"Anticipating Pathfinder's First Weather Report" Jim Murphy - June 18, 1997 Meteorologist and Mars Research Scientist NASA Ames Research Center Mountain View, California |
I've been very busy working on completing the software that's needed to convert
the signals from our wind sensor instrument, onboard the Pathfinder spacecraft, to wind
speed and wind direction.
The wind sensor sits on top of a mast about 3 feet (1 meter) tall. The mast is
currently laying down and will hopefully pop up after Pathfinder lands. The first
measurement should be completed by about 2 p.m. Pacific, July 4, while the mast is still
laying down. We'll take this measurement just to make sure things are working. Eight
hours after we land the mast should spring to an upright position and we'll start
measuring winds about 3 feet above the ground.
We expect to receive the first real wind information between 6 and 7 p.m., Pacific,
July 4. What we expect to find at the Pathfinder site is based on what Viking Lander saw
during a similar season at a nearby location (Pathfinder's site is about 1000 km (600
miles) from where Viking landed): winds on the order of 5 - 8 miles per hour. We're not
worried about dust storms because it's the middle of summer on Mars and the climate
suggests that there are not big dust storms during this time.
Our wind sensor has six wires that are spaced around a cylinder about the size of a
spool of thread. Each of those wires will get hot when we turn the electricity on. With
wind blowing past the cylinder, some of the wires are going to be in front of it, as it
sees the wind, and some will be behind it. The wires in front of the cylinder will get
cooler than the wires behind it because the wires in front will have a faster wind going
by. By measuring how hot and cold the wires are, we will get some indication of the wind
speed. And by figuring out which wires are hot and which ones are not as hot, we'll get
an indication of wind direction.
Our main science sampling will occur about every half hour when we will get about 3
minutes worth of measurements. We'll turn the sensor on for 3 minutes, collect information
and then switch it off. Thirty minutes later the process will be repeated. Each day we'll
get 51 three-minute samples.
Just because we know the temperature range on Mars, that doesn't immediately tell us
what the wind speed is. We did some testing in a wind tunnel here at Ames (run by Greg
Wilson) where we turned our instrument on at different wind speeds under conditions that
were Mars-like, e.g. very low pressures. We were able to do the calibration in which we
knew the wind speed and the temperature, so we were able to make a relationship between
the temperature of the wires and the speed of the wind.
The data we got in the tunnel aren't perfect because, as you've probably experienced,
things never go quite the way you plan them. So we also did our calculations on a piece
of paper and that told us what the approximate temperatures should be. We're now going
back and redoing those calculations to make sure that we have consistency between what
we can calculate using our hand calculator and a piece of paper and a pencil and the
temperatures we saw in the wind tunnel. After that it's just a matter of finding a
straightforward way of taking the temperature and converting it to wind speed. If we did
measurements at 3 meters per second, and at 5, 10, 20, 35 and 50 meters per second, we
could draw a curve through those points on our plot. We can then try to fit a mathematical
equation to them. So rather than using our eyes to look at every temperature and then go
to our curve and pick out what the wind speed is, we can use some math to spit out the
corresponding wind speed.
The wind-speed data will allow us to see what the winds are at a third location on
Mars compared to what the Viking landers collected 20 years ago. We'll also be able to
compare different weather processes: cold fronts and warm fronts as they move through an
area, winds that flow up and down valleys, which we saw happening at the Viking Lander 1
site. We'll also be able to tell if things are different on Mars: if there's less dust in
the atmosphere as some of the recent observations suggest, and how that affects the
weather. We're interested to see if the wind blows strong enough at the site to lift dust
off the ground. We know there's dust in the atmosphere and we know there are various
sites where dust rises. What we also want to know, is this specific site one of them?
While Viking measured wind, temperature and pressure, it only measured the wind and
temperature at single points above the surface. On Pathfinder the wind sensor sits on top
of a 3-foot-tall mast from which three wind socks hang. Pathfinder will give us new
information about how wind speed changes the closer it gets to the surface. The mast also
houses three thermal-couples, or temperature-measuring instruments hung at three
different heights. From this information we will learn how heat is transferred from the
surface vents and also how energy or momentum are transferred from the atmosphere to the
surface. Because the surface tends to drag on the atmosphere it tends to provide most of
the heat input to the atmosphere.
Understanding the climate and the weather on Mars is an absolute must before humans
can land there. This is easily accomplished by taking measurements at several different
locations. If a spaceship landed on Earth and measured temperatures in San Francisco, it
wouldn't be an accurate representation of Earth's weather. It's the same on Mars. With
Pathfinder, we'll soon have three points of data collection rather than two, so we'll
have increased our knowledge by 50 percent!
We'll receive our first Mars temperature measurement on July 4 at 7 a.m. My guess is
that it'll be 190 degrees Kelvin, which is about -110 or -120 F. By about 10 a.m.
temperatures will start to rise to 205-210 Kelvin. The temperature should peak around
1:30 or 2 p.m. at about 250 Kelvin. Then the temperature will start to drop fairly
rapidly so that by 6 p.m. it'll be down to 220 Kelvin (-60 degrees F).
There is a much larger daily temperature range on Mars than we see on Earth. Think
about it this way: If you put a small amount of water in a pot on the stove it heats up
quickly. The small amount of water is Mars' atmosphere. If you put a lot of water in the
pot it takes longer to heat up because there is more water, which is Earth's atmosphere.
I'm really excited about being one of the first people to look at the data and to
realize that things are working and that we are actually getting a glimpse of what's
happening on Mars. I actually got to see the wind sensor instrument on the spacecraft.
I didn't touch it, but it was within 12 inches of me. What's really going to be exciting
is when the rover drives off the lander and turns around to take a picture of the
spacecraft.
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