C. B. Farmer

Mars - Northern summer ice cap - Water vapor observations from Viking 2

Observations of the latitude dependence of water vapor made from the Viking 2 orbiter show peak abundances in the latitude band 70� to 80� north in the northern midsummer season (planetocentric longitude ∼ 108�). Total column abundances in the polar regions require near-surface atmospheric temperatures in excess of 200�K, and are incompatible with the survival of a frozen carbon dioxide cap at martian pressures. The remnant (or residual) north polar cap, and the outlying patches of ice at lower latitudes, are thus predominantly water ice, whose thickness can be estimated to be between 1 meter and 1 kilometer.

Composition measurements of the 1989 Arctic winter stratosphere by airborne infrared solar absorption spectroscopy

Simultaneous measurements of the stratospheric burdens of H2O, HDO, OCS, CO2, O3, N2O, CO, CH4, CF2Cl2, CFCl3, CHF2Cl, C2H6, HCN, NO, NO2, HNO3, ClNO3, HOCl, HCl, and HF were made by the Jet Propulsion Laboratory MkIV interferometer on board the NASA DC-8 aircraft during January and early February 1989 as part of the Airborne Arctic Stratosphere Experiment (AASE). Data were acquired on 11 flights at altitudes of up to 12 km over a geographic region covering the NE Atlantic Ocean, Iceland, and Greenland. Analyses of the chemically active gases reveal highly perturbed conditions within the vortex. The ClNO3 abundance was chemically enhanced near the edge of the vortex but was then depleted inside. HCl was chemically depleted near the vortex edge and became even more depleted inside. In fact, by late January deep inside the vortex, HCl was either completely removed up to 27-km altitude, or partially removed to an even greater altitude. NO2 was also severely depleted inside the vortex. In contrast to Antarctica, H2O and HNO3 were both more abundant inside the vortex than outside. While for H2O this is solely a consequence of descent (without accompanying dehydration), HNO3 additionally shows evidence for chemical enhancement inside the vortex. One exception to the high HNO3 abundances inside the vortex occurred on January 31 when stratospheric temperatures above the aircraft fell below 190 K. However, following this event, HNO3 burdens fully recovered, suggesting that if the loss on January 31 was due to temporary freeze-out of HNO3, the resulting particles reevaporated above 12 km. Taken together, these results suggest that although the Arctic vortex did not get cold enough to produce any dehydration, nor as vertically extensive denitrification as occurred in Antarctica, nevertheless, enough heterogeneous chemistry still occurred to convert over 90% of the inorganic chlorine to active forms in the 14- to 27-km altitude range by early February 1989.

High resolution infrared spectroscopy of the sun and the earth's atmosphere from space

The atmospheric trace molecule spectroscopy experiment (ATMOS) was designed to obtain high-resolution absorption spectra of the atmosphere from earth orbit, from which the vertical distributions of a large number of minor and trace molecular constituents could be retrieved. The ATMOS instrument is an FFT spectrometer covering the 600 to 5000 cm−1 frequency range and uses a double-passed, tilt-compensated optical configuration, with the two retroreflectors moving reciprocally. The scan time of 2 s gives spectra with an unapodized resolution of 0.01 cm−1, spaced 4 km apart, vertically.The first flight of ATMOS was made in April, 1985, as part of the Shuttle Spacelab-3 science payload. A total of 19 sunrise and sunset occultations were observed, which resulted in the acquisition of more than 1000 atmospheric spectra with an equal number of “solar only” scans. The spectra show absorptions of some 40 different atmospheric constituents, some of which are discernable at altitudes well into the thermosphère (i.e., up to about 150 km). The high signal/noise ratio and repeatability of the spectra have enabled a wealth of new atmospheric data to be retrieved, including the first positive identifications of such key reservoir species as COF2, HNO4, and N2O5, simultaneous vertical distributions of the minor gases from 5 to 140 km, the entire odd-nitrogen family in the stratosphere, and most of the halogen source gases with their corresponding reservoir and sink species. Measurements of the frequencies of large numbers of spectral lines has yielded Doppler shifts from which zonal winds, having a precision of the order of 2 m/s, have been retrieved throughout the stratosphere and mésosphère.

The 1985 chlorine and fluorine inventories in the stratosphere based on ATMOS observations at 30° north latitude

The set of high-resolution infrared solar observations made with the Atmospheric Trace Molecule Spectroscopy (ATMOS)-Fourier transform spectrometer from onboard Spacelab 3 (30 April-1 May 1985) has been used to evaluate the total budgets of the odd chlorine and fluorine chemical families in the stratosphere. These budgets are based on volume mixing ratio profiles measured for HCl, HF, CH3Cl, ClONO2, CCl4, CCl2F2, CCl3F, CHClF2, CF4, COF2, and SF6 near 30° north latitude. When including realistic concentrations for species not measured by ATMOS, i.e., the source gases CH3CCl3 and C2F3Cl3 below 25 km, and the reservoirs ClO, HOCl and COFCl between 15 and 40 km (five gases actually measured by other techniques), the 30° N zonal 1985 mean total mixing ratio of chlorine, Cl, was found to be equal to (2.58±0.10) ppbv (parts per billion by volume) throughout the stratosphere, with no significant decrease near the stratopause. The results for total fluorine indicate a slight, but steady, decrease of its volume mixing ratio with increasing altitude, around a mean stratospheric value of (1.15±0.12) ppbv. Both uncertainties correspond to one standard deviation. These mean springtime 1985 stratospheric budgets are commensurate with values reported for the tropospheric Cl and F concentrations in the early 1980s, when allowance is made for the growth rates of their source gases at the ground and the time required for tropospheric air to be transported into the stratosphere. The results are discussed with emphasis on conservation of fluorine and chlorine and the partitioning among source, sink, and reservoir gases throughout the stratosphere.

Water vapor diffusion in Mars subsurface environments

The diffusion coefficient of water vapor in unconsolidated porous media is measured for various soil simulants at Mars-like pressures and subzero temperatures. An experimental chamber which simultaneously reproduces a low-pressure, low-temperature, and low-humidity environment is used to monitor water flux from an ice source through a porous diffusion barrier. Experiments are performed on four types of simulants: 40–70 µm glass beads, sintered glass filter disks, 1–3 µm dust (both loose and packed), and JSC Mars–1. A theoretical framework is presented that applies to environments that are not necessarily isothermal or isobaric. For most of our samples, we find diffusion coefficients in the range of 2.8 to 5.4 cm^2 s^-1 at 600 Pascal and 260 K. This range becomes 1.9–4.7 cm^2 s^-1 when extrapolated to a Mars-like temperature of 200 K. Our preferred value for JSC Mars–1 at 600 Pa and 200 K is 3.7 ± 0.5 cm^2 s^-1. The tortuosities of the glass beads is about 1.8. Packed dust displays a lower mean diffusion coefficient of 0.38 ± 0.26 cm^2 s^-1, which can be attributed to transition to the Knudsen regime where molecular collisions with the pore walls dominate. Values for the diffusion coefficient and the variation of the diffusion coefficient with pressure are well matched by existing models. The survival of shallow subsurface ice on Mars and the providence of diffusion barriers are considered in light of these measurements.

Stratospheric profiles of heavy water vapor isotopes and CH3D from analysis of the ATMOS Spacelab 3 infrared solar spectra

Stratospheric volume mixing ratio profiles of H218O, H217O, HDO, and CH3D near latitudes of 30°N and 47°S have been retrieved from ∼0.01-cm−1 resolution infrared solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Spacelab 3 shuttle mission (April 29 to May 6, 1985). Improved heavy isotope water vapor and CH3D spectroscopic parameters determined from ∼0.005- to 0.01-cm−1 resolution room temperature laboratory spectra have been used in the retrievals. The profiles of the three water vapor isotopes show an increase in the volume mixing ratio with altitude over the range of measurements (20 to 54 km for H218O, 20 to 46 km for H217O, and 20 to 34 km for HDO). Deuterium-to-hydrogen and heavy-to-normal oxygen isotope ratio profiles have been calculated by dividing the retrieved isotopic profiles by the previously reported profiles of H216O and CH4 obtained from the same spectral data and then referencing these results to the isotopic composition of standard mean ocean water (SMOW). At 20 km the 18O/16O ratio in H2O is slightly (∼8%) depleted relative to SMOW; this ratio increases with altitude and is slightly positive above ∼36 km. No evidence has been found for the large 18O enhancements reported previously. The 17O/16O water vapor results are similar to those for 18O/16O. The ATMOS measurements show depletions of ∼63% in the D/H content of water vapor near 20 km and an increase in this ratio with altitude up to 34 km. The D/H ratio in stratospheric methane is close to the isotopic ratio in SMOW over the 18 to 28 km altitude range. No differences between the water vapor or methane isotopic compositions at 30°N and 47°S were detected. The results are compared with previously reported measurements and calculations.

Liquid water on Mars

Abstract The factors which affect fusion and evaporation of ice under a variety of Martian surface conditions are examined. It is found that a frost or ice deposit will pass through a transient liquid phase in temperate latitudes during summer, if the ice is partly or wholly dust covered. The barrier to free gaseous diffusion which the surface material presents is, under favorable (and definable) conditions, more than adequate to force the water to remain in the liquid state until its evaporation is complete. Furthermore, for a realistic range of regolith particle sizes and porosities, and depths of burial of the ice, the lifetime of the ice can be considerably longer than the duration of a single diurnal warming cycle. Current knowledge of the seosonal and diurnal behavior of the atmospheric vapor is summarized and discussed as it relates to the availability of surface ice at temperate latitudes.

Evidence for subsidence in the 1989 Arctic winter stratosphere from airborne infrared composition measurements

Simultaneous measurements of the stratospheric burdens of CO2, HCN, N2O, CH4, OCS, CF2Cl2, CFCl3, CHF2Cl and HF were made by the Jet Propulsion Laboratory MkIV interferometer on board the NASA DC-8 aircraft during January and early February 1989 as part of the Airborne Arctic Stratosphere Experiment. Data were acquired on 11 flights at altitudes of up to 12 km over a geographic region covering the NE Atlantic Ocean, Iceland and Greenland. The results obtained show large variations in the burdens of these tracers due to the effects of transport. The tropospheric source gas burdens were reduced inside the polar vortex, suggesting that the air had subsided with respect to the surrounding mid-latitude air. Increased HF burdens inside the vortex support this interpretation. The results obtained from the different tracers are highly consistent with each other and indicate that in the 15- to 20-km altitude range inside the vortex, surfaces of constant volume mixing ratio were located some 5–6 km lower in absolute altitude than outside the vortex. The results also indicate that the magnitude of this subsidence increases with altitude. These conclusions are consistent with other measurements.

Scientific objectives of the Solar Mesosphere Explorer mission

The 1981–82 Solar Mesosphere Explorer (SME) mission is described. The SME experiment will provide a comprehensive study of mesospheric ozone and the processes which form and destroy it. Five instruments will be carried on the spinning spacecraft to measure the ozone density and its altitude distribution from 30 to 80 km, monitor the incoming solar ultraviolet radiation, and measure other atmospheric constituent which affect ozone. The polar-orbiting spacecraft will be placed into a 3pm-3 am Sun-synchronous orbit. The atmospheric measurements will scan the Earths limb and measure: (1) the mesospheric and stratospheric ozone density distribution by inversion of Rayleigh-scattered ultraviolet limb radiance, and the thermal emission from ozone at 9.6 μm; (2) the water vapor density distribution by inversion of thermal emission at 6.3 μm; (3) the ozone photolysis rate by inversion of the O2(1Δg) 1.27 μm limb radiance; (4) the temperature profile by a combination of narrow-band and wide-band measurements of the 15 μm thermal emission by CO2; and, (5) theNO2 density distribution by inversion of Rayleighscattered limb radiance at 0.439 μm. The solar ultraviolet monitor will measure both the 0.2–0.31 μm spectral region and the Lyman-alpha (0.1216 μm) contribution to the solar irradiance. This combination of measurements will provide a rigorous test of the photochemical equilibrium theory of the mesospheric oxygen-hydrogen system, will determine what changes occur in the ozone distribution as a result of changes in the incoming solar radiation, and will detect changes that may occur as a result of meteorological disturbances.

Monitoring of the integrated column of hydrogen fluoride above the Jungfraujoch Station since 1977 ― the HF/HCL column ratio

The amount of hydrogen fluoride (HF) above the International Scientific Station of the Jungfraujoch (Switzerland) has been monitored during the last 8 years. The results deduced spectro-scopically from solar IR absorption measurements near 2.48 μm indicate a cumulative trend equivalent to (8.5±1)% increase per year, as well as short-term variability which appears to be strongly correlated with meridional circulation patterns during the February–April months. Based on intensified measurements made over the last three years, it is found that the integrated content of HF undergoes a seasonal change with a minimum occurring in the fall. The HF/HCl ratio derived from simultaneous HF and HCl measurements was found equal to 0.15 during the period 1977–79, and 0.24 for the 1983–85 timespan.