Atmospheric pressure measurements are unique among the meteorological variables that can be measured at a single site on the surface of Mars, since they provide the widest ranging study of phenomena and of space and time scales. Spatial processes from "dust devil" size structures, to "fronts" to regional and global dust storms, can be investigated while temporal variations from the transient dust devils to the dramatic year to year presence or absence of the global storms, can be studied by single point, long term observations. The Viking Lander observations provided the first, and most complete description of the weather at the surface of Mars forming the primary climate record of Mars (Tillman 1985, 1988, and Tillman et. al, 1993, Zurek 1988), and the meteorological context for Pathfinder and all future missions.

The bottom frame in each of the Viking lander "sol average" pressure plots, illustrates the annual CO₂ condensation -- sublimation cycle for both landers. (Solid carbon dioxide, CO₂, i.e. "dry ice", goes directly to the vapor phase, without a liquid phase, by sublimation.) Time is indicated from the landing of VL1, (VL2 landed 44 sols later), while the season is indicated at the top of the frame by the tick marks slanted down and to the right along with numeric value of L_s, where L_s 90° is summer, 180° is autumnal equinox, 270° is winter and 360 or 0°, is spring. Data from the second and subsequent years, are plotted on top of the first year.

The abscissa presents the time in Sols after landing of Viking Lander 1, Beginning on sol 0. See explanation at the bottom for details of the calculation. The right frame shows the atmospheric pressure for the 3.3 Mars years of the Viking Mission while the year long, left frame overlays the years on top each other, to better distinguish their similarities and differences. A primary example of the differences is the presence of "great" dust storms in some years and their absence in others.

Meteorological proposals for future missions generally propose networks of landers, containing from 4 to 20 comprehensive or very limited investigations, and downplay the value of any single station observations. Lacking a network of comprehensive stations, or even simple pressure measurements, the publications of Tillman, and of co-authors and collaborators, have clearly demonstrated the value of even one "climate" station. Using only pressure observations, we can characterize the daily, synoptic, seasonal, annual and inter annual variation of Martian weather and climate, while providing crucial insights into atmospheric oscillations, frontal activity, dustiness estimates, and the presence, timing and potential triggering mechanisms of "great" or "global" dust storms. Reestablishing a climate station at the Viking Lander 1, (and hopefully Lander 2), site and beginning permanent observations on Mars will greatly benefit future science, operations and engineering.

The day to day atmospheric pressure variability, low in summer and higher in fall and winter, (especially at the northern site), is due to weather "fronts" quite similar to those on Earth. Between winter and spring, and more so at VL2, the characteristics of the day to day variation are extremely different for the first, as compared with the second year: this will be described later. The pressure levels off shortly before spring and in late spring, when the multi-sol fronts become less intense, begins its decrease to repeat the cycle. The total annual pressure variation around the annual mean, is about 15%. There is no equivalent, strong cycle on Earth since the only annually variable components of our atmosphere, are water, CO₂ and other gases which are a small component.

An interesting feature of the annual condensation -- sublimation cycle is that the pressure minimum during late northern winter, is higher than the corresponding minimum during late southern winter. An interesting question is: what causes less condensation in the northern polar region and cap during winter, shown by its higher minimum pressure, than in the southern region? There is a very small clue in the plot, which, along with other information, leads to the answer.

Along with the annual cycle, meteorological fronts are indicated by the increasing variability of the pressure between fall and spring at these northern hemisphere sites as previously mentioned. The Lander 2, VL2, site is similar in latitude to Seattle, Paris or Vienna, while the southern, VL1 site, like Pathfinder, is similar in latitude to Hong Kong, Havana or Calcutta. Like on Earth, the frontal activity is less in the tropics and sub-tropics, e.g., the VL1 site. Typically, the fronts pass every few sols on Mars, often resembling those of Earth except for the lack of rain, while at other times, they are far more regular.

The major differences between the cycles from year to year are due to "great" dust storms. The most obvious differences during the six Earth year Viking mission can be seen by comparing the pressure record at VL2 during winter of the first year, blue, with that of the second year, orange. During the first winter, the pressure makes a major jump, beginning around VL1 sol 310, L_s 272, the frontal variability (rapid changes), is greatly reduced, and the pressure remains higher than during year 2 for about 100 sols. This is called the 1977 B dust storm. For the same season at VL1 (i.e., Pathfinder latitude), there also is a pressure increase and the almost complete suppression of sol to sol variations at this latitude, compared to the second and third years. The atmosphere is so dusty at this time, that less than 5% of the sunlight reached the surface directly and possibly less than 1%! Since a significant fraction of the sunlight is absorbed by the dust, (unlike water clouds on Earth), and the dust may extend to several tens of kilometers in height, (during the 1971 Mariner 9 mission, it reached heights of 31 miles, 50 km) more solar energy is being absorbed in the atmosphere than at the surface. This effectively produces an intense, deep inversion, dramatically damping out the frontal activity. A weaker dust storm, the 1977 A storm, begins earlier around VL1 sol 210. Suppression of the VL1 activity by the first year dust storms is hard to see until winter, due to the almost complete absence of observations longer than a few sols in the clearer second and third years. However, the few sol segments around year three, L_s 220, (blue) and year 2, L_s 235, show the variability that can occur and the penetration of fronts to this low latitude during clearer years.

The top trace in the pressure figure is generated from VL1 pressure data, and illustrates several phenomena, some with periods much shorter than the annual cycle, such as frontal activity, great dust storms and global oscillations. It is created by averaging the pressure readings over a sol, subtracting the average from each observation for the sol, and calculating the "variability" of these points, i.e., the standard deviation around the sol mean. Frontal activity causes pressure changes which generally take two or more sols for the front to pass through the site, and shows up as large sol to sol variability.

Atmospheric dust, (and especially great dust storms), absorbs solar radiation, which warms the air causing daily pressure variations as Mars rotates, Zurek, 1976, and Leovy and Zurek, 1979. Since it is usually deep, it takes weeks to months to fall out of the atmosphere and consequently shows little sol to sol variation.

The global oscillations, labeled as "Transient Normal Modes", occur each year at this season and appear to be caused by resonant oscillations in the atmosphere, which primarily depend on the mean atmospheric temperature and its variation around the planet. These specific oscillations, apparently recur at this season each year, and are hypothesized to be "Kelvin" modes ( Tillman 1988 ). Since Martin Kelvin modes have natural periods close to or the same as the Martian sol, 24.66 hours ( Hamilton and Garcia, 1986), Tillman suggested that their growth during summer was due to the global, seasonal temperature changes causing their period to resonate with that of the Martian day; this was supported by Zurek, 1988. Tillman also suggested that Kelvin modes might be involved in dust storm triggering later in the year, but Zurek suggests that the changing atmospheric temperature should move their resonant period away from the 24.66 hour diurnal period. Understanding why great Martian dust storms occur in some years and not others, is one of the foremost problems not only for Martian meteorology, but for manned and un-manned operations on the planet. Even better, would be a depth of understanding, and associated observations, that would allow forecasting their occurrence, as well as that of large regional, and intense local dust and sand storms.

The last half of the Viking Lander 1 meteorology record is due to Tillman's initiation and development of the Viking Computer Facility, VCF, and its staff, infrastructure and software, to provide mission operations support, by converting and building on Viking Project resources: the development of these resources is described by Tillman 1984 .

The significance of the climate data set acquired, processed, edited, summarized and made available to the community by the staff is indicated by Zurek, 1992b "The meteorological time series acquired by the Viking Landers on the surface of Mars is unprecedented for planets other than Earth in terms of its longevity and its temporal resolution."

Unless otherwise specified, Mars images are courtesy of NASA JPL or Hubble.
The assistance of George LeCompte and Neal Johnson gratefully is acknowledged.

J E Tillman