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Anticipating Pathfinder's First Weather Report

by Jim Murphy

June 18, 1997

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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