Origin of acidic surface waters and the evolution of atmospheric chemistry on early Mars

Observations from in situ experiments and planetary orbiters have shown that the sedimentary rocks found at Meridiani Planum, Mars were formed in the presence of acidic surface waters. The water was thought to be brought to the surface by groundwater upwelling, and may represent the last vestiges of the widespread occurrence of liquid water on Mars. However, it is unclear why the surface waters were acidic. Here we use geochemical calculations, constrained by chemical and mineralogical data from the Mars Exploration Rover Opportunity, to show that Fe oxidation and the precipitation of oxidized iron (Fe^(3+)) minerals generate excess acid with respect to the amount of base anions available in the rocks present in outcrop. We suggest that subsurface waters of near-neutral pH and rich in Fe^(2+) were rapidly acidified as iron was oxidized on exposure to O_2 or photo-oxidized by ultraviolet radiation at the martian surface. Temporal variation in surface acidity would have been controlled by the availability of liquid water, and as such, low-pH fluids could be a natural consequence of the aridification of the martian surface. Finally, because iron oxidation at Meridiani would have generated large amounts of gaseous H_2, ultimately derived from the reduction of H_2O, we conclude that surface geochemical processes would have affected the redox state of the early martian atmosphere

More than Mere Numbers: The Impact of Lethal Control on the Social Stability of a Top-Order Predator

Population control of socially complex species may have profound ecological implications that remain largely invisible if only their abundance is considered. Here we discuss the effects of control on a socially complex top-order predator, the dingo (Canis lupus dingo). Since European occupation of Australia, dingoes have been controlled over much of the continent. Our aim was to investigate the effects of control on their abundance and social stability. We hypothesized that dingo abundance and social stability are not linearly related, and proposed a theoretical model in which dingo populations may fluctuate between three main states: (A) below carrying capacity and socially fractured, (B) above carrying capacity and socially fractured, or (C) at carrying capacity and socially stable. We predicted that lethal control would drive dingoes into the unstable states A or B, and that relaxation of control would allow recovery towards C. We tested our predictions by surveying relative abundance (track density) and indicators of social stability (scent-marking and howling) at seven sites in the arid zone subject to differing degrees of control. We also monitored changes in dingo abundance and social stability following relaxation and intensification of control. Sites where dingoes had been controlled within the previous two years were characterized by low scent-marking activity, but abundance was similar at sites with and without control. Signs of social stability steadily increased the longer an area was allowed to recover from control, but change in abundance did not follow a consistent path. Comparison of abundance and stability among all sites and years demonstrated that control severely fractures social groups, but that the effect of control on abundance was neither consistent nor predictable. Management decisions involving large social predators must therefore consider social stability to ensure their conservation and ecological functioning

Alteration assemblages in Martian meteorites: implications for near-surface processes

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals – which comprise up to 1 vol.% of the meteorites – provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 – 150 °C) brines. This is consistent with the simple mineralogy of these assemblages – Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays – and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event post-dating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last 1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50–250 mbar of CO2 being trapped in the uppermost 200–1000 m of the martian crust. Large fractionations in 18O between igneous silicate in the meteorites and the secondary minerals (30) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (400°C) and have been taken to suggest low temperatures (e.g. 150°C) of precipitation from a hydrous fluid

Age of acquisition, not word frequency affects object recognition: Evidence from the effects of visual degradation

Four experiments examined how age of acquisition (AoA) and word frequency (WF) interact with manipulations of image quality in a picture-naming task. Experiments 1 and 2 examined the effect of overlaying the to-be-named picture with irrelevant contours. The magnitude of the AoA effect increased when the contours were added (Experiment 1), but the effect of WF remained constant (Experiment 2). Experiments 3 and 4 examined the effects of reducing the contrast of the contours defining the to-be-named picture. Both the effects of AoA (Experiment 3) and WF (Experiment 4) remained constant in the face of contrast reduction. These results provide an empirical dissociation of the effects of AoA and WF. The results are consistent with the idea that both AoA and the addition of irrelevant contours affect the efficiency of object recognition, but WF affects later processes involved in retrieval of object names. The theoretical implications of these findings in relation to accounts of AoA and frequency and their functional localisation in the lexical system are discussed. © 2008 Elsevier B.V. All rights reserved

Age-of-Acquisition Effects: A Literature Review

Age of acquisition (AoA) refers to the age at which people learn a particular item and the AoA effect refers to the phenomenon that early-acquired items are processed more quickly and accurately than those acquired later. Over several decades, the AoA effect has been investigated using neuroscientific, behavioral, corpus and computational techniques. We review the current evidence for the AoA effect stemming from a range of methodologies and paradigms and apply these findings to current explanations of how and where the AoA effect occurs. We conclude that the AoA effect can be found both in the connections between levels of representations and within these representations themselves, and that the effect itself occurs through the process of the distinct coding of early and late items, together with the nature of the connections between levels of representation. This approach strongly suggests that the AoA effect results from the construction of perceptual-semantic representations and the mappings between representation