Ground-Based Observations of the Martian Atmosphere in Support of Space Missions

Although Mars is a favored target for planetary exploration, there is still a need for complementary ground-based observing programs of the Martian atmosphere, and this need will remain in the future. Indeed, as the atmosphere is very tenuous (less than 0.01 bar at the surface), the lines are very narrow and a high spectral resolving power (above 10⁴) is required over large spectral intervals. In addition, ground-based observations of Mars allow the instantaneous mapping of the whole planet, and thus the study of diurnal effects, which cannot be achieved from an orbiter. Recent ground-based achievements about the Martian atmospheric science include the first detection of H₂O₂ in the submillimeter range, the measurement of winds from CO millimetric transitions, the first detection of CH₄ and the O₃, H₂O₂, H₂O, and CH₄ mapping in the infrared. With an ELT, it will be possible to study at high spatial resolution transient atmospheric phenomena and to search for traces of minor constituents with unprecedented sensitivity. With ALMA, it will be also possible to search for minor species and to map the mesospheric winds for better constraining the climate models.     

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.     

“Methane on Mars: A product of H2O photolysis in the presence of CO”: Response to V.A. Krasnopolsky

The detection of CH₄ in the martian atmosphere, at a mixing ratio of about 10 ppb, prompted Krasnopolsky et al. [Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537-547] and Krasnopolsky [Krasnopolsky, V.A., 2006. Icarus 180, 359-367] to propose that the CH₄ is of biogenic origin. Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322] proposed that CH₄ can be formed in the martian atmosphere by photolysis of H₂O in the presence of CO. We based our arguments on a clear demonstration that CH₄ is formed in our experiments, and on thermodynamic equilibrium calculations, which show that CH₄ formation is favored even in the presence of oxygen at a mixing ratio 1.3×¹⁰⁻³, as observed on Mars. In the present comment, Krasnopolsky [Krasnopolsky, V.A., 2007. Icarus, in press (this issue)] presents his arguments against the suggestion of Bar-Nun and Dimitrov [Bar-Nun, A., Dimitrov, V., 2006. Icarus 181, 320-322], based on the effect of O₂ on CH₄ formation, the absence of kinetic pathways for CH₄ formation and on the inadequacy of thermodynamic equilibrium calculations to describe the martian atmosphere. In this rebuttal we demonstrate that experiments with molecular oxygen at a ratio of O₂/CO₂=(8.9-17)×¹⁰⁻³, exceeding the martian ratio, still form CH₄. Thermodynamic equilibrium calculations replicate the experimental CH₄ mixing ratio to within a factor of 1.9 and demonstrate that CH₄ production is favored in the martian atmosphere, which is obviously not in thermodynamic equilibrium. Consequently, we do not find the presence of methane to be a sign of biological activity on Mars.     

Limits on the trapping of atmospheric CH4 in martian polar ice analogs

Recent detection of methane (CH₄) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH₄ on Mars - trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH₄ gas. The ices were monitored for spectroscopic evidence of CH₄ trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH₄ in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH₄ was observed in the ice analogs studied at warmer temperatures (140 K for H₂O and CO₂ clathrate, 90 K for CO₂ snow) with approximately 10 mTorr CH₄ in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH₄. If it is assumed that the trapping mechanism is linear with CH₄ partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH₄ plume, and therefore does not represent a significant sink for CH₄.     

Identification of a new band system of isotopic CO2 near 3.3 μm: Implications for remote sensing of biomarker gases on Mars

We present the discovery of a new vibrational band system of isotopic CO₂ (carbon dioxide) near 3.3 μm, with multiple strong P, Q and R lines in the prime spectral region used to search for Mars CH₄ (methane). The band system was discovered on Mars using high-resolution spectrometers (λ/δλ>40,000, CSHELL and NIRSPEC) at telescopes (NASA-IRTF and Keck-2) atop Mauna Kea, HI. The observed line intensities and frequencies agree very well with values predicted by a vibrational band model that we developed using known parameters for the molecular levels involved. Using this model, we synthesized spectra for different observing conditions (from Space and ground-based telescopes) and for different spectral resolving powers (5000 to 40,000). Although the total atmospheric burden on Mars is more than 150 times smaller than on Earth, the greater mixing ratio of CO₂ ensures that its column abundance on Mars is almost 20 times greater than on Earth. Thus, weak telluric CO₂ band systems appear much stronger on Mars. Many molecules of possible biological and geothermal interest have strong signatures at these wavelengths, in particular hydrocarbons owing to their strong ro-vibrational CH stretching modes. For example, the new isotopic CO₂ band-system encompasses lines of CH₄, C₂H₆ (ethane), CH₃OH (methanol) and H₂O (water). Implications for previous and future searches of biomarker gases are presented.     

Measurement of the isotopic signatures of water on Mars; Implications for studying methane

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 L_s=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.     

Chemistry of atmospheres formed during accretion of the Earth and other terrestrial planets

We used chemical equilibrium and chemical kinetic calculations to model chemistry of the volatiles released by heating different types of carbonaceous, ordinary and enstatite chondritic material as a function of temperature and pressure. Our results predict the composition of atmospheres formed by outgassing during accretion of the Earth and other terrestrial planets. Outgassing of CI and CM carbonaceous chondritic material produces H₂O-rich (steam) atmospheres in agreement with the results of impact experiments. However, outgassing of other types of chondritic material produces atmospheres dominated by other gases. Outgassing of ordinary (H, L, LL) and high iron enstatite (EH) chondritic material yields H₂-rich atmospheres with CO and H₂O being the second and third most abundant gases. Outgassing of low iron enstatite (EL) chondritic material gives a CO-rich atmosphere with H₂, CO₂, and H₂O being the next most abundant gases. Outgassing of CV carbonaceous chondritic material gives a CO₂-rich atmosphere with H₂O being the second most abundant gas. Our results predict that the atmospheres formed during accretion of the Earth and Mars were probably H₂-rich unless the accreted material was dominantly CI and CM carbonaceous chondritic material. We also predict significant amounts of S, P, Cl, F, Na, and K in accretionary atmospheres at high temperatures (1500-2500 K). Finally, our results may be useful for interpreting spectroscopic observations of accreting extrasolar terrestrial planets.     

Photochemistry of the martian atmosphere: Seasonal, latitudinal, and diurnal variations

There is a significant progress in the observational data relevant to Mars photochemistry in the current decade. These data are not covered by and sometimes disagree with the published models. Therefore we consider three types of models for Mars photochemistry. A steady-state model for global-mean conditions is currently the only way to calculate the abundances of long living species (H₂, O₂, and CO). However, our model does not fit the observed CO abundance using gas-phase chemistry and reasonable values of heterogeneous loss of odd hydrogen on the water ice aerosol. The second type of the calculated models is steady-state models for local conditions. The MGS/TES data on temperature profiles, H₂O, and dust are input parameters for these models. The calculations have been made for nine seasonal points spread over the martian year and for twelve latitudes with a step of 10° for each season. The only adopted heterogeneous reaction is a weak loss of H₂O₂ on water ice with probability of 5×¹⁰⁻⁴. The results are in good agreement with the recent observations of the O₂ dayglow at 1.27 μm and the O₃ and H₂O₂ abundances. Global maps of the seasonal and latitudinal behavior of these species have been made. The third type of models is a time-dependent model for local conditions. These models show that odd hydrogen quickly converts to H₂O₂ at the nighttime and the chemistry is switched off while the association of O, the heterogeneous loss of H₂O₂, and eddy diffusion continue. This requires significant changes in the global-mean and local steady-state models discussed above, and these changes have been properly done. The calculated diurnal variations of Mars photochemistry are discussed. The martian photochemistry at low and middle latitudes is significantly different in the aphelion period at LS=10°-130° from that in the remaining part of the year.     

Mapping of Ozone and Water in the Atmosphere of Mars near the 1997 Aphelion

We present absolute abundances and latitudinal variations of ozone and water in the atmosphere of Mars during its late northern spring (L_s=67.3°) shortly before aphelion. Long-slit maps of the a¹Δ_g state of molecular oxygen (O₂) and HDO, an isotopic form of water, were acquired on UT January 21.6 1997 using a high-resolution infrared spectrometer (CSHELL) at the NASA Infrared Telescope Facility. O₂(a¹Δ_g) is produced by ozone photolysis, and the ensuing dayglow emission at 1.27 μm is used as a tracer for ozone. Retrieved vertical column densities for ozone above ∼20 km ranged between 1.5 and 2.8 μm-atm at mid- to low latitudes (30°S-60°N) and decreased outside that region. A significant decrease in ozone density is seen near 30°N (close to the subsolar latitude of 23.5°N). The rotational temperatures retrieved from O₂(a-X) emissions show a mean of 172±2.5 K, confirming that the sensed ozone lies in the middle atmosphere (∼24 km). The ν₁ fundamental band of HDO near 3.67 μm was used as a proxy for H₂O. The retrieved vertical column abundance of water varies from 3 precipitable microns (pr-μm) at ∼30°S to 24 pr-μm at ∼60°N. We compare these results with current photochemical models and with measurements obtained by other methods.     

Possibilities for the detection of hydrogen peroxide-water-based life on Mars by the Phoenix Lander

The Phoenix Lander landed on Mars on 25 May 2008. It has instruments on board to explore the geology and climate of subpolar Mars and to explore if life ever arose on Mars. Although the Phoenix mission is not a life detection mission per se, it will look for the presence of organic compounds and other evidence to support or discredit the notion of past or present life.The possibility of extant life on Mars has been raised by a reinterpretation of the Viking biology experiments [Houtkooper, J. M., Schulze-Makuch, D., 2007. A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. International Journal of Astrobiology 6, 147-152]. The results of these experiments are in accordance with life based on a mixture of water and hydrogen peroxide instead of water. The near-surface conditions on Mars would give an evolutionary advantage to organisms employing a mixture of H₂O₂ and H₂O in their intracellular fluid: the mixture has a low freezing point, is hygroscopic and provides a source of oxygen. The H₂O₂-H₂O hypothesis also explains the Viking results in a logically consistent way. With regard to its compatibility with cellular contents, H₂O₂ is used for a variety of purposes in terran biochemistry. The ability of the anticipated organisms to withstand low temperatures and the relatively high water vapor content of the atmosphere in the Martian arctic, means that Phoenix will land in an area not inimical to H₂O₂-H₂O-based life. Phoenix has a suite of instruments which may be able to detect the signatures of such putative organisms.     

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).     

Atmospheric chemistry on Venus, Earth, and Mars: Main features and comparison

This paper deals with two common problems and then considers major aspects of chemistry in the atmospheres of Mars and Venus. (1) The atmospheres of the terrestrial planets have similar origins but different evolutionary pathways because of the different masses and distances to the Sun. Venus lost its water by hydrodynamic escape, Earth lost CO₂ that formed carbonates and is strongly affected by life, Mars lost water in the reaction with iron and then most of the atmosphere by the intense meteorite impacts. (2) In spite of the higher solar radiation on Venus, its thermospheric temperatures are similar to those on Mars because of the greater gravity acceleration and the higher production of O by photolysis of CO₂. O stimulates cooling by the emission at 15 μm in the collisions with CO₂. (3) There is a great progress in the observations of photochemical tracers and minor constituents on Mars in the current decade. This progress is supported by progress in photochemical modeling, especially by photochemical GCMs. Main results in these areas are briefly discussed. The problem of methane presents the controversial aspects of its variations and origin. The reported variations of methane cannot be explained by the existing data on gas-phase and heterogeneous chemistry. The lack of current volcanism, SO₂, and warm spots on Mars favor the biological origin of methane. (4) Venus’ chemistry is rich and covers a wide range of temperatures and pressures and many species. Photochemical models for the middle atmosphere (58-112 km), for the nighttime atmosphere and night airglow at 80-130 km, and the kinetic model for the lower atmosphere are briefly discussed.     

A photochemical model for the Venus atmosphere at 47–112 km

The model is intended to respond to the recent findings in the Venus atmosphere from the Venus Express and ground-based submillimeter and infrared observations. It extends down to 47 km for comparison with the kinetic model for the lower atmosphere (Krasnopolsky, V.A. [2007]. Icarus 191, 25–37) and to use its results as the boundary conditions. The model numerical accuracy is significantly improved by reduction of the altitude step from 2 km in the previous models to 0.5 km. Effects of the NUV absorber are approximated using the detailed photometric observations at 365 nm from Venera 14. The H₂O profile is not fixed but calculated in the model. The model involves odd nitrogen and OCS chemistries based on the detected NO and OCS abundances. The number of the reactions is significantly reduced by removing of unimportant processes. Column rates for all reactions are given, and balances of production and loss may be analyzed in detail for each species.The calculated vertical profiles of CO, H₂O, HCl, SO₂, SO, OCS and of the O₂ dayglow at 1.27 μm generally agree with the existing observational data; some differences are briefly discussed. The OH dayglow is ～30 kR, brighter than the OH nightglow by a factor of 4. The H + O₃ process dominates in the nightglow excitation and O + HO₂ in the dayglow, because of the reduction of ozone by photolysis. A key feature of Venus’ photochemistry is the formation of sulfuric acid in a narrow layer near the cloud tops that greatly reduces abundances of SO₂ and H₂O above the clouds. Delivery of SO₂ and H₂O through this bottleneck determines the chemistry and its variations above the clouds. Small variations of eddy diffusion near 60 km result in variations of SO₂, SO, and OCS at and above 70 km within a factor of ～30. Variations of the SO₂/H₂O ratio at the lower boundary have similar but weaker effect: the variations within a factor of ～4 are induced by changes of SO₂/H₂O by ±5%. Therefore the observed variations of the mesospheric composition originate from minor variations of the atmospheric dynamics near the cloud layer and do not require volcanism. NO cycles are responsible for production of a quarter of O₂, SO₂, and Cl₂ in the atmosphere. A net effect of photochemistry in the middle atmosphere is the consumption of CO₂, SO₂, and HCl from and return of CO, H₂SO₄, and SO₂Cl₂ to the lower atmosphere. These processes may be balanced by thermochemistry in the lower atmosphere even without outgassing from the interior, though the latter is not ruled out by our models. Some differences between the model and observations and the previous models are briefly discussed.     

Variability of the methane trapping in martian subsurface clathrate hydrates

Recent observations have evidenced traces of methane (CH₄) heterogeneously distributed in the martian atmosphere. However, because the lifetime of CH₄ in the atmosphere of Mars is estimated to be around 300-600 years on the basis of photochemistry, its release from a subsurface reservoir or an active primary source of methane have been invoked in the recent literature. Among the existing scenarios, it has been proposed that clathrate hydrates located in the near subsurface of Mars could be at the origin of the small quantities of the detected CH₄. Here, we accurately determine the composition of these clathrate hydrates, as a function of temperature and gas phase composition, by using a hybrid statistical thermodynamic model based on experimental data. Compared to the other recent works, our model allows us to calculate the composition of clathrate hydrates formed from a more plausible composition of the martian atmosphere by considering its main compounds, i.e. carbon dioxide, nitrogen and argon, together with methane. Besides, because there is no low temperature restriction in our model, we are able to determine the composition of clathrate hydrates formed at temperatures corresponding to the extreme ones measured in the polar caps. Our results show that methane enriched clathrate hydrates could be stable in the subsurface of Mars only if a primitive CH₄-rich atmosphere has existed or if a subsurface source of CH₄ has been (or is still) present.     

Remote sensing of a comet at millimeter and submillimeter wavelengths from an orbiting spacecraft

The ESA Rosetta Spacecraft, launched on March 2, 2004 with the ultimate destination being Comet 67P/Churyumov–Gerasimenko, carries a relatively small and lightweight millimeter–submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument, named Microwave Instrument for the Rosetta Orbiter (MIRO), consists of a 30-cm diameter, offset parabolic reflector telescope, which couples energy in the millimeter and submillimeter bands to two heterodyne receivers. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband, total power continuum measurements can be made in both bands. A 4096-channel spectrometer with 44 kHz resolution is connected to the submillimeter receiver. The spectral resolution is sufficient to observe individual, thermally broadened spectral lines (T⩾10 K). The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species—CO, CH₃OH, NH₃ and three isotopes of water, H₂¹⁶O, H₂¹⁷O and H₂¹⁸O. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface.     

A measurement of the 362 GHz absorption line of Mars atmospheric H2O2

The 362.156 GHz absorption spectrum of H₂O₂ in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H₂O₂ abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (L_s) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HO_x abundances above ∼10 km altitudes between the L_s=112° period of the Encrenaz et al. upper limit measurement and the current L_s=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H₂O₂ line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H₂O₂. Observation of Mars H₂O₂ also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own.     

Prebiotic organic synthesis in early Earth and Mars atmospheres: Laboratory experiments with quantitative determination of products formed in a cold plasma flow reactor

The goal of this study was to explore prebiotic chemistry in a range of plausible early Earth and Mars atmospheres. To achieve this laboratory continuous flow plasma irradiation experiments were performed on N₂/H₂/CO/CO₂ gas mixtures chosen to represent mildly reducing early Earth and Mars atmospheres derived from a secondary volcanic outgassing of volatiles in chemical equilibrium with magmas near present day oxidation state. Under mildly reducing conditions (91.79% N₂, 5.89% H₂, 2.21% CO, and 0.11% CO₂), simple nitriles are produced in the gas phase with yield (G in molecules per 100 eV), for the key prebiotic marker molecule HCN at G∼1×¹⁰⁻³ (0.1 nmol J⁻¹). In this atmosphere localized HCN concentrations possibly could approach the 10⁻² M needed for HCN oligomerization. Yields under mildly oxidizing conditions (45.5% N₂, 0.1% H₂, 27.2% CO, 27.2% CO₂) are significantly less as expected, with HCN at G∼3×¹⁰⁻⁵ (). Yields in a Triton atmosphere which can be plausibly extrapolated to represent what might be produced in trace CH₄ conditions (99.9% N₂, 0.1% CH₄) are significant with HCN at G∼1×¹⁰⁻² (1 nmol J⁻¹) and tholins produced. Recently higher methane abundance atmospheres have been examined for their greenhouse warming potential, and higher abundance hydrogen atmospheres have been proposed based on a low early Earth exosphere temperature. A reducing (64.04% N₂, 28.8% H₂, 3.60% CO₂, and 3.56% CH₄), representing a high CH₄ and H₂ abundance early Earth atmosphere had HCN yields of G∼5×¹⁰⁻³ (0.5 nmol J⁻¹). Tholins generated in high methane hydrogen gas mixtures is much less than in a similar mixture without hydrogen. The same mixture with the oxidizing component CO₂ removed (66.43% N₂, 29.88% H₂, 0% CO₂, and 3.69% CH₄) had HCN yields of G∼1×¹⁰⁻³ (0.1 nmol J⁻¹) but more significant tholin yields.     

The composition of the Martian atmosphere: Minor constituents

The Mariner 6 and 7 infrared spectrometers provided data which, in principle, determine upper limits on the possible atmospheric abundance of every gaseous substance that was undetected but which has recognized absorptions in the accessible spectral region, 1.9 to 14.4 μ. Through supporting laboratory determinations of curves of growth under pressure broadening conditions appropriate to Mars, upper limits can be specified (expressed first in cm-atm, STP, for a vertical column and then in parts per million) for the following gases: NO₂ (<0.0016, 0.23); NH₃ (<0.0031, 0.44); C₃O₂ (<0.0032, 0.45); SO₂ (?0.0037, 0.52); OCS (<0.0040, 0.56); NO (<0.0050, 0.70); O₃ (<0.0061, 0.86); CH₄ (<0.026, 3.7); N₂O (<0.13, 18); HCl (<3.7, 520); HBr (?8.2, 1150); H₂S (<27, 3800). In addition, considerations of band contours, moments of inertia, and experimental absorption coefficients permit us to place useful upper limits on twenty-seven additional substances that were not detected and for which curves of growth have not been measured: less than 1.0ppm (or, <0.0071cm-atm); SF₆, NF₃, Cl₂CO, CF₄, CHF₃, Br₂CO, SiF₄; less than 6.0ppm (or, <0.035 cm-atm); C₂F₆, CO₃, F₂CO, C₂H₄, BF₃, CS₂, CHCl₃, C₂H₆, H₂CO, CH₃F, C₆H₆, CH₂I₂; less than 50ppm (or, <0.35 cm-atm): C₂H₂, HCN, CH₂Br₂, CH₂Cl₂, CH₃Br, PH₃, CH₃I, and CH₃Cl. These upper limits are compared to earlier limits based upon terrestrial measurements. The present limits placed upon the possible atmospheric content of the molecules C₃O₂ and CO₃ are of particular interest because of their relevance to the photochemistry of the Martian atmosphere.     

Seasonal variations of photochemical tracers at low and middle latitudes on Mars: Observations and models

Mars Express observations give ozone abundances that are smaller than those from the ground-based infrared heterodyne and HST observations at low and middle latitudes. Both ground-based and Mars Express observations of the O₂ dayglow at 1.27 μm, which originates from photolysis of ozone, are in mutual agreement after correction for the local time variability. Therefore a problem appears: whether the MEX ozone data are compatible with (1) the observed O₂ dayglow intensities and (2) the photochemical model by Krasnopolsky [Krasnopolsky, V.A., 2006. Icarus 185, 153-170] within uncertainties of its reaction rate coefficients. That model involves heterogeneous loss of H₂O₂ on water ice and agrees with the observations of the O₂ dayglow, H₂O₂, and the ground-based and HST ozone. The answers are ‘yes’ to both questions. A version of the model is given that fits the MEX ozone as well as the observed O₂ dayglow and H₂O₂. Laboratory studies of two reaction rate coefficients could indicate a preferable version of the model and a preferable set of the ozone data (MEX versus the ground-based and HST). The predicted seasonal behavior of H₂O₂ is different from that in the model by Lefevre et al. [Lefevre, F., Bertaux, J.L., Clancy, R.T., Encrenaz, T., Fast, K., Forget, F., Lebonnois, S., Montmessin, F., Perrier, S., 2008. Nature 454, 971-975], and future observations may help to choose between the models.     

Martian isotopic ratios and upper limits for possible minor constituents as derived from Mariner 9 infrared spectrometer data

The Mariner 9 infrared spectrometer obtained data over a large part of Mars for almost a year beginning late in 1971. Mars' infrared emission spectrum was measured from 200 to 2000 cm^?1 with an apodized resolution of 2.4 cm^?1. No significant deviation from terrestrial ratios of carbon (¹²C/¹³C) or oxygen (¹⁶O/¹⁸O; ¹⁶O/¹⁷O) isotopes was observed on Mars. The ¹²C/¹³C isotopic ratio was found to be terrestrial with an uncertainty of 15%. Upper limits have been calculated for several minor constituents. With an effective noise equivalent radiance of 1.2 × 10^?9 W cm^?2 sr^?1/cm^?1, new upper limits in centimeter-atmospheres of 2 × 10^?5 for C₂H₂, 4 × 10^?3 for C₂H₄, 3 × 10^?3 for C₂H₆, 2 × 10^?4 for CH₄, 1 × 10^?3 for N₂O, 1 × 10^?4 for NO₂, 4 × 10^?5 for NH₃, 1 × 10^?3 for PH₃, 7 × 10^?4 for SO₂, and 1 × 10^?4 for OCS have been derived.