共查询到20条相似文献,搜索用时 15 毫秒
1.
Harold P. Klein 《Icarus》1996,120(2):431-436
Proposals for continuing the search for extant life on Mars are primarily predicated on the assumption that specialized environmental niches that could support a biota may exist on the planet. Before attempting any critical tests for extant organisms, eitherin situor on returned samples, it is imperative to determine whether any such sites actually exist. If, through remote sensing and landed instrumentation, sites of potential biological interest are discovered and characterized, biological tests can then more effectively be planned to elicit the presence of organisms that are adapted to living in these particular environments. 相似文献
2.
Methane, a potential biosignature, has recently been detected in the martian atmosphere. This Note focuses on field investigations/operational simulations and laboratory studies which resulted in successful detection of methane within arid terrestrial soils, as distinct from the usual methanogen environment, but in at least partial analogy to martian conditions. 相似文献
3.
Vladimir A. Krasnopolsky 《Icarus》2006,180(2):359-367
The following problems related to the origin of methane on Mars have been considered. (1) Laboratory simulations of the impact phenomena confirm effective heterogeneous chemistry between the products of the fireball. This chemistry lowers the fireball freezing temperature from 2000 to 750 K for methane and to 1100 K for CO/CO2. Production of methane on Mars by cometary impacts is 0.8% of the total production. A probability that the observed methane on Mars came from impact of a single comet is 0.0011. (2) The PFS observations of variations of methane on Mars require a very effective heterogeneous loss of methane. Heterogeneous effect of dust is half that of the surface rocks. Thermochemical equilibrium requires production, not loss, of methane. Existing kinetic data show a very low efficiency of heterogeneous reactions of methane. Highly reactive superoxide ions generated by the solar UV photons on the martian rocks cannot remove methane. The required efficiency of heterogeneous loss of methane on Mars is higher than that on Earth by a factor of ?1000, although the expected efficiency on Earth is stronger than that on Mars because of the liquid ocean and the abundant oxygen. All these inconsistencies may be removed if variations of the rock reflectivity contribute to the PFS observations of methane on Mars. The PFS data on H2CO, HCl, HF, and HBr also raise doubts. (3) Although geologic sources of methane are possible, the lack of current volcanism, hydrothermal activity, hot spots, and very low seepage of gases from the interior are not favorable for geologic methane. Any proposed geological source of methane on Mars should address these problems. Some weak points in the suggested geologic sources are discussed. (4) Measurements of 13C/12C and D/H in methane would be difficult because of the low methane abundance. These ratios are mostly sensitive to a temperature of methane formation and cannot distinguish between biogenic and low-temperature geologic sources. Their analysis requires the carbon isotope ratio in CO2 on Mars, which is known with the insufficient accuracy, and D/H in water, which is different in the atmosphere, polar caps, regolith and interior. Therefore, the stable isotope ratios may not give a unique answer on the origin of methane. (5) Ethane and propane react with OH much faster than methane. If their production relative to methane is similar to that on Earth, then their expected abundances on Mars are of a few parts per trillion. (6) Loss of SO2 in the reaction with peroxide on ice is smaller than its gas-phase loss by an order of magnitude. The overall results strengthen the biogenic origin of martian methane and its low variability. 相似文献
4.
Because of the ubiquity of subsurface microbial life on Earth, examination of the subsurface of Mars could provide an answer to the question of whether microorganisms exist or ever existed on that planet. Impact craters provide a natural mechanism for accessing the deep substrate of Mars and exploring its exobiological potential. Based on equations that relate impact crater diameters to excavation depth we estimate the observed crater diameters that are required to prospect to given depths in the martian subsurface and we relate these depths to observed microbiological phenomena in the terrestrial subsurface. Simple craters can be used to examine material to a depth of ∼270 m. Complex craters can be used to reach greater depths, with craters of diameters ≥300 km required to reach depths of 6 km or greater, which represent the limit of the terrestrial deep subsurface biosphere. Examination of the ejecta blankets of craters between 17.5 and 260 km in diameter would provide insights into whether there is an extant, or whether there is evidence of an extinct, deep subsurface microbiota between 500 and 5000 m prior to committing to large-scale drilling efforts. At depths <500 m some crater excavations are likely to be more important than others from an exobiological point of view. We discuss examples of impacts into putative intracrater paleolacustrine sediments and regions associated with hydrothermal activity. We compare these depths to the characteristics of subsurface life on Earth and the fossil microbiological record in terrestrial impact craters. 相似文献
5.
Rapid temporal variability of SO2 and SO in the Venus 85–100 km mesosphere (Sandor, B.J., Clancy, R.T., Moriarty-Schieven G.H. [2007]. Bull. Am. Astron. Soc. 39, 503; Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60) requires in situ sources and sinks for these molecules. While many loss mechanisms are recognized, no process for in situ production is known. Observational investigations to find, or constrain other potential sulfur reservoirs offer one method toward understanding the applicable photochemistry. Here, we report upper limits for gas-phase H2SO4 (sulfuric acid) abundances in Venus’ 85–100 km upper mesosphere, derived from 16 ground-based sub-mm spectroscopic observations in the period 2004–2008. Unlike the ubiquitous sulfuric acid solid/liquid aerosol, the gas phase would be photochemically active, potentially both source and sink for SO and SO2. H2SO4 is retrieved from sub-mm lines located in the same bandpass as the SO2 and SO lines described by Sandor et al. (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. [2010]. Icarus 208, 49–60). H2SO4 upper limits reported here are thus simultaneous and spatially coincident with measurements of SO2 and SO, providing for analysis of the three sulfur species collectively. The average H2SO4 abundance over 16 observations is 1 ± 2 ppb (i.e. <3 ppb). Upper limits for individual observations range from 3 to 44 ppb, where quality of the observing weather is the dominant constraint on measurement precision. The sum of H2SO4, SO2 and SO varies widely. In one comparison, the sum [H2SO4 + SO2 + SO] measured on one date differs by 10-σ from the sum measured 2 months later. We conclude that upper mesospheric sulfur atoms are not conserved among the three molecules, that H2SO4 is not a significant sulfur reservoir for balancing the observed variations of [SO2 + SO], and is not relevant to the (still unknown) photochemistry responsible for observed behavior of SO2 and SO. Having ruled out H2SO4, we infer that elemental sulfur is the most probable candidate for the needed third reservoir. 相似文献
6.
A spectrum of the satellite of Jupiter, Io, from 0.86 to 2.7 μm at a resolution of 3.36 cm?1 and a signal to rms noise ratio of 120 is presented. No absorptions due to any atmospheric constituents on Io could be found on the spectrum. Upper limits of 0.12 cm-atm for NH3, 0.12cm-atm for CH4, 0.4cm-atm for N2O, and 24cm-atm for H2S were determined. Laboratory spectra of ammonia frosts as a function of temperature were compared with the spectrum of Io and showed as a frost not to be present at the surface of Io. A search for possible resonance lines of carbon, silicon, and sulfur as well as the 1.08μm line of helium proved negative and upper emission limits of 60, 18, 27, and 60 kilorayleighs, respectively, were established for these lines. 相似文献
7.
A procedure with a Bayesan approach for calculating upper limits to gravitational wave bursts from coincidence experiments with multiple detectors is described, where the detection efficiency for small signals is taken into consideration. The Bayesan approach to the upper limit estimation is confronted with the unified approach for the case when no events are observed in presence of a non-zero background. 相似文献
8.
Early Mars volcanic sulfur storage in the upper cryosphere and formation of transient SO2‐rich atmospheres during the Hesperian 下载免费PDF全文
F. Schmidt E. Chassefière F. Tian E. Dartois J.‐M. Herri O. Mousis 《Meteoritics & planetary science》2016,51(11):2226-2233
In a previous paper (Chassefière et al. 2013 ), we have shown that most volcanic sulfur released to the early Mars atmosphere could have been trapped in the upper cryosphere under the form of CO2‐SO2 clathrates. Huge amounts of sulfur, up to the equivalent of an ~1 bar atmosphere of SO2, would have been stored in the Noachian upper cryosphere, then massively released to the atmosphere during the Hesperian due to rapidly decreasing CO2 pressure. It could have resulted in the formation of the large sulfate deposits observed mainly in Hesperian terrains, whereas no or little sulfates are found at the Noachian. In the present paper, we first clarify some aspects of our previous work. We discuss the possibility of a smaller cooling effect of sulfur particles, or even of a net warming effect. We point out the fact that CO2‐SO2 clathrates formed through a progressive enrichment of a pre‐existing reservoir of CO2 clathrates and discuss processes potentially involved in the slow formation of a SO2‐rich upper cryosphere. We show that episodes of sudden destabilization at the Hesperian may generate 1000 ppmv of SO2 in the atmosphere and contribute to maintaining the surface temperature above the water freezing point. 相似文献
9.
Goro Komatsu Gian Gabriele Ori Marco Cardinale James M. Dohm Victor R. Baker David A. Vaz Ryo Ishimaru Noriyuki Namiki Takafumi Matsui 《Planetary and Space Science》2011,59(2-3):169-181
We discuss in this paper possible roles of methane and carbon dioxide in geological processes on Mars. These volatiles in the martian crust may migrate upward from their sources either directly or via various traps (structural, sedimentary, ground ice, gas hydrates). They are then likely emitted to the atmosphere by seepage or through diverse vent structures. Though gas hydrates have never been directly detected on Mars, theoretical studies favor their presence in the crust and polar caps; they could have played an important role as significant gas reservoirs in the subsurface. The martian gas hydrates would possibly be a binary system of methane and carbon dioxide occupying clathrate cavities. Landforms such as mud volcanoes with well-known linkage to gas venting are extensively distributed on Earth, and methane is the primary gas involved. Thus, identification of these landforms on Mars could suggest that methane and possibly carbon dioxide have contributed to geological processes of the planet. For example, we present a newly identified field in Chryse Planitia where features closely resembling terrestrial mud volcanoes occur widely, though with no observable activity. We also present results of a preliminary search for possible recent or present-day, methane-emission zones in the regions over which enrichments of atmospheric methane have been reported. 相似文献
10.
The UCSD X-ray telescope on OSO-3 scanned Jupiter for 33 days during February and March 1968. We have searched the data for a steady Jovian flux, and for a burst component at times of decametric radio bursts. Neither component was detected at a sensitivity of ~0.1 photon (cm2sec)?1 for hv > 7.7 keV. At 4.4AU, the 3σ upper limits correspond to X-ray luminosities of 7.4 × 1019 ergs sec?1 for the steady component, and 2 × 1020 ergs sec?1 for the burst component. The observations occurred during a period of high solar activity, during which three sudden-commencement magnetic storms were observed at Earth. We compare the upper limits with several different calculations of the expected flux levels, and conclude that major improvements in X-ray detection techniques will be required before Jovian X rays can be detected with near-Earth observations. 相似文献
11.
Vladimir A. Krasnopolsky 《Icarus》2006,182(1):80-91
High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν/δν=76,000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50-112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO2 lines. The derived NO mixing ratio is 5.5±1.5 ppb below 60 km and its flux is . Predissociation of NO at the (0-0) 191 nm and (1-0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of . For a flash energy on Venus similar to that on the Earth (∼109 J), the global flashing rate is ∼90 s−1 and ∼6 km−2 y−1 which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO2 lines, and the line at 1900.076 cm−1 is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (v=1) in the lowest 20 km on Mars. However, quenching of NO (v=1) by CO2 is very effective below 40 km. Excitation of NO (v=1) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3. 相似文献
12.
The recent discovery of methane on Mars has led to much discussion concerning its origin. On Earth, the isotopic signatures of methane vary with the nature of its production. Specifically, the ratios among 12CH4, 13CH4, and 12CH3D differ for biotic and abiotic origins. On Mars, measuring these ratios would provide insights into the origins of methane and measurements of water isotopologues co-released with methane would assist in testing their chemical relationship. Since 1997, we have been measuring HDO and H2O in Mars’ atmosphere and comparing their ratio to that in Earth’s oceans. We recently incorporated a line-by-line radiative transfer model (LBLRTM) into our analysis. Here, we present a map for [HDO]/[H2O] along the central meridian (154°W) for Ls=50°. From these results, we constructed models to determine the observational conditions needed to quantify the isotopic ratios of methane in Mars’ atmosphere. Current ground-based instruments lack the spectral resolution and sensitivity needed to make these measurements. Measurements of the isotopologues of methane will likely require in situ sampling. 相似文献
13.
Daniel B. Curtis Courtney D. Hatch Christa A. Hasenkopf Owen B. Toon Margaret A. Tolbert Christopher P. McKay 《Icarus》2008,195(2):792-801
Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere. 相似文献
14.
The mechanisms that can induce short term variations of methane in the Martian atmosphere, and thus explain the observations currently available, are yet to be discovered. Seasonal exchange with the regolith, caused by reversible adsorption, is expected to induce both spatial and time variabilities without the need for additional sources and sinks, thus avoiding difficulties raised by other scenarios. However, a comprehensive view of the role of reversible exchanges with the subsurface was still lacking. We have investigated the efficiency of such a process by implementing a coupled subsurface–atmosphere transport module in a Global Climate Model, taking into account both the thermodynamics and the kinetics of the adsorption process. It is based on recent experimental data on the adsorption of methane. We show that even with an optimistic set of parameters, and although the regolith can potentially take up a large fraction of the atmospheric reservoir, the seasonal variability induced by an exchange with the subsurface is very limited. If a local plume is detected, however, the apparent decay rate of methane in the atmosphere can be affected by the regolith uptake. This study could be extended to any trace gas reacting with the regolith, to help interpret future in situ or orbital measurements. 相似文献
15.
Spectral observations have detected methane within the martian atmosphere (Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M. [2004]. Science 306, 1758–1761; Mumma, M.J. et al. [2009]. Science 323, 1041–1045), however, the origin of the methane has not been determined. Methane clathrate (also referred to as methane hydrate) has been suggested as a potential subsurface reservoir, storing and releasing biologic and/or abiogenic methane. In this study, rates of methane hydrate formation and dissociation were measured experimentally at 234–264 K and 1.4–4.7 MPa to test the clathrate reservoir hypothesis. Initial formation rates range from 4.3 × 10?6 to 8.1 × 10?5 mol m?2 s?1. Results show decreasing rates of formation over time in individual experiments, indicating initial rapid clathration, followed by diffusion-limited transport of methane into the ice through the previously formed hydrate. These experiments indicate increased pressure results in increased formation rates, likely the result of higher concentration gradients, enhancing the methane diffusion flux into the solid phase. Experiments conducted at elevated temperatures produced faster initial rates of formation, resulting from increased kinetic energy of methane molecules and/or thickening of the Quasi-Liquid Layer. Based on this temperature dependence, the activation energy for methane hydrate formation from ice was determined to be 35.9 kJ/mol. Hydrate dissociation experiments initiated by depressurization or warming at conditions between 222 K and 265 K and 0.1–2.0 MPa were conducted following each formation experiment, yielding methane hydrate dissociation rates from 3.01 × 10?6 to 9.92 × 10?5 mol m?2 s?1. While both hydrate dissociation and formation showed decreasing instantaneous rates over the course of each experiment, the transition between the initial rate of dissociation and the interpreted diffusion-limited period of continued dissociation was more abrupt than that observed in formation experiments, supporting an ice shielding effect. The initial concentration of methane in the solid phase had a significant effect on hydrate dissociation rates. Higher methane concentrations in the solid phase produce faster initial rates, likely due to increased concentration gradients, thus increasing the diffusion component of dissociation. Increased temperatures also produced faster dissociation rates, yielding an activation energy for dissociation of 32.7 kJ/mol. The rates determined within this study suggest that small near-surface methane hydrate reservoirs are a feasible source for recent methane plumes detected on Mars. Rates of methane release from gas hydrates also indicate that gas hydrate dissociation may have played a role in forming ancient chaos terrain and associated outflow channels. 相似文献
16.
Far infrared spectra (10-600 cm−1) from Cassini's Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturn's atmosphere. Three observations, comprising a total of 3088 spectra, gave 3σ upper limits on HF, HCl, HBr, and HI volume mole fractions of 8.0×10−12, 6.7×10−11, 1.3×10−10, and 1.4×10−9, respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCl, and HBr in Saturn's upper atmosphere—consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCl is 16 times lower than the tentative detection at 1.1×10−9 reported by Weisstein and Serabyn [Weisstein, E.W., Serabyn, E., 1996. Icarus 123, 23-36]. These observations are not sensitive to the deep halide abundance, which is expected to be enriched relative to the solar composition. 相似文献
17.
We investigate the ability to refine pyroxene composition and modal abundance from laboratory and remotely acquired spectra. Laboratory data including the martian meteorites, Shergotty, Zagami, MIL03346, and ALH84001 as well as additional pyroxene-rich spectra obtained from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) spectrometer for Mars are characterized using the Modified Gaussian Model (MGM), a spectral deconvolution method developed by Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S., 1990. J. Geophys. Res. 95, 6955-6966]. We develop two sensitivity tests to assess the extent to which the MGM can consistently predict (1) pyroxene composition and (2) modal abundance for a compositionally diverse suite of pyroxene spectra. Results of the sensitivity tests indicate that the MGM can be appropriately applied to remote spectroscopic measurements of extraterrestrial surfaces and can estimate pyroxene composition and relative abundance within a derived uncertainty. Deconvolved band positions for laboratory spectra of the meteorites Shergotty and Zagami are determined within ±17 nm while remotely acquired OMEGA spectra are defined within ±50 nm. These results suggest that absolute compositions can be uniquely derived from laboratory pyroxene-rich spectra and non-uniquely derived from the remote measurements of OMEGA at this time. While relative pyroxene chemistries are not assessed from OMEGA measurements at this time, relative pyroxene abundances are estimated using a normalized band strength ratio between the low-calcium (LCP) and high-calcium (HCP) endmember components and are constrained to ±10%. The fraction of LCP in a two-pyroxene mixture is the derived value from the normalized band strength ratio, LCP/(LCP + HCP). This calculation for relative abundance is robust in the presence of up to 10-15% olivine. Deconvolution results from the OMEGA spectra indicate that the ancient terrain in the Syrtis Major region is uniquely enriched in LCP (59±10% LCP) relative to HCP while the volcanics of Syrtis Major are uniquely enriched in HCP (39±10% LCP). 相似文献
18.
We calculate the amount of methane that may form via reactions catalyzed by metal-rich dust that condenses in the wake of large cometary impacts. Previous models of the gas-phase chemistry of impacts predicted that the terrestrial planets' atmospheres should be initially dominated by CO/CO2, N2, and H2O. CH4 was not predicted to form in impacts because gas-phase reactions in the explosion quench at temperatures ∼2000 K, at which point all of the carbon is locked in CO. We argue that the dust that condenses out in the wake of a large comet impact is likely to have very effective catalytic properties, opening up reaction pathways to convert CO and H2 to CH4 and CO2, at temperatures of a few hundred K. Together with CO2, CH4 is an important greenhouse gas that has been invoked to compensate for the lower luminosity of the Sun ∼4 Gyr ago. Here, we show that heterogeneous (gas-solid) reactions on freshly-recondensed dust in the impact cloud may provide a plausible nonbiological mechanism for reducing CO to CH4 before and during the emergence of life on Earth, and perhaps Mars as well. These encouraging results emphasize the importance of future research into the kinetics and catalytic properties of astrophysical condensates or “smokes” and also more detailed models to determine the conditions in impact-generated dust clouds. 相似文献
19.
20.
We have constructed a model that predicts the evolution of CO2 on Mars from the end of the heavy bombardment period to the present. The model draws on published estimates of the main processes believed to affect the fate of CO2 during this period: chemical weathering, regolith uptake, polar cap formation, and atmospheric escape. Except for escape, the rate at which these processes act is controlled by surface temperatures which we calculate using a modified version of the Gierasch and Toon energy balance model (1973, J. Atmos. Sci. 30, 1502-1508). The modifications account for the change in solar luminosity with time, the greenhouse effect, and a polar and solar equatorial energy budget. Using published estimates for the main parameters, we find no evolutionary scenario in which CO2 is capable of producing a warm (global mean temperatures>250 K) and wet (surface pressures>30 mbar) early climate, and then evolves to present conditions with approximately 7 mbar in the atmosphere, <300 mbar in the regolith, and <5 mbar in the caps. Such scenarios would only exist if the early sun were brighter than standard solar models suggest, if greenhouse gases other than CO2 were present in the early atmosphere, or if the polar albedo were significantly lower than 0.75. However, these scenarios generally require the storage of large amounts of CO2 (>1 bar) in the carbonate reservoir. If the warm and wet early Mars constraint is relaxed, then we find best overall agreement with present day reservoirs for initial CO2 inventories of 0.5-1.0 bar. We also find that the polar caps can a profound effect on how the system evolves. If the initial amount of CO2 is less than some critical value, then there is not enough heating of the poles to prevent permanent caps from forming. Once formed, these caps control how the system evolves, because they set the surface pressure and, hence, the thermal environment. If the initial amount of CO2 is greater than this critical value, then caps do not form initially, but can form later on, when weathering and escape lower the surface pressure to a point at which polar heating is no longer sufficient to prevent cap formation and the collapse of the climate system. Our modeling suggests this critical initial amount of CO2 is between 1 and 2 bar, but its true value will depend on all factors affecting the polar heat budget. 相似文献