Possible liquid water origin for Atacama Desert mudflow and recent gully deposits on Mars

Evidence of recent gully activity on Mars has been reported based on the formation of new light toned deposits within the past decade, the origin of which remains controversial. Analogous recent light toned gully features have formed by liquid water activity in the Atacama Desert on Earth. These terrestrial deposits leave no mineralogical trace of water activity but rather show an albedo difference due to particle size sorting within a fine-grained mudflow. Therefore, spectral differences indicating varying mineralogy between a recent gully deposit and the surrounding terrain may not be the most relevant criteria for detecting water flow in arid environments. Instead, variation in particle size between the deposit and surrounding terrain is a possible discriminator to identify a water-based flow. We show that the Atacama deposit is similar to the observed Mars gully deposits, and both are consistent with liquid water activity. The light-toned Mars gully deposits could have formed from dry debris flows, but a liquid water origin cannot be ruled out because not all liquid water flows leave hydrated minerals behind on the surface. Therefore, the Mars deposits could be remnant mudflows that formed on Mars within the last decade.     

Volatile retention from cometary impacts on the Moon

Impacts of comets and asteroids play an important role in volatile delivery on the Moon. We use a novel method for tracking vapor masses that reach escape velocity in hydrocode simulations of cometary impacts to explore the effects of volatile retention. We model impacts on the Moon to find the mass of vapor plume gravitationally trapped on the Moon as a function of impact velocity. We apply this result to the impactor velocity distribution and find that the total impactor mass retained on the Moon is approximately 6.5% of the impactor mass flux. Making reasonable assumptions about water content of comets and the comet size-frequency distribution, we derive a water flux for the Moon. After accounting for migration and stability of water ice at the poles, we estimate a total 1.3×10⁸-4.3×10⁹ metric tons of water is delivered to the Moon and remains stable at the poles over 1 Ga. A factor of 30 uncertainty in the estimated cometary impact flux is primarily responsible for this large range of values. The calculated mass of water is sufficient to account for the neutron fluxes poleward of 75° observed by Lunar Prospector. A similar analysis for water delivery to the Moon via asteroid impacts shows that asteroids provide six times more water mass via impacts than comets.     

Sorted stone circles in Elysium Planitia, Mars: Implications for recent martian climate

We have found sorted stone circles and polygons near the equator of Mars, using new 25 cm/pixel NASA HiRISE (High Resolution Imaging Science Experiment) images. The sorted circles occur in geologically recent catastrophic flood deposits in the equatorial Elysium Planitia region, and are diagnostic of periglacial processes: sorted polygons do not form from volcanic activity, as has been suggested for non-sorted polygons in this region. These landforms indicate that (i) a long-lived, geologically recent, active cryoturbation layer of ground ice was present in the regolith, (ii) there was some degree of freeze-thaw, and thus (iii) there were sustained period(s), likely within the last 10 Ma, in which the martian climate was 40 to 60 K warmer than current models predict.     

Lunar intrusive domes: Morphometric analysis and laccolith modelling

This study examines a set of lunar domes with very low flank slopes which differ in several respects from the frequently occurring lunar effusive domes. Some of these domes are exceptionally large, and most of them are associated with faults or linear rilles of presumably tensional origin. Accordingly, they might be interpreted as surface manifestations of laccolithic intrusions formed by flexure-induced vertical uplift of the lunar crust (or, alternatively, as low effusive edifices due to lava mantling of highland terrain, or kipukas, or structural features). All of them are situated near the borders of mare regions or in regions characterised by extensive effusive volcanic activity. Clementine multispectral UVVIS imagery indicates that they do not preferentially occur in specific types of mare basalt. Our determination of their morphometric properties, involving a combined photoclinometry and shape from shading technique applied to telescopic CCD images acquired at oblique illumination, reveals large dome diameters between 10 and more than 30 km, flank slopes below 0.9°, and volumes ranging from 0.5 to 50 km³. We establish three morphometric classes. The first class, In1, comprises large domes with diameters above 25 km and flank slopes of 0.2°-0.6°, class In2 is made up by smaller and slightly steeper domes with diameters of 10-15 km and flank slopes between 0.4° and 0.9°, and domes of class In3 have diameters of 13-20 km and flank slopes below 0.3°. While the morphometric properties of several candidate intrusive domes overlap with those of some classes of effusive domes, we show that a possible distinction criterion are the characteristic elongated outlines of the candidate intrusive domes. We examine how they differ from typical effusive domes of classes 5 and 6 defined by Head and Gifford [Head, J.W., Gifford, A., 1980. Lunar mare domes: classification and modes of origin. Moon Planets 22, 235-257], and show that they are likely no highland kipukas due to the absence of spectral contrast to their surrounding. These considerations serve as a motivation for an analysis of the candidate intrusive domes in terms of the laccolith model by Kerr and Pollard [Kerr, A.D., Pollard, D.D., 1998. Toward more realistic formulations for the analysis of laccoliths. J. Struct. Geol. 20(12), 1783-1793], to estimate the geophysical parameters, especially the intrusion depth and the magma pressure, which would result from the observed morphometric properties. Accordingly, domes of class In1 are characterised by intrusion depths of 2.3-3.5 km and magma pressures between 18 and 29 MPa. For the smaller and steeper domes of class In2 the magma intruded to shallow depths between 0.4 and 1.0 km while the inferred magma pressures range from 3 to 8 MPa. Class In3 domes are similar to those of class In1 with intrusion depths of 1.8-2.7 km and magma pressures of 15-23 MPa. As an extraordinary feature, we describe in some detail the concentric crater Archytas G associated with the intrusive dome Ar1 and discuss possible modes of origin. In comparison to the candidate intrusive domes, terrestrial laccoliths tend to be smaller, but it remains unclear if this observation is merely a selection effect due to the limited resolution of our telescopic CCD images. An elongated outline is common to many terrestrial laccoliths and the putative lunar laccoliths, while the thickness values measured for terrestrial laccoliths are typically higher than those inferred for lunar laccoliths, but the typical intrusion depths are comparable.     

Spin rates of fast-rotating asteroids and fragments in impact disruption

We present a new experimental result of fragment spin-rate in impact disruption, using a thin glass plate. A cylindrical projectile impacts on a side (edge) of the plate. Dispersed fragments are observed using a high-speed camera and the spin rates of fragments are measured. We find that the measured fragment spin-rate decreases with increasing size. Assuming that the rotational energy of fragments is supplied from the residual stress, the spin rate ω decreases with increasing fragment size r as ω∼r−1, which explains the above experimental results. This size-dependence is similar to that of the observed spin rates of small fast-rotating asteroids. Our results suggest that spin rates of fragments of small asteroids immediately after disruption may have a similar size-dependence, and can provide constraints on the subsequent spin-state evolution of small asteroids due to thermal torques.     

New models of interstellar gas–grain chemistry – III. Solid CO2

Solid CO₂ is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO₂ at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO₂ can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.     

Electron-impact rotational excitation of linear molecular ions

Molecular R -matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO⁺, HCO⁺, NO⁺ and H₂⁺ up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Δ j =1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Δ j >1 are found to have appreciable rates and are found to be entirely dominated by short-range effects.     

Hydrocarbon lakes on Titan

The Huygens Probe detected dendritic drainage-like features, methane clouds and a high surface relative humidity (∼50%) on Titan in the vicinity of its landing site [Tomasko, M.G., and 39 colleagues, 2005. Nature 438, 765-778; Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784], suggesting sources of methane that replenish this gas against photo- and charged-particle chemical loss on short (10-100) million year timescales [Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C., Fulchignoni, M., Ferri, F., Wilson, E.H., 2006. Planet. Space Sci. In press]. On the other hand, Cassini Orbiter remote sensing shows dry and even desert-like landscapes with dunes [Lorenz, R.D., and 39 colleagues, 2006a. Science 312, 724-727], some areas worked by fluvial erosion, but no large-scale bodies of liquid [Elachi, C., and 34 colleagues, 2005. Science 308, 970-974]. Either the atmospheric methane relative humidity is declining in a steady fashion over time, or the sources that maintain the relative humidity are geographically restricted, small, or hidden within the crust itself. In this paper we explore the hypothesis that the present-day methane relative humidity is maintained entirely by lakes that cover a small part of the surface area of Titan. We calculate the required minimum surface area coverage of such lakes, assess the stabilizing influence of ethane, and the implications for moist convection in the atmosphere. We show that, under Titan's surface conditions, methane evaporates rapidly enough that shorelines of any existing lakes could potentially migrate by several hundred m to tens of km per year, rates that could be detected by the Cassini orbiter. We furthermore show that the high relative humidity of methane in Titan's lower atmosphere could be maintained by evaporation from lakes covering only 0.002-0.02 of the whole surface.     

Valley formation on martian volcanoes in the Hesperian: Evidence for melting of summit snowpack, caldera lake formation, drainage and erosion on Ceraunius Tholus

Ceraunius Tholus, a Hesperian-aged volcano in the Tharsis region, is characterized by small radial valleys on its flanks, and several larger valleys originating near its summit caldera. All of these large valleys drain from near the lowest present portion of the caldera rim and down the flanks of the volcano. The largest valley debauches into Rahe Crater (an oblique impact crater), forming a depositional fan. Recent study of climate change on Mars suggests that many low-latitude regions (especially large volcanic edifices) were periodically the sites of snow accumulation, likely triggered by variations in spin orbital parameters. We apply a conductive heat flow model to Ceraunius Tholus that suggests that following magmatic intrusion, sufficient heating would be available to cause basal melting of any accumulated summit snowpack and produce sufficient meltwater to cause the radial valleys. The geometry of the volcano summit caldera suggests that meltwater would also accumulate in a volumetrically significant caldera lake. Analysis of the morphology and volumes of the largest valley, as well as depositional features at its base, suggest that fluvial erosion due to drainage of this summit caldera lake formed the large valleys, in a manner analogous to how valleys were formed catastrophically from a lake in Aniakchak caldera in Alaska. Moreover, the event which carved the largest valley on Ceraunius Tholus appears to have led to the formation of a temporary lake within Rahe Crater, at its base. The more abundant, small valleys on the flanks are interpreted to form by radial drainage of melted ice or snow from the outside of the caldera rim. Comparison of Ceraunius Tholus with the volcano-capping Icelandic ice sheet Myrdalsjokull provides insight into the detailed mechanisms of summit heating, ice-cap accumulation and melting, and meltwater drainage. These observations further underline the importance of a combination of circumstances (i.e., climate change to produce summit snowpack and an active period of magmatism to produce melting) to form the valley systems on some martian volcanoes and not on others.     

High outflow channels on Mars indicate Hesperian recharge at low latitudes and the presence of Canyon Lakes

A number of martian outflow channels were carved by discharges from large dilational fault zones. These channels were sourced by groundwater, not surface water, and when observed on high-standing plateaus they provide indicators of elevated paleo-groundwater levels. We identify three outflow channels of Hesperian age that issued from a 750-km-long fault zone extending from Candor Chasma to Ganges Chasma. Two of these channels, Allegheny Vallis and Walla Walla Vallis, have sources >2500 m above the topographic datum, too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region. The indicated groundwater levels likely required regional sources of recharge at low latitudes. The floodwaters that erupted from Ophir Cavus to form Allegheny Vallis encountered two ridges that restricted the flow, forming temporary lakes. The flow probably breached or overtopped these obstructions quickly, catastrophically draining the lakes and carving several scablands. After the last obstacle had been breached, a single main channel formed that captured all subsequent flow. We performed hydrologic analyses of this intermediate phase of the flooding, prior to incision of the channel to its present depth. Using floodwater depths of 30-60 m, we calculated flow velocities of 6-15 m s⁻¹ and discharges in the range of . Locally higher flow velocities and discharges likely occurred when the transient lakes were drained. Variable erosion at the channel and scabland crossing of MOLA pass 10644 suggests that the upper 25-30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock. We infer that an ice-covered lake with a surface elevation >2500 m probably existed in eastern Candor Chasma because this canyon is intersected by the Ophir Catenae fault system from which Allegheny Vallis and Walla Walla Vallis originated. We introduce a new hydrology concept for Mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in Tharsis by effusions of flood basalts. This model could help to reconcile the expected presence of a thick cryosphere during the Hesperian with the abundant evidence for groundwater as a source for some of the circum-Chryse outflow channels.