Photochemistry of the Martian atmosphere

A variety of models are explored to study the photochemistry of CO₂ in the Martian atmosphere with emphasis on reactions involving compounds of carbon, hydrogen, and oxygen. Acceptable models are constrained to account for measured concentrations of CO and O above 90 km, with an additional requirement that they should be in accord with observations of CO, O₂, and O₃ in the lower atmosphere. Dynamical mixing must be exceedingly rapid at altitudes above 90 km, with effective eddy diffusion coefficients in excess of 10⁷ cm² sec^?1. If recombination of CO₂ is to occur mainly by gas phase chemistry, catalyzed by trace quantities of H, OH, and HO₂, mixing must be rapid over the altitude interval 30 to 40 km. The value implied for the diffusion coefficient in this region is a function of assumptions made regarding the rates for reaction of OH with HO₂ to form H₂O and of the rate for reaction of HO₂ with itself to form H₂O₂. If rates for these reactions are taken to have values similar to rates used in current models for the Earth's stratosphere, the eddy diffusion coefficient at 40 km on Mars should be about 5 × 10⁷ cm² sec^?1, consistent with Zurek's (1976) estimate for this parameter inferred from tidal theory. Surface chemistry could have an influence on the abundances of atmospheric CO and O₂, but a major effect would imply sluggish mixing at all altitudes below 50 km and in addition would carry implications for the magnitude of the rates for reaction of OH with HO₂ and HO₂ with itself.     

High-resolution Martian atmosphere modeling

A multilayer radiative transfer, high-spectral-resolution infrared model of the lower atmosphere of Mars has been constructed to assess the effect of scattering on line profiles. The model takes into account aerosol scattering and absorption and includes a line-by-line treatment of scattering and absorption by CO₂ and H₂O. The aerosol complex indices of refraction used were those measured on montmorillonite and basalt chosen on the basis of Mars ir data from the NASA Lear Airborne Observatory. The particle sizes and distribution were estimated using Viking data. The molecular line treatment employs the AFGL line parameters and Voigt profiles. The modling results indicate that the line profiles are only slightly affected by normal aerosol scattering and absorption, but the effect could be appreciable for heavy loading. The technique described permits a quantitative approach to assessing and correcting for the effect of aerosols on lineshapes in planetary atmospheres.     

The photolytic stability of the Martian atmosphere

Several recent suggestions for stabilizing the Martian atmosphere against photolysis have proved untenable. However, downward convective transport as well as a low altitude (0–35 km) aerosol, which catalyzes two-body molecular recombination reactions, can bring about such stability. The ‘effective’ convection velocity and ‘average’ two-body reaction rate coefficients required by observed abundances are evaluated. The computed profiles of CO and O at high altitude are shown to agree well with observations.     

Solar heating of the Martian dusty atmosphere

This paper examines the solar heating of the Martian atmosphere during the 1971 global dust storm observed by Mariner 9. Radiative scattering as well as absorption is included by utilizing the delta-Eddington approximation to the full radiative transfer equation. The necessary optical parameters are generated by a Mie program which uses a size distribution and a complex refractive index inferred from a number of sources, particularly from recent analyces of Mariner 9 UVS and TV observations. Assuming uniform mixing of the dust, the solar heating per unit mass during a Martian global dust storm is remarkably uniform with height for small solar zenith angles. Heating rates may reach 80°K day^? for overhead sunlight. Overall, 20% of the direct insolation is absorbed by the dust-laden atmosphere. Even optically thin widespread dust hazes may produce heating rates of several degrees Kelvin per day.     

Photochemistry of nitrogen in the Martian atmosphere

Models are developed for the photochemistry of a CO₂H₂ON₂ atmosphere on Mars and estimates are given for the concentrations of N, NO, NO₂, NO₃, N₂O₅, HNO₂, HNO₃, and N₂O as a function of altitude. Nitric oxide is the most abundant form of odd nitrogen, present with a mixing ratio relative to CO₂ of order 10^?8. Deposition rates for nitrite and nitrate minerals could be as large as 3× 10⁵ N equivalent atoms cm^?2 sec^?1 under present conditions and may have been higher in the past.     

Planetary-scale wave structure in the Martian atmosphere

Wave-like perturbations are found in the Mariner 9 IRIS atmospheric temperature data during late Northern Hemisphere winter in a latitude band between 45°N and 65°N. The nature of the data base prevents a unique separation of spatial and temporal behavior, but Fourier analysis of the data constrains the waves to discrete combinations of planetary wavenumber and period. One major spectral component possesses a meridional amplitude cross section with a maximum near the 1-mbar level at 60°N and is strongly correlated with the circumpolar jet observed in thermal winds calculated from the mean meridional temperature cross section. This feature is consistent with the low-wavenumber baroclinic waves observed in Viking Lander data, and the vertical structure reflects the behavior anticipated for a vertically penetrating quasi-geostrophic disturbance. Other possible origins for the wave cannotbe ruled out, however. Among these is a stationary wave forced by wavenumber-2 topographic relief.     

Martian Dust Storms: A Review

A review of the dust storms observed on Mars is made. This includes the seasonal and interannual variability of planet- encircling and regional dust storms. Although there is a significant interannual variability, planet-encircling dust storms have been observed to form during the southern spring and summer seasons, while regional dust storms tend to occur more frequently. Some aspects of possible mechanisms associated with the origin, maintenance and decay of the dust storms are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ozone and photochemistry of the Martian lower atmosphere

The calculation of number densities of CO₂, H₂O and N₂ photolysis products was carried out for the Martian atmosphere at heights up to 60 km. The ozone distributed in the atmosphere as a layer of 10 km width with [O₃] max = 2.5 × 10⁹ cm³ at height of 35 km which agree well with the results of u.v. observations on the evening terminator from the Mars-5 satellite. The calculated densities of O₂, CO and H₂O are also in good agreement with the measured data. The eddy diffusion coefficient is equal to 3 × 10⁶ in the troposphere (h ? 30 km) and 10⁸ cm² s^?1 above 40 km. The dependence of the total ozone content on water vapour amount in the atmosphere is considered; the hypothesis about the influence of water ice aerosol on the ozone formation is proposed to explain the high concentrations of ozone in the morning.     

A hot film anemometer for the Martian atmosphere

A novel design of a wind sensor for the surface of Mars is described. This sensor is to be included in the Rover Environment Monitoring Station (REMS) to be launched as a part of the Mars Science Laboratory in 2009. A 2D hot film anemometer composed of four hot points and a reference point has been conceived and implemented in the preliminary design model. It uses a closed thermal feedback loop based on thermal sigma-delta modulation. In this paper, the first results obtained in a Mars-like environment are shown, and indicate that sensitivities are expected to be in the range of 0.5 m/s and 10° in wind speed and direction.     

Parameters of Dust Particles in the Martian Atmosphere

A critical analysis of the methods and results of estimating the optical thickness of the dust component in the Martian atmosphere ₀, the particle size r ₀, and the imaginary part of the refractive index n _ihas shown the following. (1) Observational data on the brightness distribution over the Martian disk as well as the phase dependences of diffusely reflected light and the azimuthal dependences of diffusely transmitted light are most appropriate to use only for verifying the reliability of the aerosol parameters determined by other methods. (2) If the morning and evening fogs in the atmosphere are disregarded, the Bouguer–Lambert–Beer method used to analyze the solar-brightness attenuation measured on the planetary surface yields overestimated extraatmospheric solar intensity I ₀and atmospheric optical depth ₀. At the Viking 1landing site, I ₀and ₀could be overestimated by a factor of 1.7 and by 0.35, respectively. (3) The aerosol size determined by analyzing measurements of the azimuthal dependences for the Martian sky brightness at low elevations of the Sun most likely corresponds to the fog particles. (4) If overestimated values of I ₀were used to standardize the observations of the solar radiation transmitted by the Martian atmosphere, then n _iwere also overestimated; using overestimated ₀also affected the reliability of the latter. (5) The problem of reliability of the available ₀and r ₀estimates for periods of high atmospheric transparency is yet to be solved. For the highest activity of the dust storm in 1971, it was found that 4.5 r ₀ 7.5 m for the lognormal particle size distribution with ²= 0.2 and the optical thickness of a dust cloud ₀ 15. (6) The spectral values of the apparent albedo of Mars measured in October 1971 at a phase angle of 42° in the spectral range 0.250 0.717 allowed the imaginary part of the refractive index to be estimated in terms of a model of a dust cloud composed of spherical particles with the lognormal size distribution with r ₀= 4.5 m and ²= 0.2.     

Helium in the Martian atmosphere: Thermal loss considerations

Helium concentrations in the Martian atmosphere are estimated assuming that the helium production on Mars, comparable to its production on Earth, via the radioactive decay of uranium and thorium, is in steady state equilibrium with its thermal escape. Although non-thermal losses would tend to reduce the estimated concentrations, these concentrations are not necessarily an upper limit since higher production rates and/or a possibly lower effective exospheric temperature over the solar activity cycle could increase them to even higher values. The computed helium concentration at the Martian exobase (200 km) is 8 × 10⁶ atoms cm^?3. Through the lower exosphere, the computed helium concentrations are 30–200 times greater than the Mariner-measured atomic hydrogen concentrations. It follows that helium may be the predominant constituent in the Martian lower exosphere and may well control the orbital lifetime of Mars-orbiting spacecraft. The estimated helium mixing ratio is greater at the Martian turbopause than at the terrestrial turbopause, and the helium column density in the lower Martian atmosphere may be comparable to that on Earth.     

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.     

Water ice clouds in the Martian atmosphere: Two Martian years of SPICAM nadir UV measurements

The SPICAM instrument onboard Mars Express has successfully performed two Martian years (MY 27 and MY28) of observations. Water ice cloud optical depths spatial and temporal distribution was retrieved from nadir measurements in the wavelength range 300–320 nm. During the northern spring the cloud hazes complex distribution was monitored. The clouds in the southern hemisphere formed a zonal belt in the latitude range 30–60°S. The edge of the retreating north polar hood merged with the northern tropical clouds in the range 250–350°E. The development of the aphelion cloud belt (ACB) started with the weak hazes formation (cloud optical thickness 0.1–0.3) in the equatorial region. At the end of the northern spring, the ACB cloud optical thickness reached already values of 0.3–1. The ACB decay in the end of the northern summer was accompanied with a presence of clouds in the north mid-latitudes. The expanded north polar hood merged with the north mid-latitude clouds in the eastern hemisphere. The interannual comparison indicates a decrease in cloud activity immediately after a strong dust storm in southern summer of MY28. The strong dust storms of the MY28 may also be a reason of the observed north polar hood edge shifting northward by 5°.     

Changes in the Martian atmosphere induced by auroral electron precipitation

Typical auroral events in the Martian atmosphere, such as discrete and diffuse auroral emissions detected by UV spectrometers onboard ESA Mars Express and NASA MAVEN, are investigated. Auroral electron kinetic energy distribution functions and energy spectra of the upward and downward electron fluxes are obtained by electron transport calculations using the kinetic Monte Carlo model. These characteristics of auroral electron fluxes make it possible to calculate both the precipitation-induced changes in the atmosphere and the observed manifestations of auroral events on Mars. In particular, intensities of discrete and diffuse auroral emissions in the UV and visible wavelength ranges (Soret et al., 2016; Bisikalo et al., 2017; Gérard et al., 2017). For these conditions of auroral events, the analysis is carried out, and the contribution of the fluxes of precipitating electrons to the heating and ionization of the Martian atmosphere is estimated. Numerical calculations show that in the case of discrete auroral events the effect of the residual crustal magnetic field leads to a significant increase in the upward fluxes of electrons, which causes a decrease in the rates of heating and ionization of the atmospheric gas in comparison with the calculations without taking into account the residual magnetic field. It is shown that all the above-mentioned impact factors of auroral electron precipitation processes should be taken into account both in the photochemical models of the Martian atmosphere and in the interpretation of observations of the chemical composition and its variations using the ACS instrument onboard ExoMars.     

Mapping methane in Martian atmosphere with PFS-MEX data

In this study we map the methane gas in the Martian atmosphere. The main goal of this work is to show the methane behaviour across the planet seasonally. To this aim, we analyze the strongest methane band in the short wavelength channel of the Planetary Fourier Spectrometer (PFS) on board ESA Mars Express (MeX) spacecraft. The optical line depth is used to derive the column density of methane. The maps thus obtained show the spatial variability of this non-condensable gas and how the gas is transported in the atmosphere due to the cycle of carbon dioxide. Moreover, the increase of methane over the north polar cap during local summer, which cannot be explained by global circulation, strongly suggests that there could be methane reservoir associated with the polar cap.     

Suprathermal oxygen and hydrogen atoms in the upper Martian atmosphere

The processes of kinetics and transport of hot oxygen and hydrogen atoms in the transition (from the thermosphere to the exosphere) region of the upper Martian atmosphere are studied. The reaction of dissociative recombination of the principal ionospheric ion O ₂ ⁺ with thermal electrons in the ionosphere of Mars served as the source of hot oxygen atoms. The process of momentum and energy transfer in elastic collisions between hot oxygen atoms and atmospheric hydrogen atoms with thermal energies was regarded as the source of hot hydrogen atoms. The kinetic energy distribution functions are determined for suprathermal oxygen and hydrogen atoms. It is shown that the exosphere is populated with a significant number of suprathermal oxygen atoms with kinetic energies ranging up to the escape energy of 2 eV (i.e., the hot oxygen Martian corona is formed). The transfer of energy from hot oxygen atoms to thermal hydrogen atoms creates an additional nonthermal flux of atomic hydrogen escaping from the Martian atmosphere.     

Theoretical interpretation of photometric properties of the Martian surface and atmosphere

Earth-based UBV photometry, high-quality photographs from the Lowell Observatory collection, and Mariner 9 data have been combined with a new radiative transfer theory to derive physical parameters for the Martian surface and atmosphere, both before and during the 1971 dust storm. We find that the dust particles of the storm had a single-scattering albedo of 0.84 ± 0.02 and an asymmetry factor of 0.35 ± 0.10 in green (V) light. The geometric albedo of Mars was 0.15 and the phase integral 1.83, which yield 0.27 for the Bond albedo. The mean optical thickness of the “clear” atmosphere averaged over the whole planet was 0.15 ± 0.05 and was not detectably dependent on wavelength. Geometric albedos for the surface are 0.25 (light areas) and 0.17 (dark areas) in V, 0.095 in B (both areas), and 0.060 in U (both areas). The soil particles are moderately backward scattering with an asymmetry factor of ?0.20, indicating them to be rather opaque. The mean surface roughness, on a scale larger than that of individual dust particles and therefore large compared with the wavelength, is 0.57. This represents the depth/radius ratio of an average hole and it is only one-half as large as values typical for the Moon and asteroids.     

Orbital observations of meteors in the Martian atmosphere using the SPOSH camera

We investigate the expected performance of a wide-angle camera in Martian orbit, which, unlike previous cameras that have flown to Mars, is capable of recording meteor activity in that planet's atmosphere. We show that, based on our current understanding of meteor physics and the interplanetary meteoroid population, several meteors will be detected by this instrument during a single nightside pass on a low Martian orbit. The instrument will also record the signatures of meteor showers expected to occur every Martian year (1.88 Earth years). The results of this investigation will test models of the flux of “large” (mm-cm) meteoroids at the orbit of Mars and their interaction with the Martian atmosphere.     

Minor constituents in the Martian atmosphere from the ISM/Phobos experiment

Global Martian atmospheric results derived from the infrared imaging spectrometer ISM flown aboard the Phobos 2 Soviet spacecraft are presented. Over low altitude regions the expected CO mixing ratio of (8 +/- 3) x 10(-4) is measured. Variations of the 2.35-micrometers feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-micrometers band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO "hole." If, however, these spectral variations at 2.35 micrometers are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-micrometers during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988-1989 could explain the enhancement of H2O in the atmosphere.     

Chemical pathway analysis of the Martian atmosphere: CO2-formation pathways

The chemical composition of a planetary atmosphere plays an important role for atmospheric structure, stability, and evolution. Potentially complex interactions between chemical species do not often allow for an easy understanding of the underlying chemical mechanisms governing the atmospheric composition. In particular, trace species can affect the abundance of major species by acting in catalytic cycles. On Mars, such cycles even control the abundance of its main atmospheric constituent CO₂. The identification of catalytic cycles (or more generally chemical pathways) by hand is quite demanding. Hence, the application of computer algorithms is beneficial in order to analyze complex chemical reaction networks. Here, we have performed the first automated quantified chemical pathways analysis of the Martian atmosphere with respect to CO₂-production in a given reaction system. For this, we applied the Pathway Analysis Program (PAP) to output data from the Caltech/JPL photochemical Mars model. All dominant chemical pathways directly related to the global CO₂-production have been quantified as a function of height up to 86 km. We quantitatively show that CO₂-production is dominated by chemical pathways involving HO_x and O_x. In addition, we find that NO_x in combination with HO_x and O_x exhibits a non-negligible contribution to CO₂-production, especially in Mars’ lower atmosphere. This study reveals that only a small number of chemical pathways contribute significantly to the atmospheric abundance of CO₂ on Mars; their contributions to CO₂-production vary considerably with altitude. This analysis also endorses the importance of transport processes in governing CO₂-stability in the Martian atmosphere. Lastly, we identify a previously unknown chemical pathway involving HO_x, O_x, and HO₂-photodissociation, contributing 8% towards global CO₂-production by chemical pathways using recommended up-to-date values for reaction rate coefficients.