Effects of low concentrations of O2 and CO on the ion-clustering reactions in the lower ionosphere of Mars

It is demonstrated that under conditions which approximate those of the Martian ionosphere traces of CO and O₂ can be effectively incorporated in ion clusters via ion-molecule reaction schemes initiated by the CO₂⁺ ion. For example, when 0.3 % CO is added to CO₂, (CO)₂⁺ and [(CO)₂CO₂]⁺ appear as the major cations (584 Å radiation, 300°K). In mixtures containing O₂ in addition to CO₂ (CO₂. O₂)⁺ and [(CO₂)₂O₂]⁺ are important species. A recently proposed mechanism to account for the low abundance of CO and O₂ in the Martian atmosphere is discussed in the light of these observations.     

Argon in the Martian atmosphere: Evidence from the Mars 6 descent module

In the descent of the automatic interplanetary station Mars 6, after parachute deployment, an analysis was carried out by means of a mass spectrometer, the analyzer of which was pumped out by a getter-ion pump. Unfortunately, mass spectra were not obtained. But during vehicle descent on a parachute tether, the pump discharge current was registered. The dynamics of the pump current at the end of descent operations imply that an inert gas is a basic component of the Martian atmosphere, along with CO₂. Laboratory post-experiment calibrations of other getter-ion pumps, performed with various mixtures of CO₂ and argon, result in a probable value of the argon content in the Martian atmosphere of 35 ± 10% by volume.     

The effects of Martian near surface conditions on the photochemistry of amino acids

In order to understand the complex multi-parameter system of destruction of organic material on the surface of Mars, step-by-step laboratory simulations of processes occurring on the surface of Mars are necessary. This paper describes the measured effects of two parameters, a CO₂ atmosphere and low temperature, on the destruction rate of amino acids when irradiated with Mars-like ultraviolet light (UV). The results show that the presence of a 7 mbar CO₂ atmosphere does not affect the destruction rate of glycine, and that cooling the sample to 210 K (average Mars temperature) lowers the destruction rate by a factor of 7. The decrease in the destruction rate of glycine by cooling the sample is thought to be predominantly caused by the slower reaction kinetics. When these results are scaled to Martian lighting conditions, cold thin films of glycine are assumed to have half-lives of 250 h under noontime peak illumination. It has been hypothesised that the absence of detectable native organic material in the Martian regolith points to the presence of oxidising agents. Some of these agents might form via the interaction of UV with compounds in the atmosphere. Water, although a trace component of Mars’ atmosphere, is suggested to be a significant source of oxidising species. However, gaseous CO₂ or adsorbed H₂O layers do not influence the photodestruction of amino acids significantly in the absence of reactive soil. Other mechanisms such as chemical processes in the Martian regolith need to be effective for rapid organic destruction.     

A new estimate of volatile outgassing on Mars

The presence of 28% argon on Mars as calculated by Levine and Riegler and indirectly inferred from Soviet Mars-6 lander data has important implications for the outgassing history of H₂O, CO₂, and N₂ on Mars. Even if the terrestrial volatile outgassing ratio is only approximately valid for Mars, then large quantities of H₂O [of the order of 10⁵ gcm^?2 (about 10⁸ more H₂O than is currently present in the Martian atmosphere)] and about 10⁴ gcm^?2 of CO₂ (about 10³ times more CO₂ than found at present in the Martian atmosphere) and some 450 gcm^?2 of N₂ may have outgassed over the history of Mars.     

Comparison between infrared Martian disk spectra and optical properties of terrestrial analogs

Medium spectral resolution (20 cm^?1) infrared measurements of the Martian disk made between 2900 and 5600 cm^?1 from the NASA Lear Airborne Observatory have been successfully compared with predictions derived from a model of the Martian soil and atmosphere. Modeling of the Martian atmosphere permitted the extraction of Martian soil reflectance in the CO₂ bands centered at 3657 cm^?1. Three previously considered acceptable Martian soil analogs, limonite, montmorillonite, and basalt, were analyzed to determine the optical complex indices of refraction in the same range as the airborne observations, for mathematical modeling. A characteristic surface particle size ～1 to 3 μm diameter is indicated. It is concluded that the Martian soil surface near-infrared optical properties are consistent with a soil composition similar to montmorillonite or limonite, mixed with a basalt.     

Sublimation of the Martian CO2 Seasonal South Polar Cap

The polar condensation/sublimation of CO₂, that involve about one fourth of the atmosphere mass, is the major Martian climatic cycle. Early observations in visible and thermal infrared have shown that the sublimation of the Seasonal South Polar Cap (SSPC) is not symmetric around the geographic South Pole.Here we use observations by OMEGA/Mars Express in the near-infrared to detect unambiguously the presence of CO₂ at the surface, and to estimate albedo. Second, we estimate the sublimation of CO₂ released in the atmosphere and show that there is a two-step process. From Ls=180° to 220°, the sublimation is nearly symmetric with a slight advantage for the cryptic region. After Ls=220° the anti-cryptic region sublimation is stronger. Those two phases are not balanced such that there is 22% ± 9 more mass the anti-cryptic region, arguing for more snow precipitation. We compare those results with the MOLA height measurements. Finally we discuss implications for the Martian atmosphere about general circulation and gas tracers, e.g. Ar.     

The Martian paleoclimate and enhanced atmospheric carbon dioxide

Current evidence indicates that the Martian surface is abundant with water presently in the form of ice, while the atmosphere was at one time more massive with a past surface pressure of as much as 1 atm of CO₂. In an attempt to understand the Martian paleoclimate, we have modeled a past CO₂H₂O greenhouse and find global temperatures which are consistent with an earlier presence of liquid surface water, a finding which agrees with the extensive evidence for past fluvial erosion. An important aspect of the CO₂H₂O greenhouse model is the detailed inclusion of CO₂ hot bands. For a surface pressure of 1 atm of CO₂, the present greenhouse model predicts a global mean surface temperature of 294°K, but if the hot bands are excluded, a surface temperature of only 250°K is achieved.     

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.     

The nature of the residual Martian polar caps

A model of the behavior of the Martian polar caps is described which incorporates the heating effects of the atmosphere, as well as insolation and conduction. This model is used to try to match the observed regression curves of the polar caps, and it predicts that all the seasonally condensed CO₂ will be lost by around the summer solstice. The implication is that the residual caps are composed of water ice which, it is found by further modeling, should be stable during the Martian summers. However, it is also argued that this model may be too simplistic, and that the effects of wind in redistributing the seasonal condensate may lead to sufficient thickness of CO₂ in the central polar region to allow the year-long existence of CO₂ without significantly changing the retreat characteristics of the cap, and it is, therefore, concluded that at the present, the nature of the residual caps cannot be reliably determined.     

A technique to deduce atmospheric temperature and constituent profiles from a planet's limb radiance profile

The concept is described of deducing the temperature and constituent profile of a planetary atmosphere from orbiter measurements of the planet's ir limb radiance profile. Expressions are derived for the weighting functions associated with the limb radiance profile for a Goody random band model. Analysis of the weighting functions for the Martian atmosphere indicates that a limb radiance profile in the 15 μm CO₂ band can be used to determine the Martian atmospheric temperature profile from 20 to 60 km. Simulation of the Martian limb radiance profile in the rotational water vapor band indicates that Martian water vapor mixing ratios can be inferred from limb radiance observations in a water vapor band.     

Polar warming in the middle atmosphere of Mars

We report ground-based laser heterodyne spectroscopy of non-thermal emission in the cores of the 10.33-μmR(8) and 10.72-μmP(32) lines of ¹²C¹⁶O₂, obtained at 23 locations on the disk of Mars during the 1984 opposition, at L_s = 130°. The data were obtained at a sub-Doppler spectral resolution, and the temperature of the middle Martian atmosphere (50–85 km) is derived from the frequency width and intensity of the R(8) emission, and from the total intensity of the P(32) emission. We find that the temperature of the middle Martian atmosphere varies with latitude. Near the subsolar latitude, the average 50- to 85-km temperature is close to the radiative equilibrium value for a CO₂ atmosphere. However, at high latitudes in both the northern (summer) and southern (winter) hemispheres the 50- to 85-km temperature exceeds the CO₂ radiative equilibrium value; a meridional gradient in the range of 0.4 – 0.9°K per degree of latitude is indicated by our data. The highest temperatures are seen at high latitudes in the winter hemisphere, reminiscent of the seasonal effects seen at the Earth's mesopause. As in the terrestrial case, this winter polar warming in the Martian middle atmosphere necessitates departures from radiative equilibrium; dynamical heating of order 4 × 10² ergs g⁻¹ sec⁻¹ is required at the edge of the winter polar night. A comparison with 2-D circulation models shows that the presence of atmospheric dust may enhance this dynamical heating at high winter latitudes, and may also account for heating at high latitudes in the summer hemisphere.     

Seasonal buffering of atmospheric pressure on Mars

An isothermal reservoir of carbon dioxide in gaseous contact with the Martian atmosphere would reduce the amplitude and advance the phase of global atmospheric pressure fluctuations caused by seasonal growth and decline of polar CO₂ frost caps. Adsorbed carbon dioxide in the upper ～10 m of Martian regolith is sufficient to buffer the present atmosphere on a seasonal basis. Available observations and related polar cap models do not confirm or refute the operation of such a mechanism. Implications for the amplitude and phase of seasonal pressure fluctuations are subject to direct test by the upcoming Viking mission to Mars.     

Methane release and the carbon cycle on Mars

It has been suggested that the present release rate of methane to the Martian atmosphere could be the result of serpentinization in the deep subsurface, followed by the conversion of H₂ to CH₄ in a CO₂-rich fluid. Making this assumption, we show that the cryosphere could act as a buffer storing, under the form of micron-size methane clathrate particles, the methane delivered from below by hydrothermal fluids and progressively releasing it to the atmosphere at the top. From an extrapolation of the present CH₄ release rate back to the past, we calculate that up to several hundred millibars (～200–2000 mbar) of CO₂, resulting from the oxidation of the released CH₄, in addition to the volcanic supply (～400 mbar), should have accumulated in the atmosphere in the absence of a CO₂ sink. We reassess the capability of escape to have removed CO₂ from the atmosphere by C non-thermal escape and show that it is not significant. We suggest that atmospheric carbon is recycled to the crust through an active subsurface hydrological system, and precipitates as carbonates within the crust. During episodic periods of magmatic activity, these carbonates are decomposed to CO₂ dissolved in running water, and CO₂ can react with H₂ formed by serpentinization to build CH₄. CH₄ is then buffered in the subsurface cryosphere, above the water table, and finally released to the atmosphere, before being recycled to the subsurface hydrological system, and converted back to carbonates. We propose a typical evolution curve of the CO₂ pressure since the late Noachian based on our hypothesis. Contrary to the steady state carbon cycle at work on Earth, a progressive damping of the carbon cycle occurs on Mars due to the absence of plate tectonics and the progressive cooling of the planet.     

Albedo models for the residual south polar cap on Mars: Implications for the stability of the cap under near-perihelion global dust storm conditions

It is uncertain whether the residual (perennial) south polar cap on Mars is a transitory or a permanent feature in the current Martian climate. While there is no firm evidence for complete disappearance of the cap in the past, clearly observable changes have been documented. Observations suggest that the perennial cap lost more CO₂ material in the spring/summer season prior to the Mariner 9 mission than in those same seasons monitored by Viking and Mars Global Surveyor. In this paper we examine one process that may contribute to these changes—the radiative effects of a planet encircling dust storm that starts during late Martian southern spring on the stability of the perennial south polar cap. To approach this, we model the radiative transfer through a dusty planetary atmosphere bounded by a sublimating CO₂ surface.A critical parameter for this modeling is the surface albedo spectrum from the near-UV to the thermal-IR, which was determined from both space-craft and Earth-based observations covering multiple wavelength regimes. Such a multi-wavelength approach is highly desirable since one spectral band by itself cannot tightly constrain the three-parameter space for polar surface albedo models, namely photon “scattering length” in the CO₂ ice and the amounts of intermixed water and dust.Our results suggest that a planet-encircling dust storm with onset near solstice can affect the perennial cap's stability, leading to advanced sublimation in a “dusty” year. Since the total amount of solid CO₂ removed by a single storm may be less than the total CO₂ thickness, a series of dust storms would be required to remove the entire residual CO₂ ice layer from the south perennial cap.     

Description of the Martian polar cap breeze

The surface temperature of the Martian polar caps is about 148 K (frost point temperature of CO₂ at a surface pressure of about 6 hPa), with the “desert” (frost-free) areas adjacent to the polar caps having much greater surface temperatures. The existence of this steep meridional gradient of temperature between the polar caps and the adjacent “desert” areas may produce in the atmosphere a baroclinic instability which generates an atmospheric circulation system similar in some aspects to the terrestrial sea breeze. We have called this circulation system the Martian polar cap breeze. In this paper, the phenomenology of the Martian polar cap breeze is developed on the basis of the indirect observational evidence. Along with friction and the Coriolis force, other factors influence the polar cap breeze: the prevailing wind, topography, irregularity of the polar cap-edge, and stability of the atmosphere. These factors are studied in a qualitative form, as well as the seasonal variations. In addition, the large-scale polar cap wind is presented as a different Martian atmospheric circulation system.     

The Martian hydrologic cycle: Effects of CO2 mass flux on global water distribution

The Martian CO₂ cycle, which includes the seasonal condensation and subsequent sublimation of up to 30% of the planet's atmosphere, produces meridional winds due to the consequent mass flux of CO₂. These winds currently display strong seasonal and hemispheric asymmetries due to the large asymmetries in the distribution of insolation on Mars. It is proposed that asymmetric meridional advection of water vapor on the planet due to these CO₂ condensation winds is capable of explaining the observed dessication of Mars' south polar region at the current time. A simple model for water vapor transport is used to verify this hypothesis and to speculate on the effects of changes in orbital parameters on the seasonal water cycle.     

An improved model of radiative transfer for the NLTE problem in the NIR bands of CO<Subscript>2</Subscript> and CO molecules in the daytime atmosphere of Mars. 1. Input data and calculation method

Advances in attacking the problem of radiative transfer in the near infrared (NIR) bands of CO₂ and CO under nonlocal thermodynamic equilibrium (NLTE) conditions depend on the accuracy of taking into account the radiation processes and inelastic collisions of CO₂ and CO molecules. The focus of the paper is to substantially improve the physical model of the problem and update the calculation method. It is the first time the surface albedo is introduced into the problem of the molecular emission under NLTE conditions. The values of the rate constants for inelastic molecular collisions and their temperature dependences have been radically updated. In some cases, since laboratory measurements of these constants are lacking, different versions are provided for them. The relative abundance of CO₂ and CO isotopologues is based on the ratios of isotope abundances for the elements C and O obtained from the measurements in the atmosphere of Mars. The intensity of extraterrestrial solar NIR radiation is specified on the base of the high-accuracy ground-based measurements. In the method for calculating the populations of vibrational states, we pioneer in completely taking into account the overlapping of spectral lines in the NIR bands of CO₂ and CO.     

Non-thermal water loss of the early Mars: 3D multi-ion hybrid simulations

In this study we analyze the non-thermal loss rates of O⁺, O₂⁺ and CO₂⁺ ions over the last 4.5 billion years (Gyr) in the Martian history by using a 3D hybrid model. For this reason we derived the past solar wind conditions in detail. We take into account the intensified particle flux of the early Sun as well as an Martian atmosphere, which was exposed to a sun's extreme ultraviolet (EUV) radiation flux 4.5 Gyr ago that was 100 times stronger than today. Furthermore, we model the evolution of the interplanetary magnetic field by a Weber & Davis solar wind model. The ‘external’ influences of the Sun's radiation flux and solar wind flux lead to the formation of an ionospheric obstacle by photoionization, charge exchange and electron impact. For the early Martian conditions we could show that charge exchange was the dominant ionization mechanism. Several hybrid simulations for different stages in the evolution of the Martian atmosphere, at 1, 2, 5, 10, 30 and 100 EUV, were performed to analyze the non-thermal escape processes by ion pick-up, momentum transfer from the solar wind to the ionosphere and detached ionospheric plasma clouds. Our results show a non-linear evolution of the loss rates. Using mean solar wind parameters the simulations result in an oxygen loss equivalent to the depth of a global Martian ocean of about 2.6 m over the last 4.5 Gyr. The induced magnetic field strength could be increased up to about 2000 nT. A simulation run with high solar wind density results in an oxygen loss of a Martian ocean up to 205 m depth during 150 million years after the sun reached the zero age mean sequence (ZAMS).     

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.     

The gas exchange experiment for life detection: The Viking Mars Lander

The Gas Exchange Experiment of the Viking mission accepts a sample of Martian soil, incubates this soil with nutrient medium, and periodically samples the enclosed atmosphere over this soil for the gases H₂, N₂, O₂, CH₄, Kr, and CO₂. These gases are analyzed by an automated gas chromatograph, and the data are transmitted to Earth. The design of the experiment and the qualitative and quantitative changes, if any, of gas composition should allow conclusions to be made on the presence of life on Mars. Data and theory substantiating this approach are presented.