Combining shock barometry with numerical modeling: insights into complex crater formation – The example of the Siljan impact structure (Sweden)

Siljan, central Sweden, is the largest known impact structure in Europe. It was formed at about 380 Ma, in the late Devonian period. The structure has been heavily eroded to a level originally located underneath the crater floor, and to date, important questions about the original size and morphology of Siljan remain unanswered. Here we present the results of a shock barometry study of quartz-bearing surface and drill core samples combined with numerical modeling using iSALE. The investigated 13 bedrock granitoid samples show that the recorded shock pressure decreases with increasing depth from 15 to 20 GPa near the (present) surface, to 10–15 GPa at 600 m depth. A best-fit model that is consistent with observational constraints relating to the present size of the structure, the location of the downfaulted sediments, and the observed surface and vertical shock barometry profiles is presented. The best-fit model results in a final crater (rim-to-rim) diameter of ~65 km. According to our simulations, the original Siljan impact structure would have been a peak-ring crater. Siljan was formed in a mixed target of Paleozoic sedimentary rocks overlaying crystalline basement. Our modeling suggests that, at the time of impact, the sedimentary sequence was approximately 3 km thick. Since then, there has been around 4 km of erosion of the structure

A 3-D study of mineral inclusions in chromite from ordinary chondrites using synchrotron radiation X-ray tomographic microscopy-Method and applications

Abstract– A method is described for imaging in 3-D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X-ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long-term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment-dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non-destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two-thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth

An Early Jurassic age for the Puchezh-Katunki impact structure (Russia) based on 40Ar/39Ar data and palynology

The Puchezh-Katunki impact structure, 40–80 km in diameter, located ~400 km northeast of Moscow (Russia), has a poorly constrained age between ~164 and 203 Ma (most commonly quoted as 167 ± 3 Ma). Due to its relatively large size, the Puchezh-Katunki structure has been a prime candidate for discussions on the link between hypervelocity impacts and extinction events. Here, we present new 40Ar/39Ar data from step-heating analysis of five impact melt rock samples that allow us to significantly improve the age range for the formation of the Puchezh-Katunki impact structure to 192–196 Ma. Our results also show that there is not necessarily a simple relationship between the observed petrographic features of an impact melt rock sample and the obtained 40Ar/39Ar age spectra and inverse isochrons. Furthermore, a new palynological investigation of the postimpact crater lake sediments supports an age significantly older than quoted in the literature, i.e., in the interval late Sinemurian to early Pliensbachian, in accordance with the new radioisotopic age estimate presented here. The new age range of the structure is currently the most reliable age estimate of the Puchezh-Katunki impact event

Identifying Shocked Feldspar on Mars Using Perseverance Spectroscopic Instruments : Implications for Geochronology Studies on Returned Samples

The Perseverance rover (Mars 2020) mission, the first step in NASA’s Mars Sample Return (MSR) program, will select samples for caching based on their potential to improve understanding Mars’ astrobiological, geological, geochemical, and climatic evolution. Geochronologic analyses will be among the key measurements planned for returned samples. Assessing a sample’s shock history will be critical because shock metamorphism could influence apparent sample age. Shock effects in one Mars-relevant mineral class, plagioclase feldspar, have been well-documented using various spectroscopy techniques (thermal infrared reflectance, emission, and transmission spectroscopy, Raman, and luminescence). A subset of these data will be obtained with the SuperCam and SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instruments onboard Perseverance to inform caching decisions for MSR. Here, we review shock indicators in plagioclase feldspar as revealed in Raman, luminescence, and IR spectroscopy lab data, with an emphasis on Raman spectroscopy. We consider how this information may inform caching decisions for selecting optimal samples for geochronology measurements. We then identify challenges and make recommendations for both in situ measurements performed with SuperCam and SHERLOC and for supporting lab studies to enhance the success of geochronologic analyses after return to Earth

Ground penetrating radar observations of the contact between the western delta and the crater floor of Jezero crater, Mars

The delta deposits in Jezero crater contain sedimentary records of potentially habitable conditions on Mars. NASA's Perseverance rover is exploring the Jezero western delta with a suite of instruments that include the RIMFAX ground penetrating radar, which provides continuous subsurface images that probe up to 20 meters below the rover. As Perseverance traversed across the contact between the Jezero crater floor and the delta, RIMFAX detected a distinct discontinuity in the subsurface layer structure. Below the contact boundary are older crater floor units exhibiting discontinuous inclined layering. Above the contact boundary are younger basal delta units exhibiting regular horizontal layering. At one location, there is a clear unconformity between the crater floor and delta layers, which implies that the crater floor experienced a period of erosion before the deposition of the overlying delta strata. The regularity and horizontality of the basal delta sediments observed in the radar cross sections indicate that they were deposited in a low-energy lake environment.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Stratigraphic relationships in Jezero crater, Mars: constraints on the timing of fluvial‐lacustrine activity from orbital observations

On February 18, 2021 NASA's Perseverance rover landed in Jezero crater, located at the northwestern edge of the Isidis basin on Mars. The uppermost surface of the present-day crater floor is dominated by a distinct geologic assemblage previously referred to as the dark-toned floor. It consists of a smooth, dark-toned unit overlying and variably covering light-toned, roughly eroded deposits showing evidence of discrete layers. In this study, we investigated the stratigraphic relations between materials that comprise this assemblage, the main western delta deposit, as well as isolated mesas located east of the main delta body that potentially represent delta remnants. A more detailed classification and differentiation of crater floor units in Jezero and determination of their relative ages is vital for the understanding of the geologic evolution of the crater system, and determination of the potential timeline and environments of habitability. We have investigated unit contacts using topographic profiles and DEMs as well as the distribution of small craters and fractures on the youngest portions of the crater floor. Our results indicate that at least some of the deltaic deposition in Jezero postdates emplacement of the uppermost surface of the crater floor assemblage. The inferred age of the floor assemblage can therefore help to constrain the timing of the Jezero fluviolacustrine system, wherein at least some lake activity postdates the age of the uppermost crater floor. We present hypotheses that can be tested by Perseverance and can be used to advance our knowledge of the geologic evolution of the area

Perseverance rover reveals ancient delta-lake system and flood deposits at Jezero crater, Mars

International audienceObservations from orbital spacecraft have shown that Jezero crater, Mars, contains a prominent fan-shaped body of sedimentary rock deposited at its western margin. The Perseverance rover landed in Jezero crater in February 2021. We analyze images taken by the rover early in the mission, finding that the fan has outcrop faces that were invisible from orbit, which record the hydrological evolution of Jezero crater. We interpret the presence of inclined strata in these outcrops as evidence for deltas that advanced into a lake. By contrast, the uppermost fan strata are composed of boulder conglomerates, which imply deposition by episodic high-energy floods. This sedimentary succession indicates a transition from a sustained hydrologic activity in a persistent lake environment to highly energetic, short-duration fluvial flows

Sedimentary and stratigraphic observations at the Jezero western delta front using Perseverance cameras: initial constraints on palaeoenvironments

International audienceNASA’s Mars 2020 Perseverance rover mission is seeking signs of ancient life in Jezero crater and is collecting a cache of Martian rock and soil samples for planned return to Earth by a future mission. A key exploration target for the mission is a prominent sedimentary fan deposit at the western margin of Jezero crater that has been interpretated to be a river delta that built into an ancient lake basin during the Late Noachian-Early Hesperian epochs on Mars (~3.6-3.8 Ga) [1, 2]. Long distance observations of a remnant butte (informally named Kodiak) related to the western fan demonstrated that it comprised two distinct Gilbert-type delta units [2, 3].In her approach to the western fan, Perseverance drove alongside the east-facing scarp of the western fan and arrived at a key location called Three Forks - a setting off point for delta exploration - in April 2022. Images from the Mastcam-Z and SuperCam Remote Micro-Imager instrumentsprovide new views of the stratigraphy exposed in the erosional front of the western Jezero delta; in particular, showing sections of the delta previously not visible from long distance observations and at much higher resolution. These observations provide the first direct evidence of delta geometries in the main western fan deposit. Here, we report its stratigraphy and sedimentology, which providenew constraints on the nature of the fan deposits, and therefore paleoenvironmental implications

SIGNIFICANCE OF THE VARIATIONS IN FLUVIAL INPUT WITHIN JEZERO CRATER FROM PERSEVERANCE ROVER OBSERVATIONS

International audienceThe Perseverance rover landed on the floor of Jezero crater on 18 February 2021. The landing site, named “Octavia E. Butler” is located ~2.2 km from the SE-facing erosional scarp of the western fan deposits, which are of strong interest for the mission [1-2]. Images obtained using the Mastcam-Z camera and the Remote Micro-Imager (RMI) of the SuperCam instrument provided the first Mars ground-based observations of this western fan (Fig. 1). At the distance images were taken, the RMI images offer a pixel resolution of 2.2 cm, thus enabling identification of objects of typically 7-8 cm (3-4 pixels). Observations of the residual butte Kodiak confirmed the presence of a lake within Jezero crater, but also showed that the lake deduced from the deltaic architecture at Kodiak had a level ~100 m lower than expected (-2495/-2500 m), and was thus a closed system for a significant period [3]. In addition, the coarser deposits (boulder conglomerates and pebbly sandstones) observed near the top of all of the scarps are typical of fluvial floods with high energy, reflecting a change in hydrology of the fluvial system. Here, we focus on the hydrological characteristics of fluvial deposits observed within the scarps of the delta, both as topsets and as boulder conglomerates