With the Sun setting and polar twilight beginning and an unexpected dust storm at the end of October, the solar-powered Phoenix rwas unable to take in enough photon fuel to keep its batteries charged. It last phoned home on November 2 and has since shut down. Later in the Martian winter, the spacecraft will become buried in carbon dioxide ice. (Image Credit: NASA, JPL-Caltech, University of Arizona)
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The seasonal decline in sunshine at the lander’s arctic site simply cannot provide enough photon fuel for the solar arrays to produce the power necessary to charge the batteries that operate the spacecraft’s instruments states A.J.S. Rayl in an article for the Planetary Society.
But it isn’t over until it’s over, as Yankee philosopher Yogi Berra says. Although the spacecraft is no longer functioning, analysis and interpretation of the mission’s science investigations are really only beginning. "Phoenix has given us some surprises, and I'm confident we will be pulling more gems from this trove of data for years to come," said Principal Investigator Peter Smith, of the University of Arizona, the first public university to lead a NASA mission.
Unlike the Mars Exploration Rovers (MER), Phoenix is a lander and, therefore, is stationary and cannot rove to another location to improve its odds for a longer life. Even so, the mission exceeded its planned operational life of three months, to conduct and return science data for two additional months.
“We knew this would happen eventually,” said Project Manager Barry Goldstein, of NASA’s Jet Propulsion Laboratory (JPL), during a press teleconference yesterday. “We always anticipated we would brown out. The situation we experienced is almost play-by-play of what we expected except it happened three weeks earlier."
At a cost of about $475 million – a price tag that does not include the weather station, which was provided by the Canadian Space Agency -- “NASA's gotten what it wanted out of this mission,” Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington, told reporters. “We achieved full mission success in August, obviously the objective and exciting to do. Its demise is a little earlier than we hoped, but we've certainly gone much longer than the initial 90-day mission proposed.”
The team had been hoping Phoenix would survive through this month, collecting weather data and taking an occasional image here and there, but on October 27 – the lander’s Sol 151 -- one of Mars’ notorious dust storms whipped up “out of the blue,” and dashed those hopes, bringing a quick end to the last of the mission’s “high-power science days,” Goldstein added.
The spacecraft first put itself into a low-power safe mode, but it soon stopped communicating. Beyond the fading Sun and reduced hours of daylight, Phoenix recently began seeing more clouds and experiencing colder temperatures, which began dropping from the “balmy” 32 degrees Fahrenheit (0 Celsius) to -22 degrees F (-30 degrees Celsius) in which it basked in the primary phase of the mission, Goldstein said.
During the final days of October, Phoenix did continue to “wake-up” as the Sun rose, but only for long enough to communicate. The solar panels apparently generated enough power for the spacecraft to arise, then the batteries drained. Mission engineers last received a signal from Phoenix on November 2 and have not heard from it since. “Unfortunately that high condition of dust lasted for some days and it became increasingly harder and harder for vehicle to wake up and increasingly colder in the morning,” Goldstein explained. It was the dust storm, he said, that ultimately sealed its fate earlier than hoped.
Over the next few weeks, the Phoenix team will continue listening carefully as Mars Odyssey and Mars Reconnaissance Orbiter (MRO) pass over the lander to hear if Phoenix revives and phones home. However, engineers now believe that's unlikely because of the deteriorating weather conditions on Mars. “At this time we're pretty convinced the vehicle is no longer available to use, so we are actually ceasing operations and declaring end of mission,” said Goldstein.
Phoenix – which Smith conceived in February 2002 – launched on August 4, 2007 and made a picture perfect landing May 25, 2008, during Memorial Day weekend. The spacecraft flew into the history books the moment it touched down safely, landing farther north than any previous spacecraft to land on the surface of Mars.
"Phoenix not only met the tremendous challenge of landing safely, it accomplished scientific investigations on 149 of its 152 Martian days as a result of dedicated work by a talented team," Goldstein pointed out.
During the last five months, Phoenixdug, scooped, baked, sniffed, and tasted the Red Planet's soil. Among early results, it verified the presence of water-ice in the Martian subsurface, which Bill Boynton, of the University of Arizona’s Lunar and Planetary Laboratory, and team first detected remotely with the Gamma Ray Spectrometer on the Odyssey orbiter in 2002. Boynton, who joined the Phoenix team as principal investigator of the Thermal and Evolved Gas Analyzer (TEGA), and his team confirmed the discovery of that water-ice on this mission.
“We landed, looked around and saw a field of dirt and rock spread out to the horizon,” Smith recounted. “We didn't see ice right away and it wasn't until we looked under the spacecraft that we found out we were standing on it. This has been a thrill for everybody.” The study of ice, he added, kept the Phoenix scientists busy for the last 5 months. “We’ve excavated the ice and we know its depth and know how it changes over the surface and seen different types of ice. That's going to keep us busy for some time as we try and understand what we've got.”
In fact, Phoenix succeeded in excavating soil above the ice table, revealing at least two distinct types of ice deposits. “By measurements from Odyssey that shows so much concentration of ice, it's hard to believe there’s much open space [in this area] that does not have ice near the surface. I would bet if you had a broom, you could make an ice rink there where we landed,” said Smith.
In the last five months, Phoenix's cameras have returned more than 25,000 pictures, from sweeping panoramic vistas to pictures of the telltale, “which tells us the wind directions and little bit about the speed,” to images near the atomic level, taken by the first atomic force microscope ever used on another planet. “We watched clouds develop, used the Robotic Arm Camera to look at the trenches and material in the scoop,” Smith chronicled. “We've had a microscope on deck look at the size and color of the grains. And we've even had an atomic force microscope that allows us to go almost down to a level of resolution never seen before, 100 nanometers resolution.”
The arctic lander also continuously monitored the weather on Mars with the Canadian Space Agency’s weather station providing a mission-long record, with data on temperature, pressure, humidity and wind, observations of haze, clouds, frost, whirlwinds, and even snow descending from clouds. Phoenix and its weather station also coordinated with MRO to perform simultaneous ground and orbital observations of the Martian weather.
“We've got a complete weather record of the entire time we've been there with very sophisticated instruments that tell us about the clouds, pressure, temperatures, winds humidity, and so forth, because you cannot study a surface in an ice layer without knowing the atmosphere above it,” Smith pointed out. “We have a very wonderful description now for this entire time and a huge volume of data. It's one of the major accomplishments of the mission to retrieve this weather record.”
Phoenix’s preliminary science accomplishments have already advanced the goal of studying whether the Martian arctic environment has ever been favorable for microbes, documenting a mildly alkaline soil environment unlike any found by earlier Mars missions and finding small concentrations of salts that could be nutrients for life, as well as discovering perchlorate salt, which has implications for ice and soil properties, and finding calcium carbonate, a marker of effects of liquid water.
“We know the soil is alkaline and filled with carbonates and clays,” said Smith. “On the Earth, we would conclude immediately that there was liquid water in this soil. For Mars, we have to be a little more careful and we're going to develop this story as we interpret our data. But definitely liquid water has been part of this soil. We also see salts and perchlorate molecules which is totally unexpected and that has a profound implication for Mars. Perchlorate is an energy source for microbes on Earth,” he told reporters.
“If you had a liquid water environment, you could imagine the salts would flow to the lowest point and concentrate, so it's theoretically possible you could find brine at this landing site with the perchlorate that we've seen,” Smith suggested. “That's something we were looking for and hoping to find, and as we analyze data we'll be looking for any indication of that.”
Finding nutrients and energy sources leads, of course, to the question about habitability and whether the arctic area could have or does host microbial life. Answering the question of habitability was a primary objective for the Phoenix mission.
“Have we found such a thing on Mars?” asked Smith, anticipating reporters’ inquiring minds. “Even if not now, as polar spin axis tilts more toward the Sun and the climate warms, could this place be a place where life exists? Now that operation phase is over, our science team can go back to labs and look at data in great detail and do analysis and interpretation that will lead to final results of our mission,” he said.
“We think that right now at this epic in Martian history it's certainly too cold for organisms to be alive, at least in the sense of Earth organisms,” Smith elaborated later. “The ice is just too cold. But we do think over time, as Mars’ climate changes, it could get warm enough that perhaps we could get films of liquid water or dampness in soil and that could create an environment where life could exist. That would be within the last few million years, recent in Mars’ history. We think that there’s nutrients and energy sources that it’s certainly possible that at a warmer, wetter period in Mars’ history this could be a habitable zone.”
“There are a lot of lessons learned in this mission,” said McCuistion. “We learned a lot about handling of soils samples and soil consistency and how unexpected it could be and how difficult it is to handle ice when it sublimes quickly, how to manage assets of laboratories sometimes competing labs and operating in dust storms that have led to the earlier-than-expected demise of Phoenix.”
While the research from the Phoenix mother lode will continue for years, the mission’s scientists are at work now on articles slated for Science, Smith added. “We’re talking to editors now and in next few weeks we hope we will be in the review process.”
Currently, science team members are working to “recreate the signals” Phoenix collected from soil samples. “We were expecting acidic soils with sulfates and other things indicative of volcanic environments," Smith said. "We didn’t spend much time on alkaline [soils] or perchlorate [prior to arrival], so the first thing is to go back and try and recreate the signals [collected] on Mars to see if we are understanding properly that the soils are giving the signals we see.”
The scientists will also be looking for organics – that is, anything that is or related to or derived from or possessing properties characteristic of living organisms or any chemical compounds featuring a carbon basis -- another objective on the team’s wish list of discoveries. Since organics would offer up “subtle signatures” in data collected by Phoenix’s science instruments, “it takes a fair amount of work to tease out organics,” said Smith. “Until we do the work, I can't say we didn't find organics. Really it’s a question of what is the truth on Mars and we're trying to get the right answer and not come rushing out with a quick analysis. This is very tricky stuff and the data sets quite complex with regard to organics.”
There has been talk about resurrecting Phoenix once winter passes and the arctic regions warms in spring and summer. It’s a long shot and not probable but possible,” Goldstein said. “The Sun goes down for approximately 3 months and will begin to rise again in on April 1. Probably around mid October Sun will be high enough in the sky that we could conceive of vehicle waking up. We always speculated that it's highly unlikely vehicle will come back.”
For one thing, temperatures are expected to drop to an unfathomable -238 degrees Fahrenheit (-150 Celsius) as winter takes hold in the Martian arctic, something that will cause the lander to become “encased” in carbon dioxide (CO2) ice. The unbelievably cold temperatures will cause “the electronics coatings to become brittle like glass and break,” challenging wiring boards and other essential parts. “But,” he added, “this vehicle has been so superlative in the way it's been behaving since we landed that nothing would surprise me.”
“If Phoenix miraculously came back to life,” Smith said, they probably could make use of the one TEGA oven that was not used. In finding that the soil in the arctic region was stickier than expected, the TEGA team had a heck of a time at first getting soil samples into the tiny ovens, which are the size of a pencil lead and if they had the mission to do over again, Smith said they’d probably put bigger holes in the screen.
Throughout the mission, the Phoenix team has had its ups and downs and confronted confounding challenges – not the least of which was trying to get the unexpectedly sticky Martian soil into the tiny TEGA ovens. Getting down on Mars in one piece, however, is still the hardest part and for many the most memorable moment. For Smith, after seven years of investment, “the day of landing and seeing those pictures coming back that meant everything was working” remains, for him, the singular highpoint.
Landing also ranked at the top for Goldstein. “For me, ripping up all the contingency procedures we needed for failing EDL [entry, descent and landing]. That was the best moment for me without question.”
Phoenix was the first of NASA’s Scout missions, smaller, focused operations selected from proposals submitted by members of the science community, and “it set standard for Scout missions to come,” offered McCuistion. The University of Arizona led the mission with project management at JPL and a partnership with Lockheed Martin Corporation. The Canadian Space Agency provided the weather station, and the University of Neuchatel, Switzerland, the universities of Copenhagen and Aarhus in Denmark, the Max Planck Institute in Germany, the Finnish Meteorological Institute, and Imperial College of London also contributed to the mission in significant ways, making the Phoenix mission an international team effort.
“It's certainly been a grand adventure -- the first university-led mission to Mars, so hats off to Arizona,” said McCuistion. “Phoenix has truly has epitomized the program's interconnectedness of missions and played on the best attributes of those missions. Odyssey's discovery of large volumes of water-ice at the poles inspired the mission. MRO was critical for choosing the landing site for Phoenix and both Odyssey and MRO were absolutely crucial for providing communication and even European Space Agency’s Mars Express helped out with communication,” he noted.
"Phoenix provided an important step to spur the hope that we can show Mars was once habitable and possibly supported life," McCuistion summed up. “”It has been an excellent exploration mission into uncharted territory -- the Martian arctic -- and it has fulfilled dreams of touching water for first time. While we're losing a spacecraft, it's really an Irish wake rather than funeral we're looking forward to here. We should celebrate what Phoenix and the Phoenix team have done and where that’s going to take us in the future.”
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