Antarctic Diving Robot Practices for Europa Mission

Evidencefound by NASA's Galileo spacecraft of an ocean on Europaput the giant Jovian moon on the A-list of worlds worth investigating for signsof extraterrestrial life.

Exploringthat ocean won't be easy: It's covered by ice perhaps as much as 100 kilometers(62 miles) thick. But late last year, a group of NASA-funded scientists andengineers took an important step toward figuring out howit might be done.

The ENDURANCEproject (ENDURANCE stands for Environmentally Non-Disturbing Under-IceRobotic Antarctic Explorer) wrapped up its second field season at Lake Bonney, an ice-covered lake in Antarctica's Dry Valleys.

Lake Bonney is composed of two lobes, connected by a narrowchannel. At the west end of the west lobe, Taylor Glacier dips down to andbelow the lake's ice cover. The lip of the glacier, a small underwater iceshelf, sticks out into the lake some 17 meters (56 feet) below its surface.

As they haddone during the project's first field season in 2008, EUDURANCE team membersbegan the 2009 season by melting a hole 2 meters (6.6 feet) wide in Lake Bonney's ice cover and constructing a temporaryheadquarters around it, complete with a crane capable of lowering the1300-kilogram (1.4-ton) robot into the lake.

Over thecourse of a month, sometimes performing multiple runs in a day, ENDURANCEstudied the west lobe of LakeBonney, including the underwater face of TaylorGlacier. Using sonar, it produced a high-resolution bathymetric (depth) map ofthe west lobe. Simultaneously, it stopped every 100 meters (328 feet) to lowera package of scientific instruments, known as a "sonde,"down through the water column to collect data on temperature, salinity, pH, andseveral biological indicators. Near the glacier face, it sampled even morefrequently.

ENDURANCEhad collected a partial set of similar data during the project's first seasonin 2008. In 2009, ENDURANCE successfully explored the entire west lobe. It alsonavigated through the channel connecting East and West Lake Bonney,and for the first time explored portions of the east lobe.

The resultis perhaps the most extensive 3-D biogeochemical map of any lake on Earth."There's not a data set like this for any other lake in the world that Iknow of," said John Priscu, a professor atMontana State University and a member of the ENDURANCE science team. "Theonly way we could have done this study is robotically," he added.

Over thecourse of the next year, Priscu and Peter Doran,professor of earth and environmental sciences at the University of Illinois atChicago (UIC) and principal investigator for the ENDURANCE project, will workwith the Electronic Visualization Lab at UIC to produce 3-D representations ofthe Lake Bonney data that can be studied by viewerssitting in front of 10-by-20-foot screen, able to navigate through the data asthough they were swimming through the lake.

The lab willproduce individual "color 3-D plots ? of all the differentparameters," Doran said, that will highlight the structure of chemical,temperature and biological gradients within the lake. Further, combiningmultiple data sets into composite plots will enable the science team tovisualize relationships between parameters. For example, how the amount oflight available at various points in the lake affects the presence ofchlorophyll, an indicator of photosynthetic activity; or, for parts of the lakewhere data was collected in both 2008 and 2009, "how some parameters havechanged from year to year."

Ofparticular interest to the team was the area beneath the underwater TaylorGlacier ice shelf, where ENDURANCE found evidence of a stream of frigid waterflowing into the lake that had not been present the previous year.

But the tipof the glacier dips down to about 20 meters (66 feet) below the lake surface.At about 17 meters (about 56 feet) below the surface, a chemocline,an increase in salinity, occurs in Lake Bonney'swater, and this saltier water is more buoyant than the overlying layer offresher water.  In order for ENDURANCE to sink down into the salty water,more weight needed to be added.

"If wewanted to go under the glacier, we'd have to come up with some way to reballast the vehicle to go below this chemocline,because the density went up by almost 20 percent," said Bill Stone ofStone Aerospace, who heads the company that designed and built the robot.Without the added weight, ENDURANCE "would bob on that chemoclinelike a cork."

The extraweight would cause problems for most operations, so it couldn't be added to thevehicle at operation central, where every day the massive crane pulled ENDRANCEin and out of the hole in the ice.  Instead, the team melted a second holein the ice near the glacier and set up a small portable gantry to lift the bot just far enough out of the water to pile on 120kilograms (265 pounds) of lead.  ENDURANCE was then lowered back down intothe water, now heavy enough to sink into the saltier layer.  Afterexploring this denser region, ENDURANCE returned to this second hole and thelead weights were removed, allowing the vehicle to return unencumbered to themain access hole.

Ideas arealready in development, Stone said, to reconfigure the ENDURANCErobot to explore beneath Europa's ice.Miniaturization will be critical to that effort. He envisions a parent craftthat carries "a redundant collection of sub-bots." The parent craft,he says, "images the gross topography" and "maneuvers towardplaces of interest," but stays "well away from the dangerousobstacles." When it discovers a feature worth exploring in more detail, itspins off a sub-bot to takeclose-up images, collect higher-resolution chemical and biological data, andretrieve samples for chemical and biological analysis by scientific instrumentson-board the parent craft.

One criticalprocedural question that will need to be addressed before launching aspacecraft to explore Europa's ocean is to what degree it will operateautonomously. ENDURANCE is capable of performing its tasks without humanintervention, but operating that way took a toll on the science return.

In 2008,Doran says, when ENDURANCE operated fully autonomously, "we couldn't seethe science data live. The thing would be out collecting data and we wouldn'tknow what it collected until it came back. And sometimes the sensors were notworking properly and they needed some kind of treatment and we couldn't do thatuntil after it got back and we'd wasted a whole day." In 2009, however,during most of its runs, ENDURANCE was connected via an optical fiber that feddata to a computer screen. "We were there on the site tweaking it thewhole time," Doran says. "We would [have gotten] a data set if weweren't there, but it would [have been] nowhere near as good a data set."

Maintainingcommunication with a sub-ice explorer on Europa willbe difficult. In addition to the delays introduced by the time it takes for radiosignals to travelbetween Earth and Europa will be the difficultyof keeping up a communications link between the moon's surface, throughkilometers of ice, to a vehicle continuously changing position in the oceanbelow. 

Despite thechallenge, Priscu says, having that feedback loop maymake the difference between scientific success and failure. "If we sendsomething to Europa, having a data feed come back sowe can make decisions on the fly, in near-real time," he argues, will be"invaluable."

ENDURANCE isfunded by NASA, through its ASTEP (Astrobiology Science and Technology forExploring Planets) program.

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