to surface-water management, where future foci may include reaches that were not the focus of the original capture analysis. Herein, we use the concept of a response matrix to generalize the theory of the capture-map approach to estimate spatially discrete streamflow depletion potential. We also use first-order, second-moment uncertainty estimation techniques with the concept of “risk shifting” to place capture maps and streamflow depletion potential in a stochastic, risk-based framework. Our approach is demonstrated for an integrated groundwater/surface-water model of the lower San Antonio River, Texas, USA.  相似文献   

The Effects of Fault-Zone Cementation on Groundwater Flow at the Field Scale     

Tyler G. Sproule  Glenn A. Spinelli  John L. Wilson  Michael D. Fort  Peter S. Mozley  Jared Ciarico 《Ground water》2021,59(3):396-409

Fault zones are an important control on fluid flow, affecting groundwater supply, contaminant migration, and carbon storage. However, most models of fault seal do not consider fault zone cementation, despite the recognition that it is common and can dramatically reduce permeability. In order to study the field-scale hydrogeologic effects of fault zone cementation, we conducted a series of aquifer pumping tests in wells installed within tens of meters of the variably cemented Loma Blanca Fault, a normal fault in the Rio Grande Rift. In the southern half of the study area, the fault zone is cemented by calcite; the cemented zone is 2-8 m wide. In the center of the study area, the cemented fault zone is truncated at a buttress unconformity that laterally separates hydrostratigraphic units with a ∼40X difference in permeability. The fault zone north of the unconformity is not cemented. Constant rate pumping tests indicate that where the fault is cemented, it is a barrier to groundwater flow. This is an important demonstration that a fault with no clay in its core and similar sediment on both sides can be a barrier to groundwater flow by virtue of its cementation; most conceptual models for the hydrogeology of faults would predict that it would not be a barrier to groundwater flow. Additionally, the lateral permeability heterogeneity across the unconformity imposes another important control on the local flow field. This permeability discontinuity acts as either a no-flow boundary or a constant head boundary, depending on the location of pumping.  相似文献   

Channel response to a dam-removal sediment pulse captured at high-temporal resolution using routine gage data     

Matthew J. Cashman  Allen C. Gellis  Eric Boyd  Mathias J. Collins  Scott W. Anderson  Brett D. McFarland  Ashley M. Ryan 《地球表面变化过程与地形》2021,46(6):1145-1159

In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge (~7.8 km yr⁻¹) and peak (~2.6 km yr⁻¹) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre-pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later. The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment-transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process-factors largely independent of high flow. In contrast, persistent changes occurred to out-of-channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Service-indicated flood stages being attained by 3–43% smaller discharges compared to earlier in the study period. This study captured a two-signal response from the sediment pulse: (1) short- to medium-term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer-term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales.  相似文献   

Modelling of sediment supply from torrent catchments in the Western Alps using the sediment contributing area (SCA) approach     

Moritz Altmann  Florian Haas  Tobias Heckmann  Frédéric Liébault  Michael Becht 《地球表面变化过程与地形》2021,46(5):889-906

The production of coarse sediment in mountain landscapes depends mainly on the type and activity of geomorphic processes and topographic and natural conditions (e.g. vegetation cover) of these catchments. The supply of sediment from these slopes to mountain streams and its subsequent transport lead to sediment connectivity, which describes the integrated coupled state of these systems. Studies from the Northern Calcareous Alps show that the size of the sediment contributing area (SCA), a subset of the drainage area that effectively delivers sediment to the channel network, can be used as a predictor of sediment delivery to mountain streams. The SCA concept is delineated on a digital elevation model (DEM) using a set of rules related to the steepness and length of slopes directly adjacent to the channel network, the gradient of the latter and the vegetation cover. The present study investigates the applicability of this concept to the Western Alps to identify geomorphologically active areas and to estimate mean annual sediment yield (SY) in mainly debris-flow-prone catchments. We use a statistical approach that shows a parameter optimisation and a linear regression of SY on SCA extent. We use a dataset of ~25 years of assessed coarse sediment accumulation in 35 sediment retention basins. In the investigated catchments, sediment transport is governed by several factors, mainly by the extent of vegetation-free areas with a minimum slope of 23° that is coupled to the channel network with a very low gradient of the latter. With our improved framework, we can show that the SCA approach can be applied to catchments that are widely distributed, in a large spatial scale (hectare area) and very heterogeneous in their properties. In general, the investigated catchments show high connectivity, resulting in significant correlations between long-term average yield and the size of the SCA.  相似文献   

Effects of pipe outlet blocking on hydrological functioning in a degraded blanket peatland     

Taco H. Regensburg  Pippa J. Chapman  Michael G. Pilkington  David M. Chandler  Martin G. Evans  Joseph Holden 《水文研究》2021,35(3):e14102

Peatland restoration practitioners are keen to understand the role of drainage via natural soil pipes, especially where erosion has released large quantities of fluvial carbon in stream waters. However, little is known about pipe-to-stream connectivity and whether blocking methods used to impede flow in open ditch networks and gullies also work on pipe networks. Two streams in a heavily degraded blanket bog (southern Pennines, UK) were used to assess whether impeding drainage from pipe networks alters the streamflow responses to storm events, and how such intervention affects the hydrological functioning of the pipe network and the surrounding peat. Pipeflow was impeded in half of the pipe outlets in one stream, either by inserting a plug-like structure in the pipe-end or by the insertion of a vertical screen at the pipe outlet perpendicular to the direction of the predicted pipe course. Statistical response variable η² showed the overall effects of pipe outlet blocking on stream responses were small with η² = 0.022 for total storm runoff, η² = 0.097 for peak discharge, η² = 0.014 for peak lag, and η² = 0.207 for response index. Both trialled blocking methods either led to new pipe outlets appearing or seepage occurring around blocks within 90 days of blocking. Discharge from four individual pipe outlets was monitored for 17 months before blocking and contributed 11.3% of streamflow. Pipe outlets on streambanks with headward retreat produced significantly larger peak flows and storm contributions to streamflow compared to pipe outlets that issued onto straight streambank sections. We found a distinctive distance-decay effect of the water table around pipe outlets, with deeper water tables around pipe outlets that issued onto straight streambanks sections. We suggest that impeding pipeflow at pipe outlets would exacerbate pipe development in the gully edge zone, and propose that future pipe blocking efforts in peatlands prioritize increasing the residence time of pipe water by forming surface storage higher up the pipe network.  相似文献   

New snow metrics for a warming world     

Anne W. Nolin  Eric A. Sproles  David E. Rupp  Ryan L. Crumley  Mariana J. Webb  Ross T. Palomaki  Eugene Mar 《水文研究》2021,35(6):e14262

Snow is Earth's most climatically sensitive land cover type. Traditional snow metrics may not be able to adequately capture the changing nature of snow cover. For example, April 1 snow water equivalent (SWE) has been an effective index for streamflow forecasting, but it cannot express the effects of midwinter melt events, now expected in warming snow climates, nor can we assume that station-based measurements will be representative of snow conditions in future decades. Remote sensing and climate model data provide capacity for a suite of multi-use snow metrics from local to global scales. Such indicators need to be simple enough to “tell the story” of snowpack changes over space and time, but not overly simplistic or overly complicated in their interpretation. We describe a suite of spatially explicit, multi-temporal snow metrics based on global satellite data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) and downscaled climate model output for the U.S. We describe and provide examples for Snow Cover Frequency (SCF), Snow Disappearance Date (SDD), At-Risk Snow (ARS), and Frequency of a Warm Winter (FWW). Using these retrospective and prospective snow metrics, we assess the current and future snow-related conditions in three hydroclimatically different U.S. watersheds: the Truckee, Colorado Headwaters, and Upper Connecticut. In the two western U.S. watersheds, SCF and SDD show greater sensitivity to annual differences in snow cover compared with data from the ground-based Snow Telemetry (SNOTEL) network. The eastern U.S. watershed does not have a ground-based network of data, so these MODIS-derived metrics provide uniquely valuable snow information. The ARS and FWW metrics show that the Truckee Watershed is highly vulnerable to conversion from snowfall to rainfall (ARS) and midwinter melt events (FWW) throughout the seasonal snow zone. In comparison, the Colorado Headwaters and Upper Connecticut Watersheds are colder and much less vulnerable through mid- and late-century.  相似文献   

Multi-wavelength observations of Asteroid 2100 Ra-Shalom     

Michael K. Shepard  Beth Ellen Clark  Lance A.M. Benner  Jon D. Giorgini  Kandy Jarvis  Lucy F. Lim  James Bell  Andrew Rivkin  Daniel Scheeres 《Icarus》2008,193(1):20-38

We observed near-Earth asteroid (NEA) 2100 Ra-Shalom over a six-year period, obtaining rotationally resolved spectra in the visible, near-infrared, thermal-infrared, and radar wavelengths. We find that Ra-Shalom has an effective diameter of Deff=2.3±0.2 km, rotation period P=19.793±0.001 h, visual albedo pv=0.13±0.03, radar albedo , and polarization ratio μc=0.25±0.04. We used our radar observations to generate a three-dimensional shape model which shows several structural features of interest. Based on our thermal observations, Ra-Shalom has a high thermal inertia of ∼10³ J m⁻² s^−0.5 K⁻¹, consistent with a coarse or rocky surface and the inferences of others [Harris, A.W., Davies, J.K., Green, S.F., 1998. Icarus 135, 441-450; Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116-130]. Our spectral data indicate that Ra-Shalom is a K-class asteroid and we find excellent agreement between our spectra and laboratory spectra of the CV3 meteorite Grosnaja. Our spectra show rotation-dependent variations consistent with global variations in grain size. Our radar observations show rotation-dependent variations in radar albedo consistent with global variations in the thickness of a relatively thin regolith.  相似文献