Thursday's Classroom, September 9, 1999: Life on the Edge

Can life exist in boiling water? Is the largest ocean in the solar system on one of Jupiter's moons? How many sled dogs does it take to carry 50 lb of microbes to a high mountain peak? This week's episode of Thursday's Classroom tackles these questions and more with a set of educational lesson plans and activities about nature's extremophiles.

NASA Space Science News for Sept. 9, 1999

Divining Water on Europa: Jupiter's icy moon Europa could harbor the solar system's largest ocean beneath its frigid crust. As circumstantial evidence for water continues to mount, JPL scientists are analyzing data from an ingenious experiment to find hexagonal ice crystals on Europa's surface. If they succeed, it could greatly strengthen the case for underground water on that alien world.

May 21, 1998: A day on Europa

APRIL 11, 1997


Galileo's close flyby of the Jovian moon Europa in February has provided solid evidence for liquid water or slush under at least part of the moon's ice crust. The best images show a broken jumble of giant slabs that have been "rafted" and tipped. Geologists estimate that all this could have taken place as recently as one million years ago. Overall, the scene looks like the disrupted pack ice found in arctic seas on Earth, and it's thought that a large reservoir of water lies as little as 1 or 2 km below the moon's surface. Calculations suggest the tides raised by Jupiter are heating Europa's interior, enough to keep water from freezing. In fact, there could be more liquid water under Europa's skin than on the entire Earth.

The combination of a subsurface ocean and continuous interior warmth has heightened speculation that life is -- or at least was -- possible in Europa's murky waters. Oceanographers point out that vast microbial colonies thrive on Earth under the sea floor and deep underground, completely cut off from the Sun. Europa seems to possess similar conditions. But Galileo is unlikely to settle whether life exists on Europa -- that will be the task for future exploration.


April 9, 1997


Chunky ice rafts and relatively smooth, crater-free patches on the surface of Jupiter's frozen moon Europa suggest a younger, thinner icy surface than previously believed, according to new images from Galileo's spacecraft released today.

The images were captured during Galileo's closest flyby of Europa on February 20, when the spacecraft came within 586 kilometers (363 miles) of the Jovian moon. These features, which lend credence to the idea of hidden, subsurface oceans, are also stirring up controversy among scientists who disagree about the age of Europa's surface.

Dr. Ronald Greeley, an Arizona State University geologist and Galileo imaging team member, said the ice rafts reveal that Europa had, and may still have, a very thin ice crust covering either liquid water or slush.

"We're intrigued by these blocks of ice, similar to those seen on Earth's polar seas during springtime thaws," Greeley said. "The size and geometry of these features lead us to believe there was a thin icy layer covering water or slushy ice, and that some motion caused these crustal plates to break up."

"These rafts appear to be floating and may, in fact, be comparable to icebergs here on Earth," said another Galileo imaging team member, Dr. Michael Carr, a geologist with the U.S. Geological Survey. "The puzzle is what causes the rafts to rotate. The implication is that they are being churned by convection."

The new images of Europa's surface have also sparked a lively debate among scientists. Galileo imaging team member Dr. Clark Chapman is among those who believe the smoother regions with few craters indicate Europa's surface is much younger than previously believed. In essence, Chapman, a planetary scientist at Southwest Research Institute, Boulder, CO, believes the fewer the craters, the younger the region. Chapman based his estimate on current knowledge about cratering rates, or the rate at which astronomical bodies are bombarded and scarred by hits from comets and asteroids.

"We're probably seeing areas a few million years old or less, which is about as young as we can measure on any planetary surface besides Earth," said Chapman. "Although we can't pinpoint exactly how many impacts occurred in a given period of time, these areas of Europa have so few craters that we have to think of its surface as young."

Chapman added, "Europa's extraordinary surface geology indicates an extreme youthfulness -- a very alive world in a state of flux."

However, Carr sees things differently. He puts Europa's surface age at closer to one billion years old.

"There are just too many unknowns," Carr said. "Europa's relatively smooth regions are most likely caused by a different cratering rate for Jupiter and Earth. For example, we believe that both Earth's moon and the Jovian moon, Ganymede, have huge craters that are 3.8 billion years old. But when we compare the number of smaller craters superimposed on these large ones, Ganymede has far fewer than Earth's moon. This means the cratering rate at Jupiter is less than the cratering rate in the Earth-moon system."

Scientists hope to find answers to some of the questions surrounding Europa and its possible oceans as the Galileo spacecraft continues its journey through the Jovian system.

"We want to look for evidence of current activity on Europa, possibly some erupting geysers," Greeley said. "We also want to know whether Europa's surface has changed since the Voyager spacecraft flyby in 1979, or even during the time of the Galileo flybys."

The craft will return for another Europa flyby on November 6, 1997, the final encounter of Galileo's primary mission. However, eight more Europa flybys are planned as part of Galileo's two-year extended mission, which will also include encounters with two other Jovian moons, Callisto and Io.

The Jet Propulsion Laboratory manages the Galileo mission for NASA's Office of Space Science, Washington, DC.

Images and other data received from Galileo are posted on the Galileo mission home page on the World Wide Web.

CNN: New pictures of Europa suggest ocean, possible life.

Astronomy Picture of the Day: Europa -- The Latest From Galileo.

NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA

January 17, 1997


Ice-spewing volcanoes and the grinding and tearing of tectonic plates have reshaped the chaotic surface of Jupiter's frozen moon Europa, images from NASA's Galileo spacecraft reveal.

The images, captured when Galileo flew within just 430 miles (692 kilometers) of Europa on Dec. 19, were released at a news briefing today at NASA Headquarters, Washington, DC.

Although the images do not show currently active ice volcanoes or geysers, they do reveal flows of material on the surface that probably originated from them, said Galileo imaging team member Dr. Ronald Greeley of Arizona State University, Tempe.

"This is the first time we've seen actual ice flows on any of the moons of Jupiter," said Greeley. "These flows, as well as dark scarring on some of Europa's cracks and ridges, appear to be remnants of ice volcanoes or geysers."

The new images appear to enhance Europa's prospects as one of the places in the Solar System that could have hosted the development of life, said Greeley.

"There are three main criteria to consider when you are looking for the possibility of life outside the Earth -- the presence of water, organic compounds and adequate heat," said Greeley. "Europa obviously has substantial water ice, and organic compounds are known to be prevalent in the Solar System. The big question mark has been how much heat is generated in the interior.

"These new images demonstrate that there was enough heat to drive the flows on the surface. Europa thus has a high potential to meet the criteria for exobiology," Greeley added.

"This doesn't prove that there is an ocean down there under the surface of Europa, but it does demonstrate that it is a scientifically exciting place," said Galileo imaging team member Dr. Robert Sullivan, also of Arizona State University.

The images also reveal a remarkable diversity in the geological age of various regions of Europa's surface. Some areas appear relatively young, with smooth, crater-free terrain, while others contain large craters and numerous pits, suggesting that they are much older.

The icy crust bears the signs of having been disrupted by the motion of tectonic plates. "There appear to be signs of different styles of tectonism," said Greeley. "In many areas we see that the crust was pulled apart in a spreading similar to the processes on the sea floor on Earth. This is different from the tectonic processes at work on, say, Jupiter's moon Ganymede. This suggests that Europa's interior may be different from Ganymede's."

Galileo scientists will have a better chance to understand Europa's interior when the spacecraft gathers gravity data on another flyby next November. The gravity field is measured by tracking how the frequency of Galileo's radio signal changes as it flies past the moon. This was not possible during the recent flyby because radio conditions were degraded as Jupiter passed behind the Sun from Earth's point of view.

Europa is crisscrossed by an amazingly complex network of ridges, according to Sullivan. "Ridges are visible at all resolutions," he explained. "Closely paired ridges are most common. With higher resolution, ridges seen previously as singular features are revealed to be double."

Some of the ridges may have formed by tension in the icy crust: as two plates pull apart slightly, warmer material from below might push up and freeze to form a ridge. Other ridges may have been formed by compression: as two plates push together, the material where they meet might crumple to form the ridge.

In addition to ice flows and tectonics, Greeley and Sullivan noted that some areas on Europa seem to have been modified by unknown processes that scientists are still debating. Greeley said that some areas, for example, seem to have been modified by "sublimation erosion" -- the evaporation of water and other volatiles such as ammonia and methane into the vacuum of space. "Something is destroying the topography," said Greeley, "and this sublimation erosion is a good candidate for what is at work."

During last month's encounter, Galileo flew more than 200 times closer to Europa than the Voyager 2 spacecraft did in 1979. After a swing past Jupiter next week in what mission engineers call a "phasing orbit," Galileo's next targeted flyby will take it again past Europa as it passes within 364 miles (587 kilometers) on Feb. 20.

The Jet Propulsion Laboratory, Pasadena, CA, manages the Galileo mission for NASA's Office of Space Science, Washington, DC.


Launched in October 1989, Galileo entered orbit around Jupiter on December 5, 1995, and is currently in the middle of its 2 year orbital tour around the solar system's largest planet. Galileo's orbital tour consists of 11 elliptical orbits around Jupiter, and each orbit (except one) will include a close flyby and gravity assist of one of the Galilean moons (Ganymede, Callisto or Europa). In addition to closeup observations of a Galilean moon, distant scientific encounters with additional satellites are scheduled and Io will be observed at medium range on every orbit.

The first three orbits have been completed and the next close encounter is scheduled for Europa on December 19, 1996 06:53 UT (or December 18 at 22:53 PST). With a diameter of 3,138 km, Europa is slighty smaller than our own Moon and is the smoothest object in the solar system. Galileo will attempt to find evidence of a liquid ocean underneath Europa's icy crust and look for signs of active volcanism on the moon's young surface. Galileo's closest approach to Europa will be 698 km, which is nearly 300 times closer than Voyager's closest approach. Also scheduled for the Europa 4 encounter are observations of the other satellites of Jupiter: Io, Amalthea, Thebe and Adrastea. Observations of Jupiter of also planned.

NASA Headquarters, Washington, DC
Jet Propulsion Laboratory, Pasadena, CA

August 13, 1996


Tantalizing new images of Jupiter's moon Europa from NASA's Galileo spacecraft indicate that "warm ice" or even liquid water may have existed, and perhaps still exists today beneath Europa's cracked icy crust.

The Europa results are one of several new Galileo findings, including an image of a huge erupting geyser-like volcano on Jupiter's moon Io and new information about Jupiter's Great Red Spot, released today in a news briefing at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

Galileo scientists are poring over images that show places on Europa resembling ice floes in Earth's polar regions, along with suggestions of geyser-like eruptions and details of long dark bands centered with white stripes that stretch like interstate highways across Europa's face.

"This moon is a marvelous place," said Dr. Ronald Greeley, a Galileo imaging team scientist and a geologist at Arizona State University, Tempe, AZ. "We're seeing evidence of a lot of geological activity on Europa."

"In some areas, the ice is broken up into large pieces that have shifted away from one another, but obviously fit together like a jigsaw puzzle," said Greeley. "This shows the ice crust has been or still is lubricated from below by warm ice or maybe even liquid water."

The results bring scientists a step closer to determining whether Europa has environmental "niches" warm enough and wet enough to meet the requirements to host life, Greeley said.

Europa is about the size of Earth's Moon and is covered largely with smooth white and brownish-tinted ice, instead of large craters like so many other bodies in the Solar System. Scientists believe its cracked cue-ball appearance is due to stressing caused by the contorting tidal effects of Jupiter's strong gravity. They speculate that the warmth generated by tidal heating may have been sufficient to soften or even liquefy some portion of Europa's icy covering.

Europa has long been considered by scientists and celebrated in science fiction as one of the handful of places in the Solar System (along with Mars and Saturn's moon Titan) that could possess an environment where primitive forms of life could possibly exist.

"A major goal of Galileo's studies of Europa is to search for signs of current or past activity to help answer the question: Is there a liquid zone on Europa?" said Greeley. "We are interested in identifying the time and places on Europa where liquid water might exist. We want to go back to some of these areas that suggest soft ice or liquid water under the ice and test some of the questions we're asking now."

The current images, taken from a distance of about 95,700 miles (155,000 kilometers), show features about one mile across (1.6 kilometer per pixel resolution). Moon flybys later in the mission will bring the Galileo spacecraft to within 370 miles (600 kilometers) of Europa's surface. During those flybys, the best resolution from the camera will average about 72 to 98 feet (22 to 30 meters- per-pixel) and as fine as 36 feet (11 meters) per pixel, so that objects the size of buildings on Earth could be discerned, Greeley said.

Galileo's close flybys of Europa will occur Dec. 19, 1996, Feb. 20, 1997 and Nov. 6, 1997. Additional non-flyby observations will be made during this September and November, and in April, June and September of 1997.

Galileo's detailed images are shedding new light on the nearly global, highway-like stripes on Europa that scientists call "triple bands" because of their dark-bright-dark appearance. Originally discovered in data from NASA's Voyager spacecraft in 1979, the cracks are thought to reflect tidal stressing in Europa's icy crust. "The scale of fracture patterns -- extending a distance equivalent to the width of the western United States -- dwarf the San Andreas fault in length and width," said Greeley.

Planetary geologists have proposed several models that could be responsible for creating the banded roadway look of these features. One set of models calls for combinations of tectonic faulting and flooding caused by liquid water or warm ice mixed with darker silicates that well up through cracks and then freeze over.

Galileo scientists say the new data suggest another model where "dirty geysers" erupt along a line, ejecting a mixture of ice and darker silicate debris along the surface. This event may be followed by a more gentle, continuous flow of cleaner water ice that paints the white stripe down the center of the feature.

In other mission results, one new Galileo image of the moon Io shows a huge new blue-colored volcanic plume extending about 60 miles (about 100 kilometers) into space. Scientists believe the blue color of the plume coming from the feature, called Ra Patera, is probably due to tiny particles of sulfur dioxide "snow" that condense from the gas as the plume expands and cools.

"This is very different from what we see with volcanic eruptions on Earth," said Galileo project scientist Dr. Torrence V. Johnson of JPL. "Terrestrial eruptions cannot throw materials to such high altitudes. We believe that on Io we are seeing geyser-like eruptions that are driven by sulfur dioxide or sulfur gas that erupts and freezes in Io's extremely tenuous atmosphere."

Galileo images have also shown that the Ra Patera plume glows in the dark, perhaps due to the fluorescence of sulfur and oxygen ions created by the breaking apart of sulfur dioxide molecules by energetic particles in the Jovian magnetosphere.

Comparing the Galileo Io images with Voyager images taken 17 years ago, scientists have found that Ra Patera is the site of dramatic surface changes. An area around the volcano about the size of the state of New Jersey (about 40,000 square kilometers) has been covered by new volcanic deposits since 1979.

Fresh Galileo images of Jupiter's Great Red Spot show new detail in the hurricane, which has been observed for at least 300 years. Winds blow counterclockwise around the Great Red Spot at about 250 miles per hour (400 kilometers per hour). The size of the storm is more than one Earth diameter (8,000 miles or 13,000 kilometers) in the north-south direction and more than two Earth diameters in the east-west direction. Galileo images are allowing scientists to detect varying altitudes of clouds within and surrounding the storm, and are showing new details in the structure of the giant storm.

Galileo's next flyby of a moon of Jupiter occurs Sept. 6, 1996, when the spacecraft will come within 155 miles (250 kilometers) of the surface of Ganymede -- the largest moon in the Solar System. The spacecraft accomplished its first encounter of Ganymede on June 27 and found remarkable tectonic features on the big moon. September's flyby of Ganymede will be about 370 miles (about 600 kilometers) closer than Galileo's first encounter of that moon in the previous flyby, when the spacecraft flew 531 miles (855 kilometers) above the surface.

The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC.


August 12, 1996

The first Galileo image of Europa has been released on the Galileo home page.

The caption file for the Europa image is appended below.


Europa's Broken Ice

Jupiter's moon Europa, as seen in this image taken June 27, 1996 by NASA's Galileo spacecraft, displays features in some areas resembling ice floes seen in Earth's polar seas. Europa, about the size of Earth's moon, has an icy crust that has been severely fractured, as indicated by the dark linear, curved, and wedged-shaped bands seen here. These fractures have broken the crust into plates as large as 30 kilometers (18.5 miles) across. Areas between the plates are filled with material that was probably icy slush contaminated with rocky debris. Some individual plates were separated and rotated into new positions. Europa's density indicates that it has a shell of water ice as thick as 100 kilometers (about 60 miles), parts of which could be liquid. Currently, water ice could extend from the surface down to the rocky interior, but the features seen in this image suggest that motion of the disrupted icy plates was lubricated by soft ice or liquid water below the surface at the time of disruption.

This image covers part of the equatorial zone of Europa and was taken from a distance of 156,000 kilometers (about 96,300 miles) by the solid-state imager camera on the Galileo spacecraft. North is to the right and the sun is nearly directly overhead. The area shown is about 360 by 770 kilometers (220-by-475 miles or about the size of Nebraska), and the smallest visible feature is about 1.6 kilometers (1 mile) across.

NASA Headquarters, Washington, DC

August 13, 1996


"These fantastic new images of an icy moon of Jupiter are reminiscent of the ice-covered Arctic Ocean on our planet. The lack of craters, the cracks and signs of movement, all indicate that this might be young ice on a dynamic surface. It raises the possibility of a liquid ocean on Europa, the only other place in our solar system where we suspect such an ocean might exist.

"These images, dramatic as they are, are not the best Galileo will provide. These are distant snapshots taken during Galileo's encounter with a different moon, Ganymede. We'll shoot a whole photo album when Galileo takes its targeted pass at Europa in December.

"We're not going to jump the gun. These pictures do not prove the existence of liquid water on Europa, and the higher-resolution pictures yet to come may not prove it. A few days ago, I greeted the possibility of ancient microbial life on Mars with skeptical optimism, and invited further scientific examination and debate. I greet the new pictures of Europa in the same light.

"The pictures are exciting and compelling, but not conclusive. The potential for liquid water on Europa is an intriguing possibility, and another step in our quest to explore the solar system, the stars, and the answer to the great mystery of whether life exists anywhere else in the cosmos.

"We won't wait for all the answers. We'll release the data as soon as it's available, and share the excitement of discovery not only with scientists, but with the American public, with educators, and especially with children. This is their space program, the American space program, and they should share in the awe and wonder of exploration.

"Once again, NASA will ask the scientific process to work. We'll ask the best minds in the world to analyze these pictures, and the pictures from our targeted pass at Europa. Then we'll ask the scientific community to suggest the best way to follow up on these fascinating findings. As always, NASA will seek to continue to expand knowledge, and find the fastest, best and most efficient ways to further this research."

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