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Planetary News: Mars (2005)Mars Global Surveyor Spots Viking 2 and, Probably, Mars Polar LanderBy Emily LakdawallaMay 6, 2005
The sharp-eyed Mars Orbiter Camera may finally have spotted the final resting place of Mars Polar Lander. If the little white dot in the middle of a black smudge near Mars' south pole really is Mars Polar Lander, the image represents the first hard forensic evidence for how the spacecraft met its end. Mars Polar Lander went missing on December 3, 1999. The spacecraft had been in excellent health, with no reported problems, as it approached its landing. But after the predicted landing time passed, no signal was ever received from the spacecraft. It was a tragic loss for the Mars science community, as well as for The Planetary Society, who had built the Mars Microphone for the lander in the hopes of returning the first recorded sound from another planet. The orbiting Mars Global Surveyor had been waiting to serve as a communications relay for Mars Polar Lander. Instead, the orbiter began a campaign on December 16, 1999, to search for the missing lander. The search would be a challenging one: the lander itself, at 1.5 meters in diameter, would be smaller than one pixel in even the highest-resolution Mars Orbiter Camera (MOC) image. And there was a large area to search. The spacecraft could be anywhere within a landing ellipse covering a thousand square kilometers (350 square miles), and each MOC image could only cover a fraction of that area. "We took pictures through early February of 2000," says Mike Malin, the principal investigator on MOC. "After examining them carefully for almost anything, we didn’t see anything." To narrow the search, the MOC team identified strict criteria for candidate crash sites: "The criteria we used in searching for Mars Polar Lander required a bright feature of irregular or elongated shape (the parachute) within about 1 kilometer (0.62 miles) of a location that included a dark area (rocket-disturbed martian dirt) and a small, bright spot near its center (the lander)," the team stated in the caption associated with the released image. No further action was taken for four years. Finally, in 2004, Spirit and Opportunity landed on Mars. Because of the terrible lesson learned from the loss of Mars Polar Lander, both rovers broadcast a carrier signal throughout their entry, descent, and landing on Mars, and both landers carred a Descent Image Motion Estimation System (DIMES) camera that captured several images of the surface on the way down. Thanks to these precautions, the location of both rover landing sites were quickly identified after their arrival on Mars, and MOC captured images of both rovers shortly afterward. The MOC images were captured through a tricky technique known as cPROTO imaging, in which the Mars Global Surveyor spacecraft pitches and rolls to cancel out its orbital motion as the MOC captures an image. (Click here for a more thorough explanation of cPROTO imaging.) cPROTO images can have resolutions as high as 50 centimeters per pixel, so even small landers span several pixels in such images. But each cPROTO image covers only a few square kilometers; mapping the entire Mars Polar Lander landing ellipse using this technique would take 60 years, according to the MOC team.
Upon seeing those Mars Exploration Rover landing site images, Malin says, "I had this tickle in the back of my mind that I’d seen something that was not unlike those things in one of those pictures that we had taken back in 1999-2000. So I went back and looked in my notes and we had, in fact, identified this area back in 2000 in January. But because there was nothing to distinguish it--when you look at it in the raw images, it’s really not convincing at all. So we didn’t do anything then. But I think what we learned from MER was at least permissive that this was a reasonable candidate." The candidate site lies at the very edge of the elliptical zones of terrain that were considered the most probable landing location for Mars Polar Lander, based upon the last telemetry received from the spacecraft. The sequence of images below zooms in from regional coverage of the area obtained from Viking Orbiter images to the highest level of detail available in the MOC images that were acquired of the area in 1999. MOC scientists studied the rover landing site images to determine the likely reflectance properties of Mars Polar Lander's parachute and the blast zone created on the Martian surface by Mars Polar Lander's retrorockets. Returning to the candidate site, they determined that the bright spot and the dark smear indeed had the expected properties for a Mars Polar Lander landing site. The candidate site will now become a target for cPROTO imaging once summer arrives at Mars's south pole, improving illumination conditions. But the five intervening Earth years since the loss of Mars Polar Lander could have changed the landing site. In fact, the imaging campaign can't start immediately, because "the area is just starting to defrost now," Malin says. As the pole defrosts, black spots develop atop the carbon dioxide frost, making it quite tough to pick out the spot that would mark the lander. Eventually, the frost will disappear. "We will probably go ahead and take a couple of pictures along the way because the real problem with cPROTOs is that the vast majority of them don’t go where we target them. So we’re probably going to have to shoot a number of them to bracket spatially the area where we need to target it so it will fall where we want it to fall. And I want to get that out of the way before we actually start taking pictures for real. The pictures for real will probably start some time in late July or August, when the area is defrosted." Although the frosting and defrosting of two Martian winters may not have affected the vehicle much, another Martian weather hazard -- dust -- could have had more effect. "In between now and when it landed, there was a global dust event. Also, the south polar region has lots of dust storms. So it’s possible that, for example, the reflectivity of the parachute may have changed. The picture we took was within a few days or a month of it landing. Now [Mars Polar Lander] has been on Mars for five-plus years, and I suspect it’s not going to be as distinctive as it was in January 2000." The wind and dust could even have obliterated the dark "rocket blast zone" that the MOC team identified in the image. But that would be good news. "If the dark area went away, but the bright thing at the center remained, that might be an argument that it was in fact an ephemeral dark thing, and that might actually confirm that it was the blast area," Malin says. "We have been monitoring the effects of the Mars Exploration Rover landing in an attempt to determine how rapidly these things fade." If the candidate site does turn out to be the real deal, forensic investigators can already draw some conclusions. The presence of a parachute, separated from the lander, and the evidence of a blast zone around the lander suggests that "Mars Polar Lander's descent proceeded more-or-less successfully through parachute jettison and terminal rocket firing. The relative location of the candidate parachute and lander is consistent with the slight west-to-east wind seen in dust cloud motion in the area around the date of landing," the MOC team's image caption states. "The blast-disturbed area is consistent with the engines continuing to fire until the vehicle was close to the ground. How close is not known. The larger Mars Exploration Rover retrorockets fired at about 100 meters altitude and continued firing until the engines were about 20-25 meters above the surface; the candidate Mars Polar Lander disturbance is roughly the same size, but whether this means the engines were firing as close to the ground as the Mars Exploration Rover rockets cannot be determined." Viking 1, Viking 2, and Mars Pathfinder Also Located (Maybe)In addition to finding Spirit and Opportunity, the MOC team has announced that they have also bagged all three of the other successful Mars landers. Images of Viking 1 and Mars Pathfinder were released earlier this year, and Viking 2 was finally spotted in an image released this week. The MOC team states that the locations of these landers "were determined by using sight lines from the landers to near and far objects seen in the pictures acquired by the landers, and then matching these to locations in earlier, 1.5 to 3.0 meter per pixel MOC images. Then, the cPROTO images were acquired by Mars Global Surveyor so that the actual landers, sitting on the martian surface, might be resolved."
Until now, the only way to localize Mars landers was by performing careful geographical studies, taking sights to topographic features visible to the landers on the horizon, and correlating them with topographic features visible in orbital photos. Geologist Tim Parker, of the Jet Propulsion Laboratory, has been the expert in this field since Viking, and he's not totally sure about Malin's identification of Viking Lander 2. Parker says, "I can't correlate the ridge that’s blocking the view to the south and the southwest of the lander from the ground with any obvious topographic feature in the vicinity of that location that Malin has now proposed in the MOC image. There’s other subtle topographic features that are visible from the ground -- particularly, to the north, there’s a slope that’s 100 or 200 meters from the lander that doesn’t quite line up with a feature that is north of the lander from Malin’s proposed location. So although his candidate is a strong one, because it’s clearly an anomalous object, whatever it is, I’m still having difficulty with it." Parker is quick to admit that the site he had picked out based on his study of much lower resolution Viking Orbiter images hasn't panned out either. "I have looked at his cPROTO image at the area I proposed the lander to be at, and I can’t see anything diagnostic or unusual there either. I think Malin's got a strong candidate there, but we’re going to have a tough time settling it one way or another. Presumably, HiRISE, with its 25 cm/pixel images, will be able to see the lander itself, and maybe recognize it." Malin's spot is only 300 meters away from Parker's initial choice, so the difference may be an academic one.
If the Mars Polar Lander sighting proves true, that will leave Beagle 2 as the only Mars lander that MOC has not yet spotted. MOC targeted the Beagle 2 landing site with an intense imaging campaign in January of 2004, but the only candidate site identified in normal, 1.5 to 3.0 meter resolution MOC images turned out to be a false lead. The MOC team has not given up hope, stating that "the landing ellipse will remain a MOC target, indefinitely, until either the ellipse is covered or the Mars Global Surveyor MOC mission ends, whichever comes first." What use are these images to Mars scientists? Seeing a little fuzzy dot of a lander on Mars has coolness value, but what's more important is that the aerial photographs place the landers, and all of the images and other data that they have captured, in their proper geologic context. Even with the long treks accomplished by the Mars Exploration Rovers, each landed mission sees just an infinitesimal fraction of the planet. But if the data that they capture can be placed in context, the results of lander missions can be extrapolated out to other environments that are similar to the rover sites. Every landed mission has as one of its scientific aims the intent to "ground truth" orbital data in this fashion. By imaging landers on Mars, MOC has extended the reach of each lander's scientific data across a wide reach of Martian terrain. |
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