After more than four months on the arctic plains of the red planet, NASA's Phoenix Mars Lander's days are finally numbered.As the sun begins to set for the frigid Martian winter, the spacecraft will lose its energy supply, freeze and eventually fall into a mechanical coma from which it will likely never wake up.
Phoenix's mission has been to dig up samples of Martian dirt and the subsurface layer of rock-hard water ice at its landing site in Mars' Vastitas Borealis plains. The lander has been scanning the samples for signs of the region?s past potential for habitability.
Phoenix landed on Mars on May 25, late spring in the Martian northern hemisphere. The mission was originally slated to last three months, to the end of August, but was extended twice; first to the end of September and recently through the end of December.
But whether or not Phoenix will survive that long is uncertain and depends on how the spacecraft's systems handle its ever-dwindling energy supply and the harsh conditions of the Martian winter.
"We're at the mercy of Mars," said Phoenix project manger Barry Goldstein, of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Winter sets in
As winter descends on the Martian arctic, two important things will happen: The sun will sink below the horizon, and "it's going to get cold," said Phoenix meteorological team member Peter Taylor of York University in Toronto, Canada.
Of course, Mars is never warm by Earth standards (it is further from the sun and lacks our planet's thick, heat-trapping atmosphere), but summer above the Martian arctic circle is downright balmy compared to the winter.
Midday temperatures at Phoenix's landing site hit about -4 degrees Fahrenheit (-20 degrees Celsius) in the summer (as measured by the lander's meteorological mast thermometer). Nighttime temperatures then still dropped to -112 F (-80 C). Currently, those daytime temperatures have started dipping down to -22 F (-30 C), with nighttime temperatures hitting about -130 F (-90 C).
By mid-November, those night temperatures are expected to plummet to -184 F (-120 C).
The reason of course, is that setting sun.
The sun is constantly above the horizon during the arctic summer, just as it is on Earth. Come fall, it starts to dip below the horizon more and more each day until winter, when it sets for good and doesn?t rise again until the spring.
The sun has already begun to sink below the horizon for part of the day at Phoenix's location, Goldstein said. Phoenix's landing site is at a latitude similar to northern Alaska on Earth.
The colder temperatures and setting sun combined will diminish the energy available to Phoenix for its science operations.
During the summer, there is plenty of sunlight hitting Phoenix's wing-like solar arrays, its sole source of power on the planet. But once the sun is gone, so is its energy supply.
"The sun is going down, so there's less and less energy being fed into the batteries through the solar panels, and that really is the biggest problem" facing the mission, Taylor said.
Specifically, the orientation of Phoenix's solar arrays limits how much sunlight it can take in as the sun changes its position in the sky.
"The problem is that the solar panels are horizontal, and we can't tilt them, so as the sun gets lower and lower on the horizon, there's less and less power being generated," Taylor explained.
At the beginning of the mission, Goldstein said, Phoenix's solar arrays were generating about 3300 Watt-hours per sol, or Martian day (about 24 hours, 39 minutes) ? that's enough energy to light a 60-Watt light bulb for 55 hours.
On Sept. 13, or 109th sol since landing, energy generation had already dipped down to 2400 Watt-hours per sol.
"And we're steadily decreasing," Goldstein said.
The energy cut-off point for the lander will come around the time the arrays can only generate about 1000 Watt-hours, which is "the absolute minimum amount of energy that it takes for the spacecraft to wake up in the morning," Goldstein told SPACE.com.
"Not surprisingly, we kind of hit that number at about the middle to end of November" according to model projections, Goldstein added. Those models are somewhat conservative in their estimates he said, so it's possible the lander could hold on for a few extra days, "but it's not going to be much," he said.
Mission leaders are planning to have all their science operations, such as gathering and analyzing samples, completed by that cut-off date.
Mission scientists had planned to gather all the remaining samples (for the one unused cell in the wet chemistry lab and the four unused ovens in the Thermal and Evolved-Gas Analyzer, both of which analyze the composition of samples) by the end of September. Since that didn't happen, they are now aiming to complete sample gathering by mid-October.
Goldstein said he changed the strategy of sample analysis to focus on gathering all the remaining samples and then analyzing them instead of processing them one-by-one. This shift was made because moving the robotic arm and scraping up the dirt and ice is still a tricky process that involves using an unknown amount energy, whereas sample analysis involves a known, discrete amount of energy and will be easier to budget as energy supplies diminish. (Another complication is the energy that must be used to keep the instruments warm enough to function, which takes a chunk out of the remaining energy supplies.)
"So the sooner we get as many of the samples into the cells, the better," Goldstein said.
After October, the lander will essentially transition into a weather station, observing the transition into winter for as long as it can hold on.
"We'll keep measuring temperatures and pressures as long as we can," Taylor said. If possible the team will also try to use the lander's lidar to take measurements of the clouds that have been gathering overhead as the atmosphere has cooled and its camera to take images of the frost that has already started forming on the ground.
The frost that has already begun to accumulate in the area around Phoenix's landing site, as well as the sheer cold temperatures will also affect the lander, though their impact will mostly come after Phoenix ceases operations.
Images taken recently with the lander's Surface Stereo Imager have shown pockets of frost forming on the ground, especially in the trenches that Phoenix has been digging, Taylor said.
So far, the frost hasn't formed on the lander ? except for on the small mirror used to view the wind telltale at the top of the meteorological mast ? because Phoenix stays warmer than the ground around it.
"In general the lander itself is designed to absorb as much solar radiation as it can, and to emit relatively little radiation in the infrared. So the lander deck has been much hotter than the surrounding ground surface, for instance," Taylor explained. "It's a bit like the top of a relatively warm computer, if you like."
The lander will likely stay warmer than its surroundings for awhile after Phoenix loses the energy it needs to operate, "so it'll be pretty late on when frost actually starts to form on the lander," Taylor said. So Phoenix isn't likely to get any pictures of itself coated in frost.
Right now the frost that is forming is all water ice because it is not yet cold enough at Phoenix's latitude for carbon dioxide ice to form, though it eventually will. Whether the frost will come as a thin coating or a thick sheet, like Mars' polar ice caps, isn't known.
"We're not sure how much CO2 will deposit at this latitude ? most of it is on the polar cap," Taylor said.
Taylor tends to think the frost won't build up as much as at higher latitudes. "We'll see little flakes of ice, we'll see ice crystals and frost, but it won't be [like] the ice that freezes on your windscreen on a winter's morning," he said.
After Phoenix shuts down, the only way to observe the mounting frost will be through NASA?s Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) camera. Scientists aren't sure just how much HiRISE will be able to see, but the team is hoping it will be able to shed some light on Phoenix fate, since communication with the lander will be lost.
"We won't be able to talk, but we'd sure like to watch," Goldstein said.
The big question will be what happens when Phoenix emerges on the other side of winter: Can it live up to its name and come back to life when spring brings back the sun?
Not likely, Goldstein and Taylor said.
Phoenix does have a built in reboot program that its designers call a "Lazarus mode," "where when energy comes back into the vehicle from the solar arrays ? if and when energy comes ? it'll automatically try to reboot and try to communicate," Goldstein explained. But he doubts that will happen.
"I would be overjoyed to hear something come back from Phoenix; I'm extremely ? I find it very, very unlikely," Goldstein said.
The reason Goldstein, and others on the Phoenix team, think it unlikely that Phoenix will make a comeback is simple: The lander has entered conditions on the surface beyond what it was built and tested to withstand.
"We passed the warranty a long time ago," Goldstein said.
The build-up of frost is part of the problem. While the amount of ice reaching down to Phoenix's latitude may only be a thin layer, it could also be enough to encase the lander in ice. Phoenix's engineering team tested the lander's survivability under many scenarios, but "that was a test I refused to do during development, survivability if encased in CO2 ice," Goldstein said. ("I was going to call that the Ted Williams test," he joked, referring to the legendary Boston Red Sox player who had his body cryogenically frozen after he died.)
But even without being entombed in carbon dioxide ice, Phoenix likely won't survive the harsh winter because even in the summer, the lander needs heaters to keep its electronics warm enough to function.
Phoenix's circuit boards and wiring are generally regulated to about -40 degrees Fahrenheit (-40 degrees Celsius) for optimum performance.
"So will they survive past that? Yeah they will, but at some point they're going to get so cold that they won't survive," Goldstein said.
Most electronics can only last down to about -148 or -193 F (-100 or -125 C), after which some of the materials that make them up go below their glassification temperature.
Goldstein explains glassification this way: "Think about a rubbery substance or a plastic substance becoming brittle like glass, and once that happens, it starts to crack," Goldstein said.
If Phoenix's electronic components crack, it's unlikely the lander will be able to resurrect itself even when sunlight returns to the northern hemisphere in the spring.
"The kind of temperatures we're talking about with no energy to keep the vehicle warm, it's pretty difficult to imagine," Goldstein said.
Come spring on Mars (summer on Earth, as the Martian year is longer), when sunlight has been streaming down long enough to potentially re-awaken the spacecraft, NASA will likely listen for any beeps coming from Phoenix, though Goldstein doesn't think they'll hear anything. So once Phoenix dips below its energy threshold around the end of November, that will likely be all she wrote for the mission.
"It's a fun project but we're getting near the end," Goldstein said.
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