Scenarios for the formation of Chasma Boreale, Mars

The martian polar caps feature large chasmata and smaller trough systems which have no counterpart in terrestrial ice sheets. Chasma Boreale cuts about 500 km into the western part of the north-polar cap, is up to 100 km wide and up to 2 km deep. One possible formation mechanism is by a temporary heat source under the ice due to tectono-thermal or volcanic activity, which melts the ice from below. It is demonstrated by model simulations that this process is feasible, a moderately increased heat flux of 0.5-1 W m⁻², sustained over at least tens of thousands of years, producing a topographic depression which resembles the real chasma. Associated meltwater discharge rates are small (), but can exceed 10 km³ a⁻¹ if a stronger heat flux of 10 W m⁻² is assumed. Local ice-flow velocities during the process of chasma formation can exceed 1 m a⁻¹ at the head and scarps of the chasma. However, if the thermal anomaly shuts down, glacial flow quickly decreases, so that the chasma can stay open for an indefinite amount of time without an ongoing, sustaining process under the climate conditions of the most recent millions of years.     

Evidence for ice flow prior to trough formation in the martian north polar layered deposits

The relative importance of surface mass fluxes and ice flow in shaping the north polar layered deposits (NPLD), now or in the past, remains a fundamental and open question. Motivated by observation of an apparent ice divide on Gemina Lingula (also known as Titania Lobe), we propose a two-stage evolution leading to the present-day topography on that lobe of the NPLD. Ice flow approximately balances surface mass fluxes in the first stage, but in the second stage ice flow has minimal influence and topography is modified predominantly by the formation of troughs. We focus here on evidence for the first stage, by testing the fit of topography between troughs to an ice-flow model. We find that independent model fits on distinct flow paths closely match inter-trough topography, uniformly over a broad region on Gemina Lingula, with mutually consistent and physically reasonable fitting parameters. However, our model requires ice to occupy and flow in spaces where troughs currently incise the ice. We therefore infer that the troughs (and the distribution of mass balance that caused them) post-date deposition of the inter-trough material and its modification by flow. Because trough formation has apparently altered inter-trough topography very little, we infer that trough formation must have been rapid in comparison to the (still unknown) time-scale of flow since troughs began to form. We view the evidence for past flow as strong, but we do not think that topographic evidence alone can be conclusive. Observations of englacial stratigraphy using orbital sounding radars will yield conclusive tests of our inferred mechanism for the formation of inter-trough topography.     

A historical search for habitable ice at the Phoenix landing site

A time-resolved energy balance model in the latitude range targeted by Phoenix, and extending back in time over the past 10 Ma, has been developed and used to predict the time-varying temperature field in ground ice over scales ranging from minutes to millions of years. The temperature history is compared to the population doubling times of terrestrial psychrophiles as a function of temperature, and the lifetime of analog microbe spores against de-activation by galactic cosmic rays (GCR), in order to assess the habitability of ground ice and surrounding materials that may be sampled by Phoenix. Metrics are derived to quantify “habitability” and compare different model configurations, including total and maximum continuous time, per year, that ground ice temperatures exceed various thresholds, maximum and average dormancy periods, and maximum and average consecutive growing seasons. The key unknowns in assessing the position, and hence the temperature, of the ground ice table at high northern latitude is the fate of the perennial north polar cap at high obliquity. If enough H₂O ice can persist at polar latitudes to buffer at least the high-latitude atmosphere at all orbital configurations, ground ice is found to be relatively shallow over much of the past 10 Ma, and regularly achieves temperatures in excess of those required for the growth of terrestrial psychrophiles. The dry overburden expected at the landing site can easily be sampled by Phoenix, and includes the “sweet spot” that is characterized by the optimal habitability metrics over the past 10 Ma. If the atmosphere is buffered only by low-latitude ice deposits at obliquities greater than about 30°, the frequency and duration of habitable ice is considerably diminished, and the intervening dormancy periods, during which cosmic ray damage accumulates, are correspondingly longer. In all cases, the maximum dormancy period that must be survived by putative martian psychrophiles is at least an order of magnitude greater than the amount of time required to reduce terrestrial psychrophile spore viability by 10⁻⁶ (∼7×¹⁰⁴ years). Depending on the fate of high-obliquity polar ice, the maximum dormancy period can exceed 4×¹⁰⁶ years, a factor of 60 longer than terrestrial psychrophile spore lifetimes. Habitability of martian ground ice is therefore dependent on putative martian psychrophiles developing robustness against GCR deactivation at least an order of magnitude greater than their terrestrial counterparts. Simulations of ground ice throughout the 65° N-72° N latitude range accessible to Phoenix suggest that higher-latitude ground ice has better habitability metrics, although the discrepancy is less than an order of magnitude for all metrics and across the entire latitude range.     

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (Abrt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian (1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, Abb_c) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, Abb₁). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (Abb_c). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.     

Albedo models for the residual south polar cap on Mars: Implications for the stability of the cap under near-perihelion global dust storm conditions

It is uncertain whether the residual (perennial) south polar cap on Mars is a transitory or a permanent feature in the current Martian climate. While there is no firm evidence for complete disappearance of the cap in the past, clearly observable changes have been documented. Observations suggest that the perennial cap lost more CO₂ material in the spring/summer season prior to the Mariner 9 mission than in those same seasons monitored by Viking and Mars Global Surveyor. In this paper we examine one process that may contribute to these changes—the radiative effects of a planet encircling dust storm that starts during late Martian southern spring on the stability of the perennial south polar cap. To approach this, we model the radiative transfer through a dusty planetary atmosphere bounded by a sublimating CO₂ surface.A critical parameter for this modeling is the surface albedo spectrum from the near-UV to the thermal-IR, which was determined from both space-craft and Earth-based observations covering multiple wavelength regimes. Such a multi-wavelength approach is highly desirable since one spectral band by itself cannot tightly constrain the three-parameter space for polar surface albedo models, namely photon “scattering length” in the CO₂ ice and the amounts of intermixed water and dust.Our results suggest that a planet-encircling dust storm with onset near solstice can affect the perennial cap's stability, leading to advanced sublimation in a “dusty” year. Since the total amount of solid CO₂ removed by a single storm may be less than the total CO₂ thickness, a series of dust storms would be required to remove the entire residual CO₂ ice layer from the south perennial cap.     

区域金融空间集聚与金融城市体系研究——基于美国外资银行的分析

周期性金融危机表明．大国的区域金融系统越来越融于全球资本流通和金融服务网络体系中,外资银行对东道国区域金融网络的空间组织发挥了重要作用,在促进区域金融流动的同时放大区域金融风险。本文总结了区域金融空间集聚与扩散的理论基础,以外资银行为例,论述了美国金融城市体系的规模和等级结构,分析了美国中心城市金融集聚特征与网络化趋势。     

Analysis of Environmental Bonding System for Oil and Gas Projects

This paper is concerned with potential financial impacts of different bonding instruments on oil and gas projects. An algorithm was prepared in order to assist decision makers, both regulators and industry, evaluate potential Net Present Value (NPV) impacts of financial instruments used to satisfy bonding requirements. Instrument option is the main variable for the proposed model. The user will be able to select between four instruments (Letters of Credit, Prepaid Collateral Closure Accounts, Leasing Specific Closure Accounts, and Ex-post Insurance Policies). This study includes simulations for three producing fields of different economically recoverable reserves (9 MMbbl, 53 MMbbl, and 148 MMbbl⁵), where four financial instruments, in addition to a no instrument scenario, are tested under a proposed bonding regime. Sensitivity analysis of NPV and Government Take (GT) value indicate ex-post insurance policies and letters of credit cause fewer impacts yielding significantly better payoffs. Preliminary simulations also confirm that small projects, around 9 MMbbl, can be severely affected when collateral account instruments are used.     

浅谈地质调查项目绩效评价

随着我国公共财政框架的建立，研究建立符合国家财政改革总体要求的地质调查项目绩效评价制度以及组织实施地质调查项目绩效评价工作已成当务之急。从地质调查项目绩效评价涵义及内容、重要意义、工作思路等方面进行了阐述，并提出相关建议。     

Surcharge preloading consolidation of reclaimed land with distributed sand caps

Abstract

For land reclamation using the conventional surcharge preloading method, a sand cap layer is often fully placed at the ground surface of ultra-soft compressible estuary or marine clays. A novel strategy of distributed sand caps is proposed to save cost associated with sand materials. At an early stage of consolidation, the drainage capacity of distributed sand caps is less than the drainage demand, whereas at a later stage, the capacity exceeds the demand. Hence, the overall drainage capacity of distributed sand caps is equivalent to the drainage demand. A numerical model is developed to study the effectiveness of the proposed technique, and calculations are compared against theoretical solutions. A parametric study is conducted to optimize design parameters. It has been found that distributed sand caps should be uniformly placed with a size of 3% of the total width and a spacing of 1 times the size, such that sand materials can be saved by up to 50% without compromising the consolidation efficiency (an increase of consolidation time by less than 5%). The use of distributed sand caps will be effective regardless of soil properties, including hydraulic conductivity, elastic modulus and Poisson’s ratio.