Pressure
Surface pressure, in units of millibars, measured by the Mars
Pathfinder lander on the Martian surface. Oscillations in measured pressure during a day
are caused by the warming/expansion of the atmosphere which follows the sun around the
planet and the cooling/contraction which follows the night. Similar atmospheric processes
occur on Earth. We interpret the observed variations to indicate the presence of suspended
dust over a large portion of the planet and mixed upwards to several tens of kilometers in
depth. We resumed complete diurnal coverage with our meteorology measurements at 7 AM on
Sol 18, following the successful uplink of a flight software patch.
We believe we have seen the minimum annual daily-averaged pressure at the landing site, which apparently occurred on either sol 14, 15 or 16. Our lack of complete diurnal coverage makes this estimation a bit uncertain. We had predicted that the minimum would occur between sols 15-20 of the mission (I erred several days ago in saying we expected an additional 15 sols of decline before reaching the minimum). Our estimate was based upon Viking lander 1 measurements from 1976-1982 and upon results from numerical models. The data shown in this and the additional three ASI/MET plots span the time from landing ( ~3 AM on Sol 1) through our most recent data (2:12 PM PM on Sol 23).
Atmospheric densities from
the Mars Pathfinder Atmospheric Structure Instrument: This figure presents a preliminary
evaluation of the atmospheric density structure encountered by Mars Pathfinder during its
descent through the Martian atmosphere on July 4, 1997. The deceleration of the probe
during its entry is directly related to atmospheric density. The Atmospheric Structure
Instrument measured the probe's deceleration using accelerometers and these measurements
were used in the derivation of this density profile. The density profile measured by the
Viking 1 lander during its descent to the surface of Mars on July 20, 1976 is shown for
comparison. Atmospheric densities above 60 km altitude at the time of the Mars Pathfinder
entry are much lower (up to about a factor of 5) than at the time of the Viking 1 entry.
These observations of upper atmospheric densities will be important in planning the
aerobraking maneuver to put the Mars Global Surveyor spacecraft in its intended orbit
starting in September 1997. The densities at lower altitudes also appear to be somewhat
lower than those at the time of Viking. Further analysis of these observations will allow
the determination of the variation of temperature with altitude. These results provide
valuable detailed information on the Martian climate and by comparison with Viking results
can provide information on the variation with time of the Martian atmosphere.
The Mars
Pathfinder ASI/MET experiment is measuring the atmospheric pressure at the Martian
surface. The pressures measured during the first three Martian days (Sols) of the mission
have average values near 6.75 millibars. Typical sea-level pressures on Earth are about
150 times larger (1013.25 millibars). The Pathfinder pressures are 10 to 20% smaller than
those recorded 21 years ago during the same Martian season (middle northern hemisphere
summer) by the Viking Lander 1. These differences may result from differences in the
elevations of the Pathfinder and Viking 1 landing sites. Despite these differences, the
Pathfinder and Viking 1 pressure measurements display a similar daily cycle, with minimum
pressures near 4AM and 6PM, and maximum values near midnight and 10AM. These daily
pressure variations are due primarily to the atmosphere's response to daytime heating and
nighttime cooling. The much finer resolution of the Pathfinder measurements (one
one-thousandth of a millibar) will aid in studies of small scale weather phenomena which
have small pressure signatures.
The Mars Pathfinder ASI/MET
experiment is measuring the atmospheric pressure on the Martian surface. Pressures
measured during the first two Martian days (sols) of the mission average 6.9 millibars.
This pressure can be compared to the average sea-level pressure on Earth of 1013
millibars. Daily variations are very similar to those measured by the Viking 1 lander 21
years ago at the same Martian season (middle northern hemisphere summer). The daily
variations in pressure are due in part to the atmosphere's response to daytime heating and
nighttime cooling. One distinction between the Pathfinder and Viking lander 1 pressure
measurements is the finer resolution provided by Pathfinder, which allows for the
determination of very small differences in pressure (one one-thousandth of a millibar).
This will aid in studying small scale weather phenomena which have small pressure
signatures.
This figure compares preliminary atmospheric surface
pressures recorded by the Pathfinder MET experiment with pressures acquired at the same
season and time of day by the Viking 1 lander. Pressure is given in millibars and time is
local time of day at the lander site.