Evidence of atmospheric sulphur in the martian regolith from sulphur isotopes in meteorites

Sulphur is abundant at the martian surface, yet its origin and evolution over time remain poorly constrained. This sulphur is likely to have originated in atmospheric chemical reactions, and so should provide records of the evolution of the martian atmosphere, the cycling of sulphur between the atmosphere and crust, and the mobility of sulphur in the martian regolith. Moreover, the atmospheric deposition of oxidized sulphur species could establish chemical potential gradients in the martian near-surface environment, and so provide a potential energy source for chemolithoautotrophic organisms. Here we present measurements of sulphur isotopes in oxidized and reduced phases from the SNC meteorites--the group of related achondrite meteorites believed to have originated on Mars--together with the results of laboratory photolysis studies of two important martian atmospheric sulphur species (SO2 and H2S). The photolysis experiments can account for the observed sulphur-isotope compositions in the SNC meteorites, and so identify a mechanism for producing large abiogenic 34S fractionations in the surface sulphur reservoirs. We conclude that the sulphur data from the SNC meteorites reflects deposition of oxidized sulphur species produced by atmospheric chemical reactions, followed by incorporation, reaction and mobilization of the sulphur within the regolith.     

Water and the martian landscape.

Over the past 30 years, the water-generated landforms and landscapes of Mars have been revealed in increasing detail by a succession of spacecraft missions. Recent data from the Mars Global Surveyor mission confirm the view that brief episodes of water-related activity, including glaciation, punctuated the geological history of Mars. The most recent of these episodes seems to have occurred within the past 10 million years. These new results are anomalous in regard to the prevailing view that the martian surface has been continuously extremely cold and dry, much as it is today, for the past 3.9 billion years. Interpretations of the new data are controversial, but explaining the anomalies in a consistent manner leads to potentially fruitful hypotheses for understanding the evolution of Mars in relation to Earth.     

Groundwater formation of martian valleys

The martian surface shows large outflow channels, widely accepted as having been formed by gigantic floods that could have occurred under climatic conditions like those seen today. Also present are branching valley networks that commonly have tributaries. These valleys are much smaller than the outflow channels and their origins and ages have been controversial. For example, they might have formed through slow erosion by water running across the surface, either early or late in Mars' history, possibly protected from harsh conditions by ice cover. Alternatively, they might have formed through groundwater or ground-ice processes that undermine the surface and cause collapse, again either early or late in Mars' history. Long-duration surface runoff would imply climatic conditions quite different from the present environment. Here we present high-resolution images of martian valleys that support the view that ground water played an important role in their formation, although we are unable as yet to establish when this occurred.     

Santa Catharina and the origin of cloudy taenite in meteorites

CLOUDY taenite appears as a dark-etching rim that occurs at the high nickel border of taenite fields in iron meteorites and mesosiderites. Transmission electron microscopy (TEM) shows it to consist of a foam-like association of interpenetrating single crystals of taenite and kamacite when well developed(1) with an orientation relationship between those of the Kurdjumow-Sachs and the Nishiyama(2). It occurs between compositional limits that have been reported as approximately 25-40% Ni(1) and 30-40% Ni(3). Observed taenite particle sizes range from 0.4 microm in the Estherville mesosiderite(1) to considerably less than 0.1 microm for many iron meteorites (unpublished data). The mechanism of cloudy taenite formation has not been previously explained, although it is believed(2) to be a phenomenon related to the Invar effect, which occurs in Fe-35% Ni (Invar) and several other alloy systems. Santa Catharina is a wholly taenitic anomalous iron meteorite that contains 35% Ni and 0.6% Co (ref. 4). This composition, which is very similar to that of Invar, places it well inside the range where cloudy taenite is known to form. The optical microstructure of Santa Catharina suggested that the matrix phase of the entire meteorite may consist of cloudy taenite, so I have now examined it by TEM and the results are reported here.     

A topographically forced asymmetry in the martian circulation and climate

Large seasonal and hemispheric asymmetries in the martian climate system are generally ascribed to variations in solar heating associated with orbital eccentricity. As the orbital elements slowly change (over a period of >104 years), characteristics of the climate such as dustiness and the vigour of atmospheric circulation are thought to vary, as should asymmetries in the climate (for example, the deposition of water ice at the northern versus the southern pole). Such orbitally driven climate change might be responsible for the observed layering in Mars' polar deposits by modulating deposition of dust and water ice. Most current theories assume that climate asymmetries completely reverse as the angular distance between equinox and perihelion changes by 180 degrees. Here we describe a major climate mechanism that will not precess in this way. We show that Mars' global north-south elevation difference forces a dominant southern summer Hadley circulation that is independent of perihelion timing. The Hadley circulation, a tropical overturning cell responsible for trade winds, largely controls interhemispheric transport of water and the bulk dustiness of the atmosphere. The topography therefore imprints a strong handedness on climate, with water ice and the active formation of polar layered deposits more likely in the north.     

Compositional differences between meteorites and near-Earth asteroids

Understanding the nature and origin of the asteroid population in Earth's vicinity (near-Earth asteroids, and its subset of potentially hazardous asteroids) is a matter of both scientific interest and practical importance. It is generally expected that the compositions of the asteroids that are most likely to hit Earth should reflect those of the most common meteorites. Here we report that most near-Earth asteroids (including the potentially hazardous subset) have spectral properties quantitatively similar to the class of meteorites known as LL chondrites. The prominent Flora family in the inner part of the asteroid belt shares the same spectral properties, suggesting that it is a dominant source of near-Earth asteroids. The observed similarity of near-Earth asteroids to LL chondrites is, however, surprising, as this meteorite class is relatively rare ( approximately 8 per cent of all meteorite falls). One possible explanation is the role of a size-dependent process, such as the Yarkovsky effect, in transporting material from the main belt.     

Orbital forcing of the martian polar layered deposits

Since the first images of polar regions on Mars revealed alternating bright and dark layers, there has been speculation that their formation might be tied to the planet's orbital climate forcing. But uncertainties in the deposition timescale exceed two orders of magnitude: estimates based on assumptions of dust deposition, ice formation and sublimation, and their variations with orbital forcing suggest a deposition rate of 10(-3) to 10(-2) cm yr(-1) (refs 5, 6), whereas estimates based on cratering rate result in values as high as 0.1 to 0.2 cm yr(-1) (ref. 7). Here we use a combination of high-resolution images of the polar layered terrains, high-resolution topography and revised calculations of the orbital and rotational parameters of Mars to show that a correlation exists between ice-layer radiance as a function of depth (obtained from photometric data of the images of the layered terrains) and the insolation variations in summer at the martian north pole, similar to what has been shown for palaeoclimate studies of the Earth. For the best fit between the radiance profile and the simulated insolation parameters, we obtain an average deposition rate of 0.05 cm yr(-1) for the top 250 m of deposits on the ice cap of the north pole of Mars.     

An integrated view of the chemistry and mineralogy of martian soils

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.     

Phyllosilicates on Mars and implications for early martian climate

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected-phyllosilicates and sulphates--result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed.     

The Borealis basin and the origin of the martian crustal dichotomy

The most prominent feature on the surface of Mars is the near-hemispheric dichotomy between the southern highlands and northern lowlands. The root of this dichotomy is a change in crustal thickness along an apparently irregular boundary, which can be traced around the planet, except where it is presumably buried beneath the Tharsis volcanic rise. The isostatic compensation of these distinct provinces and the ancient population of impact craters buried beneath the young lowlands surface suggest that the dichotomy is one of the most ancient features on the planet. However, the origin of this dichotomy has remained uncertain, with little evidence to distinguish between the suggested causes: a giant impact or mantle convection/overturn. Here we use the gravity and topography of Mars to constrain the location of the dichotomy boundary beneath Tharsis, taking advantage of the different modes of compensation for Tharsis and the dichotomy to separate their effects. We find that the dichotomy boundary along its entire path around the planet is accurately fitted by an ellipse measuring approximately 10,600 by 8,500 km, centred at 67 degrees N, 208 degrees E. We suggest that the elliptical nature of the crustal dichotomy is most simply explained by a giant impact, representing the largest such structure thus far identified in the Solar System.     

Extraterrestrial meteors: a martian meteor and its parent comet

Regular meteor showers occur when a planet approaches the orbit of a periodic comet--for example, the Leonid shower is evident around 17 November every year as Earth skims past the dusty trail of comet Tempel-Tuttle. Such showers are expected to occur on Mars as well, and on 7 March last year, the panoramic camera of Spirit, the Mars Exploration Rover, revealed a curious streak across the martian sky. Here we show that the timing and orientation of this streak, and the shape of its light curve, are consistent with the existence of a regular meteor shower associated with the comet Wiseman-Skiff, which could be characterized as martian Cepheids.     

Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations--particularly the suggested seasonal and latitudinal variations--and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet's surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.     

Evidence for an ancient martian ocean in the topography of deformed shorelines

A suite of observations suggests that the northern plains of Mars, which cover nearly one third of the planet's surface, may once have contained an ocean. Perhaps the most provocative evidence for an ancient ocean is a set of surface features that ring the plains for thousands of kilometres and that have been interpreted as a series of palaeoshorelines of different age. It has been shown, however, that topographic profiles along the putative shorelines contain long-wavelength trends with amplitudes of up to several kilometres, and these trends have been taken as an argument against the martian shoreline (and ocean) hypothesis. Here we show that the long-wavelength topography of the shorelines is consistent with deformation caused by true polar wander--a change in the orientation of a planet with respect to its rotation pole--and that the inferred pole path has the geometry expected for a true polar wander event that postdates the formation of the massive Tharsis volcanic rise.     

Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum

The high-pressure solubility in silicate liquids of moderately siderophile 'iron-loving' elements (such as nickel and cobalt) has been used to suggest that, in the early Earth, an equilibrium between core-forming metals and the silicate mantle was established at the bottom of a magma ocean. But observed concentrations of the highly siderophile elements--such as the platinum-group elements platinum, palladium, rhenium, iridium, ruthenium and osmium--in the Earth's upper mantle can be explained by such a model only if their metal-silicate partition coefficients at high pressure are orders of magnitude lower than those determined experimentally at one atmosphere (refs 3-8). Here we present an experimental determination of the solubility of palladium and platinum in silicate melts as a function of pressure to 16 GPa (corresponding to about 500 km depth in the Earth). We find that both the palladium and platinum metal-silicate partition coefficients, derived from solubility, do not decrease with pressure--that is, palladium and platinum retain a strong preference for the metal phase even at high pressures. Consequently the observed abundances of palladium and platinum in the upper mantle seem to be best explained by a 'late veneer' addition of chondritic material to the upper mantle following the cessation of core formation.     

Implications of an impact origin for the martian hemispheric dichotomy

The observation that one hemisphere of Mars is lower and has a thinner crust than the other (the 'martian hemispheric dichotomy') has been a puzzle for 30 years. The dichotomy may have arisen as a result of internal mechanisms such as convection. Alternatively, it may have been caused by one or several giant impacts, but quantitative tests of the impact hypothesis have not been published. Here we use a high-resolution, two-dimensional, axially symmetric hydrocode to model vertical impacts over a range of parameters appropriate to early Mars. We propose that the impact model, in addition to excavating a crustal cavity of the correct size, explains two other observations. First, crustal disruption at the impact antipode is probably responsible for the observed antipodal decline in magnetic field strength. Second, the impact-generated melt forming the northern lowlands crust is predicted to derive from a deep, depleted mantle source. This prediction is consistent with characteristics of martian shergottite meteorites and suggests a dichotomy formation time approximately 100 Myr after martian accretion, comparable to that of the Moon-forming impact on Earth.     

Iron meteorites as remnants of planetesimals formed in the terrestrial planet region

Iron meteorites are core fragments from differentiated and subsequently disrupted planetesimals. The parent bodies are usually assumed to have formed in the main asteroid belt, which is the source of most meteorites. Observational evidence, however, does not indicate that differentiated bodies or their fragments were ever common there. This view is also difficult to reconcile with the fact that the parent bodies of iron meteorites were as small as 20 km in diameter and that they formed 1-2 Myr earlier than the parent bodies of the ordinary chondrites. Here we show that the iron-meteorite parent bodies most probably formed in the terrestrial planet region. Fast accretion times there allowed small planetesimals to melt early in Solar System history by the decay of short-lived radionuclides (such as 26Al, 60Fe). The protoplanets emerging from this population not only induced collisional evolution among the remaining planetesimals but also scattered some of the survivors into the main belt, where they stayed for billions of years before escaping via a combination of collisions, Yarkovsky thermal forces, and resonances. We predict that some asteroids are main-belt interlopers (such as (4) Vesta). A select few may even be remnants of the long-lost precursor material that formed the Earth.