December 20, 2004
Jack Frost Hits Mars
As the evidence for past martian water becomes stronger, what can be said about the hydrology today on the red planet? Looking for images showing weather on Mars requires some patience, but one may have to look no further than the rovers themselves to see what a cold, martian morning might bring.
Astrobiology Magazine -- Is Mars a dry, dead place?
An unlikely black knob(or 'gnomon') is used foremost for calibrating the rovers' panoramic cameras, but also serves a dual function as a traditional sundial marking the passage of the Sun across the sky. In a newly discovered role, the sundial also is providing a good test background for monitoring the morning temperatures.
The inscriptions on the martian sundials foreshadowed this need to understand better the complex martian environment:
People launched this spacecraft from Earth in our year 2003. It arrived on Mars in 2004. We built its instruments to study the martian environment and to look for signs of water and life.
"Now, two of these inscriptions are on their way to Mars," said Bill Nye, one of the key figures in making the sundial project possible. "It gives me chills."
What Nye may not have fully appreciated was how literally the sundials might give chills not only to students and martian timekeepers, but also to the scientists seeking direct evidence of the red planet's relative humidity.
As shown in the banner image, the martian winter is now putting a layer of frost on the sundials. The sundial's normally black surfaces are covered in an icy morning image, giving visual evidence of what many have considered to be a complex water cycle in the martian atmosphere.
Frost can form on surfaces if enough water is present and the temperature is sufficiently low. On each of NASA's Mars Exploration Rovers, the sundials provide a good place to look for such events. A thin frost was observed by Opportunity's panoramic camera on the rover's 257th sol (Oct. 13, 2004), 11 minutes after sunrise (left image).
The presence of the frost is most clearly seen on the post in the center of the target, particularly when compared with the unsegmented outer ring of the target, which is white. The post is normally black. For comparison, note the difference in appearance in the image on the right, taken about three hours later, after the frost had dissipated. Frost has not been observed at Spirit, where the amount of atmospheric water vapor is observed to be appreciably lower.
Frost shows up some mornings not only on the sundial, but on the rover itself. The possibility that frost has a clumping effect on the accumulated dust on solar panels is under consideration as a factor in unexpected boosts of electric output from the panels.
Clocks on Wheels
One of the important and novel features of the laboratories on Mars is their mobility. It's as if planetary explorers have reinvented the wheel, given their enthusiasm about traversing to intriguing spots that their cameras might highlight on the horizon. But according to those who have patiently watched the shadows on Mars, the rovers' mission to find evidence of past water may also be advanced by checking the rover's timekeepers. "In a reasonable sense," said Nye, "the invention of clocks has had a greater effect on our lives than the invention of wheels."
"Sundials are so much a part of human history," Nye told Astrobiology Magazine: "I attended the meeting in the Cornell Space Sciences building, where Carl Sagan had worked. I took one look at the 'photometric calibration target,' and exclaimed or quickly proposed with a weird urgency, 'That's got to be a sundial,' or something like that."
These calibration instruments, positioned on the solar panels of both Spirit and the Mars Exploration Rover Opportunity, are tools for both scientists and educators. Scientists use the sundial to adjust the rovers' panoramic cameras, while students participating in NASA's 'Red Rover Goes to Mars' program will monitor the dial to track time on Mars.
University of Washington professor Woody Sullivan, another key figure in designing the sundial project, concluded: "It is a great way to get people to think about their place in the cosmos...fortunately, except for a very few high, thin clouds, most days on Mars are sunny."
Each sundial is inscribed with the words "Two Worlds, One Sun" and bears the name "Mars" in 17 languages, including Bengali, Inuktituk, Lingala and Malay-Indonesian, as well as ancient Sumerian and Mayan.
Perhaps another scientific language spoken to meteorologists watching Mars is the sign of changing seasons. Judging by the frost on the sundial, it is indeed now martian winter.
Rover science-team member Dr. Michael Wolff of the Brookfield, Wisconsin branch of the Boulder, Colorado-based Space Science Institute is reporting frost, clouds and other atmospheric observations. "We're seeing some spectacular clouds," Wolff said. "They are a dramatic reminder that you have weather on Mars. Some days are cloudy. Some are clear."
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