Influences of atmospheric variations on Mars's record of small craters

Planetary atmospheres influence cratering rates at small diameters (∼2-250 m) by filtering impactor populations via ablation, aerobraking and breakup of entering objects. The atmosphere of Mars undergoes rapid and drastic obliquity-driven variations in density, corresponding to pressure variations between zero and several tens of millibars. Here a simulation is used to assess the fate of a large population of impactors interacting with the present and predicted past and future martian atmospheres. We find that even Mars's present atmosphere significantly reduces crater production rates at small diameters (<30 m) and past denser atmospheres would have affected cratering even more strongly, and to considerably larger diameters. These effects are increased if the inner Solar System's small impactor population contains significant numbers of icy, cometary bodies. Evidence of recent atmospheric density variations may be detectable in the martian small cratering record with future planned imaging capabilities. Because of martian atmospheric effects and variations, surface ages derived from counts of craters of less than about 250 m on Mars may be underestimated.     

Neutron Capture Isotopes in the Martian Regolith and Implications for Martian Atmospheric Noble Gases

Impact-produced glasses in some martian meteorites have trapped significant amounts of the recent martian atmosphere. From literature data, we estimate that ∼9% of the trapped ⁸⁰Kr in these meteorites was produced from neutron capture on ⁷⁹Br. Estimates of neutron fluences made from ⁸⁰Kr and ¹⁴⁹Sm for bulk samples of meteorite EET79001 indicate that ⁸⁰Kr excesses in the impact glass were not produced in situ. Theoretical calculations independently predict production of a large neutron-capture component of ⁸⁰Kr and ³⁶Ar in the martian regolith, and part of this component presumably escaped into the martian atmosphere. These calculations were made by using the Los Alamos High-Energy Transport Code to calculate the fluxes of galactic cosmic ray (GCR)-produced thermal neutrons as a function of depth in the uppermost 500 g cm⁻² of the martian surface, and by adopting average Cl, Br, and I concentrations of the upper martian surface of ∼0.3%, ∼20 ppm, and ∼0.5 ppm, respectively. Combining these data with the appropriate neutron-capture cross sections, we calculate Mars global production rates of ⁸⁰Kr_n=2.4×10¹⁶atoms sec⁻¹, ³⁶Ar_n=5.5×10¹⁸ atoms sec⁻¹, and ¹²⁸Xe_n=3×10¹³ atoms sec⁻¹. Calculated global production rates of spallogenic ⁸⁰Kr_sp, and ³⁶Ar_sp, are smaller by factors of ∼770 and ∼29, respectively. It would require ∼330 Myr to produce an amount of ⁸⁰Kr_n equivalent to the amount inferred to be present today in the martian atmosphere (∼2.5×10³² atoms). Production of these neutron-capture components probably has occurred over the past ∼4 Gyr, as only an atmospheric pressure substantially higher than today's would appreciably decrease the neutron flux in the regolith. Thus, most of the neutron-capture noble gases produced over time probably remain in the martian regolith and would make sensitive indicators of the time period a sample has resided near the martian surface. Assuming mixing of the martian surface to an average depth of 100 m, the predicted average regolith concentrations of ⁸⁰Kr_n, ³⁶Ar_n, and ¹²⁸Xe_n are ∼4×10⁻⁹ cm⁻³ g⁻¹, ∼1×10⁻⁶ cm³ g⁻¹, and ∼5×10⁻¹² cm³ g⁻¹, respectively. If similar fractions of these neutron-capture isotopes have escaped into the atmosphere, they would comprise ∼3% and ∼0.2% of the present atmospheric inventories of ³⁶Ar and ¹²⁸Xe, respectively. The fractional excess of ⁸⁰Kr_n in ancient martian meteorite ALH84001 appears similar to that in shock-glass phases of young shergottite meteorites. If ALH84001 acquired its atmospheric gases ∼4 Gyr ago, this implies that, prior to that time, halogens were greatly concentrated at the martian surface by crustal formational and weathering processes, impacts efficiently degassed the regolith, and Mars did not have a significant atmosphere to shield the surface.     

Investigation of the water sorption properties of Mars-relevant micro- and mesoporous minerals

Encouraged by recent results of the Mars Odyssey spacecraft mission and the OMEGA team (Mars Express) concerning water in equatorial latitudes between ±45° on Mars and the possible existence of hydrated minerals, we have investigated the water sorption properties of natural zeolites and clay minerals close to martian atmospheric surface conditions as well as the properties of Mg-sulfates and gypsum. To quantify the stability of hydrous minerals on the martian surface and their interaction with the martian atmosphere, the water adsorption and desorption properties of nontronite, montmorillonite, chabazite and clinoptilolite have been investigated using adsorption isotherms at low equilibrium water vapor pressures and temperatures, modeling of the adsorption equilibrium data, thermogravimetry (TG), differential scanning calorimetry (DSC), and proton magic angle spinning nuclear magnetic resonance measurements (¹H MAS NMR). Mg-sulfate hydrates were also analyzed using TG/DSC methods to compare with clay mineral and zeolites. Our data show that these microporous minerals can remain hydrated under present martian atmospheric conditions and hold up to 2.5-25 wt% of water in their void volumes at a partial water vapor pressure of 0.001 mbar in a temperature range of 333-193 K. Results of the ¹H MAS NMR measurements suggest that parts of the adsorbed water are liquid-like water and that the mobility of the adsorbed water might be of importance for adsorption-water-triggered chemistry and hypothetical exobiological activity on Mars.     

Martian xenon components in Shergotty mineral separates: Locations,sources, and trapping mechanisms

Abstract— Isotopic signatures and concentrations of xenon have been measured in Shergotty mineral separates by laser step heating. Martian atmosphere and ‘martian interior’ xenon are present, as is a spallation component. Martian atmospheric xenon is 5–10 times more concentrated in opaque minerals (magnetite, ilmenite, and pyrrhotite) and maskelynite than in pyroxenes, perhaps reflecting grain size variation. This is shown to be consistent with shock incorporation. A component consisting of solar xenon with a fission contribution, similar to components previously identified in martian meteorites and associated with the martian interior, is best defined in the pyroxene‐dominated separates. This component exhibits a consistent ¹²⁹Xe (¹²⁹Xe/¹³²Xe ?1.2) excess over solar/planetary (¹²⁹Xe/¹³²Xe ?1.04). We suggest that gas present in the melt, perhaps a mixture of interior xenon and martian atmosphere, was incorporated into the pyroxenes in Shergotty as the minerals crystallized.     

The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres

The transmission of light through a planetary atmosphere can be studied as a function of altitude and wavelength using stellar or solar occultations, giving often unique constraints on the atmospheric composition. For exoplanets, a transit yields a limb-integrated, wavelength-dependent transmission spectrum of an atmosphere. When scattering haze and/or cloud particles are present in the planetary atmosphere, the amount of transmitted flux not only depends on the total optical thickness of the slant light path that is probed, but also on the amount of forward-scattering by the scattering particles. Here, we present results of calculations with a three-dimensional Monte Carlo code that simulates the transmitted flux during occultations or transits. For isotropically scattering particles, like gas molecules, the transmitted flux appears to be well-described by the total atmospheric optical thickness. Strongly forward-scattering particles, however, such as commonly found in atmospheres of Solar System planets, can increase the transmitted flux significantly. For exoplanets, such added flux can decrease the apparent radius of the planet by several scale heights, which is comparable to predicted and measured features in exoplanet transit spectra. We performed detailed calculations for Titan’s atmosphere between 2.0 and 2.8 μm and show that haze and gas abundances will be underestimated by about 8% if forward-scattering is ignored in the retrievals. At shorter wavelengths, errors in the gas and haze abundances and in the spectral slope of the haze particles can be several tens of percent, also for other Solar System planetary atmospheres. We also find that the contribution of forward-scattering can be fairly well described by modelling the atmosphere as a plane-parallel slab. This potentially reduces the need for a full three-dimensional Monte Carlo code for calculating transmission spectra of atmospheres that contain forward-scattering particles.     

The chemical variability at the surface of Mars: Implication for sediment formation and rock weathering

Compositional data analysis was performed on chemical compositions of martian surface materials in order to unravel scenarios of past and present weathering and to evaluate the role of meteoritic accumulation. The observed chemical variability is analyzed by means of principal component analysis. Potential reservoirs that may have contributed primary material to soil formation are assessed. Chemical alteration in the course of in situ weathering is described in terms of alteration vectors that link the compositions of fresh rocks and their weathering crusts. The interplay of localized chemical alteration and global scale re-distribution and mixing of fines material is documented through the identification of different soil forming branches. These branches emanate from distinct compositional domains, which comprise basaltic and basalt-andesitic primary materials, and they converge to a global dust composition, which represents the product of chemical and physical disintegration and subsequent global mixing. Mass balance considerations applied to localized weathering phenomena are in line with findings from experimental acid-sulfate weathering on olivine-bearing basalts and the persistence of secondary silica in evaporitic rocks. In addition the composition and oxidation state of involved volcanic gases is deduced. Our findings corroborate the past activity of volcanic exhalation products in combination with liquid water. We conclude that average martian crust is dominated by basaltic materials at its topmost level and that the amount of meteoritic accumulation may contribute about 6 wt% to the martian fines. From the meteoritic contribution minimum soil formation rates of 60±20 cm/Gyr are derived. Sequestration of atmospheric oxygen during weathering of primary materials may account for the oxygen deficiency of the martian atmosphere. A 4-14-m-thick layer of oxidized martian fines may account for the estimated deficit of 1.7×¹⁰¹⁸ mol O₂ in the martian atmosphere depending on the primary oxidation state of volatile volcanic emanations.     

Mars’ atmospheric 40Ar: A tracer for past crustal erosion

Noble gas ⁴⁰Ar may be used as a tracer of the past evolution of volatiles in Mars’ crust, mantle and atmosphere. ⁴⁰Ar is formed by the radioactive decay of ⁴⁰K in the mantle and in the crust and is released from the mantle to the atmosphere due to volcanism and from the crust by erosion such as eolian and hydrothermal erosion. Furthermore, ⁴⁰Ar can escape from the atmosphere into space via atmospheric escape mechanisms. The evolution of the atmospheric abundance of ⁴⁰Ar thus depends on these three processes whose efficiencies vary with time.In the present study we reconsider atmospheric escape mechanism efficiencies and describe various possible scenarios of the evolution of ⁴⁰Ar with a model describing the three main reservoirs of ⁴⁰Ar, the mantle, crust and atmosphere. First, we show that atmospheric escape, which is stronger in the early evolution, does not significantly influence the present abundance of the atmospheric ⁴⁰Ar. In the early evolution the atmospheric concentration of ⁴⁰Ar is very low as the outgassing of ⁴⁰Ar from the mantle occurs relatively late in the martian evolution. Thus, the atmospheric ⁴⁰Ar concentration is essentially a tracer of Mars’ outgassing history and not of the escape processes. Second, using the results of the most recent published crustal formation models, the calculated present ⁴⁰Ar atmospheric abundance is smaller than its observed value. This discrepancy may be explained by a significant ⁴⁰Ar supply from the crust by erosion (16–30% of the ⁴⁰Ar content of the upper first 10 km of crust). The knowledge of the fraction of crustal ⁴⁰Ar outgassed to the atmosphere is an important constraint for any future global modelling of past Mars’ hydrothermal activity aiming at better characterizing the role of subsurface aqueous alteration processes in Mars climate evolution. One of the main sources of the uncertainty of these results is the present uncertainty in the measured atmospheric ⁴⁰Ar value (±20%). More precise measurements of ⁴⁰Ar and ³⁶Ar in the martian atmosphere are therefore required to better constrain the model.     

Minimum estimates of the amount and timing of gases released into the martian atmosphere from volcanic eruptions

Volcanism has been a major process during most of the geologic history of Mars. Based on data collected from terrestrial basaltic eruptions, we assume that the volatile content of martian lavas was typically ∼0.5 wt.% water, ∼0.7 wt.% carbon dioxide, ∼0.14 wt.% sulfur dioxide, and contained several other important volatile constituents. From the geologic record of volcanism on Mars we find that during the late Noachian and through the Amazonian volcanic degassing contributed ∼0.8 bar to the martian atmosphere. Because most of the outgassing consisted of greenhouse gases (i.e., CO₂ and SO₂) warmer surface temperatures resulting from volcanic eruptions may have been possible. Our estimates suggest that ∼1.1 × 10²¹ g (∼8 ± 1 m m⁻²) of juvenile water were released by volcanism; slightly more than half the amount contained in the north polar cap and atmosphere. Estimates for released CO₂ (1.6 × 10²¹ g) suggests that a large reservoir of carbon dioxide is adsorbed in the martian regolith or alternatively ∼300 cm cm⁻² of carbonates may have formed, although these materials would not occur readily in the presence of excess SO₂. Up to ∼120 cm cm⁻² (2.2 × 10²⁰ g) of acid rain (H₂SO₄) may have precipitated onto the martian surface as the result of SO₂ degassing. The hydrogen flux resulting from volcanic outgassing may help explain the martian atmospheric D/H ratio. The amount of outgassed nitrogen (∼1.3 mbar) may also be capable of explaining the martian atmospheric ¹⁵N/¹⁴N ratio. Minor gas constituents (HF, HCl, and H₂S) could have formed hydroxyl salts on the surface resulting in the physical weathering of geologic materials. The amount of hydrogen fluoride emitted (1.82 × 10¹⁸ g) could be capable of dissolving a global layer of quartz sand ∼5 mm thick, possibly explaining why this mineral has not been positively identified in spectral observations. The estimates of volcanic outgassing presented here will be useful in understanding how the martian atmosphere evolved over time.     

Does Io have an ammonia atmosphere?

An atmosphere containing 0.5 cm atm of ammonia is assumed on Io. Such an atmosphere will be frozen at the unilluminated pole during the solstices, but will evaporate at the equinoctial seasons. The ammonia atmosphere will explain: (1) the posteclipse brightenings and their observed times of occurrence and nonocurrence; (2) the observed departure from a two-layer model beating curve upon emergence from eclipse; (3) the discordant temperatures obtained at 10 and 20 μm; and (4) discordant temperatures obtained at 10 and 20 μm during the total phase of an eclipse by Jupiter.In order to explain items 3 and 4 above, a proton flux in Jupiter's magnetosphere of 1.1 × 10⁹ cm^?2s^? at an energy of 0.5MeV at io's distance from Jupiter is assumed. This flux is 40 times the flux in Divine's (1972) “upper-limit” model of the Jovian radiation belts, while the proton energy is eight times less. The proton flux, plus the solar ultraviolet and infrared flux absorbed by the ammonia, will heat the atmosphere to 245 ± 10°K. At this temperature the occultation atmospheric upper limit allows the addition of 4 cmatm of nitrogen.     

Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years

Abstract— We analyzed noble gases from 18 samples of weathering products (“iddingsite”) from the Lafayette meteorite. Potassium‐argon ages of 12 samples range from near zero to 670 ± 91 Ma. These ages confirm the martian origin of the iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric ⁴⁰Ar. In any case, because iddingsite formation requires liquid water, this data requires the presence of liquid water near the surface of Mars at least as recently as 1300 Ma ago, and probably as recently as 650 Ma ago. Krypton and Xe analysis of a single 34 μg sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas. Our strongest conclusion is that the iddingsite in Lafayette formed on Mars, in agreement with the microstratigraphic arguments of Gooding et al. (1991) and Treiman et al. (1993). A preterrestrial origin of the iddingsite is required both by the many non‐zero K‐Ar ages and by the presence of Xe that is isotopically distinct from any terrestrial Xe. The Xe is accompanied by Kr, but the Kr and Xe have been fractionated if they are derived from the present martian atmosphere. This is presumably the result of either incorporation via interaction with liquid water (Drake et al., 1994; Bogard and Garrison, 1998) or by adsorption from the martian atmosphere, perhaps accompanied by shock (see also Gilmour et al., 1998, 1999). Although the iddingsite is enriched in Kr and Xe compared to whole‐rock analyses, it is not clear whether iddingsite is the dominant carrier of the atmospheric‐derived gas (Drake et al., 1994) or merely a minor carrier (Gilmour et al., 1999). Our ⁴⁰Ar‐³⁹Ar experiment was disappointing, in that it mostly served to confirm that the iddingsite, which contains fine‐grained clays, is susceptible to recoil loss of ³⁹Ar during irradiation. Only one sample of five gave a clear signal of radiogenic or extraterrestrial ⁴⁰Ar, and that was only by 3°. Potassium‐argon ages of the second set of samples were more successful, ranging from near 0 to 670 ± 91 Ma. It is not clear whether any or all of the ages represent iddingsite formation, as opposed to later alteration. The fact that a Rb‐Sr experiment (Shih et al., 1998) gave an apparent age for iddingsite of 679 ± 66 Ma (2a) suggests that perhaps formation of iddingsite occurred (or began) ～650 Ma ago and that some samples either formed, or were thermally altered, later. The ages could be even younger than 650 Ma, if the samples have incorporated martian atmospheric ⁴⁰Ar. This means that liquid water was certainly present on Mars in the last 1300 Ma (the formation age of Lafayette), and probably within the last 650 Ma.     

Water sorption on martian regolith analogs: Thermodynamics and near-infrared reflectance spectroscopy

The near-infrared reflectance spectra of the martian surface present strong absorption features attributed to hydration water present in the regolith. In order to characterize the relationships between this water and atmospheric vapor and decipher the physical state of water molecules in martian regolith analogs, we designed and built an experimental setup to measure near-IR reflectance spectra under martian atmospheric conditions. Six samples were studied that cover part of the diversity of Mars surface mineralogy: a hydrated ferric oxide (ferrihydrite), two igneous samples (volcanic tuff, and dunite sand), and three potential water rich soil materials (Mg-sulfate, smectite powder and a palagonitic soil, the JSC Mars-1 regolith stimulant). Sorption and desorption isotherms were measured at 243 K for water vapor pressure varying from 10⁻⁵ to ∼0.3 mbar (relative humidity: 10⁻⁴ to 75%). These measurements reveal a large diversity of behavior among the sample suite in terms of absolute amount of water adsorbed, shape of the isotherm and hysteresis between the adsorption and desorption branches. Simultaneous in situ spectroscopic observations permit a detailed analysis of the spectral signature of adsorbed water and also point to clear differences between the samples. Ferric (oxy)hydroxides like ferrihydrite or other phases present in palagonitic soils are very strong water adsorbent and may play an important role in the current martian water cycle by allowing large exchange of water between dust-covered regions and atmosphere at diurnal and seasonal scales.     

Loss of water from Mars:: Implications for the oxidation of the soil

The evolution of the martian atmosphere with regard to its H₂O inventory is influenced by thermal loss processes of H, H₂, nonthermal atmospheric loss processes of H⁺, H₂⁺, O, O⁺, CO₂, and O₂⁺ into space, as well as by chemical weathering of the surface soil. The evolution of thermal and nonthermal escape processes depend on the history of the intensity of the solar XUV radiation and the solar wind density. Thus, we use actual data from the observation of solar proxies with different ages from the Sun in Time program for reconstructing the Sun's radiation and particle environment from the present to 3.5 Gyr ago. The correlation between mass loss and X-ray surface flux of solar proxies follows a power law relationship, which indicates a solar wind density up to 1000 times higher at the beginning of the Sun's main sequence lifetime. For the study of various atmospheric escape processes we used a gas dynamic test particle model for the estimation of the pick up ion loss rates and considered pick up ion sputtering, as well as dissociative recombination. The loss of H₂O from Mars over the last 3.5 Gyr was estimated to be equivalent to a global martian H₂O ocean with a depth of about 12 m, which is smaller than the values reported by previous studies. If ion momentum transport, a process studied in detail by Mars Express is significant on Mars, the water loss may be enhanced by a factor of about 2. In our investigation we found that the sum of thermal and nonthermal atmospheric loss rates of H and all nonthermal escape processes of O to space are not compatible with a ratio of 2:1, and is currently close to about 20:1. Escape to space cannot therefore be the only sink for oxygen on Mars. Our results suggest that the missing oxygen (needed for the validation of the 2:1 ratio between H and O) can be explained by the incorporation into the martian surface by chemical weathering processes since the onset of intense oxidation about 2 Gyr ago. Based on the evolution of the atmosphere-surface-interaction on Mars, an overall global surface sink of about 2×10⁴² oxygen particles in the regolith can be expected. Because of the intense oxidation of inorganic matter, this process may have led to the formation of considerable amounts of sulfates and ferric oxides on Mars. To model this effect we consider several factors: (1) the amount of incorporated oxygen, (2) the inorganic composition of the martian soil and (3) meteoritic gardening. We show that the oxygen incorporation has also implications for the oxidant extinction depth, which is an important parameter to determine required sampling depths on Mars aimed at finding putative organic material. We found that the oxidant extinction depth is expected to lie in a range between 2 and 5 m for global mean values.     

A search for neutrons of solar origin using balloon borne detectors 1967–69

A series of telescopes having approximately a 30° half opening angle and responding to neutrons in the energy range 50 MeV to 350 MeV has been flown to the top of the atmosphere on balloons released from an equatorial launching site at Kampala, Uganda, between 1967 and 1969. The aim of the experiment was to attempt to detect solar neutrons during periods of enhanced solar activity. No neutrons of solar origin were detected, but an upper limit of the order of 30 neutrons m^–2 s^–1 at the Earth has been placed on the continuous solar neutron flux in the above energy range, and a limit of four photons m^–2 s^–1 has also been placed on the corresponding -ray flux above 80 MeV. Limits have likewise been placed on the total emission from various flares. For a 1B flare the values were 23 × 10⁴ neutrons m^–2 and 6 × 10⁴ photons m^–2.     

Atmospheric variations and meteorite production on Mars

Through a combination of aerobraking (drag deceleration) and ablation, meteoroids which enter planetary atmospheres may be slowed sufficiently to soft-land as meteorites. Results of an earlier study suggest that the current 6 mbar atmosphere of Mars is sufficient to aerobrake significant numbers of small (<10 kg) asteroidal-type meteoroids into survivable, low-velocity (<500 m s⁻¹) impacts with the planet's surface. Since rates of meteorite production depend upon the density of Mars's atmosphere, they must also change as the martian climate changes. However, to date, martian meteorite production has received relatively little attention in the literature Here we expand upon our previous work to study martian meteorite production rates and how they depend upon variations of the martian atmosphere, and to estimate the ranges of mass, velocity and entry-angle that produce meteorites. We find that even the current atmosphere of Mars is sufficient to soft-land significant fractions of incident stony and iron objects, and that these fractions increase dramatically for denser martian atmospheres. Therefore, like impact cratering, meteorite populations may preserve evidence of past martian climates.     

On the influence of neutral interstellar matter on the upper atmosphere

Neutral interstellar matter entering the solar system has been considered in respect to its influences on the upper atmosphere. Calculations show that in consequence of the focussing effect due to the sun's gravitational field the incoming neutral hydrogen and helium under special, but possible conditions will represent a semi-annually varying density along the earth's orbit. The particle fluxes amounting at least to some 10⁷ cm^?2 sec^?1, which are connected with these density-profiles and reach the upper atmosphere, show annual periodicities and so will cause annual variations of the densities of the light, atmospheric gas constituents. Especially it is to be expected, that so produced density variations of atmospheric hydrogen are important. Temperature increases caused by the energy flux of interstellar particles should in general only amount to a few thousandths of the CIRA-temperatures.     

Ejection ages from krypton‐81‐krypton‐83 dating and pre‐atmospheric sizes of martian meteorites

Abstract— Cosmic‐ray exposure (CRE) ages and Mars ejection times were calculated from the radionuclide ⁸¹Kr and stable Kr isotopes for seven martian meteorites. The following ⁸¹Kr‐Kr CRE ages were obtained: Los Angeles = 3.35 ± 0.70 Ma; Queen Alexandra Range 94201 = 2.22 ± 0.35 Ma; Shergotty = 3.05 ± 0.50 Ma; Zagami = 2.98 ± 0.30 Ma; Nakhla = 10.8 ± 0.8 Ma; Chassigny = 10.6 ± 2.0 Ma; and Allan Hills 84001 = 15.4 ± 5.0 Ma. Comparison of these ages with previously obtained CRE ages from the stable noble gas nuclei ³He, ²¹Ne, and ³⁸Ar shows excellent agreement. This indicates that the method for the production rate calculation for the stable nuclei is reliable. In all martian meteorites we observe effects induced by secondary cosmic‐ray produced epithermal neutrons. Epithermal neutron fluxes, φ_n (30–300 eV), are calculated based on the reaction ⁷⁹Br(n, γβ)⁸⁰Kr. We show that the neutron capture effects were induced in free space during Mars‐Earth transfer of the meteoroids and that they are not due to a pre‐exposure on Mars before ejection of the meteoritic material. Neutron fluxes and slowing down densities experienced by the meteoroids are calculated and pre‐atmospheric sizes are estimated. We obtain minimum radii in the range of 22–25 cm and minimum masses of 150–220 kg. These results are in good agreement with the mean sizes reported for model calculations using current semiempirical data.     

Cosmic rays and terrestrial life: A brief review

“The investigation into the possible effects of cosmic rays on living organisms will also offer great interest.” – Victor F. Hess, Nobel Lecture, December 12, 1936High-energy radiation bursts are commonplace in our Universe. From nearby solar flares to distant gamma ray bursts, a variety of physical processes accelerate charged particles to a wide range of energies, which subsequently reach the Earth. Such particles contribute to a number of physical processes occurring in the Earth system. A large fraction of the energy of charged particles gets deposited in the atmosphere, ionizing it, causing changes in its chemistry and affecting the global electric circuit. Remaining secondary particles contribute to the background dose of cosmic rays on the surface and parts of the subsurface region. Life has evolved over the past ∼3 billion years in presence of this background radiation, which itself has varied considerably during the period [1], [2], [3]. As demonstrated by the Miller–Urey experiment, lightning plays a very important role in the formation of complex organic molecules, which are the building blocks of more complex structures forming life. There is growing evidence of increase in the lightning rate with increasing flux of charged particles. Is there a connection between enhanced rate of cosmic rays and the origin of life? Cosmic ray secondaries are also known to damage DNA and cause mutations, leading to cancer and other diseases. It is now possible to compute radiation doses from secondary particles, in particular muons and neutrons. Have the variations in cosmic ray flux affected the evolution of life on earth? We describe the mechanisms of cosmic rays affecting terrestrial life and review the potential implications of the variation of high-energy astrophysical radiation on the history of life on earth.     

Upper limit for the solar neutron flux in the energy interval 20–120 MeV

An experiment has been performed to search for the existence of a flux of solar neutrons at the earth using a detector sensitive to neutrons in the energy region 20–120 MeV. The instrument was carried by balloon to an atmospheric depth of 4 g/cm², from Palestine, Texas on the morning of November 2, 1967 and flown through sunrise and for about 7 hours into the day. Numerous flares of importance 1B or less occurred during the float period. By comparison of night and day counting rates we have deduced that the upper limit to the continuous emission of solar neutrons at the earth is 2 × 10^–2 neutrons/cm² sec in the above energy region. Using a theoretical form for the neutron differential energy spectrum we have expressed this result as an upper limit differential solar neutron flux. If neutrons were emitted in association with any of the small flares then the maximum flux at the earth was less than 4 × 10^–2 neutron/cm² sec in the same energy region. The minimum detectable flux with the present instrument is therefore well below the predicted flux from a 3B flare (e.g., Nov. 12, 1960) of 550 neutrons/cm² sec.     

Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere

Recent observations suggest methane in the martian atmosphere is variable on short spatial and temporal scales. However, to explain the variability by loss reactions requires production rates much larger than expected. Here, we report results of laboratory studies of methane adsorption onto JSC-Mars-1, a martian soil simulant, and suggest that this process could explain the observations. Uptake coefficient (γ) values were measured as a function of temperature using a high-vacuum Knudsen cell able to simulate martian temperature and pressure conditions. Values of γ were measured from 115 to 135 K, and the data were extrapolated to higher temperatures with more relevance to Mars. Adsorptive uptake was found to increase at lower temperatures and larger methane partial pressures. Although only sub-monolayer methane surface coverage is likely to exist under martian conditions, a very large mineral surface area is available for adsorption as atmospheric methane can diffuse meters into the regolith. As a result, significant methane may be temporarily lost to the regolith on a seasonal time scale. As this weak adsorption is fully reversible, methane will be re-released into the atmosphere when surface and subsurface temperatures rise and so no net loss of methane occurs. Heterogeneous interaction of methane with martian soil grains is the only process proposed thus far which contains both rapid methane loss and rapid methane production mechanisms and is thus fully consistent with the reported variability of methane on Mars.     

Neutrino-Astronomy with the Macro Experiment at Gran Sasso

The MACRO experiment at Gran Sasso is collecting muons since February 1989. These muons are induced by charged current interactions of atmospheric and, possibly, astrophysical muon neutrinos below the apparatus. A search has been made for discrete astrophysical sources of VHE and UHE neutrinos using the upward going muons direction information. Given the large area and the long period of data taking MACRO is, at the moment, able to set very competitive upper limits to the flux of neutrinos from most of the sources considered in the search. This revised version was published online in July 2006 with corrections to the Cover Date.