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Planetary News: 2001 Mars Odyssey (2007)The 2007 Martian Dust Storm: Crisis for Some, Opportunity for OthersBy Emily LakdawallaJuly 20, 2007
Mars' 2007 dust storm season is shaping up to be one to remember. Unlike the mild (and more typical) 2005 season, which spawned only local dust storms, the 2007 season has witnessed a storm blanketing Mars' entire southern hemisphere, and dust is now crossing the equator to the north. Dust kicked high into the atmosphere renders the sky opaque, frustrating orbital cameras and endangering the solar-powered rovers. However, some orbital instrument teams are enjoying the change in the weather and the opportunity to study a recurring and rapidly evolving Martian weather pattern. Although the storm appears alarming to the rovers, it is not as great as past storm events, according to Mark Lemmon, a member of the Mars Exploration Rover science team. "This one is smaller than 2001, smaller than the worst Viking storm, and pales compared to the worst we've seen" from spacecraft -- the worst being the one that greeted Mariner 9 and Mars 2 and 3 when they arrived to become Mars' first artificial satellites in 1971. Dark Skies for Spirit and OpportunityWhile the three orbiters at Mars float above the storm, the rovers are caught
in the middle of it, with no shelter anywhere on Mars' broad plains. Although
the storm is accompanied by high winds, the rovers cannot come to physical
harm from it; they'll remain solidly upright on their wheels. The problem
is the dust. Dust now fills Mars' atmosphere, preventing most of the
sun's light from reaching the rovers' solar panels. Although the situation
is dire, particularly for Opportunity, the rover team has taken steps to preserve
the health of both rovers, and are optimistic that both will successfully
ride out the storm. Atmospheric scientists quantify the amount of dust in the atmosphere using a parameter called optical depth, which is usually referred to by its Greek symbol, "tau." Tau can range from zero to infinity, where zero means the sky is perfectly clear. The amount of light that can pass through the atmosphere drops exponentially with increasing tau. The rovers typically experience atmospheric optical depths or tau values ranging from zero to one. Until now, neither rover had ever measured a tau above two; during this storm, Opportunity has been under skies with tau as high as 5.5, according to Lemmon. The exponential decrease in sunlight with increasing tau means that Opportunity's skies have become almost unbelievably dark, even at noon.
With such high optical depths, no direct sunlight reaches the rover; the only light impinging on its solar panels is indirect, coming weakly in all directions from the gloomy haze. Before the dust storms began, Opportunity's solar panels were producing a robust 700 watt hours of power, according to a Jet Propulsion Laboratory release issued today. When solar panel output dropped to 400 watt hours, the rover team suspended most operations. On Tuesday, July 17, the output dropped to 148 watt hours, and on Wednesday, to 128 watt hours. At these levels, Opportunity is in a "power-negative" situation, meaning that no matter how much it curtails its activities, it consumes more power than it generates -- a situation that can only last as long as the stored battery power does.
Fortunately, the rover team went in to this situation with their eyes open, Lemmon says. "The initial reaction to this storm was to keep the expectation that these things come and go. However, we never plan based on such optimistic assumptions, so battery levels have been kept high at all times. When it became clear from the orbiters that there were many storms (lifitng areas) and that one was going hemispheric, the storm looked scary. It still does. However, Opportunity can survive the worst weather she has seen so far for many sols. "My expectation now is to see dust levels at that site to come down for a little while. Nonetheless, she will sleep through the weekend (her first holiday weekend on Mars, I think). Monday we will evaluate things, but expect very conservative plans." It's a very good thing that Opportunity had not yet driven into Victoria crater, which was the next activity on the rover's itinerary. "If we had entered the crater, we'd be in a different situation. Direct sunlight is currently insignificant. The energy is all from diffuse sky light. Sitting on a slope in a crater reduces the amount (solid angle) of sky, and therefore power." Opportunity's "holiday" means that it will be, for the first time in its mission, skipping communication sessions with Earth in order to conserve energy; no communications were scheduled for yesterday or today. This step reduces Opportunity's daily energy use to less than 130 watt hours. The situation is also serious for Spirit, but not quite as bad as for Opportunity. Where Opportunity is experiencing dust raised by both the hemispheric storm and a local storm, Spirit is seeing lower dust levels. "There is a significant risk of tau increase there," Lemmon says, so "Spirit just started a conservative plan." And for both rovers, the storm has a silver lining: the strong winds that accompany the stormy weather have thoroughly cleaned both rovers' decks, making their solar arrays quite efficient at capturing the few available photons. Once the storm abates, both rovers should have power to spare, provided that the abatement of the storm isn't accompanied by much dust settling on the panels. The View from AboveAll three of Mars' orbiters -- Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter -- are now shifting plans to study the storm. The extreme atmospheric opacity means that attempting high-resolution imaging of most areas on the surface of Mars is pointless. Orbiter science teams are scrambling to cancel bandwidth-intensive imaging activity, and spend more of their resources studying the storm.
Mars Express, for instance, is "rescheduling our science operations in view of the major dust storm going on at Mars at the moment," says Agustin Chicarro, the mission's project scientist. "This means that the HRSC camera and OMEGA mineralogical spectrometer are basically not observing, but full priority is being given to the PFS atmospheric spectrometer, as they had never observed such a dust storm before and they need to complete a number of gaps in their global coverage." ESA scrambled to switch into this mode, Chicarro says: "Normally all operations are 'frozen'" [i.e. planned] "about six to eight weeks in advance, but we are 'breaking the rules' for PFS." Mars Odyssey THEMIS has also shifted modes, performing global mapping in a way that helps them track the storm's development. The THEMIS team is sharing their maps on the Web as they are generated. The animation above right begins on July 1, almost a week after the dust storm began, so atmospheric opacity is already high. Opportunity is nearly at the center of the map, while Spirit is located at the extreme right edge of the map. The instrument that is in the best position to study the rise and fall of the dust storm is Mars Climate Sounder, which is always operated in a mode that is suited for long-term monitoring of Mars' weather. Mars Climate Sounder rarely points down at Mars. Instead, it spends most of its time staring forward along Mars Reconnaissance Orbiter's path, looking at Mars' limb, the edge of the planet's disk. By staring at the limb, the instrument almost exclusively detects radiation that has interacted only with Mars' atmosphere, not with its surface. Mars Climate Sounder caught the storm in the process of formation, and is closely monitoring the dust's effect on the atmosphere over time.
Mars Climate Sounder investigation scientist David Kass explains the chronology. "We saw the first dust storm-related thermal signature on June 25, although there is a hint of something on June 24; we haven't had the time to look to see whether it is the first hint of the storm or just the standard weather for the season. By June 27, the signature in the south is very strong and the dynamical response in the north is also very noticeable. By the first of July, the entire atmosphere at [an elevation of] 30 kilometers [20 miles] has warmed up significantly. This warming has continued since then, with some regions warming by more than 40 kelvins (40 degrees Celsius or 70 degrees Fahrenheit) and most of the atmosphere warming at least 20 kelvins (20 degrees Celsius or 35 degrees Fahrenheit)." Just as on Earth, warming of Mars' atmosphere is equivalent to increasing the amount of energy available in the atmosphere to power storm winds. So storms can be self-sustaining for some time, powered by the heat that they produce by filling the atmosphere with dust. Another of Mars Reconnaissance Orbiter's instruments that was designed to monitor global events is the Mars Color Imager, or MARCI. MARCI continually acquires color images along the orbiter's path with a wide field of view. Thirteen image strips, comprising one day's worth of data, can be combined to create a global color view of Mars for every day of the orbiter's mission. The image below shows two such daily snapshots, taken before the storm began (top) and after the storm developed into a hemispheric event (bottom). Animations of data over several days are avilable from the Malin Space Science Systems website.
These images are used every day for planning by Mars Reconnaissance Orbiter's other instrument teams. A post on the HiRISE team weblog states that "The CRISM instrument can measure the composition of the dust…so we’ve helped them acquire extra images by canceling a number of our own images that would have been obstructed by the dust anyway." CRISM -- the Compact Reconnaissance Imaging Spectrometer for Mars -- is taking full advantage of the opportunity to observe a dust storm in action, says Scott Murchie, the instrument's principal investigator. "There's a part of the team that never wanted to have a big dust season and a part of the team that was hoping to have a really big dust season," he laughs. "We are completely using up our data allocation taking measurements of the dust storm." Murchie explains that their instrument usually runs in one of three operating modes: targeted, high-resolution observations; global mapping at lower spatial and spectral resolution; and globally gridded high-resolution observations that are designed to study Mars' "emission phase function," which allows the team to characterize the ice, dust, and trace gases in Mars' atmosphere. "In normal times, what we do is a lot of global mapping (which is beginning to wind down, because we've achieved more than 50% coverage), a lot of targeted observations, and every two and a half weeks we do a global grid of emission phase functions." One of these global grids "actually caught the dust storm as it was starting up. That was lucky, and it is going to be scientifically interesting." Long before Mars Reconnaissance Orbiter departed Earth, the CRISM team planned for a dust storm season by defining a "storm operating mode" for their instrument. In this mode, they do fewer targeted observations and instead do nadir-pointed mapping and emission phase function observations on alternate orbits. Although different from their usual mode, it does not mean less science, Murchie says. "We're not standing down our science, we're just switching gears and taking advantage of the opportunity. The every-other-orbit interleaving of mapping and emission phase functions allows us to measure different aspects of the story. Mapping mode lets us track the spectral properties with high spatial resolution, and the emission phase functions determine the total dust in the atmosphere. "For the latitude band that's affected by the data, we do dust storm mode. Presently we're doing that from -5 to -75 [latitude]. For the next planning cycle, which starts Monday, we're doing it basically over all latitudes below five kilometers [three miles] elevation." Above that elevation, which includes many of Mars' volcanoes -- and the cave skylights on their flanks -- the skies are clear enough to permit imaging. The south pole is also relatively clear; the north pole is probably clear but is in winter darkness. In addition to science, the CRISM team is getting some housekeeping done. "It's hard to use up Mars Reconnaissance Orbiter's data volume right now, because it has a relatively high rate" and high-data-volume imaging observations are being canceled. "So we are doing a really, really thorough flat field." By pointing their instrument at a bland but sunlit dusty area, the CRISM team can develop detailed models of any imperfections that there may be in the instrument's optics. Once the storm abates, it will take some time for dust to settle out of the atmosphere, and even that is an event that Mars atmospheric scientists look forward to studying: which size dust particles settle at which rates? How long will the atmosphere remain anomalously warm or opaque? For most, the subject is merely of academic interest, but for the rover teams, the interest is more urgent. "We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense," said Alan Stern, associate administrator of NASA's Science Mission Directorate. Around the world, rover fans are hoping that Spirit and Opportunity, which have already survived so much, will ride out this storm. |
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