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Hess, S. L.

Publications and source records attributed to Hess, S. L..

16 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Band-pass filtering of one year of daily mean pressures on Mars

The sol-mean atmospheric pressures measured at the two Viking landers exhibit fluctuations ranging in period from seasonal to a few days. The time series are highly nonstationary, which severely limits standard methods such as power-spectrum analysis. Advantage is taken of the rather precise annual periodicity of these data to design sharp numerical filters and use them to split the series at each lander into six time series, each containing oscillations associated with a discrete frequency band. This analysis reveals 2- to 4-sol waves during the cold seasons at both landers, and 8-sol waves during the spring at Lander 2, confirming previous results from spectral analysis. As previously predicted, high-frequency waves appear suppressed during the second dust storm. Certain low-frequency signals, probably associated with the global dust storms, are detected and demonstrated to be significant for the first time.

Niver, D. S.↗
NASA NTRS1980

The annual cycle of pressure on Mars measured by Viking landers 1 and 2

Daily mean atmospheric pressures at the two Viking landers are presented for slightly more than a Martian year. The seasonal variation of pressure owing to exchange of CO2 with the polar caps is quite evident and contradicts, in part, earlier theoretical results. Day-to-day variations are the result of passage of synoptic-scale high and low pressure systems and are an important clue to the general circulation of the atmosphere. The effects of global dust storms on the general circulation and on the diurnal variation of pressure are detected and interpreted.

Hess, S. L.↗
NASA NTRS1979

The seasonal variation of atmospheric pressure on Mars as affected by the south polar cap

The daily mean pressures at two locations on Mars, observed over 57% of a Martian year, reveal a semiannual oscillation with a peak-to-peak difference that is 26% of the mean pressure. This intrinsically Martian phenomenon is caused by exchange of CO2 between the atmosphere and the winter polar caps. Evidence is presented that the difference in pressure at the two landers varies with season and that the seasonal variation is not completely removed by hydrostatic correction for the difference in elevation. The mass CO2 sublimed from the south polar cap is estimated to be greater than or equal to 7.9 x 10 to the 12th metric tons, corresponding to a mean thickness of solid CO2 over the maximum extent of that cap of greater than or equal to 23 cm. Estimates are formed of the meridonal wind speed conveying gas out of the dissipating cap and the associated zonal geostrophic wind, both averaged over longitude. The results are approximately 2.3 m/sec and 14 m/sec, respectively.

Hess, S. L.↗
NASA NTRS1979

Frontal systems during passage of the Martian north polar hood over the Viking Lander 2 site prior to the first 1977 dust storm

Analysis of a 12-sol period of wind speed, wind direction, temperature, pressure, and optical depth at the Viking Lander 2 site presents the first in situ evidence of high- and low-pressure systems, complete with fronts, on the surface of Mars. The discovery of these systems in the Lander data occurred while analyzing a period during which the north polar hood was advected over the site at midday, dramatically decreasing the surface illumination and surface-to-atmospheric heat flux. This obscuration immediately preceded a global dust storm in the southern hemisphere and low latitudes of the northern hemisphere. The direct effects of the dust storm, reached 48 N, the Lander 2 latitude, later more gradually than they reached the 22 deg N latitude of Lander 1. The system responsible for the polar hood passages is a major disturbance, and it appears that radiational damping is inadequate to stop strong frontal formation. The front analyzed is characteristic of a repetitive series of systems that pass roughly every 3.3 sols. These systems are similar to those predicted by theoretical analyses and by general circulation modeling of the Martian atmosphere and those observed in laboratory experiments.

Tillman, J. E.↗
NASA NTRS1979

Static stability and thermal wind in an atmosphere of variable composition Applications to Mars

Radiometric measurements of the temperature of the south polar cap of Mars in winter have yielded values significantly below the expected 148 K. One proposed explanation for this result is a substantial reduction in the CO2 content of the atmosphere and a lowering of the mean molecule weight near the surface. The meteorological consequences of this explanation are explored by deriving a criterion for vertical static stability and a thermal wind law for an atmosphere of variable composition. The atmosphere proves to be statically unstable unless the anomaly in the CO2 mixing ratio extends to heights of tens of kilometers. The effect of varying molecular weight exceeds the effect of temperature gradient, producing shears with height of reversed sign. The shears are baroclinically unstable, and this instability would eradicate the latitudinal gradient of molecular weight. This inconsistency can be resolved by invoking a reasonable elevation of the central polar cap and by imposing an adequate zonal wind. It is concluded that if the explanation requiring a change in atmospheric composition is correct, it must be accompanied by other special circumstances to make it meteorologically consistent.

Hess, S. L.↗
NASA NTRS1978

Mars meteorology - Three seasons at the surface

We summarize some meteorological results from Viking for northern summer, autumn and winter. Little Sol-to-Sol meteorological change was observed during summer, except for secular pressure change. However, a regular sequence of weather disturbances, interpreted as baroclinic waves, appeared primarily at VL-2 during autumn and winter. The number of Rossby waves present at a given instant is calculated to be 4 and 6 for these seasons. The extreme regularity and low wave number make these systems closely resemble the baroclinic waves of rotating annulus experiments.

Ryan, J. A.↗
NASA NTRS1977

Meteorological results from the surface of Mars - Viking 1 and 2

The meteorology equipment carried by the Viking landers was intended to measure atmospheric temperature, wind speed, wind direction, and pressure. During the summer months, the winds were a few meters per second, with a complex hodograph and the Lander-1 site, dominated by counterclockwise turning of the wind, and a simpler hodograph at the Lander-2 site, marked by clockwise turning of the wind. With advancing season, the repetitive wind pattern began to break down, and protracted northeasterly winds were recorded on several occasions (some of which are associated with lower than normal temperatures). Examples are given of wind and temperature traces over short periods, illustrating the effects of convection, static stability, and lander interference. A theoretical argument, based on the horizontal scale dictated by heating of slopes and on vertical mixing of momentum, is presented to explain the different sense of wind rotation at the two lander sites.

Hess, S. L.↗
NASA NTRS1976

Early meteorological results from the Viking 2 lander

Early results from the meteorological instruments on the Viking 2 lander are presented. As on lander 1, the daily patterns of temperature, wind, and pressure have been highly repetitive during the early summer period. The average daily maximum temperature was 241 K and the diurnal minimum was 191 K. The wind has a vector mean of 0.7 meter per second from the southeast with a diurnal amplitude of 3 meters per second. Pressure exhibits both diurnal and semidiurnal oscillations, although of substantially smaller amplitude than those of lander 1. Departures from the repetitive diurnal patterns begin to appear on sol 37.

Hess, S. L.↗
NASA NTRS1976

Mars climatology from Viking 1 after 20 sols

Results from the meteorology instruments on the Viking 1 lander are presented for the first 20 sols of operation. The daily patterns of temperature, wind, and pressure have been highly consistent during the period. Hence, these have been assembled into 20-sol composistes and analyzed harmonically. Maximum temperature was 241.8 K and minimum was 187.2 K. The composite wind vector has a mean diurnal magnitude of 2.4 meters per second with prevailing wind from the south and counterclockwise diurnal rotation. Pressure exhibits diurnal and semidiurnal oscillations. The diurnal is ascribed to a combination of effects, and the semidiurnal appears to be the solar semidiurnal tide. Similarities to earth are discussed. A major finding is a continual secular decrease in diurnal mean pressure. This is ascribed to carbon dioxide deposition at the south polar cap.

Hess, S. L.↗
NASA NTRS1976

Viking-1 meteorological measurements - First impressions

A preliminary evaluation is given of in situ meteorological measurements made by Viking 1 on Mars. The data reported show that: (1) the atmosphere has approximate volume mixing ratios of 1.5% argon, 3% nitrogen, and 95% carbon dioxide; (2) the diurnal temperature range is large and regular, with a sunrise minimum of about 188 K and a midafternoon maximum near 244 K; (3) air and ground temperatures coincide quite closely during the night, but ground temperature exceeds air temperature near midday by as much as 25 C; (4) the winds exhibit a marked diurnal cycle; and (5) a large diurnal pressure variation with an afternoon minimum and an early-morning maximum parallels the wind pattern. The variations are explained in terms of familiar meteorological processes. It is suggested that latent heat is unlikely to play an important role on Mars because no evidence has been observed for traveling synoptic-scale disturbances such as those that occur in the terrestrial tropics.

Hess, S. L.↗
NASA NTRS1976

Preliminary meteorological results on Mars from the Viking 1 lander

The results from the meteorology instruments on the Viking 1 lander are presented for the first 4 sols of operation. The instruments are working satisfactorily. Temperatures fluctuated from a low of 188 K to an estimated maximum of 244 K. The mean pressure is 7.65 millibars with a diurnal variation of amplitude 0.1 millibar. Wind speeds averaged over several minutes have ranged from essentially calm to 9 meters per second. Wind directions have exhibited a remarkable regularity which may be associated with nocturnal downslope winds and gravitational oscillations, or to tidal effects of the diurnal pressure wave, or to both.

Hess, S. L.↗
NASA NTRS1976

The vertical distribution of water vapor in the atmosphere of Mars

Calculations are performed of the vertical distribution of water vapor and condensate in an adiabatic atmosphere on Mars taking into account turbulent diffusion and terminal velocity. The distributions are found to be substantially different when terminal velocity is included. The eddy-diffusion coefficient in the troposphere cannot be much greater than 100,000 sq cm/sec if optical depths are to be kept low enough to be consistent with observations. Processes in the boundary layer are also discussed. It is concluded that virtually all the water vapor is to be found in the lowest 6-10 km and that the lowest 2 km should have a greater concentration than the rest of that layer. Some observational tests of these ideas and conclusion can be performed by the Viking missions to Mars.

Hess, S. L.↗
NASA NTRS1972

Meteorology experiments - The Viking Mars Lander.

The purposes, procedures, and nature of the planned meteorology experiment of Viking, 1976 are described. The elements to be measured are pressure, temperature, wind speed, wind direction, and water vapor content of the atmosphere. The interactions with other Viking experiments are outlined and candidate sensors are described.

Hess, S. L.↗