ALBERT

Description: Changes in observed photometric intensity on a planetary surface are caused by variations in local viewing geometry defined by the radiance incidence, emission, and solar phase angle coupled with a wavelength-dependent surface phase function f(α, λ) which is specific for a given terrain. In this paper we provide preliminary empirical models, based on data acquired inflight, which enable the correction of Moon Mineralogy Mapper (M3) spectral images to a standard geometry with the effects of viewing geometry removed. Over the solar phase angle range for which the M3 data were acquired our models are accurate to a few percent, particularly where thermal emission is not significant. Our models are expected to improve as additional refinements to the calibrations occur, including improvements to the flatfield calibration; improved scattered and stray light corrections; improved thermal model corrections; and the computation of more accurate local incident and emission angles based on surface topography.

Description: The origin mechanisms and geologic evolution of chaotic terrain on Mars are poorly constrained. Iani Chaos, located at the head Ares Vallis, is among the most geomorphologically complex of the chaotic terrains. Its morphology is defined by (1) multiple, 1 to 2 km deep basins, (2) flat-topped, fractured plateaus that are remnants of highland terrain, (3) knobby, fractured remnants of highland terrain, (4) plateaus with a knobby surface morphology, (5) interchaos grooved terrain, (6) interior layered deposits (ILDs), and (7) mantling material. Topography, the observed geomorphology, and measured fracture patterns suggest that the interchaos basins formed as a result of subsurface volume loss and collapse of the crust, likely owing to effusion of groundwater to the surface. Regional patterns in fracture orientation indicate that the basins developed along linear zones of preexisting weakness in the highland crust. Multiple overlapping basins and fracture systems point to multiple stages of collapse at Iani Chaos. Furthermore, the total estimated volume loss from the basins (104 km3) is insufficient to explain erosion of 104–105 km3 of material from Ares Vallis by a single flood. Comparisons with the chronology of Ares Vallis indicate multiple water effusion events from Iani Chaos that span the Hesperian, with termination of activity in the early Amazonian. Recharge of groundwater through preexisting fracture systems may explain this long-lived, but likely episodic, fluvial activity. Late-stage, early to middle Amazonian aqueous processes may have deposited the ILDs. However, the topography data indicate that the ILDs did not form within lacustrine environments.

Description: Carbonaceous chondrites are considered to be the most primitive meteorites that contain some of the earliest formed solar system phases, e.g., Ca-Al-rich inclusions and chondrules. The parent bodies of the carbonaceous chondrites are traditionally known to have experienced aqueous alteration. However, the recent paleomagnetic records of the CV chondrite, Allende, indicate remnant magnetism that suggests that these chondrites possibly evolved on the crustal region of a partially differentiated asteroid having a convective molten iron core. We present results based on our comprehensive numerical simulations of the scenario involving melting and planetary differentiation of asteroids. The possibility of a sizable carbonaceous chondritic crust on these differentiated bodies is explored. The simulations indicate that it could be possible to explain the paleomagnetic records of the chondrites by the dynamo-generated magnetic field from the convective molten iron core. Apart from the slow linear accretion scenario, we envisage a scenario involving two episodes of accretion of the asteroids. An early rapid accretion of the asteroids in the initial stage, perhaps within the initial ∼2 Myr, would produce a sizable molten iron core subsequent to differentiation to produce the required magnetic fields. This would be followed by a slow accretion of consolidated chondritic crust, perhaps over several million years. There is also a possibility of the disrupted expulsion of the chondritic crust by the internal pressures generated by gases released during hydration/dehydration reactions.

Description: In the near-infrared from about 2 μm to beyond 3 μm, the light from the Moon is a combination of reflected sunlight and emitted thermal emission. There are multiple complexities in separating the two signals, including knowledge of the local solar incidence angle due to topography, phase angle dependencies, emissivity, and instrument calibration. Thermal emission adds to apparent reflectance, and because the emission's contribution increases over the reflected sunlight with increasing wavelength, absorption bands in the lunar reflectance spectra can be modified. In particular, the shape of the 2 μm pyroxene band can be distorted by thermal emission, changing spectrally determined pyroxene composition and abundance. Because of the thermal emission contribution, water and hydroxyl absorptions are reduced in strength, lowering apparent abundances. It is important to quantify and remove the thermal emission for these reasons. We developed a method for deriving the temperature and emissivity from spectra of the lunar surface and removing the thermal emission in the near infrared. The method is fast enough that it can be applied to imaging spectroscopy data on the Moon.

Description: Mars was warmer and wetter during the early to middle Noachian, before a hydrologic and climatic transition in the late Noachian led to a decrease in erosion rates, a change in valley network morphology, and a geochemical shift from phyllosilicate to sulfate formation that culminated in the formation of widespread sulfate-rich sedimentary deposits in Meridiani Planum and the surrounding Arabia Terra region. This secular evolution was overprinted by episodic and periodic variability, as recorded in the fluvial record, sedimentary layering, and erosional discontinuities. We investigate the temporal evolution of Martian groundwater hydrology during the Noachian and early Hesperian epochs using global-scale hydrological models. The results suggest that the more active hydrological cycle in the Noachian was a result of a greater total water inventory, causing a saturated near-surface and high precipitation rates. The late Noachian hydrologic, climatic, and geochemical transition can be explained by a fundamental shift in the hydrological regime driven by a net loss of water due to impact and solar wind erosion of the atmosphere. Following this transition, the water table retreated deep beneath the surface, except in isolated regions of focused groundwater upwelling and evaporation, producing the playa evaporites in Meridiani Planum and Arabia Terra. This long-term evolution was modulated by shorter-term climate forcing in the form of periodic and chaotic variations in the orbital parameters of Mars, resulting in changes in the volume of water sequestered in the polar caps and cryosphere. This shorter-term forcing can explain the observed periodic and bundled sedimentary layering, erosional unconformities, and evidence for a fluctuating water table at Meridiani Planum.

Description: The Mars Exploration Rovers Spirit and Opportunity investigated the physical properties of Martian regolith in 7 wheel trenches and 20 wheel scuffs distributed along traverses at Gusev crater and Meridiani Planum. Specialized wheel-trenching sequences allowed analysis of wheel motor and suspension telemetry to determine regolith friction angle $\phi$ and cohesion c at trench sites. Friction angles were 30°–37°, and cohesions were 0–2 kPa. Simpler wheel-scuff maneuvers were analyzed for cohesion by assuming the range of $\phi$ determined from trenches; cohesions in wheel-scuffed regoliths were from 0 to 11 kPa. Regolith $\phi$ and c can be related to regolith origins. Grain sorting, compaction, shape, size, and angularity influence $\phi$. Impact cratering and aeolian processes have affected grain angularity and sorting of Martian regolith at both Mars Exploration Rover (MER) landing sites and contend in opposing ways to determine grain characteristics in the regolith. Friction angles are consistent with dry, rigid, nonplaty grains with particle size frequencies dominated by very fine sand (as seen by the Microscopic Imager or MI) with at least some grain rounding (unresolved by MI), reflecting physical weathering from aeolian saltation. Friction angle results from MER trenches therefore indicate that regolith states are between fully mature aeolian materials and impact debris. MI and color Pancam views show trench tailings and trench floors are redder, brighter, and have more intermixed extremely fine (unresolved) grains than regolith closer to the surface disturbed and exposed only by rolling tracks.

Description: A hydrologic routing model has been applied to the Noachian cratered highlands of Mars to establish the climatic conditions required to maintain exit breached lakes on early Mars and the likely fraction of the upland surface that would have hosted lakes whether they overflowed or not. The climatic conditions were expressed as a ratio of net evaporative loss from lakes to the surface runoff from uplands (the “X ratio”). Simulations were conducted using 16 different X ratios. The lake area, volume, and number of overflowing lakes decrease as climate becomes drier (larger X ratio). The modal frequency of the X ratio for the overflow of highland basins with eroded exit breaches was 5.0, which is comparable to that of the Great Basin region in the western United States during the Last Glacial Maximum (LGM). This indicates that lakes on early Mars were likely to have been at least as extensive as those in the Great Basin region during the LGM.

Description: Many regions near the lunar poles are currently cold enough that surface water ice would be stable against sublimation losses for billions of years. However, most of these environments are currently too cold to efficiently drive ice downward by thermal diffusion, leaving impact burial as the primary means of protection from surface loss processes. In this respect, most of the present near-surface thermal environments on the Moon may actually be quite poor traps for water ice. This was not always the case. Long-term orbital changes have dramatically altered the lunar polar thermal environment. We develop a simple model of the evolution of the lunar orbit and spin axis to examine the thermal environments available for volatile deposition and retention in the past. Our calculations show that some early lunar polar environments were in the right temperature regime to have collected subsurface ice if a supply were available. However, a high-obliquity period, which occurred when the Moon was at about half its present distance from the Earth, would either have driven this ice out into space or deep into the lunar subsurface. Since that time, as the lunar obliquity has slowly decreased to its present value, environments have undergone their own thermal evolution, and each of the current cold traps experienced a period when they were most efficient at thermally burying ice. We examine the thermal history of a lunar polar crater to provide a framework for examining other processes effecting volatiles in the Moon's near-surface cold traps.

Description: We model the atmospheric response to a chaos-forming event at Juventae Chasma, north of Valles Marineris, Mars, using the Mars Regional Atmospheric Modeling System (MRAMS). Interactions between lake-driven convergence, topography, and the regional wind field steer lake-induced precipitation to the southwest. Mean snowfall reaches a maximum of 0.9 mm/h water equivalent (peak snowfall 1.7 mm/h water equivalent) on the SW rim of the chasm. More than 80% of vapor released by the lake is trapped in or next to the lake as snow. Radiative effects of the thick cloud cover raise mean plateau surface temperature by up to 18 K locally. We find that the area of maximum modeled precipitation corresponds to the mapped Juventae plateau channel networks. At Echus Chasma, modeled precipitation maxima also correspond to mapped plateau channel networks. This is consistent with the earlier suggestion that Valles Marineris plateau layered deposits and interbedded channel networks result from localized precipitation. However, snowpack thermal modeling shows temperatures below freezing for the 12 mbar CO2 atmosphere used in our MRAMS simulations. This is true even for the most favorable orbital conditions, and whether or not the greenhouse effect of the lake storm is included. Moderately higher CO2 pressures, or non-CO2 greenhouse forcing, is very likely required for melting and plateau channel network formation under a faint young Sun. Required warming is ≤10 K: global temperatures need not be higher than today. In these localized precipitation scenarios, the rest of the planet remains dry.