A model for the temperature-dependence of tidal dissipation in convective plumes on icy satellites: Implications for Europa and Enceladus

To explain the formation of surface features on Europa, Enceladus, and other satellites, many authors have postulated the spatial localization of tidal heating within convective plumes. However, the concept that enhanced tidal heating can occur within a convective plume has not been rigorously tested. Most models of this phenomenon adopt a tidal heating with a temperature-dependence derived for an incompressible, homogeneous (zero-dimensional) Maxwell material, but it is unclear whether this formulation is relevant to the heterogeneous situation of a warm plume surrounded by cold ice. To determine whether concentrated dissipation can occur in convective plumes, we develop a two-dimensional model to compute the volumetric dissipation rate for an idealized, vertically oriented, isolated convective plume obeying a Maxwellian viscoelastic compressible rheology. We apply the model to the Europa and Enceladus ice shells, and we investigate the consequences for partial melting and resurfacing processes on these bodies. We find that the tidal heating is strongly temperature dependent in a convective ice plume and could produce elevated temperatures and local partial melting in the ice shells of Europa and Enceladus. Our calculation provides the first quantitative verification of the hypothesis by Sotin et al. [Sotin, C., Head, J.W., Tobie, G., 2002. Geophys. Res. Lett. 29. 74-1] and others that the tidal dissipation rate is a strong function of temperature inside a convective plume. On Europa, such localized heating could help allow the formation of domes and chaos terrains by convection. On Enceladus, localized tidal heating in a thermal plume could explain the concentrated activity at the south pole and its associated heat transport of 2-7 GW.     

Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates

We present the first redundant detection of sporadic impact flashes on the Moon from a systematic survey performed between 2001 and 2004. Our wide-field lunar monitoring allows us to estimate the impact rate of large meteoroids on the Moon as a function of the luminous energy received on Earth. It also shows that some historical well-documented mysterious lunar events fit in a clear impact context. Using these data and traditional values of the luminous efficiency for this kind of event we obtain that the impact rate on Earth of large meteoroids (0.1-10 m) would be at least one order of magnitude larger than currently thought. This discrepancy indicates that the luminous efficiency of the hypervelocity impacts is higher than 10⁻², much larger than the common belief, or the latest impact fluxes are somewhat too low, or, most likely, a combination of both. Our nominal analysis implies that on Earth, collisions of bodies with masses larger than 1 kg can be as frequent as 80,000 per year and blasts larger than 15-kton could be as frequent as one per year, but this is highly dependent on the exact choice of the luminous efficiency value. As a direct application of our results, we expect that the impact flash of the SMART-1 spacecraft should be detectable from Earth with medium-sized telescopes.     

Time-resolved studies of hypervelocity vertical impacts into porous particulate targets: Effects of projectile density on early-time coupling and crater growth

The depth and duration of energy and momentum coupling in an impact shapes the formation of the crater. The earliest stages of crater growth (when the projectile transfers its energy and momentum to the target) are unrecoverable when the event is described by late stage parameters, which collapse the initial conditions of the impact into a singular point in time and space. During the coupling phase, the details of the impact are mapped into the ejecta flow field. In this experimental study, we present new experimental and computational measurements of the ejecta distribution and crater growth extending from early times into main-stage ballistic flow for hypervelocity impacts over a range of projectile densities. Specifically, we assess the effect of projectile density on coupling depth and location in porous particulate (sand) targets. A non-invasive high-speed imaging technique is employed to capture the velocity of individual ejecta particles very early in the cratering event as a function of both time and launch position. These data reveal that the effects of early-stage coupling, such as non-constant ejection angles, manifest not only in early-time behavior but also extend to main-stage crater growth. Time-resolved comparisons with hydrocode calculations provide both benchmarking and insight into the parameters controlling the ejection process. Measurements of the launch position and metrics for the transient diameter to depth ratio as a function of time demonstrate non-proportional crater growth throughout much of excavation. Low-density projectiles couple closer to the surface, thereby leading to lower ejection angles and larger effective diameter to depth ratios. These results have implications for the ballistic emplacement of ejecta on planetary surfaces, and are essential to interpreting temporally resolved data from impact missions.     

Infrared spectroscopy of Wild 2 particle hypervelocity tracks in Stardust aerogel: Evidence for the presence of volatile organics in cometary dust

Abstract— Infrared spectroscopy maps of some tracks made by cometary dust from 81P/Wild 2 impacting Stardust aerogel reveal an interesting distribution of organic material. Out of six examined tracks, three show presence of volatile organic components possibly injected into the aerogel during particle impacts. When particle tracks contained volatile organic material, they were found to be ‐CH₂‐rich, while the aerogel is dominated by the ‐CH₃‐rich contaminant. It is clear that the population of cometary particles impacting the Stardust aerogel collectors also includes grains that contained little or none of this organic component. This observation is consistent with the highly heterogeneous nature of collected grains, as seen by a multitude of other analytical techniques.     

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild‐2

Abstract— Metallic aluminum alloy foils exposed on the forward, comet‐facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl‐shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild‐2, with a known and constant spacecraft‐particle relative velocity and effective surface‐perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well‐documented particle size range from 10 μm to nearly 100 μm. Light gas gun buckshot firings of these particles at approximately 6 km s^?1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild‐2, independent of the active impact detector instruments aboard the Stardust spacecraft.     

Reaction between H2, CO,and H2S over Fe,Ni metal in the solar nebula: Experimental evidence for the formation of sulfur‐bearing organic molecules and sulfides

Abstract— Detailed laboratory studies have been carried out in order to simulate the interaction between nanometer‐sized kamacite metal particles and different gas mixtures consisting of H₂:H₂S (250:0.1), H₂:CO (250:1), and H₂:CO:H₂S (250:1:0.1) under nebular‐type conditions (5 × 10^?4 atm and 473 K). Reaction of H₂ + H₂S with kamacite particles for 1000 h leads to the formation of pyrrhotite. Incorporation of CO into the gaseous reactant mixture results in the formation of both sulfide and carbide phases. At the same time, amorphous C is deposited onto the metal particles and organic molecules are evolved, namely hydrocarbons and thiols in the C₁‐C₅ and C₁‐C₂ range, respectively. Carbon deposition and production of organics are enhanced with respect to experiments performed with H₂ + CO, where a carbide phase is formed. There is no evidence for the existence of S‐poisoning effects on the metal‐catalysed hydrogenation of CO through Fischer‐Tropsch‐type reactions in nebular environments. In fact, it is experimentally demonstrated that S‐containing organic species could be synthesized by such reactions from nebular gas.     

Validation of numerical codes for impact and explosion cratering: Impacts on strengthless and metal targets

Abstract— Over the last few decades, rapid improvement of computer capabilities has allowed impact cratering to be modeled with increasing complexity and realism, and has paved the way for a new era of numerical modeling of the impact process, including full, three‐dimensional (3D) simulations. When properly benchmarked and validated against observation, computer models offer a powerful tool for understanding the mechanics of impact crater formation. This work presents results from the first phase of a project to benchmark and validate shock codes. A variety of 2D and 3D codes were used in this study, from commercial products like AUTODYN, to codes developed within the scientific community like SOVA, SPH, ZEUS‐MP, iSALE, and codes developed at U.S. National Laboratories like CTH, SAGE/RAGE, and ALE3D. Benchmark calculations of shock wave propagation in aluminum‐on‐aluminum impacts were performed to examine the agreement between codes for simple idealized problems. The benchmark simulations show that variability in code results is to be expected due to differences in the underlying solution algorithm of each code, artificial stability parameters, spatial and temporal resolution, and material models. Overall, the inter‐code variability in peak shock pressure as a function of distance is around 10 to 20%. In general, if the impactor is resolved by at least 20 cells across its radius, the underestimation of peak shock pressure due to spatial resolution is less than 10%. In addition to the benchmark tests, three validation tests were performed to examine the ability of the codes to reproduce the time evolution of crater radius and depth observed in vertical laboratory impacts in water and two well‐characterized aluminum alloys. Results from these calculations are in good agreement with experiments. There appears to be a general tendency of shock physics codes to underestimate the radius of the forming crater. Overall, the discrepancy between the model and experiment results is between 10 and 20%, similar to the inter‐code variability.     

Comparison of FT‐IR spectra of bulk and acid insoluble organic matter in chondritic meteorites: An implication for missing carbon during demineralization

Past studies of the various separable carbonaceous fractions have been unable to account for all of C in primitive chondrites. In particular, up to 20–50% of the C is lost during acid leaching of bulk samples even after the C in carbonates and soluble organic matter is accounted for. To try to better characterize the nature of this “missing C,” we have compared the bulk infrared (IR) absorption spectra of a number of primitive chondrites with those of their previously reported insoluble organic matter (IOM). The aliphatic C–H stretching bands, in particular, allow us to compare the molecular structures of bulk C with that of IOM. The spectral differences between bulk C and IOM reflect “missing C” phases that were lost during acid leaching, although we cannot completely exclude the possibility that the OM was modified after demineralization. Comparing IR spectra of bulk meteorite powder and IOM suggests that the missing C varies in its molecular structure, and that mildly thermally metamorphosed type 3 chondrites tend to be richer in an aliphatic fraction with lower CH₂/CH₃ ratios, relative to IOM, compared to aqueously altered carbonaceous chondrites (CI/CM/CR). The missing C is most likely released from acid‐labile functional groups, such as esters, acetals, and amides, during demineralization, although it cannot be ruled out that some fraction of the missing C is in small grains that are difficult to recover from suspension, or in water‐soluble compounds trapped in phyllosilicates.     

Survival of endospores of Bacillus subtilis on spacecraft surfaces under simulated martian environments: implications for the forward contamination of Mars

Experiments were conducted in a Mars simulation chamber (MSC) to characterize the survival of endospores of Bacillus subtilis under high UV irradiation and simulated martian conditions. The MSC was used to create Mars surface environments in which pressure (8.5 mb), temperature (-80, -40, -10, or +23 degrees C), gas composition (Earth-normal N2/O2 mix, pure N2, pure CO2, or a Mars gas mix), and UV-VIS-NIR fluence rates (200-1200 nm) were maintained within tight limits. The Mars gas mix was composed of CO2 (95.3%), N2 (2.7%), Ar (1.7%), O2 (0.2%), and water vapor (0.03%). Experiments were conducted to measure the effects of pressure, gas composition, and temperature alone or in combination with Mars-normal UV-VIS-NIR light environments. Endospores of B. subtilis, were deposited on aluminum coupons as monolayers in which the average density applied to coupons was 2.47 x 10(6) bacteria per sample. Populations of B. subtilis placed on aluminum coupons and subjected to an Earth-normal temperature (23 degrees C), pressure (1013 mb), and gas mix (normal N2/O2 ratio) but illuminated with a Mars-normal UV-VIS-NIR spectrum were reduced by over 99.9% after 30 sec exposure to Mars-normal UV fluence rates. However, it required at least 15 min of Mars-normal UV exposure to reduce bacterial populations on aluminum coupons to non-recoverable levels. These results were duplicated when bacteria were exposed to Mars-normal environments of temperature (-10 degrees C), pressure (8.5 mb), gas composition (pure CO2), and UV fluence rates. In other experiments, results indicated that the gas composition of the atmosphere and the temperature of the bacterial monolayers at the time of Mars UV exposure had no effects on the survival of bacterial endospores. But Mars-normal pressures (8.5 mb) were found to reduce survival by approximately 20-35% compared to Earth-normal pressures (1013 mb). The primary implications of these results are (a) that greater than 99.9% of bacterial populations on sun-exposed surfaces of spacecraft are likely to be inactivated within a few tens of seconds to a few minutes on the surface of Mars, and (b) that within a single Mars day under clear-sky conditions bacterial populations on sun-exposed surfaces of spacecraft will be sterilized. Furthermore, these results suggest that the high UV fluence rates on the martian surface can be an important resource in minimizing the forward contamination of Mars.     

Chandrayaan-1 observation of distant secondary craters of Copernicus exhibiting central mound morphology: Evidence for low velocity clustered impacts on the Moon

Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20-1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30-335 m diameter and 10-40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters.     

Experimental investigation of impacts by solar cell secondary ejecta on silica aerogel and aluminum foil: Implications for the Stardust Interstellar Dust Collector

Abstract– We have shown in laboratory experiment that hypervelocity impacts on a solar cell produce ejecta that can be captured on aluminum (Al 1100) foil or in low density (33 kg m^?3) aerogel. The origin of the secondary impacts can be determined by either analysis of the residue in the craters in the foils (which preserve an elemental signature of the solar cell components) or by their pointing direction for tracks in the aerogel (which we show align with the impact direction to ± 0.4°). This experimental evidence explains the observations of the NASA Stardust mission which has reported that the majority of tracks in the aerogel collector used to collect interstellar dust actually point at the spacecraft’s solar panels. From our results, we suggest that it should also be possible to recognize secondary ejecta craters in the Stardust mission aluminum foils, also used as dust sampling devices during the mission.     

S.E.M.A.Ph.Or.E COMETAIRE,a tool for the study of the photochemical decomposition of probable large organic molecules in comets. first application: Polyoxymethylene

This paper presents a current experimental program concerning the study of the photochemical evolution of the organic matter ejected from the cometary nucleus. The aim of the work is to better understand, using laboratory simulations, the mechanisms which are involved in the degradation of the high molecular weight organics in cometary ices and dust when they are submitted to the warming up and to the bombardment of photons in the surrounding area of the Sun. This experimental study will establish correlations between the nucleus and the molecular composition of the coma. Furthermore, experimental data will provide useful information to bring to a close the question of the origin of the extended sources of H₂CO and CO. Polyoxymethylene, suspected to be present in the cometary nucleus, is often mentioned as a possible parent molecule for the extended source of H₂CO. In order to test this hypothesis, irradiation of POM has been performed at 147 nm. The preliminary results show effectively H₂CO as one photodegradation product as well as CO. C0₂ and HCOOH. Tentative detections of CH₃0CH₃, CH₃OCH₂OCH₃, CH₃OCHO and C₃H₆O₃ are also presented.     

NanoSIMS analysis of organic carbon from the Tissint Martian meteorite: Evidence for the past existence of subsurface organic‐bearing fluids on Mars

Two petrographic settings of carbonaceous components, mainly filling open fractures and occasionally enclosed in shock‐melt veins, were found in the recently fallen Tissint Martian meteorite. The presence in shock‐melt veins and the deuterium enrichments (δD up to +1183‰) of these components clearly indicate a pristine Martian origin. The carbonaceous components are kerogen‐like, based on micro‐Raman spectra and multielemental ratios, and were probably deposited from fluids in shock‐induced fractures in the parent rock of Tissint. After precipitation of the organic matter, the rock experienced another severe shock event, producing the melt veins that encapsulated a part of the organic matter. The C isotopic compositions of the organic matter (δ¹³C = ?12.8 to ?33.1‰) are significantly lighter than Martian atmospheric CO₂ and carbonate, providing a tantalizing hint for a possible biotic process. Alternatively, the organic matter could be derived from carbonaceous chondrites, as insoluble organic matter from the latter has similar chemical and isotopic compositions. The presence of organic‐rich fluids that infiltrated rocks near the surface of Mars has significant implications for the study of Martian paleoenvironment and perhaps to search for possible ancient biological activities on Mars.     

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3-6 log units below the iron-wüstite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO₂ being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core-mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe-S-Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe-S liquid, poor in Si; (ii) a middle layer of Fe-Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.