| The Latest Weather Report from Mars
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| Martian air temperature,
in Kelvin (200 Kelvin = -100 Fahrenheit; 260 Kelvin = +8 Fahrenheit), measured
by the three thermocouples (electronic temperature measuring devices) located
at different heights on the Mars Pathfinder ASI/MET mast. The top mast
thermocouple is located one meter above the outer edge of one of Pathfinder's
three solar panels. The top of the solar panel is 0.4 meters (16 inches)
above the martian surface, which puts the top mast thermocouple 1.4 meters
(55 inches) above the surface. The middle mast thermocouple is located
0.5 meters above the solar panel; the bottom thermocouple at 0.25 meters.
Since air temperature generally decreases with increasing distance from
the sun-warmed ground during daylight hours, the bottom mast thermocouple
measures the warmest temperature at those times of day. During the night,
the ground rapidly cools and air temperatures are coldest nearest the ground.
Thus, at night, the top mast thermocouple measures the warmest temperature. |
Surface pressure, in
units of millibars, measured by the 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. |
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| Wind direction at the
Mars Pathfinder site is indicated as the direction from which the wind
is coming: i.e. a southerly wind is a wind from the south. During the initial
sols of the mission, wind direction rotated in a clockwise manner with
time: southerly at night, westerly in the morning, northerly during early
afternoon, and from the east in the evening. Winds from the south at night
are consistent with flow down the Ares Valley at whose northern end Pathfinder
is located. The change in the daily wind variation seen to begin late on
sol 7 occurs in conjunction with an apprent change in the measured surface
pressure. We continue to analyze these changes. |
Wind speeds have still
not been absolutely determined from the signals we are receiving from the
wind sensor located at the top of the ASI/MET mast. However, our wind sensor
signals are strong, and do provide some qualitative information about wind
speed (i.e., when are the winds apparently strongest...). During the mission's
initial 10 sols, winds were strong during early morning hours. The time
around noon also was a preferred time for relatively strong winds. Weakest
winds occur during late afternoon/early evening on most sols. |
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Atmospheric temperatures from the Mars Pathfinder Atmospheric Structure
Instrument: This figure presents a preliminary evaluation of the atmospheric
temperature 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. Pressures
and temperatures can be derived from the density using well-established
physical principles. The Atmospheric Structure Instrument measured the
probe's deceleration using high precision accelerometers. The temperature
profile measured by the Viking 1 lander during its descent to the surface
of Mars on July 20, 1976 is shown for comparison. A key debate in the Martian
atmospheric sciences community has been over whether Martian climate has
changed significantly since the era of the Viking missions. The issue has
been whether the lower and middle atmosphere of Mars (altitudes less than
50 km) are "cold" relative to Viking or whether they are "warm"
as at the time of Viking. The temperature profile in this figure shows
the upper atmosphere of Mars to be quite cold relative to Viking. In fact
at about 80 km altitude, the temperature is the lowest ever measured on
Mars-- a brisk -275 degrees Fahrenheit. The cold temperatures in this region
are not surprising since Mars Pathfinder entered the atmosphere at 3 AM
Mars Local Time when the upper atmosphere cools due to the lack of solar
heating. Below 60 km altitude, the temperatures measured by Pathfinder
are quite close to those measured by Viking. Therefore, the measurements
by Mars Pathfinder, which represent one slice through the atmosphere at
one location and time, show that the atmosphere is "warm" as
it was at the time of Viking.
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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.
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| 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 Lander measures air temperature
with sensors located on the Meteorological mast at 10, 20, and 40 inches,
(0.25, 0.50, 1.0 meters) above the solar panel. Air temperatures measured
during the first three days of the mission show an afternoon high near
+14 degrees Fahrenheit (264 Kelvin). [The 270 degree Kelvin temperatures
measured at noon on Sol 1 were obtained while the mast was still lying
down just above a warm solar panel]. During the day, the air temperatures
measured by the lowest of the 3 sensors are 9 to 10 degrees Fahrenheit
(5 to 6 Kelvin) warmer than those measured by the top sensor because the
atmosphere is heated by the sunlit surface below it. At night, the ground
cools more quickly than the atmosphere, and the warmest air temperatures
are measured by the uppermost temperature sensor. These temperature differences
provide information about the exchange of heat between the surface and
the atmosphere. The Viking landers could not determine this quantity directly
because they measured the air temperature at only one height (64 inches
or 1.6 meters). The air temperatures measured by Pathfinder are slightly
warmer than those seen 21 years ago at the nearby Viking Lander 1 site
at the same Martian season. This may result because the somewhat darker
surface at the Pathfinder landing site absorbs more sunlight than the Viking
1 Landing site. |
The Mars Pathfinder Atmospheric Structure/Meteorology
Experiment measures wind speed and direction with a sensor mounted on the
top of the 40-inch (1 meter) tall meteorology mast. Measurements made during
the late evenings and early mornings of the second and third Sols after
landing indicate prevailing winds from the south-south-east. The wind direction
then changed in the early afternoon, such that the prevailing winds blew
from the north to north-east. These results are similar to those obtained
by the Viking Lander 1 at this season. Wind speeds have not yet been precisely
determined, but early estimates show that the winds are quite light (a
few miles per hour) during the day, and somewhat stronger (about 10 miles
per hour, or 16 kilometers per hour) at night. |
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| Air temperatures
measured by the Mars Pathfinder Lander during the first two days of the
mission show an afternoon high near +9 degrees Fahrenheit (260 Kelvin).
[The 270 degree Kelvin temperatures measured at noon on Sol 1 were obtained
while the mast was still lying down just above a warm solar panel]. During
the day, the air temperatures measured by the lowest of the 3 sensors on
the meterology mast are 9 to 10 degrees Fahrenheit (5 to 6 Kelvin) warmer
than those measured by the top sensor because the atmosphere is heated
by the sunlit surface below it. These temperature differences provide information
about the exchange of heat between the surface and the atmosphere. The
Viking landers could not determine this quantity directly because they
measured the air temperature at only one height. The air temperatures measured
by Pathfinder are slightly warmer than those seen at the nearby Viking
Lander 1 site at the same season. This may result because the somewhat
darker soil at the Pathfinder landing site may absorb more sunlight than
the Viking 1 Landing site. We hope to obtain morning minimum temperature
measurements in the early hours of Sol 3. |
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. |
Mars Pathfinder's measurements
of wind direction and speed suggest values quite similar to those measured
by Viking Lander 1 at this season. Afternoon wind directions at the Mars
Pathfinder landing site show winds blowing from the southwest (from 230
degrees) during early afternoon and then turning to the northwest and then
to the north (from 350 degrees) by 3 PM local Mars time. Wind speeds have
not yet been definitively determined, but initial indications are that
the winds are quite light (a few miles per hour) near noon but increase
as the afternoon progresses and might attain values of 10-15 miles per
hour. Stay tuned for our first well determined wind speeds... |
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| 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. |
This figure compares preliminary atmospheric temperatures
recorded by the Pathfinder MET experiment with temperatures acquired at
the same season and time of day by the Viking 1 lander. Temperature is
given in Kelvin and time is local time of day at the lander site. |
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| This figure shows the entry vehicle deceleration in g's
along its flight path during the entry phase of the mission. As the probe
enters at 7.5 km/second, the deceleration forces produced by the atmosphere
range from less than 10 micro g at time 0 (160 km) to about 20 g at 100
seconds (25 km). The deceleration pulse produced by parachute deployment
is clearly seen at 185 seconds (10 km). |
This figure shows entry vehicle deceleration in g along
its flight path during the entry phase of the mission. Deceleration is
plotted in logarithmic coordinates to reveal accelerations in the range
0.00001 to 20 g. These measurements allow the atmospheric density, pressure
and temperature profiles to be calculated. |
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