Thermal Alteration of Labile Elements in Carbonaceous Chondrites

Carbonaceous chondrite meteorites are some of the oldest Solar System planetary materials available for study. The CI group has bulk abundances of elements similar to those of the solar photosphere. Of particular interest in carbonaceous chondrite compositions are labile elements, which vaporize and mobilize efficiently during post-accretionary parent-body heating events. Thus, they can record low-temperature alteration events throughout asteroid evolution. However, the precise nature of labile-element mobilization in planetary materials is unknown. Here we characterize the thermally induced movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous chondrites by conducting experimental simulations of volatile-element mobilization during thermal metamorphism. This process results in appreciable loss of some elements at temperatures as low as 500 K. This work builds on previous laboratory heating experiments on primitive meteorites and shows the sensitivity of chondrite compositions to excursions in temperature. Elements such as S and Hg have the most active response to temperature across different meteorite groups. Labile element mobilization in primitive meteorites is essential for quantifying elemental fractionation that occurred on asteroids early in Solar System history. This work is relevant to maintaining a pristine sample from asteroid (101955) Bennu from the OSIRIS-REx mission and constraining the past orbital history of Bennu. Additionally, we discuss thermal effects on surface processes of near-Earth asteroids, including the thermal history of "rock comets" such as (3200) Phaethon. This work is also critical for constraining the concentrations of contaminants in vaporized water extracted from asteroid regolith as part of future in situ resource utilization for sustained robotic and human space exploration.Comment: 12 pages of text, 3 tables, 7 figures, accepted by Icaru

Can an Off-Nominal Landing by an MMRTG-Powered Spacecraft Induce a Special Region on Mars When No Ice Is Present?

This work aims at addressing whether a catastrophic failure of an entry, descent, and landing event of a Multimission Radioisotope Thermoelectric Generator-based lander could embed the heat sources into the martian subsurface and create a local environment that (1) would temporarily satisfy the conditions for a martian Special Region and (2) could establish a transport mechanism through which introduced terrestrial organisms could be mobilized to naturally occurring Special Regions elsewhere on Mars. Two models were run, a primary model by researchers at the Lawrence Berkeley National Laboratory and a secondary model by researchers at the Jet Propulsion Laboratory, both of which were based on selected starting conditions for various surface composition cases that establish the worst-case scenario, including geological data collected by the Mars Science Laboratory at Gale Crater. The summary outputs of both modeling efforts showed similar results: that the introduction of the modeled heat source could temporarily create the conditions established for a Special Region, but that there would be no transport mechanism by which an introduced terrestrial microbe, even if it was active during the temporarily induced Special Region conditions, could be transported to a naturally occurring Special Region of Mars

Petrology and geochemistry of the Northwest Africa 3368 eucrite

Analysis of the mineralogy, isotopic, and bulk compositions of the eucrite meteorites is imperative for understanding their origin on the asteroid 4 Vesta, the proposed parent body of the HED meteorites. We present here the petrology, mineral compositions, and bulk chemistry of several lithic components of the new brecciated basaltic eucrite Northwest Africa (NWA) 3368 to determine if all the lithologies reflect formation from one rock type or many rock types. The meteorite has three main lithologies: coarse- and fine-grained clasts surrounded by a fine-grained recrystallized silicate matrix. Silicate compositions are homogeneous, and the average rare earth element pattern for NWA 3368 is approximately 10× CI chondrites with a slight negative Eu anomaly. Major and trace element data place NWA 3368 with the Main Group-Nuevo Laredo trend. High-Ti chromites with ilmenite exsolution lamellae provide evidence of NWA 3368’s history of intense metamorphism. We suggest that this meteorite underwent several episodes of brecciation and metamorphism, similar to that proposed by Metzler et al. (1995). We conclude that NWA 3368 is a monomict basaltic eucrite breccia related to known eucrites in texture and in mineral, bulk, and oxygen isotopic composition

Taphonomy of the fossil insects of the middle Eocene Kishenehn Formation

The lacustrine oil shales of the Coal Creek Member of the Kishenehn Formation in northwestern Montana comprise a relatively unstudied middle Eocene fossil insect locality. Herein, we detail the stratigraphic position of the fossiliferous unit, describe the insect fauna of the Coal Creek locality and document its bias towards very small but remarkably pre-served insects. In addition, the depositional environment is examined and the mineral constituents of the laminations that comprise the varves of the Kishenehn oil shale are defined. Fifteen orders of insects have been recorded with the majority of all insects identified as aquatic with the families Chironomidae (Diptera) and Corixidae (Hemiptera) dominant. The presence of small aquatic insects, many of which are immature, the intact nature of >90% of the fossil insects and the presence of Daphnia ephippia, all indicate that the depositional environment was the shallow margin of a large freshwater lake. The fossil insects occur within fossilized microbial mat layers that comprise the bedding planes of the oil shale. Unlike the fossiliferous shales of the Florissant and Okanagan Highlands, the mats are not a product of diatomaceous algae nor are diatom frustules a component of the sediments or the varve structure. Instead, the varves are composed of very fine eolian siliciclastic silt grains overlaid with non-diatomaceous, possibly cyanobacteria-derived microbial mats which contain distinct traces of polyaromatic hydrocarbons. A distinct third layer composed of essentially pure calcite is present in the shale of some exposures and is presumably derived from the seasonal warming-induced precipitation of carbonate from the lake’s waters. The Coal Creek locality presents a unique opportunity to study both very small middle Eocene insects not often preserved as compression fossils in most Konservat-Lagerstätte and the processes that led to their preservation

Can an Off-Nominal Landing by an MMRTG-Powered Spacecraft Induce a Special Region on Mars When No Ice Is Present?

This work aims at addressing whether a catastrophic failure of an entry, descent, and landing event of a Multimission Radioisotope Thermoelectric Generator-based lander could embed the heat sources into the martian subsurface and create a local environment that (1) would temporarily satisfy the conditions for a martian Special Region and (2) could establish a transport mechanism through which introduced terrestrial organisms could be mobilized to naturally occurring Special Regions elsewhere on Mars. Two models were run, a primary model by researchers at the Lawrence Berkeley National Laboratory and a secondary model by researchers at the Jet Propulsion Laboratory, both of which were based on selected starting conditions for various surface composition cases that establish the worst-case scenario, including geological data collected by the Mars Science Laboratory at Gale Crater. The summary outputs of both modeling efforts showed similar results: that the introduction of the modeled heat source could temporarily create the conditions established for a Special Region, but that there would be no transport mechanism by which an introduced terrestrial microbe, even if it was active during the temporarily induced Special Region conditions, could be transported to a naturally occurring Special Region of Mars

Overview of Perseverance’s Upper Fan Campaign

International audienceThe Mars 2020 Perseverance rover’s mission objective is to collect diverse suites of samples for return to Earth via Mars Sample Return and characterize their geologic and environmental context. Jezero crater was selected as the landing site for Perseverance partially due to the opportunity to explore the western fan deposits, which include water-lain sediments from a diverse ancient watershed that have been lithified and exposed by wind. The Upper Fan Campaign, described here, is the third campaign of the mission; it follows the crater floor campaign and the delta front campaign that explored ~40 m of sedimentary deposits at the base of two lobes of the fan. The Upper Fan Campaign includes the rover traverse from the top of the delta front campaign to the margin unit lining the inner crater rim.Prior to landing, two major geologic units were identified on the Upper Fan: the truncated curvilinear unit and the blocky unit. Our goals are to characterize the depositional context and major lithologies present in each unit and acquire samples. At time of writing, we have collected two samples and are planning 1-2 more sample acquisitions in the Upper Fan.The Melyn sample is a fine-grained sandstone from the light toned bands of the truncated curvilinear unit at Tenby, part of the Skrinkle Haven member of the Tenby formation. The sandstone lacks internal structure and decimeter-thick layers have steep (>20°) dip angles. It is interpreted to have formed in subaqueous downslope avalanches, likely on a downstream bar or delta foreset. Melyn contains detrital primary and altered silicates, minor sulfates, and Mg, Fe carbonate grains, coatings, and possible cement.The Otis_Peak sample is a cross-stratified conglomerate interpreted as part of a higher-energy delta plain braided river deposit in the Carew Castle member of the Tenby formation, overlying the Skrinkle Haven member. Proximity science and remote sensing indicate Otis_Peak contains coarse (up to 4 mm) detrital primary and altered silicates with sulfates and Fe, Mg carbonates.Perseverance is characterizing the most commonly observed boulder lithologies in the blocky unit, including boulders with dominant olivine and pyroxene spectral signatures. We plan to collect 1-2 more samples in the campaign, including a lithology representative of the latest phase of delta deposition

Overview of Perseverance’s Upper Fan Campaign

International audienceThe Mars 2020 Perseverance rover’s mission objective is to collect diverse suites of samples for return to Earth via Mars Sample Return and characterize their geologic and environmental context. Jezero crater was selected as the landing site for Perseverance partially due to the opportunity to explore the western fan deposits, which include water-lain sediments from a diverse ancient watershed that have been lithified and exposed by wind. The Upper Fan Campaign, described here, is the third campaign of the mission; it follows the crater floor campaign and the delta front campaign that explored ~40 m of sedimentary deposits at the base of two lobes of the fan. The Upper Fan Campaign includes the rover traverse from the top of the delta front campaign to the margin unit lining the inner crater rim.Prior to landing, two major geologic units were identified on the Upper Fan: the truncated curvilinear unit and the blocky unit. Our goals are to characterize the depositional context and major lithologies present in each unit and acquire samples. At time of writing, we have collected two samples and are planning 1-2 more sample acquisitions in the Upper Fan.The Melyn sample is a fine-grained sandstone from the light toned bands of the truncated curvilinear unit at Tenby, part of the Skrinkle Haven member of the Tenby formation. The sandstone lacks internal structure and decimeter-thick layers have steep (>20°) dip angles. It is interpreted to have formed in subaqueous downslope avalanches, likely on a downstream bar or delta foreset. Melyn contains detrital primary and altered silicates, minor sulfates, and Mg, Fe carbonate grains, coatings, and possible cement.The Otis_Peak sample is a cross-stratified conglomerate interpreted as part of a higher-energy delta plain braided river deposit in the Carew Castle member of the Tenby formation, overlying the Skrinkle Haven member. Proximity science and remote sensing indicate Otis_Peak contains coarse (up to 4 mm) detrital primary and altered silicates with sulfates and Fe, Mg carbonates.Perseverance is characterizing the most commonly observed boulder lithologies in the blocky unit, including boulders with dominant olivine and pyroxene spectral signatures. We plan to collect 1-2 more samples in the campaign, including a lithology representative of the latest phase of delta deposition