A model of Titan's haze of fractal aerosols constrained by multiple observations

We use Titan's geometric albedo to constrain the vertical distribution of the haze. Microphysical models incorporating fractal aggregates do not readily fit the methane features at 0.62 μm band and the dark 0.88 μm of the albedo spectrum simultaneously. We take advantage of this apparent discrepancy to constrain the haze vertical profile.We used the geometric albedo and several results and constraints from other works to better constrain the vertical haze extinction profile, especially in the low stratosphere. The objective of this model is to give a solution that simultaneously fits the main constraints known to apply to the haze.We find that the haze extinction increases with decreasing altitude with a scale height about equal to the atmospheric scale height down to 100 km. Below this altitude, extinction must decrease down to 30 km. This is necessary in order to have enough haze to sustain a relatively high albedo (0.076) in the dark 0.88 μm methane band and to show the 0.62 μm band in the haze continuum. We set the haze production rate around 7×10⁻¹⁴ kgm⁻² s⁻¹, and the aerosols production altitude around 400 km (or at pressure 1.5 Pa).The physical processes which generate such a profile are not clear. However, purely one-dimensional effects such as condensation, sedimentation, and rainout can be ruled out, and we believe that this relative clearing in Titan's troposphere and lower stratosphere is due to particle horizontal transport by the mean circulation.     

Formation of methane in comet impacts: implications for Earth, Mars, and Titan

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.     

PSR J1740−3052: a pulsar with a massive companion

We report on the discovery of a binary pulsar, PSR J1740−3052, during the Parkes multibeam survey. Timing observations of the 570-ms pulsar at Jodrell Bank and Parkes show that it is young, with a characteristic age of 350 kyr, and is in a 231-d, highly eccentric orbit with a companion whose mass exceeds 11 M_⊙. An accurate position for the pulsar was obtained using the Australia Telescope Compact Array. Near-infrared 2.2-μm observations made with the telescopes at the Siding Spring observatory reveal a late-type star coincident with the pulsar position. However, we do not believe that this star is the companion of the pulsar, because a typical star of this spectral type and required mass would extend beyond the orbit of the pulsar. Furthermore, the measured advance of periastron of the pulsar suggests a more compact companion, for example, a main-sequence star with radius only a few times that of the Sun. Such a companion is also more consistent with the small dispersion measure variations seen near periastron. Although we cannot conclusively rule out a black hole companion, we believe that the companion is probably an early B star, making the system similar to the binary PSR J0045−7319.     

Can rapid loss and high variability of Martian methane be explained by surface H2O2?

It has been reported by several groups that methane in the Martian atmosphere is both spatially and temporally variable. Gough et al. (2010) suggested that temperature dependent, reversible physical adsorption of methane onto Martian soils could explain this variability. However, it is also useful to consider if there might be chemical destruction of methane (and compensating sources) operating on seasonal time scales. The lifetime of Martian methane due to known chemical loss processes is long (on the order of hundreds of years). However, observations constrain the lifetime to be 4 years or less, and general circulation models suggest methane destruction must occur even faster (<1 year) to cause the reported variability and rapid disappearance. The Martian surface is known to be highly oxidizing based on the Viking Labeled Release experiments in which organic compounds were quickly oxidized by samples of the regolith. Here we test if simulated Martian soil is also oxidizing towards methane to determine if this is a relevant loss pathway for Martian methane. We find that although two of the analog surfaces studied, TiO₂·H₂O₂ and JSC-Mars-1 with H₂O₂, were able to oxidize the complex organic compounds (sugars and amino acids) used in the Viking Labeled Release experiments, these analogs were unable to oxidize methane to carbon dioxide within a 72 h experiment. Sodium and magnesium perchlorate, salts that were recently discovered at the Phoenix landing site and are potential strong oxidants, were not observed to directly oxidize either the organic solution or methane. The upper limit reaction coefficient, α, was found to be <4×10^?17 for methane loss on TiO₂·H₂O₂ and <2×10^?17 for methane loss on JSC-Mars-1 with H₂O₂. Unless the depth of soil on Mars that contains H₂O₂ is very deep (thicker than 500 m), the lifetime of methane with respect to heterogeneous oxidation by H₂O₂ is probably greater than 4 years. Therefore, reaction of methane with H₂O₂ on Martian soils does not appear to be a significant methane sink, and would not destroy methane rapidly enough to cause the reported atmospheric methane variability.     

The surface energy balance at the Huygens landing site and the moist surface conditions on Titan

The Huygens Probe provided a wealth of data concerning the atmosphere of Titan. It also provided tantalizing evidence of a small amount of surface liquid. We have developed a detailed surface energy balance for the Probe landing site. We find that the daily averaged non-radiative fluxes at the surface are 0.7 W m^?2, much larger than the global average value predicted by McKay et al. (1991) of 0.037 W m^?2. Considering the moist surface, the methane and ethane detected by the Probe from the surface is consistent with a ternary liquid of ethane, methane, and nitrogen present on the surface with mole fractions of methane, ethane, and nitrogen of 0.44, 0.34, and 0.22, respectively, and a total mass load of ～0.05 kg m^?2. If this liquid is included in the surface energy balance, only a small fraction of the non-radiative energy is due to latent heat release (～10^?3 W m^?2). If the amount of atmospheric ethane is less than 0.6×10^?5, the surface liquid is most likely evaporating over timescales of 5 Titan days, and the moist surface is probably a remnant of a recent precipitation event. If the surface liquid mass loading is increased to 0.5 kg m^?2, then the liquid lifetime increases to ～56 Titan days. Our modeling results indicate a dew cycle is unlikely, given that even when the diurnal variation of liquid is in equilibrium, the diurnal mass variation is only 3% of the total liquid. If we assume a high atmospheric mixing ratio of ethane (>0.6×10^?5), the precipitation of liquid is large (38 cm/Titan year for an ethane mixing ratio of 2×10^?5). Such a flux is many orders of magnitude in excess of the photochemical production rate of ethane.     

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.     

Possibilities for methanogenic life in liquid methane on the surface of Titan

Photochemically produced compounds on Titan, principally acetylene, ethane and organic solids, would release energy when consumed with atmospheric hydrogen, at levels of 334, 57, and 54 kJ mol⁻¹, respectively. On Earth methanogenic bacteria can survive on this energy level. Here we speculate on the possibility of widespread methanogenic life in liquid methane on Titan. Hydrogen may be the best molecule to show the affects of such life because it does not condense at the tropopause and has no sources or sinks in the troposphere. If life is consuming atmospheric hydrogen it will have a measurable effect on the hydrogen mixing ratio in the troposphere if the biological consumption is greater than . Life could develop strategies to overcome the low solubility of organics in liquid methane and use catalysts to accelerate biochemical reactions despite the low temperature. The results of the recent Huygens probe could indicate the presence of such life by anomalous depletions of acetylene and ethane as well as hydrogen at the surface.