Divining Water on Europa

Marshall Space Flight Center

http://science.nasa.gov/newhome/headlines/ast09sep99_1.htm

Circumstantial evidence for water on Europa mounts as JPL scientists try an ingenious experiment to find hexagonal water-ice crystals on the frigid surface of Jupiter's iciest moon.

September 9, 1999: A future guide book for Solar System vacationers might describe Jupiter's moon Europa this way: "Europa is cold -- really cold. The surface temperature is a chilly -260 deg F, so bring your space parka. The entire planet is covered with a relatively smooth layer of frozen water and ice skating is allowed in most regions. Travelers should bring their own air, as Europa's oxygen atmosphere is a million times thinner than Earth's. Other items of note include cold water volcanoes, gigantic ice rafts, and an underground ocean. Ice fishing is not encouraged."

It may sound like science fiction, but most of this fanciful description is true. NASA's Voyager and Galileo spacecraft have shown Europa to be a frigid world dotted with remnants of "cryo-volcanoes" and rafts of ice similar in appearance to those seen on Earth's polar seas during springtime thaws. As astonishing as the surface sounds, Europa may be even more interesting underground. Many scientists think that tidal friction from nearby Jupiter heats the interior of the moon to temperatures where liquid water is possible. Beneath its icy crust, Europa could harbor the solar system's largest ocean!

Aside from the novelty of oceans on another planet, scientists are fascinated by the possibility of Europan seas because they could be sites for extraterrestrial life. Here on Earth, undersea volcanoes and hydrothermal vents create environments that sustain rich colonies of microbes. If similar systems are active on Europa, scientists reason, life might be present there too. If there is an ocean on Europa, will it contain life? Chris Chyba of the SETI Institute responds, "Can an ocean of liquid water persist for 4.5 billion years and not have life in it?" [ref]. We simply don't know.

Mounting evidence...but still not enough

Circumstantial evidence for subterranean seas continues to mount. New Galileo images released on August 27, 1999, show large chaotic areas on Europa's surface where a liquid ocean or warm ice may have welled up and disrupted the moon's icy shell. Thera and Thrace are two rust-colored areas, each over 50 miles wide, consisting of remarkably jumbled terrain. The latest images show that Thera lies slightly below the level of the surrounding plains. Curved fractures along its boundaries suggest that the whole region collapsed some time in the past.

Cracking, shifting ice rafts. Cold water volcanoes. Gigantic surface melts. How long ago did these events take place? No one knows for sure. Some planetary scientists think less than 50 million years ago based on Europan cratering statistics. That may seem like a long time, but to a geologist, it's a brief interval.

"Fifty million years is only 1% of the age of Europa," says Dr. Bob Pappalardo, a planetary geologist at Brown University. "Geologically speaking, if something was happening there 50,000,000 years ago then it's probably still happening today. Still, we can't rule out the possibility that Europa's oceans have 'recently' frozen. For example, there might be some cyclic tidal effects associated with Jupiter and the other Galilean satellites that cause Europa to warm up and then cool off with a period of a hundred million years. At this point it's speculative."

One way to get to the bottom of the mystery without actually drilling through the ice is to watch for changes on Europa's surface. Photos of, say, an active cryo-volcanic geyser would prove that liquid water exists. So far, after numerous encounters with Europa Galileo has found no direct evidence for liquid water -- just tantalizing hints.

"Cynthia Phillips, a University of Arizona graduate student working with Dr. Alfred McEwen, is also looking for changes on Europa by comparing Voyager and Galileo images," adds Pappalardo. "It's difficult work because many of the images are low resolution, there are lighting differences to contend with, etc. So far they haven't found any definitive evidence for changes across a 20 year interval."

Now that Galileo has completed its scheduled flybys of Europa, researchers are poring over the data for additional clues about what might lie below the moon's surface.

"There's lots of research going on," says Pappalardo. "What I and others are doing is refining and testing models of how various surface features form. When the Galileo images first came out we developed models based on warm ice to explain features like we see around Thera and Thrace. Combined with NIMS [Near Infrared Mapping Spectrometer] measurements, these models support the idea that the surface may be salty in places. The best spectral match is to magnesium sulfate, better known as Epsom salts. When warm convecting ice rises up and hits a salty area it melts at a lower temperature than the surrounding plains. This sort of process might explain regions like Thrace where we see what appears to be melting and liquid-like flows across the surface."

Researchers aren't certain what causes the brown discoloration of regions like Thera and Thrace and around cryo-volcanic ridges. NIMS data indicate a correlation between brown areas and salty spots.

"The problem," continues Pappalardo, "is that Epsom salt isn't brown. The best candidates for the brown material are iron compounds or some sort of sulfur compound. Both iron and sulfur are relatively abundant in the solar system and have a red appearance visually. Plus, we know that there's SO2 on Europa."

"We may not be able to answer all the questions now," says Pappalardo, "but framing the problems is important because of the planned Europa Orbiter mission."

Still under development, the Europa Orbiter will use a radar sounder to measure the thickness of Europa's icy crust and possibly determine whether liquid water exists below the ice. An on board altimeter will gather topography data and characterize the tidal response of the surface. Tidal distortions of Europa's crust will be large if there is an underlying layer of liquid water and smaller if the water is in the form of ice. The mission could launch in 2003 and would serve as a precursor to spacecraft that might send undersea explorers into the Europan oceans.

A Ring Around The Moon

Unwilling to wait years for an answer, Dr. Robert Carlson and colleagues at JPL recently tried an ingenious experiment to solve the mystery now. According to Carlson, a crucial clue to the state of affairs below Europa's crust lies in the crystalline form of ice on the surface.

In theory, three different forms of ice can exist at the low temperatures and pressures on Europa's surface: amorphous ice, cubic crystals and hexagonal crystals. High pressure ices can take on other forms, but they are not relevant to Europa.

"Ice that forms from water vapor at 100 [Kelvin] is amorphous," explains Carlson. "At 140K, ice formed from water vapor has a cubic crystalline form. At temperatures above 170K, ice formed from liquid water or from 'warm' water vapor has a hexagonal crystal, just like ice on Earth. If we could find evidence for hexagonal ice crystals on Europa, it would mean that the ice formed fairly recently from liquid water or warm vapor."

On Earth, hexagonal ice crystals are responsible for a sight well-known to stargazers: a 22 degree halo around the Sun or Moon when it is viewed through humid cirrus clouds. Similar halos can be seen from sunlight glinting off snowy surfaces, especially from freshly fallen snow.

Carlson and colleagues reasoned that if hexagonal ice crystals are present on Europa, they might be able to see a similar effect using Galileo's Near Infrared Mapping Spectrometer. When sunlight hits the surface of Europa at near grazing incidence it penetrates the surface and refracts through the ice crystals. Hexagonal crystals will produce a ring-like glow in the infrared much like the familiar "ring around the Moon."

"NIMS is a spectrometer, but we're not using its spectral capability for this work," says Carlson. "The search for hexagonal ice on Europa involves a search for light intensity variations as a function of phase angle."

"Geometry was crucial for this experiment," he continued, "There was only one orbit when we could do it when the Sun was only 22 degrees above the grazing incidence reflection point. The spot we observed doesn't have a particular name, it was a small patch of ice surrounded by darker material at about 48 degrees longitude on Europa. The patch was about 100 km across, and we could detect hexagonal crystals even if they make up as little as 10% of the ice."

The observations were played back earlier this year and Carlson et al are busy analyzing the data.

"If we don't see the halo, it doesn't necessarily mean anything," says Carlson. "That's because crystalline ice exposed to energetic particle radiation over a period of time becomes amorphous, and there is a strong source of radiation in the neighborhood -- Jupiter's magnetosphere. No one knows what's the time scale for transformation. There has been some lab work at temperatures less than 80K, but Europa's surface is warmer than that and the time scales are sure to be longer at higher temperatures."

If hexagonal ice is discovered, it will certainly spur new lab experiments that measure the transformation time, which will in turn set limits on the age of the hexagonal ice. In that way scientists will be able to "date" cryo-volcanic activity on Europa more precisely than ever before, and possibly draw definitive conclusions about modern-day Europan oceans.

Meanwhile, back on Earth....

Many scientists would love to travel to Europa to study conditions there first-hand, but regular flights to Jupiter probably won't begin for some time. Meanwhile, there is an environment right here on Earth with significant similarities to Europa, a place called Lake Vostok.

In 1974, a team of scientists conducting airborne research passed over the Soviet research station Vostok in Antarctica. Their sounding instruments detected an expanse of water beneath the ice roughly the size of Lake Ontario. Although Antarctica records some of the coldest temperatures on Earth, Lake Vostok is buried under four kilometers of ice. The ice sheet acts as a blanket, shielding the lake from cold temperatures on the surface. It is also thought that geothermal heat helps keep the water liquid.

Last year, Richard Hoover of NASA's Marshall Space Sciences Lab and Dr. S.S. Abyzov of the Russian Academy of Sciences used an Environmental Scanning Electron Microscope to examine ice cores from above Lake Vostok for evidence of microbiotic life. What they found surprised them.

"We've found some really bizarre things - things that we've never seen before," said Hoover. "There are all sorts of microorganisms in the ice. Some are readily recognizable as cyanobacteria, bacteria, fungi, spores, pollen grains, and diatoms, but some are not recognizable as anything we've ever seen before."

Hoover is about to begin new studies of the deep ice microorganisms in collaboration with Academician Mikhail V. Ivanov, Director of the Institute of Microbiology of the Russian Academy of Sciences to search for microorganisms in very deep samples of Vostok ice. The extremely deep ice samples from approximately 100 meters above the surface of the lake (about 400,000 years old) were obtained by an international team of US, Russian, and French scientists.

It's possible that Europa and Lake Vostok share a number of remarkable properties, including a kilometers-thick covering of ice over liquid water, an environment where life may have developed along unique evolutionary paths and subterranean waterways accessible to remote observation via radio sounding techniques and in situ observations by means of melting probes. A group of scientists and engineers from JPL, The University of Nebraska, and the Woods Hole Oceanographic Institute have begun a research program to identify technologies needed for the exploration of both Lake Vostok and Europa. This program benefits researchers interested in both areas - Vostok is very difficult to explore with current technology, but with the infusion of NASA technology a safe entry may be possible for the first time. At the same time, Europa explorers will benefit from experience obtained at Vostok and similar places. The program is studying designs for a cryobot vehicle that would penetrate a thick icy crust and a companion hydrobot vehicle that would explore subterranean waterways looking for signs of microbial life.

One of the highest mission priorities for any exploration of Lake Vostok or Europa is to avoid environmental contamination.

"Lake Vostok is an incredibly precious resource," Richard Hoover said in a Feb. 1999, interview, "and it would be a colossal mistake to take samples before it can be done without contaminating the waters with chemicals or surface microorganisms."

Lake Vostok is clearly a valuable new laboratory for astrobiologists, and they intend to proceed very carefully. Whatever we learn from this mysterious Antarctic lake will undoubtedly influence future missions to search for and explore the waterways of Europa.

Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997. The spacecraft is currently near the end of a two-year extended mission that will culminate in two daring flybys of volcanoes on Io later this year. More information about the Galileo mission is available at: http://www.jpl.nasa.gov/galileo/

JPL manages Galileo for NASA' s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.