Martian volatiles: Their degassing history and geochemical fate

Observations of Mars and cosmochemical considerations imply that the total inventory of degassed volatiles on Mars is 10² to 10³ times that present in Mars' atmosphere and polar caps. The degassed volatiles have been physically and chemically incorporated into a layer of unconsolidated surface rubble (a “megaregolith”) up to 2km thick. Tentative lines of evidence suggest a high concentration (～5g/cm²) of ⁴⁰ Ar in the atmosphere of Mars. If correct, this would be consistent with a degassing model for Mars in which the Martian “surface” volatile inventory is presumed identical to that of Earth but scaled to Mars' smaller mass and surface area. The implied inventory would be: (⁴⁰Ar) = 4g/cm², (H₂O) = 1 × 10⁵g/cm², (CO₂) = 7 × 10³g/cm², (N₂) = 3 × 10²g/cm², (Cl) = 2 × 10³g/cm², and (S) = 2 × 10²g/cm². Such a model is useful for testing, but differences in composition and planetary energy history may be anticipated between Mars and Earth on theoretical grounds. Also, the model demands huge regolith sinks for the volatiles listed.If the regolith were in physical equilibrium with the atmosphere, as much as 2 × 10⁴g/cm² of H₂O could be stored in it as hard-frozen permafrost, or 5 × 10⁴g/cm² if equilibrium with the atmosphere were inhibited. Spectral measurements of Martian regolith material and laboratory measurement of weathering kinetics on simulated regolith material suggest large amounts of hydrated iron oxides and clay minerals exist in the regolith; the amount of chemically bound H₂O could be from 1 × 10⁴ to 4 × 10⁴g/cm². In an Earth-analogous model, a 2 km mixed regolith must contain the following concentrations of other volatile-containing compounds by weight: carbonates = 1.5%, nitrates = 0·3%, chlorides = 0.6%, and sulfates = 0.1%. Such concentrations would be undetectable by current Earth-based spectral reflectance measurements, and (except the nitrates) formation of the “required” amounts of these compounds could result from interaction of adsorbed H₂O and ice with primary silicates expected on Mars. Most of the CO₂ could be physically adsorbed on the regolith.Thus, maximum amounts of H₂O and other volatiles which could be stored in the Mars regolith are marginally compatible with those required by an Earth-analogous model, although a lower atmospheric ⁴⁰Ar concentration and regolith volatile inventory would be easier to reconcile with observational constraints. Differences in the ratios of H₂O and other volatiles to ⁴⁰Ar between surface volatiles on the real Mars and on an Earth-analogous Mars could result from and reflect differences in bulk composition and time history of degassing between Mars and Earth. Models relating Viking-observable parameters, e.g., (⁴⁰Ar) and (³⁶Ar), to the time history and overall intensity of Mars degassing are given.     

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.     

Global distribution and migration of subsurface ice on mars

A thermal/diffusive model of H₂O kinetics and equilibrium was developed to investigate the long-term evolution and depth distribution of subsurface ice on Mars. The model quantitatively takes into account (1) obliquity variations; (2) eccentricity variations; (3) long-term changes in the solar luminosity; (4) variations in the argument of subsolar meridian (in planetocentric equatorial coordinates); (5) albedo changes at higher latitudes due to seasonal phase changes of CO₂ and the varying extent of CO₂ ice cover; (6) planetary internal heat flow; (7) temperature variations in the regolith as a function of depth, time, and latitude due to the above factors; (8) atmospheric pressure variations over a 10⁴-year time scale; (9) the effects of factors (1) through (5) on seasonal polar cap temperatures; and (10) Knudsen and molecular diffusion of H₂O through the regolith. The migration of H₂O into or out of the regolith is determined by two boundary conditions, the H₂O vapor pressure at the subsurface ice boundary and the annual average H₂O concentration at the base of the atmosphere. These are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap. Starting from an arbitrary initial uniform depth distribution of subsurface ice, H₂O fluxes into or out of the regolith are calculated for 100 selected obliquity cycles, each representing a different epoch in Mars' history. The H₂O fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time. The results show that obliquity variations influence annual average regolith temperatures in varying degrees, depending on latitude, with the greatest effect at the poles and almost no effect at 40° lat. Insolation changes at the pole, due to obliquity, argument of subsolar meridian, and eccentricity variations can produce enormous atmospheric H₂O concentration variations of ≈6 orders of magnitude over an obliquity cycle. Superimposed on these cyclic variations is a slow, monotonic change due to the increasing solar luminosity. Albedo changes at the polar cap due to seasonal phase changes of CO₂ and the varying thickness of the CO₂ ice cover are critically important in determining annual average atmospheric H₂O concentrations. Despite the strongly oscillating character of the boundary conditions, only small amounts of H₂O are exchanged between the regolith and the atmosphere per obliquity cycle (<10 g/cm²). The net result of H₂O migration is that the regolith below 30–40° lat is depleted of subsurface ice, while the regolith above 30–40° lat contains permanent ice due to the depth of penetration of the annual thermal wave. This result is supported by recent morphological studies. The rate of migration of H₂O is strongly dependent on average pore/capillary radius for which we have assumed values of 1 and 10 μm. We estimate that the H₂O ice removed from the regolith would produce a permanent ice cap with a volume between 2 × 10⁶ and 6 × 10⁶ km³. This generally agrees with estimates deduced from deflationary features at lower latitudes, depositional features at higher latitudes, and the mass of the polar caps.     

Mars: Photodesorption from mineral surfaces and its effects on atmospheric stability

A mechanism has been proposed for uv-accelerated desorption from Fe²⁺ sites on mineral surfaces that satisfies kinetic constraints determined in the laboratory by Huguenin. The process is an integral step of the photochemical weathering mechanism for producing dust on Mars, and it now appears that it may play primary roles in stabilizing CO₂ against dissociation by sunlight and in controlling the oxidation state of the atmosphere. We propose that adsorption occurs at octahedrally coordinated Fe²⁺ surface sites to form seven-coordinate transition-state complexes. These complexes acquire 16–18 kcal mole^?1 of ligand field stabilization energy. During illumination (λ ≤ 0.35 μm), electrons are photoemitted from the surfaced Fe²⁺, temporarily oxidizing them to Fe³⁺. Fe³⁺ has no ligand field stabilization energy, and the complexes lose 16–18 kcal mole^?1 of stabilization energy. This is a large fraction of the 19- to 28-kcal mole^?1 activation energy for dissociating the complexes, and desorption should proceed spontaneously. The gases that were observed to undergo adsorption-photodesorption include O₂, CO₂, CO, H₂O, N₂, and Ar. Photodesorption can drive several catalytic reactions, one of which is the oxidation of CO to CO₂. The rate of this reaction should be limited by the supply of CO and O₂ to the surface to ～2 × 10¹² cm^?2 sec^?1 (column photodissociation rate of CO₂). By including this surface reaction in models of Martian atmospheric CO₂ chemistry, CO₂ can be stabilized against photodissociation with eddy diffusion coefficients of only 3 × 10⁵?1 × 10⁷ cm² sec^?1 below 40 km, raising to ～ 10⁹ cm² sec^?1 at 140 km. Odd hydrogen is not needed to catalyze the oxidation of CO below 40 km, and odd hydrogen mixing ratios need only to be $\text{f}{}_{\mathrm{<mtext>H</mtext>}}\text{? 10}{}^{\mathrm{?10}}$ to depress ozone concentrations below the observed upper limit in equatorial regions. Another catalytic reaction that should be driven by photodesorption on Mars is $\text{20}\text{H}{}^{\mathrm{?}}{}_{\mathrm{<mtext>(</mtext><mtext>ads</mtext><mtext>)</mtext>}}\text{→}\text{H}{}_2\text{O}\text{+}\text{1}\text{2}\text{O}{}_{\mathrm{2(g)}}\text{+ 2e}{}^{\mathrm{?}}{}_{\mathrm{<mtext>crystal</mtext>}}$. This is an important source of atmospheric O₂, amounting to 7 × 10¹³?2 × 10¹⁷ O₂ molecules cm^?2 yr^?1, and it could have a significant effect on atmospheric oxidation state.     

Postperihelion interference filter photometry of the “annual” comet P/Encke

Interference filter photometry was taken of Comet Encke on June 14, 1974 (1.07 AU heliocentric distance, postperihelion) at the CTIO (Cerro Tololo Interamerican Observatory) 150-cm reflector. Production rates were calculated of 4.1 × 10²³ mol sec^?1 of CN, 5.3 × 10²³ mol sec^?1 of C₃, and 4.3 × 10²⁴ mol sec^?1 of C₂. These are about three times smaller than at comparable heliocentric distance preperihelion, assuming a value of 100 for the ratio H₂O/ (C₂ + C₃ + CN). An upper limit was placed on the production of nonvolatiles at about one-third that of volatiles in mass by assuming a bulk density of 1 g cm^?3, a particle geometric albedo of 0.1, and a phase function of 0.2.     

Nonthermal escape of hydrogen and deuterium from Venus and implications for loss of water

Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H⁺ with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O⁺ and H₂. However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 10¹⁰ cm^?2 sec^?1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10^?3. This corresponds to an H₂O mixing ratio of 1 × 10^?3 at the cold trap and ～15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ～1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H₂O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ～4.0 × 10^?6.     

The evolution of an impact-generated atmosphere

Shock wave and thermodynamic data for rock-forming and volatile-bearing minerals are used to determine minimum impact velocities (v_cr) and minimum impact pressures (p_cr) required to form a primary H₂O atmosphere during planetary accretion from chondritelike planetesimals. The escape of initially released water from an accreting planet is controlled by the dehydration efficiency. Since different planetary surface porosities will result from formation of a regolith, v_cr and p_cr can vary from 1.5 to 5.8 km/sec and from 90 to 600 kbar, respectively, for target porosities between 0 and ～45%. On the basis of experimental data, hydration rates for forsterite and enstatite are derived. For a global regolith layer on the Earth's surface, the maximum hydration rate equals 6 × 10¹⁰ g H₂O sec^?1 during accretion of the Earth. Attenuation of impact-induced shock pressure is modeled to the extent that the amount of released water as a function of projectile radius, impact velocity, weight fraction of water in the target, target porosity, and dehydration efficiency can be estimated. The two primary processes considered are the impact release of water bound in hydrous minerals (e.g., serpentine) and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the Earth. Parameters which lead to a primary atmosphere/hydrosphere are: an accretion time of ? 1.6 × 10⁸years, the use of an accretion model defined by Weidenschilling (1974, 1976), a mean planetesimal radius of 0.5 km, a hydration rate of 6 × 10¹⁰ g H₂O sec^?1 inferred from a mean porosity of ～ 10% for the upper 1 km of the accreting Earth, and values for the dehydration efficiency, DE, of 0.55 and 0.07 for the maximum and minimum pressure decay model, respectively. Conditions which prohibit the formation of a primary atmosphere include an accretion time much longer than 1.6 × 10⁸ years, a hydration rate for forsterite and enstatite well in excess of 6 × 10¹⁰ g H₂O sec^?1, and a dehydration efficiency DE < 0.07. We conclude that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation.     

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.     

Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH₃ in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH₃ and hydrocarbon photochemistry, specifically the C₂H₂ catalyzed photodissociation of CH₄. PH₃ is rapidly depleted with altitude (scale height ～35 km) in the upper troposphere when K～10⁴cm²sec^?1; an upper limit for K at the tropopause is estimated at ～10⁵cm²sec^?1. If there is no gas phase P₂H₄ because of sublimation, P₂ and P₄ formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H₂ + M → PH₃ + M is exceedingly slow. An upper limit P₄ column density of ～2×10¹⁵cm^?2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P₂H₄, P₂ and P₄ may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T?100°K. Potentially observable amounts of the organophosphorus compounds CH₃P₂H₂ and HCP are predicted, with column densities of >10¹⁷ cm^?2 and production rates of ～2×10⁸cm^?2sec^?1. The possible importance of electronically excited states of PH_x and additional PH₃/hydrocarbon photochemical coupling paths are also considered.     

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.     

On the structure of Mars' atmosphere at 120–220 km

Altitude dependences of [CO₂] and [CO₂⁺] are deduced from Mariner 6 and 7 CO₂⁺ airglow measurements. CO₂ densities are also obtained from n_e radio occultation measurements. Both [CO₂] profiles are similar and correspond to the model atmosphere of Barth et al. (1972) at 120 km, but at higher altitudes they diverge and at 200–220 km the obtained [CO₂] values are three times less the model. Both the airglow and radio occultation observations show that a correction factor of 2.5 should be included into the values for solar ionization flux given by Hinteregger (1970). The ratio of [CO₂⁺]/n_e is 0.15–0.2 and, hence, [O]/[CO₂] is ～3% at 135 km. An atmospheric and ionospheric model is developed for 120–220 km. The calculated temperature profile is characterized by a value of T ≈ 370°K at h ? 220 km, a steep gradient (～2°/km) at 200-160 km, a bend in the profile at 160 km, a small gradient (～0.7°/km) below and a value of T ≈ 250°K at 120 km. The upper point agrees well with the results of the Lyman-α measurements; the steep gradient may be explained by molecular viscosity dissipation of gravity and acoustical waves (the corresponding energy flux is 4 × 10^?2 erg cm^?2sec^?1 at 180 km). The bend at 160 km may be caused by a sharp decrease of the eddy diffusion coefficient and defines K ≈ 2 × 10⁸cm²sec^?1; and the low gradient gives an estimate of the efficiency of the atmosphere heating by the solar radiation as ? ≈ 0.1.     

A search for H2O and Ch4 in Comet Kohoutek

A spectroscopic search for H₂O and CH₄ in Comet Kohoutek (1973f) was made using a Pepsios interferometer. No evidence was found for either molecule, allowing us to set an upper limit on their production rates (on about 21 January 1974) of Q(H₂O) < 6.2 × 10²⁸ sec^?1 and Q(CH₄) < 2.0 × 10³⁰ sec^?1. If the cometary surface is water-ice, this production rate leads to a product (1 ? A)·(πR₀²) < 2.2 km², where A is the Bond albedo, R₀ is the nuclear radius, and we assume that all the absorbed solar energy is used to evaporate H₂O.     

Some problems in Venus' aeronomy

Models are presented for the height distribution of various photochemically active gases in Venus' upper atmosphere. Attention is directed to the chemistry and vertical transport of odd hydrogen (H, OH, HO₂, H₂O₂), odd oxygen (O, O₃), free chlorine (Cl, ClO, ClOO, Cl₂), CO, O₂, H₂ and H₂O. Supply of O₂ may play a limiting role in the formation of a possible H₂SO₄ cloud on Venus. The supply rate is influenced by both chemical and dynamical processes in the stratosphere, and an analysis of recent spectroscopic data for O₂ implies a lower limit to the appropriate eddy coefficient of about 3 × 10⁵ cm²/sec. The abundances of thermospheric O and CO are determined largely by vertical mixing, and an analysis of Mariner 10 measurements of Venus' Lyman α airglow suggests that the eddy coefficient in the lower thermosphere may be as large as 5 × 10⁷cm²sec. The corresponding values for the mixing ratios of O and CO at the ionospheric peak are approximately 1 per cent. The Lyman α data could be reconciled with larger values for thermospheric O, and smaller values for the vertical eddy coefficient, if non-thermal loss processes were to play a dominant role in hydrogen escape, and if the corresponding flux were to exceed 10⁷ atoms/cm²/sec. A sink of this magnitude would imply major depletion of Venus' atmospheric water over geologic time, and would appear to require mixing ratios of H₂O in the lower atmosphere in excess of 4 × 10^?4. The extensive component to the Lyman α emission measured by Mariner 5 may be due to resonance scattering of sunlight by hot atoms formed by charge transfer with O⁺. The H scale height, therefore, may reflect the temperature of positive ions in Venus' topside ionosphere.     

Distribution and state of H2O in the high-latitude shallow subsurface of mars

A quantitative model of the state, distribution, and migration of water in the shallow Martian regolith is presented. Reported results are confined to the region of the planet greater than 40° lat. The calculations take into account (1) expected thermal variations at all depths, latitudes, and times resulting from seasonal and astronomically induced insolation variations; (2) variations in atmospheric P_H2O and P_CO2 resulting from polar insolation variations and regolith adsorptive equilibria; (3) feedback effects related to latent heat and albedo variations resulting from condensation of atmospheric constituents; (4) two possible regolith mineralogies; (5) variable total H₂O content of the regolith; (6) kinetics of H₂O transport through the Martian atmosphere and regolith; and (7) equilibrium phase partitioning of H₂O between the condensed, adsorbed, and vapor phases. Results suggest that the adsorptive capacity of the regolith is important in controlling the state and distribution of high-latitude H₂O; unweathered mafic silicates favor the development of shallow ground ice at all temperate and polar latitudes, while heavily weathered clay-like regolith materials leads to a deeper ground ice interface and far more extensive quantities of adsorbed H₂O. The capacity of the high-latitude regolith for storage of H₂O and the total mass of H₂O exchanged between the atmosphere, polar cap, and subsurface over an obliquity cycle is found to be relatively independent of mineralogy. The maximum exchanged volume is found to be 3.0 × 10⁴ km³ of ice per cycle. Implications for the history of the polar caps and the origin of the layered terrain are discussed. Results also suggest that seasonal thermal waves act to force adsorbed H₂O into the solid phase over a wide variety of latitude/obliquity conditions. Seasonal phase cycling of regolith H₂O is most common at high latitudes and obliquities. Such phase behavior is highly dependent on regolith mineralogy. In a highly weathered regolith, adsorbed H₂O is annually forced into the solid phase at all latitudes ≥40° at obliquities greater than approximately 25°. Seasonal adsorption-freezing cycles which are predicted here may produce geomorphologic signatures not unlike those produced by terrestrial freeze-thaw cycles.     

Near-infrared laboratory spectra of solid H2O/CO2 and CH3OH/CO2 ice mixtures

We present near-IR spectra of solid CO₂ in H₂O and CH₃OH, and find they are significantly different from that of pure solid CO₂. Peaks not present in either pure H₂O or pure CO₂ spectra become evident when the two are mixed. First, the putative theoretically forbidden CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹), that is absent from our spectrum of pure solid CO₂, is prominent in the spectra of H₂O/CO₂=5 and 25 mixtures. Second, a 2.74-μm (3650 cm⁻¹) dangling OH feature of H₂O (and a potentially related peak at 1.89 μm) appear in the spectra of CO₂-H₂O ice mixtures, but are probably not diagnostic of the presence of CO₂. Other CO₂ peaks display shifts in position and increased width because of intermolecular interactions with H₂O. Warming causes some peak positions and profiles in the spectrum of a H₂O/CO₂=5 mixture to take on the appearance of pure CO₂. Absolute strengths for absorptions of CO₂ in solid H₂O are estimated. Similar results are observed for CO₂ in solid CH₃OH. Since the CO₂ (2ν₃) overtone near 2.134 μm (4685 cm⁻¹) is not present in pure CO₂ but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO₂ is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies.     

The astronomical theory of climatic change on Mars

We examine the response of Martian climate to changes in solar energy deposition caused by variations of the Martian orbit and obliquity. We systematically investigate the seasonal cycles of carbon dioxide, water, and dust to provide a complete picture of the climate for various orbital configurations. We find that at low obliquity (15°) the atmospheric pressure will fall below 1 mbar; dust storms will cease; thick permanent CO₂ caps will form; the regolith will release CO₂; and H₂O polar ice sheets will develop as the permafrost boundaries move poleward. At high obliquity (35°) the annual average polar temperature will increase by about 10°K, slightly desorbing the polar regolith and causing the atmospheric pressure to increase by not more than 10 to 20 mbar. Summer polar ground temperatures as high as 273°K will occur. Water ice caps will be unstable and may disappear as the equilibrium permafrost boundary moves equatorward. However, at high eccentricity, polar ice sheets will be favored at one pole over the other. At high obliquity dust storms may occur during summers in both hemispheres, independent of the eccentricity cycle. Eccentricity and longitude of perihelion are most significant at modest obliquity (25°). At high eccentricity and when the longitude of perihelion is close to the location of solstice hemispherical asymmetry in dust-storm generation and in polar ice extent and albedo will occur.The systematic examination of the relation of climate and planetary orbit provides a new theory for the formation of the polar laminae. The terraced structure of the polar laminae originates when eccentricity and/or obliquity variations begin to drive water ice off the dusty permanent H₂O polar caps. Then a thin (meters) layer of consolidated dust forms on top of a dirty, slightly thicker (tens of meters) ice sheet and the composite is preserved as a layer of laminae composed predominately of water ice. Because of insolation variation on slopes, a series of poleward- and equatorward-facing scarps are formed where the edges of the laminae are exposed. Independently of orbital variations, these scarps propagate poleward both by erosion of the equatorward slopes and by deposition on the poleward slopes. Scarp propagation resurfaces and recycles the laminae forming the distinctive spiral bands of terraces observed and provides a supply of water to form new permanent ice caps. The polar laminae boundary marks the furthest eqautorward extension of the permanent H₂O caps as the orbit varies. The polar debris boundary marks the furthest equatorward extension of the annual CO₂ caps as the orbit varies.The Martian regolith is now a significant geochemical sink for carbon dioxide. CO₂ has been irreversibly removed from the atmosphere by carbonate formation. CO₂ has also benn removed by regolith adsorption. Polar temperature increases caused by orbital variations are not great enough     

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.     

Regolith-atmosphere exchange of water and carbon dioxide on Mars: Effects on atmospheric history and climate change

Exchange of CO₂ and H₂O between the Mars regolith and the atmosphere-cap system plays an important role in governing the evolution of the martian atmosphere and the martian climate. Most of the exchangeable CO₂ (perhaps one or two orders of magnitude more than the atmospheric inventory) is currently adsorbed on the deep regolith, and can be “cryopumped” to a large quasipermanent CO₂ cap (not now present) during lowest Mars obliquity (θ). During the obliquity driven regolith-cap CO₂ exchange cycle, the atmospheric pressure varies harmonically between ~0.1 mb (lowest Θ) and ? 20 mb (highest Θ). The regolith buffer plays only a small or negligible role in the seasonal CO₂ pressure variations caused by atmosphere-cap exchange because adsorption greatly inhibits diffusion of the seasonal “pressure wave” into the regolith. In contrast, thermally driven H₂O seasonal exchange between the atmosphere and regolith appears to be in large part responsible for observed seasonal variations in the small atmospheric H₂O inventory. Long term exchange of H₂O may be dominated by transfer between the polar caps and ice in the regolith. Available and potential tests of regolith-atmospheric-cap volatile exchange models using ground-based and spacecraft-based techniques are discussed.     

Photochemistry of minor constituents in the troposphere

Altitude profiles for the number densities of NO, NO₂, NO₃, N₂O₅, HNO₂, CH₃O, CH₃O₂, H₂CO, OH, and HO₂ are calculated as a function of time of day with a steady-state photochemical model in which the altitude profiles for the number densities of H₂O, CH₄, H₂, CO, O₃, and the sum of NO and NO₂ are fixed at values appropriate to a summer latitude of 34°. Average daily profiles are calculated for the long-lived species, HNO₃, H₂O₂, and CH₃O₂H.The major nitrogen compound HNO₃ may have a number density approaching 5 × 10¹¹ molecules cm^?3 at the surface, although an effective loss path due to collisions with particulates could greatly reduce this value.The number density of OH remains relatively unchanged in the first 6 km and reaches 1 × 10⁷ molecules cm^?3 at noon, while the number density of HO₂ decreases throughout the lower troposphere from its noontime value of 8 × 10⁸ molecules cm^?3 at the surface.H₂O₂ and H₂CO both have number densities in the ppb range in the lower troposphere.Owing to decreasing temperature and water concentration, the production of radicals and their steady-state number densities decrease with altitude, reaching a noontime minimum of 1 × 10⁸ molecules cm^?3 for OH and 3 × 10⁷ molecules cm^?3 for HO₂ at the tropopause. The related minor species show even sharper decreases with increasing altitude.The primary path for interconverting OH and HO₂ serves as the major sink for CO and leads to a tropospheric lifetime for CO of ~0.1 yr.Another reaction cycle, the oxidation of CH₄, is quite important in the lower troposphere and leads to the production of H₂CO along with the destruction of CH₄ for which a tropospheric lifetime of ~2 yr is estimated.The destruction of H₂CO that was produced in the CH₄ oxidation cycle provides the major source of CO and H₂ in the atmosphere.     

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.