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Planetary News: Mars Exploration Rovers (2004)A MarsDial Covered in Dust: The Mars Exploration Rovers' 50,000th ImageBy Emily Lakdawalla5 November 2004
Since the landing of Spirit on January 4, 2004, the Mars Exploration Rover mission has swiftly racked up impressive numbers of images returned from the surface of Mars. On September 25, the image odometer ticked over to 50,000 -- and kept right on counting. The 50,000th image was captured by Spirit at 4:31 p.m. Local Solar Time on the 260th Martian day after she landed. The 50,000th image happens to be of Spirit's Panoramic Camera (Pancam) Calibration Target, otherwise known as the "MarsDial." In an article he wrote for The Planetary Report, Pancam Principal Investigator Jim Bell explained how the color calibration target for his instrument became the first sundial sent to another planet. "During a fateful airplane flight in 1998, [I] noticed television writer and entertainer Bill Nye (the Science Guy) and struck up a conversation about Mars missions and Pancam in particular. Bill was intrigued by the mission and yearned for more details about the instruments themselves. When he learned about Pancam and its stick-casting-a-shadow calibration target, he had an epiphany: “It’s a sundial!” Bill’s eyes lit up as he foresaw an opportunity to merge science, education, his own personal interest in sundials (Bill’s father wrote a book on the sundials of Maryland and Virginia), and space exploration into an exciting new project. We could make that mundane little object into the first sundial on another planet! Wouldn’t it be great if we could tell time on Mars by reading the post’s shadow?"
Throughout the first two months of the Mars Exploration Rover mission, The Planetary Society's Student Astronauts did just that, employing software to impose hour markings on the face of the MarsDials and thus tell time. They also created movies showing the shadow of the MarsDial's gnomon (vertical post) moving across the face of the Dial over time. Later, the Student Astronauts also performed careful measurements on the MarsDial images to help the Pancam team figure out the necessary calibrations to perform on their images in order to show Mars in its true colors. As of Sol 77, some dust had accumulated, but the colors on the chips in the corners of the MarsDial face are still quite clear (green, yellow, red, and blue), and the center circle on the Dial is clearly separated into three different levels of gray. But in the top image, the red dust nearly obscures the color and gray chips. The only relatively dust-free area in the whole image is in the center of the "Sweep Magnet," the small object to the right of the MarsDial. This device has a powerful ring-shaped magnet that attracts the magnetic Martian dust, which has built up into a thick red ring, but repels magnetic dust from the center of the ring, leaving it relatively clean. "Isn't it amazing?" Pancam Team Member Jascha Sohl-Dickstein asks about the amount of dust that has built up over the course of the mission. It's Sohl-Dickstein's job to figure out how to use the MarsDial to color-calibrate the Pancam images. The job became more difficult as more dust accumulated, but Sohl-Dickstein says that his job is getting easier again. "We know what the dust looks like very well, so unless the dust changes color, the closer [the MarsDial] comes to looking just like dust, the more believable our calibration will be." Because Spirit and Opportunity take photos of the MarsDials for color calibration every single time they capture a sequence of images, the MarsDials have become the most-photographed objects on Mars. According to Jim Bell, images of the MarsDials account for 18% of all Pancam images acquired throughout the mission to date (nearly 10,000 images by now!), but because the MarsDial occupies only a small portion of one image frame, they account for only 3.5% of the entire Pancam data volume. Some facts about the Mars Exploration Rovers' imaging activities up to September 25, the date of the 50,000th image:
To put the 50,000 images in perspective, consider the fact that, taken together, the Viking landers collected only 7,000 images over nearly 7 years, while Mars Pathfinder managed almost 17,000 during her 12 weeks of operation. The 50,000-and-growing image catalog for the Mars Exploration Rover mission boggles the mind and presents a formidable challenge to scientists attempting to understand what they all mean for Mars. "We didn't know what we were getting into!" Bell says. But he certainly wouldn't give it up! |
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