¹Logan Jaeger et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13747]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, 94720 USA
Published by arrangement with John Wiley & Sons

NASA’s Stardust mission utilized a sample collector composed of aerogel and aluminum foil to return cometary and interstellar particles to Earth. Analysis of the aluminum foil begins with locating craters produced by hypervelocity impacts of cometary and interstellar dust. Interstellar dust craters are typically less than one micrometer in size and are sparsely distributed, making them difficult to find. In this paper, we describe a convolutional neural network based on the VGG16 architecture that achieves high specificity and sensitivity in locating impact craters in the Stardust interstellar collector foils. We evaluate its implications for current and future analyses of Stardust samples.

¹S. M. R. Turner et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13748]
¹AstrobiologyOU, School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Data returned by NASA’s Mars Science Laboratory Curiosity rover showed evidence for abundant secondary materials, including Fe-oxides, phyllosilicates, and an amorphous component on and below Vera Rubin ridge in the Murray formation. We used equilibrium thermochemical modeling to test the hypothesis that altered sediments were deposited as detrital igneous grains and subsequently underwent diagenesis. Chemical compositions of the Murray formations’ altered components were calculated using data returned by the chemistry and mineralogy X-ray diffraction instrument and the alpha particle X-ray spectrometer on board Curiosity. Reaction of these alteration compositions with a CO2-poor and oxidizing dilute aqueous solution was modeled at 25–100 °C, with 10–50% Fe3+/Fetot of the host rock. The modeled alteration assemblages included abundant phyllosilicates and Fe-oxides at water-to-rock ratios >100. Modeled alteration abundances were directly comparable to observed abundances of hematite and clay minerals at a water-to-rock ratio of 10,000, for system temperatures of 50–100 °C with fluid pH ranging from 7.9 to 9.3. Modeling results suggest that the hematite–clay mineral assemblage is primarily the result of enhanced groundwater flow compared to the Sheepbed mudstone observed at Yellowknife Bay, and underwent further, localized alteration to produce the mineralogy observed by Curiosity.

¹Matthew C.Brennan,¹Rebecca A.Fischer,²Francis Nimmo,³David P.O’Brien
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.09.022]
¹Department of Earth and Planetary Sciences, Harvard University (20 Oxford Street, Cambridge, MA 02138, USA)
²Department of Earth and Planetary Sciences, University of California Santa Cruz (1156 High Street, Santa Cruz, CA 95064, USA)
³Planetary Science Institute (1700 East Fort Lowell, Tucson, AZ 85719-2395, USA)
Copyright Elsevier

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars.

We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ∼5 million years of solar system formation to reproduce the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.