A primer on the spacecraft's instruments and results

"Good Morning! It's 7:05 a.m. here in California, August 26,1997. On Mars it's Sol 52 and it's 2:15 p.m., local time. Winds are light and out of the north. We expect they'll shift around the compass as the day progresses. Right now it's 14 degrees Fahrenheit, pretty pleasant for Mars, but we're forecasting that by Sol 54, the Martian day after tomorrow, it'll have been as low as minus 103 degrees F. But during the days we may have edged up a few degrees to 17 above. Pressure is 6.81 millibars. That's it for me, Tim Schofield here at JPL. Now back to more news about those rocks."

That was more or less the Martian weathercast given by the leader of the Atmospheric Structure Instrument/Meteorology (ASI/MET) team at a press conference at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, CA.

If it sounds familiar, there are two good reasons. The first is that the Mars Pathfinder spacecraft (renamed the Sagan Memorial Station in honor of the late astronomer, Carl Sagan) is equipped with weather instruments very similar to those commonly used down here on Earth, by the National Weather Service, by local TV weathercasters, and by schools whose students record daily weather conditions!

The key measurements are essentially the same: temperature, pressure, and wind speed and direction.

The second reason is that, unlike Jupiter or Saturn, gas giant planets with no solid surfaces on which to land and from which to send back data and images, Mars is a place in many ways much like Earth. It has rocks and sand dunes, a day and night cycle with regularly varying temperatures in a pattern much like that in Earth's desert regions. There are morning and evening clouds, dust storms and seasons similar to Earth. Though there are many significant differences (an atmosphere only about 100th as dense as Earth's, temperatures on average far lower than those across most of our home planet), it's a place where the weather would be pretty familiar to all of us.

Through the Weather Worlds collaborative online activity, students were challenged to gather data here on Earth to compare with those returned from Mars. In doing so, they came to understand, hands-on, what's the same and what's very different. The purpose of this brief overview of Pathfinder's weather instruments was to provide a simple-to-understand description of how Pathfinder gathered data, and to headline some key features of Martian weather. The following section contains hot links to several sites that provide more detailed descriptions.

(This overview was drawn, with thanks and explicit acknowledgment, from three main online resources: (1) JPL's description of the spacecraft and its instruments; (2) JPL's images and data released since July 4, 1997, and (3) overview and background information and temperature data published by the University of Washington's LIVE FROM EARTH AND MARS Project. See below for URLs to take you directly to these sources. Any inaccuracies brought to this digest are the responsibility of PASSPORT TO KNOWLEDGE / LIVE FROM MARS and not the original sources and will be corrected as soon as detected!)

Pathfinder's Weather Instruments

Pathfinder recorded familiar kinds of data, but its instruments are more than a little different from those we use on Earth, designed in ways that have to do a lot with having to deal with very little weight. Every aspect of the Pathfinder/Sojourner mission was influenced by constraints of time and budget, just as students' work on Weather Worlds was down here on Earth.

Students were free to chose to build terrestrial versions of Pathfinder's distinctive conical windsocks to measure aeolian characteristics (named after Aeolus, the Greek God of the winds!), or to create other ingenious designs, or even to use traditional wind vanes and anemometers.

Many of the instruments were also used during the Entry, Descent and Landing phase of the mission, providing a record of atmospheric density and temperature as the spacecraft plunged downward during the last four minutes of its journey. We concentrated on the "landed" phase of the mission which began when Pathfinder woke up on the surface of the Red Planet.

Temperature

Temperature was measured by thin wire thermocouples (explained below) mounted on a meteorological mast (in shorthand, the "met mast") which lay flat on top of one of the solar panels during the cruise phase of the mission. The mast measures 1.1 meter high with struts 1 cm in diameter. It is situated at the very end of one of the spacecraft's solar panels to minimize the effects of wind blowing over the structure of the lander, and small temperature variations caused by the operation of the spacecraft itself.

Spacecraft engineers have used imagery from the lander and Sojourner to determine that the top of the solar panel supporting the met mast resting on rocks is actually about 0.4 meters above the Martian surface. This means that the top detector is at 1.4 meters [55 inches] above the surface, almost the same height as Viking's one and only sensor.

Thermocouples work because when two different metals are joined together they produce a characteristic electrical signal (voltage) as their temperature varies. Onboard Pathfinder, as on Viking, the two metals used are chromel and constantan. Each one degree (centigrade) variation produced a change of 60 millionths of a volt. Though this is obviously very small, with proper design and data collection, it gave results that are accurate to within 0.1 degree C.

There are four sensors in all, one used to measure temperature during descent and three at different heights on the met mast. This placement was chosen because scientists know that temperatures on Mars vary greatly by even small changes in altitude above the surface. The Viking Orbiters, using their infrared detectors from high above the planet, found temperatures on the surface of Mars to be more than 80 degrees Fahrenheit, but the Viking Landers, with temperature detectors fixed at 1.5 meters above the surface, gave results ranging from a low of -178 degrees F (-107 C) and a high of 1 F (-17.2 C).

Pathfinder's designers wanted to get more detail on this phenomenon and so the thermocouples were placed at 25, 50 and 100 cms above the spacecraft. (The University of Washington's LIVE FROM EARTH AND MARS site provides a detailed record of all results returned from the lander, in both graphic and tabular format. You can clearly see the expected temperature variations in the different red, green and blue figures at http://www-k12.atmos.washington.edu/k12/mars/LOPS_Pathfinder_temperatures.cgi. For the tabular data, see
http://www-k12.atmos.washington.edu/k12/mars/LOPS_MPF_allavg_data.html

Tim Schofield said that there are similar differences down here on Earth, with detectors at similar heights, especially with an experiment over a surface like that of a parking lot.

Pressure

Pressure was measured by a Tavis magnetic reluctance diaphragm sensor, just as on Viking. This is more of a "black box" (as far as we can tell), and we don't think there's too much we can add to elucidate its workings! Some students used barometers, some of which might have used Galileo's principles (how a column of air weighs down on a surface and pushes up a column of liquid), while other students used diaphragm barometers, somewhat similar to that on the spacecraft.

Wind

There are two different kinds of wind detectors aboard Pathfinder, using two different technologies. The wind sensor on top of Pathfinder's met mast uses six hot wire elements on a round, tin-can-like detector that sits on the top of the mast, 100 cm above the surface. The differential cooling and heating effects of wind blowing over these detectors, as well as variations from side to side, were transformed into measurements of speed and direction.

Also on the met mast are three aluminum wind socks; little conical detectors freely hanging on a stalk with a pivot attached to the mast. (LIVE FROM MARS "borrowed" a test model and astronaut Kathy Sullivan at the Columbus COSI science center showed how it works during the July 6 program.)

The wind socks were imaged by the IMP camera (Imager for Mars Pathfinder), which has also sent back the wonderful pictures of the rocks and dunes around the landing site. There were initially some problems with the exact calibration of the wind sensors, so the experimenters began reporting results in relative terms such as light, strong, etc., rather than absolutes. However, they now feel comfortable with interpreting the varying positions of the wind socks as indicating speeds around 12 kms per hour, very much in line with the results obtained from the wire sensors atop the met mast.

Just as with temperature, the experimenters wanted to know how wind varied with height above the surface, to try and find out how much "roughness," turbulence and local variation there might be with height. Viking had only one set of wind sensors, so again, Pathfinder's instrument package is a significant advance.

Wind is a very important force in shaping the Martian surface. These results were used to help determine the age of features around the landing site by analyzing how much dust has built up.

Imager for Mars Pathfinder (IMP)

It may seem strange to list the camera as a weather instrument, but it returned at least four sets of very interesting meteorological data. First, it showed much more detail about morning and evening clouds than did the Viking cameras. The dramatic sunset images are impressive, but to scientists the high, early morning wispiness is just as fascinating. They think they may be seeing carbon dioxide clouds, formed overnight when temperatures plummet and dispersing as the atmosphere warms up.

For sunset images see: http://mpfwww.jpl.nasa.gov/ops/clouds_at_sunset.jpg
For sunrise images see: http://mpfwww.jpl.nasa.gov/ops/sunrise_stills.gif

For more images, see the extended mission archive including:
http://mpfwww.jpl.nasa.gov/ops/83616f_th.jpg and http://mpfwww.jpl.nasa.gov/ops/ss24_0.jpg
Also see the wonderful skies recorded on Sol 45-46, August 18, the only images available for that date.

IMP also imaged the Sun in order to determine the volume of particles in the atmosphere and used special filters to assess just how much water vapor exists.
See http://mpfwww.jpl.nasa.gov/ops/suns.gif

Even at night, in the early days of the mission (before the batteries deteriorated), IMP was at work, targeting Deimos (one of Mars' two small moons) to assay dust particles in the atmosphere. See http://mpfwww.jpl.nasa.gov/ops/deimos.gif

Pathfinder also has a set of magnets attached to the rover deployment ramps. (See the LIVE FROM MARS Teacher's Guide, page 45, for an activity tracking magnetic particles.) IMP used a special diopter (producing a kind of close-up lens which can be switched on and off) to image the accumulation of what's assumed to be, in part, iron-rich wind-blown dust. The cameras used other filters to help figure out in greater detail the composition of these mineral deposits. And how fast the piles build up sheds light on how quickly wind can move dust around the planet. See: http://mpfwww.jpl.nasa.gov/ops/magnets.gif

So, those are the instruments onboard the spacecraft. What did they see?

Patterns Seen in the Pathfinder Results

Temperature

Just as on Earth, temperature on Mars varies by day and night. Pathfinder's results are pretty similar to those from Viking 1, whose landing site is only about 870 kilometers away to the northwest. But while terrestrial and space-based telescopes led researchers to conclude that Mars had on average cooled by about 20 degrees in the two decades since Viking, Pathfinder's results seem to show a planet much closer to Viking temperatures.

Initial analysis (by Schofield and others) ascribes this in part to local factors around the landing site. For example, the rocks and surface around Pathfinder are darker than at the Viking site, meaning the Sun's warmth is retained more efficiently. But on Mars giant dust storms produce the greatest variations in temperature, and in its first months Pathfinder did not experience anything like the huge storm of 1977.

As expected, during the day the bottom sensor was warmest and the top-most one the coolest. During the first Sols there was a 9-10 degrees F. difference between the two detectors. But during the night the pattern reverses, and the bottom stays warmest and the top becomes the coolest because the ground cools more quickly than the atmosphere.

Pathfinder scientists were pleased that they obtained just the kinds of height differences they expected, and which they could not measure on Viking.

(For actual sol-by-sol temperature data, see the LIVE FROM EARTH AND MARS site referenced above, for the most comprehensive set of cumulative data returned from any of Pathfinder's instruments, both in graphic and tabular format.)

Pressure

Just as on Earth, solar heating during the day causes the atmosphere of Mars to expand and contract at night. But there are also seasonal variations. Pathfinder scientists think they saw the lowest annual pressures of 1997 (by then using data from Global Surveyor) on Sols 14, 15 and 16. See: http://mpfwww.jpl.nasa.gov/ops/ss034.gif

During the first three days of the mission, pressure on Mars averaged 6.75 millibars: sea-level pressures on Earth are about 150 times greater (1013.25 millibars.) The average Martian pressures recorded by Pathfinder are some 10-20% lower than those from Viking, recorded during the same Martian season 21 years ago. Nevertheless the Pathfinder scientists report finding the same daily cycle of variations: daily minimums are found near 4:00 a.m. and 6:00 p.m., and maximums near midnight and 10:00 a.m.
See: http://mpfwww.jpl.nasa.gov/ops/ss026.gif

Scientists have also been very pleased with how Pathfinder's greater resolution, measuring pressure changes as little as one one-thousandth of a millibar, which should result in study of much smaller-scale phenomena. They think they've already seen the pressure signatures of a couple of dust devils passing by, though they weren't able to capture one on camera.

Wind Speed

In the early days of Pathfinder's landed mission, wind direction rotated throughout the day: from the south at night, westerly in the mornings, northerly in late afternoon, and from the east in the evening. Researchers think the southerly night-time winds are caused by air flowing down Ares Vallis, at whose northern end Pathfinder landed. On Sol 7, a decrease in surface pressure resulted in a change in this daily pattern, which subsequently resumed and continued.
See: http://mpfwww.jpl.nasa.gov/ops/ss036.gif

In general, winds were strongest in the early morning hours and were relatively strong around noon. The lightest winds were seen in late afternoon and early evening. The Pathfinder team thinks the winds are just a few miles per hour in the morning, rising to perhaps 10 to 15 m.p.h. (16 to 24 kms.) at night.

Conversion Factors

One challenge that Weather Worlds classes experienced is something that still bedevils much of the Martian data provided on NASA and related Web sites -- different sets of data use different measurements. For example:

- Temperatures appear in degrees Kelvin, Centigrade and Fahrenheit.

- Time appears in Earth days and hours: Pacific and Eastern, Greenwich and Sidereal and then sometimes in Martian Sols, sometimes expressed in decimals (e.g. Sol 76.75).

- Size is sometimes metric, Imperial (feet and inches), kilometers or miles.

Only pressure and latitude and longitude seem, thankfully, to be uniform.