Stormclouds above Cape Canaveral pushed back the launch of the space shuttle Endeavour on Thursday. NASA meteorologists had been predicting bad weather all week, so the scrub came as no surprise. The launch was pushed back to Friday evening, but even that launch date is at risk, as the stormy weather is expected to last through the weekend. When Endeavour does get into space, it will deliver Expedition 5 to the International Space Station, and return the previous crew back to Earth.
Endeavour Launch Time Announced
Space shuttle managers announced the launch time for mission STS-111 on Wednesday afternoon – Endeavour will launch from Cape Canaveral on Thursday evening at 2344 GMT (7:44pm EDT). Although it was known that the launch was scheduled for Thursday, the exact time was kept secret for security reasons. Endeavour will carry the crew of Expedition 5 to the International Space Station as well as install the new Mobile Remote Servicer Base System. The weather doesn’t look good though, and could very well scrub the launch.
Solar Flare Silences Japanese Mars Probe
A solar flare has struck Nozomi, Japan’s first probe to Mars, cutting off its communications with Earth. Japanese scientists believe that the spacecraft can repair the damage within 6-months, hopefully before it arrives at the Red Planet in December 2003. The solar flare happened over a month ago, but the damage to the spacecraft was only recently admitted to the public. Nozomi will study Mars’ upper atmosphere and magnetic field.
Spotlight on the 2MASS Sky Survey
Image credit: 2MASS
One of the most groundbreaking work in astronomy being done right now is the Two Micron All-Sky Survey which is producing a high-resolution survey of the entire sky in the Infrared spectrum. Two telescopes, one in Arizona, the other in Cerro Tololo, Chile have been working non-stop for 4 years to take over 100 million individual images; these have been stitched together by computer. Infrared light has a much longer spectrum than visible light, so the images reveal objects which would normally be obscured by thick clouds of gas and dust.
Would it be possible to see the entire sky without ever stepping outside? Well, if you have access to a computer, the answer would be ?yes,? thanks to the Two Micron All-Sky Survey (2MASS), the most detailed digital map of the heavens ever made.
“These telescopes have given us the first detailed global view of our Milky Way Galaxy and the galaxies that lie beyond,” said Dr. Michael Skrutskie of the University of Virginia, the survey?s principal investigator. “The resulting databases and source catalogues are a treasure trove which will be mined for discovery by scientists and the public alike for decades to come.”
A Huge Undertaking
This tremendous task was accomplished by dividing the sky into nearly 60 thousand strips, each covering roughly the area of a toothpick held at arm?s length. Two dedicated telescopes, one in Arizona, the other in Cerro Tololo, Chile, patiently scanned these strips of sky every night, weather permitting, for nearly four years. The telescopes were built and observations conducted by the University of Massachusetts at Amherst.
While observations concluded in February 2001, the massive data reduction efforts have continued at the Infrared Processing and Analysis Center on the Caltech campus.
The 2MASS software automated what would have taken astronomers decades to do using conventional techniques. The system processed over 100 million individual frames, stitching them together into larger images while simultaneously identifying and measuring the properties of all the stars and galaxies seen within them.
An Infrared Perspective
The ?Two Micron? part of the sky survey refers to the part of the spectrum viewed by the survey?s digital eyes. Near-infrared light has a wavelength about four times longer than visible light, well beyond the limits of human vision. The survey telescopes employed sophisticated electronic cameras cooled to temperatures not far above absolute zero to see into this part of the spectrum.
The cameras simultaneously captured the sky in three different colors of infrared light. By remapping these colors into the visible colors of red, green, and blue, astronomers have produced full color images of the sky that look similar to ordinary visible light images, but show different features. Even well-known objects like the Orion Nebula take on striking new appearances.
Near-infrared light offers several advantages to astronomers. It more easily penetrates clouds of dust like those found across the Milky Way, revealing stars and galaxies that are completely hidden in visible light. It is also more sensitive to the largest population of stars in the Galaxy, the ones that are smaller and cooler than the Sun. The Two Micron All-Sky Survey observations open up the universe for studies of previously unknown stars and lay bare the internal structures of distant galaxies.
The Galaxy Inside-Out
One stunning product of the survey is an ?inside-out? view of our own Milky Way Galaxy. Constructed from the database of half a billion stars automatically identified by the processing software, it gives an unparalleled census of the Milky Way?s geography and population.
Evident in striking detail is the flat disk, punctuated by thin lanes of dense dust clouds. Towards the center we find the galactic bulge surrounding the inner nucleus of the Milky Way, long thought to harbor a supermassive black hole.
The view also extends beyond just the local Milky Way stars. Just beneath and right of the galactic center we can see the star clouds associated with our nearest neighbors: the Large and Small Magellanic clouds. A very sharp eye can even pick out a faint ?finger? of stars in the lower left side of the galactic bulge; this is the first direct image ever made of a small dwarf galaxy recently discovered to be in the process of merging with the Milky Way.
Getting the Pictures
The Two Micron All-Sky Survey has opened up new views of the universe, literally allowing people to see the whole sky in a different light. And anyone with access to a web browser can enjoy the visual feast.
The catalogues and images are distributed freely to the astronomical community and the general public via the Internet. Nearly half of the sky is currently available, and processing is ongoing for the final data release, expected in late summer 2002.
When the final sets of catalogues and images come online, they will consist of about 2 terabytes (that’s 2,000 gigabytes, or 2 million megabytes!) of computer data.
The final release will include catalogues of about half a billion stars and 2 million galaxies. Over one million research-quality images of the whole sky will be available in each of the three infrared colors observed by 2MASS. Such huge data sets represent part of astronomy’s future, as scientists learn interesting new things by analyzing terabyte-sized sets of data.
Fortunately the astronomy enthusiast has more direct access to this fantastic sky imagery. A new ?2MASS Showcase? gallery contains a selection of the very finest full color images from the survey, some at resolutions suitable for printing full posters!
In addition, the existing Image Gallery and Picture of the Week archives contain hundreds of images of interesting objects throughout the Galaxy and beyond. Even the most demanding visitors should be able to find the infrared views of their favorite objects now that the sky is online for everyone to enjoy!
Original Source: NASA News Release
Galileo’s Last Look at Io
Image credit: NASA
The final images that the Galileo spacecraft will take of Jupiter’s moon Io were released today. They showcase crumbling crater slopes and the surface deposits from recent eruptions. Galileo also discovered 13 previously unknown hotspots on the moon’s surface, bringing the total number to 120; many more than anticipated. Galileo will make one final pass of another moon, Amalthea, before crashing into Jupiter in September, 2003.
The final images are in, and the resulting portrait of Jupiter’s moon Io, after a challenging series of observations by NASA’s Galileo spacecraft, is a peppery world of even more plentiful and diverse volcanoes than scientists imagined before Galileo began orbiting Jupiter in 1995.
Now that Galileo’s observations of Io have ended, scientists are focusing on trying to understand the big picture of how Io works by examining details.
Thirteen previously unknown active volcanoes dot infrared images from Galileo’s final successful flyby of Io, volcanologist Dr. Rosaly Lopes of NASA’s Jet Propulsion Laboratory reported today at the spring meeting of the American Geophysical Union in Washington, D.C.
That brings the total number of known Ionian hot spots to 120. Galileo images revealed 74 of them.
“We expected maybe a dozen or two,” said Dr. Torrence Johnson, Galileo project scientist at JPL in Pasadena, Calif. That expectation was based on discoveries by NASA’s Voyager spacecraft in 1979 and 1980, and subsequent ground-based observations.
“The volcanoes on Io have displayed an assortment of eruption styles, but recent observations have surprised us with the frequency of both giant plumes and crusted-over lakes of molten lava,” said planetary scientist Dr. Alfred McEwen of the University of Arizona, Tucson.
Galileo’s latest images, which also show tall slopes crumbling and surface deposits from two eruptions’ recent giant plumes, are available online from JPL at http://www.jpl.nasa.gov/images/io and from the University of Arizona Lunar and Planetary Laboratory at http://pirlwww.lpl.arizona.edu/Galileo/Releases .
Some high-resolution views taken as Galileo skimmed past Io on Oct. 16, 2001, are aiding analysis of the connection between volcanism and the rise and fall of mountains on Io. Few of Io’s volcanoes resemble the crater-topped volcanic peaks seen on Earth and Mars, said planetary scientist Dr. Elizabeth Turtle of the University of Arizona. Most of Io’s volcanic craters are in relatively flat regions, not near mountains, but nearly half of the mountains Io does have sit right beside volcanic craters.
“It appears that the process that drives mountain-building — perhaps the tilting of blocks of crust — also makes it easier for magma to get to the surface,” Turtle said. She showed a new image revealing that material slumping off a mountain named Tohil Mons has not piled up in a crater below, suggesting that the crater floor has been molten more recently than any landslides have occurred. Galileo’s infrared-mapping instrument has detected heat from the crater, indicating an active or very recent eruption.
From the analysis of Galileo’s observations, scientists are developing an understanding of how that distant world resurfaces itself differently than our world does.
“On Earth, we have large-scale lateral transport of the crust by plate tectonics,” McEwen said. “Io appears to have a very different tectonic style dominated by vertical motions. Lava rises from the deep interior and spreads out over the surface. Older lavas are continuously buried and compressed until they must break, with thrust faults raising the tall mountains. These faults also open new pathways to the surface for lava to follow, so we see complex relations between mountains and volcanoes, like at Tohil.”
“Io is a weird place,” Johnson said. “We’ve known that since even before Voyager, and each time Galileo has given us a close look, we get more surprises. Galileo has vastly increased our understanding of Io even though the mission was not originally slated to study Io.”
Extensions to Galileo’s original two-year orbital mission included six swings close to Io, where exposure to Jupiter’s intense radiation belts stresses electronic equipment on board the spacecraft. Researchers presented some results today from two Io encounters in the second half of 2001. Observations were not made successfully during Galileo’s final Io flyby, in January 2002, because effects of the radiation belts put the spacecraft into a precautionary standby mode during the crucial hours of the encounter.
Galileo will make its last flyby of a moon when it passes close to Amalthea, a small inner satellite of Jupiter, on November 5. No imaging is planned for that flyby. With fuel for altering its course and pointing its antenna nearly depleted, the long-lived spacecraft will then loop one last time away from Jupiter and perish in a final plunge into Jupiter’s atmosphere in September 2003.
Additional information about Galileo, Jupiter and Jupiter’s moons is available online at http://galileo.jpl.nasa.gov . JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA’s Office of Space Science, Washington, D.C.
Original Source: NASA/JPL News Release
Evidence of Vast Quantities of Water Ice on Mars
Image credit: NASA
As predicted last week, NASA scientists announced that they have discovered evidence of vast deposits of water ice under the rocky surface of Mars. Special detectors on the Mars Odyssey spacecraft have found strong signals of enough ice to fill up Lake Michigan. As we’ve found on Earth, wherever there’s water and heat, there’s life, so this is encouraging for the search for life on Mars. This is also encouraging for possible future human missions to the Red Planet, as astronauts will have easy access to water for drinking, as well as hydrogen and oxygen.
Using instruments on NASA’s 2001 Mars Odyssey spacecraft, surprised scientists have found enormous quantities of buried treasure lying just under the surface of Mars — enough water ice to fill Lake Michigan twice over. And that may just be the tip of the iceberg.
Images are available at http://www.jpl.nasa.gov/images/mars and http://mars.jpl.nasa.gov/odyssey.
“This is really amazing. This is the best direct evidence we have of subsurface water ice on Mars. We were hopeful that we could find evidence of ice, but what we have found is much more ice than we ever expected,” said Dr. William Boynton, principal investigator for Odyssey’s gamma ray spectrometer suite at the University of Arizona, Tucson.
Scientists used Odyssey’s gamma ray spectrometer instrument suite to detect hydrogen, which indicated the presence of water ice in the upper meter (three feet) of soil in a large region surrounding the planet’s south pole. “It may be better to characterize this layer as dirty ice rather than as dirt containing ice,” added Boynton. The detection of hydrogen is based both on the intensity of gamma rays emitted by hydrogen, and by the intensity of neutrons that are affected by hydrogen. The spacecraft’s high-energy neutron detector and the neutron spectrometer observed the neutron intensity.
The amount of hydrogen detected indicates 20 to 50 percent ice by mass in the lower layer. Because rock has a greater density than ice, this amount is more than 50 percent water ice by volume. This means that if one heated a full bucket of this ice-rich polar soil it would result in more than half a bucket of water.
The gamma ray spectrometer suite is unique in that it senses the composition below the surface to a depth as great as one meter. By combining the different type of data from the instrument, the team has concluded the hydrogen is not distributed uniformly over the upper meter but is much more concentrated in a lower layer beneath the top-most surface.
The team also found that the hydrogen-rich regions are located in areas that are known to be very cold and where ice should be stable. This relationship between high hydrogen content with regions of predicted ice stability led the team to conclude that the hydrogen is, in fact, in the form of ice. The ice-rich layer is about 60 centimeters (two feet) beneath the surface at 60 degrees south latitude, and gets to within about 30 centimeters (one foot) of the surface at 75 degrees south latitude.
“Mars has surprised us again. The early results from the gamma ray spectrometer team are better than we ever expected,” said Dr. R. Stephen Saunders, Odyssey’s project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “In a few months, as we get into martian summer in the northern hemisphere, it will be exciting to see what lies beneath the cover of carbon dioxide dry-ice as it disappears.”
“The signature of buried hydrogen seen in the south polar area is also seen in the north, but not in the areas close to the pole. This is because the seasonal carbon dioxide (dry ice) frost covers the polar areas in winter. As northern spring approaches, the latest neutron data indicate that the frost is receding, revealing hydrogen-rich soil below,” said Dr. William Feldman, principal investigator for the neutron spectrometer at Los Alamos National Laboratories, New Mexico.
“We have suspected for some time that Mars once had large amounts of water near the surface. The big questions we are trying to answer are, ‘where did all that water go?’ and ‘what are the implications for life?’ Measuring and mapping the icy soils in the polar regions of Mars, as the Odyssey team has done, is an important piece of this puzzle, but we need to continue searching, perhaps much deeper underground, for what happened to the rest of the water we think Mars once had,” said Dr. Jim Garvin, Mars Program Scientist, NASA Headquarters, Washington, D.C.
Another new result from the neutron data is that large areas of Mars at low to middle latitudes contain slightly enhanced amounts of hydrogen, equivalent to several percent water by mass. Interpretation of this finding is ongoing, but the team’s preliminary hypothesis is that this relatively small amount of hydrogen is more likely to be chemically bound to the minerals in the soil, than to be in the form of water ice.
JPL manages the 2001 Mars Odyssey mission for NASA’s Office of Space Science, Washington, D.C. Investigators at Arizona State University, Tempe; the University of Arizona, Tucson; and NASA’s Johnson Space Center, Houston, operate the science instruments. The gamma-ray spectrometer was provided by the University of Arizona in collaboration with the Russian Aviation and Space Agency, which provided the high-energy neutron detector, and the Los Alamos National Laboratories which provided the neutron spectrometer. Lockheed Martin Astronautics, Denver, developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL, a division of the California Institute of Technology in Pasadena.
Additional information about the 2001 Mars Odyssey and the gamma-ray spectrometer is available on the Internet at: http://mars.jpl.nasa.gov/odyssey/ and http://grs.lpl.arizona.edu.
Original Source: NASA/JPL News Release
Microwave View of the Universe’s Oldest Light
Image credit: NSF
Astronomers from the National Science Foundation and Caltech have created the most detailed images ever made of the oldest light emitted by the Universe. The team used the Cosmic Background Imager, an array of sensitive microwave detectors in the Chilean desert, to gather light that had traveled 14 billion years to reach the Earth; it shows us the Universe at only 300,000 years old, just as seeds of matter had started to form, eventually becoming galaxies, stars, planets, and us.
Astronomers operating from a remote plateau in the Chilean desert have produced the most detailed images ever made of the oldest light emitted by the universe, providing independent confirmation of controversial theories about the origin of matter and energy.
Pushing the limits of available technology, the Cosmic Background Imager (CBI) funded by the National Science Foundation (NSF) and California Institute of Technology (Caltech) detected minute variations in the cosmic microwave background, the radiation that has traveled to Earth over almost 14 billion years. A map of the fluctuations shows the first tentative seeds of matter and energy that would later evolve into clusters of hundreds of galaxies.
The measurements also provide independent evidence for the long-debated theory of inflation, which states that the universe underwent a violent expansion in its first micro-moments. After about 300,000 years it cooled enough to allow the seeds of matter to form and became “transparent,” allowing light to pass through. CBI observed remnants of that early radiation. The data are also helping scientists learn more about the repulsive force called “dark energy” that appears to defy gravity and force the universe to accelerate at an ever-increasing pace.
“This is basic research at its finest and most exciting,” said NSF Director Rita Colwell. “Each new image of the early universe refines our model of how it all began. Just as the universe grows and spreads, humankind’s knowledge of our own origins continues to expand, thanks to the technical expertise and patient persistence of scientists such as these.”
“We have seen, for the first time, the seeds that gave rise to clusters of galaxies, thus putting theories of galaxy formation on a firm observational footing,” said team leader Anthony Readhead of Caltech. “These unique high-resolution observations provide a new set of critical tests of cosmology, and provide new and independent evidence that the universe is flat and is dominated by dark matter and dark energy.”
Readhead, with Caltech colleagues Steve Padin and Timothy Pearson and others from Canada, Chile and the United States, generated the finest measurements to date of the cosmic microwave background. Cosmic microwave background (CMB) is a record of the first photons that escaped from the rapidly cooling, coalescing universe about 300,000 years after the cosmic explosion known as the Big Bang that is commonly believed to have given birth to the universe.
Data from the CBI on temperature distributions in the CMB support a modification of the Big Bang theory; that modification is called inflation theory. Inflation states that the hot plasma of the initial universe underwent an extreme and rapid expansion in its first 10 -32 second. The variations in temperature measured by the CBI are as small as 10 millionths of a degree.
By plotting the peaks of temperature distribution, the scientists showed that the precise CBI data are entirely consistent with inflation and confirm earlier findings by other scientists. In April 2000, an international team of cosmologists led by Caltech’s Andrew Lange announced the first compelling evidence that the universe is flat-that is, its geometry is such that parallel lines will neither converge or diverge. Lange’s team observed at a different frequency from CBI, using a high-altitude balloon flown over Antarctica.
Since then, two other teams — using independent methods — have revealed their analyses of the very faint variations in temperature among the cosmic microwaves. The four instruments have conducted precise measurements of parameters that cosmologists have long used to describe the early universe. Each set of data has offered new clues to the form of the embryonic plasma and has drawn scientists closer to definitive answers. NSF has supported the work of all four teams and their instruments, some of them for more than 15 years.
Five papers on the CBI data were submitted today to the Astrophysical Journal for publication.
The CBI consists of 13 interferometers mounted on a 6-meter-diameter platform, operating at frequencies from 26 GHz to 36 GHz. Located in the driest desert in the world — the Atacama — CBI takes advantage of the low humidity at an altitude of 5,080 meters (16,700 feet). NSF has supported the CBI research since 1995. The National Council of Science and Technology of Chile provided the CBI site.
Original Source: NSF News Release
Weather on Brown Dwarf Stars
Image credit: NASA
A team of astronomers from UCLA have found cloudy, stormy atmospheres on brown dwarfs – objects larger than gas giants like Jupiter, but not large enough to ignite into full stars. They believe the discovery of these storms could provide insights into some strange observations of brown dwarfs. Instead of steadily cooling, the objects have been seen to get brighter for brief periods, so this could be accounted for by breaks in the cloudy atmosphere.
For the first time, researchers have observed planet-like weather acting as a major influence on objects outside our solar system.
A team of scientists from NASA and the University of California, Los Angeles (UCLA), has found cloudy, stormy atmospheres on brown dwarfs, celestial bodies that are less massive than stars but that have more mass than giant planets like Jupiter. The discovery will give scientists better tools for interpreting atmospheres and weather on brown dwarfs or on planets around other stars.
“The best analogy to what we witness on these objects are the storm patterns on Jupiter,” said Adam Burgasser, astronomer at UCLA and lead author of the study. “But I suspect the weather on these more massive brown dwarfs makes the Great Red Spot look like a small squall.” Jupiter?s Great Red Spot is a massive storm more than 15,000 miles across and with winds of up to 270 miles per hour. Burgasser teamed up with planetary scientist Mark Marley, meteorologist Andrew Ackerman of NASA Ames Research Center in California’s Silicon Valley, and other collaborators to propose how weather phenomena could account for puzzling observations of brown dwarfs.
“We had been thinking about what storms might do to the appearance of brown dwarfs,? Marley said. “And when Adam showed us the new data, we realized there was a pretty good fit.” The team calculated that using a model with breaks or holes in the cloudy atmosphere solved the mysterious observations of cooling brown dwarfs.
Brown dwarfs, only recently observed members of the skies, are “failed stars at best,” said Ackerman. Not massive enough to sustain the burning of hydrogen like stars, brown dwarfs go through cooling stages that scientists observe with infrared energy-detecting telescopes. They appear as a faint glow, like an ember from a fire that gives off both heat and light energy as it dims.
Astronomers expected brown dwarfs, like most objects in the universe, to grow steadily fainter as they cool. However, new observations showed that during a relatively short phase brown dwarfs appear to get brighter as they cool. The explanation lies in the clouds.
At least 25,000 times fainter than the sun, brown dwarfs are still incredibly hot, with temperatures as high as 2,000 degrees Kelvin (3,140 F). At such high temperatures, things like iron and sand occur as gases. As brown dwarfs cool, these gases condense in the atmosphere into liquid droplets to form clouds, similar to water clouds on Earth. As the brown dwarf cools further, there is a rapid clearing of the clouds caused by atmospheric weather patterns. As the clouds are whisked away by the storms, bright infrared light from the hotter atmosphere beneath the clouds escapes, accounting for the unusual brightening of the brown dwarfs.
“The model developed by the group for the first time matches the characteristics of a very broad range of brown dwarfs, but only if cloud clearing is considered,” said Burgasser. “While many groups have hinted that cloud structures and weather phenomena should be present, we believe we have actually shown that weather is present and can be quite dramatic.”
By using Earth’s weather as a starting point, Ackerman helped the team work storms?that include wind, downdrafts and iron rain?into their calculations. “The astrophysicists needed some help understanding rain because it’s not an important process in most stars,? Ackerman said. “We used observations and simulations of terrestrial clouds to estimate the effect of iron rain on the thickness of an iron cloud.”
The team’s study, to be published in the June 1 issue of Astrophysical Journal Letters, will help researchers determine the make-up of atmospheres outside our solar system. “Brown dwarfs have traditionally been studied like stars, but it’s more of a continuum,” Marley said. “If you line a mug shot of Jupiter up with these guys, it is just a very low-mass brown dwarf.” Brown dwarfs are a training ground for scientists to learn how to interpret observations of planet-like objects around other stars, he said. “Everybody wants to find brown dwarfs that are even colder and have water clouds just like Earth. Once we find those, that will be a good test of our understanding.”
Original Source: NASA News Release
Europa Could be Very Thick Skinned
Image credit: NASA
The evidence is mounting that Europa, one of the moons of Jupiter, has an ocean of water covered by a sheet of ice. Scientists are now speculating about how thick that ice is by measuring the size and depth of 65 impact craters on the moon’s surface – from what they can tell, it’s 19 km. The thickness of Europa’s ice will have an impact on the possibility of finding life there: too thick and sunlight will have trouble reaching photosynthetic organisms.
Detailed mapping and measurements of impact craters on Jupiter?s large icy satellites, reported in the May 23, 2002, issue of the journal Nature, reveal that Europa?s floating ice shell may be at least 19 kilometers thick. These measurements, by Staff Scientist and geologist Dr. Paul Schenk, at Houston?s Lunar and Planetary Institute, indicate that scientists and engineers will have to develop new and clever means of searching for life on the frozen world with a warm interior.
The Great Europa Pizza Debate: “Thin Crust or Thick Crust?”
Geologic and geophysical evidence from Galileo support the idea that a liquid water ocean exists beneath the icy surface of Europa. The debate now centers on how thick this icy shell is. An ocean could melt through a thin ice shell only a few kilometers thick exposing water and anything swimming in it to sunlight (and radiation). A thin ice shell could melt through, exposing the ocean to the surface, and granting easy access of photosynthetic organisms to sunlight. A thick ice shell tens of kilometers thick would be very unlikely to melt through.
Why is the thickness of Europa’s icy shell important?
The thickness is an indirect measure of how much tidal heating Europa is getting. Tidal heating is important for estimating how much liquid water is on Europa and whether there is volcanism on Europa?s sea floor but it must be derived; it cannot be measured. The new estimate of a 19 kilometer thickness is consistent with some models for tidal heating, but requires much additional study.
The thickness is important because it controls how and where biologically important material in Europa’s ocean can move to the surface, or back down to the ocean. Sunlight cannot penetrate more than a few meters into the icy shell, so photosynthetic organisms require easy access to Europa’s surface to survive. More on this subject later.
The thickness will also ultimately determine how we can explore Europa’s ocean and search for evidence of any life or organic chemistry on Europa. We cannot drill or sample the ocean directly through such a thick crust and must develop clever ways to search for ocean material that may have been exposed on the surface.
How do we estimate the thickness of Europa?s ice shell?
This study of impact craters on the large icy Galilean satellites of Europa is based on a comparison of the topography and morphology of impact crater on Europa with those on its sister icy satellites Ganymede and Callisto. Over 240 craters, 65 of them on Europa, have been measured by Dr. Schenk using stereo and topographic analysis of images acquired from NASA?s Voyager and Galileo spacecraft. Galileo is currently orbiting Jupiter and heading toward its final plunge into Jupiter in late 2003. Although both Ganymede and Callisto are believed to have liquid water oceans inside, they are also inferred to be rather deep (roughly 100-200 kilometers). This means that most craters will be unaffected by the oceans and can be used for comparison with Europa, where the depth to the ocean is uncertain but likely to be much shallower.
The estimate of the thickness of Europa?s ice shell is based on two key observations. The first is that the shapes of Europa?s larger craters differ significantly from similar sized craters on Ganymede and Callisto. Dr. Schenk?s measurements show that craters larger 8 kilometers across are fundamentally differ from those on Ganymede or Callisto. This is due to the warmth of the lower part of the ice shell. The strength of ice is very sensitive to temperature and warm ice is soft and flows rather quickly (think glaciers).
The second observation is that morphology and shape of craters on Europa change dramatically as crater diameters exceed ~30 kilometers. Craters smaller than 30 kilometers are several hundred meters deep and have recognizable rims and central uplifts (these are standard features of impact craters). Pwyll, a crater 27 kilometers across, is one of the largest of these craters.
Craters on Europa larger than 30 kilometers, on the other hand, have no rims or uplifts and have negligible topographic expression. Rather they are surrounded by sets of concentric troughs and ridges. These changes in morphology and topography indicate a fundamental change in the properties of the icy crust of Europa. The most logical change is from solid to liquid. The concentric rings in large Europan craters are probably due to the wholesale collapse of the crater floor. As the originally deep crater hole collapses, the material underlying the icy crust rushes in to fill in the void. This inrushing material drags on the overlying crust, fracturing it and forming the observed concentric rings.
Where does the 19 to 25 kilometer value come from?
Larger impact craters penetrate more deeply into the crust of a planet and are sensitive to the properties at those depths. Europa is no exception. The key is the radical change in morphology and shape at ~30 kilometers crater diameter. To use this, we must estimate how big the original crater was and how shallow a liquid layer must be before it can affect the final shape of the impact crater. This is derived from numerical calculations and laboratory experiments into impact mechanics. This ?crater collapse model? is then used to convert the observed transition diameter to a thickness for the layer. Hence, craters 30 kilometers wide are sensing or detecting layers 19-25 kilometers deep.
How certain are these estimates of Europa?s ice shell thickness?
There is some uncertainty in the exact thickness using these techniques. This is due mostly to uncertainties in the details of impact cratering mechanics, which are very difficult to duplicate in the laboratory. The uncertainties are probably only between 10 and 20%, however, so we can be reasonably sure that Europa’s ice shell is not a few kilometers thick.
Could the ice shell have been thinner in the past?
There is evidence in the crater topography that the thickness of ice on Ganymede has changed over time, and the same might be true for Europa. The estimate for ice shell thickness of 19 to 25 kilometers is relevant to the icy surface we now see on Europa. This surface has been estimated to be 30 to 50 million years or so. Most surface materials older than this have been destroyed by tectonism and resurfacing. This older icy crust could have been thinner than today?s crust, but we presently have no way of knowing.
Could the ice shell on Europa have thin spots now?
The impact craters Dr. Schenk studied were scattered across Europa?s surface. This suggests that the ice shell is thick everywhere. There could be local areas where the shell is thin due to higher heat flow. But the ice at the base of the shell is very warm and as we see in glaciers here on Earth, warm ice flows fairly rapidly. As a result, any ?holes? in Europa?s ice shell will be filled in quickly by flowing ice.
Does a thick ice shell mean there is no life on Europa?
No! Given how little we know about the origins of life and conditions inside Europa, life is still plausible. The probable presence of water under the ice is one of the key ingredients. A thick ice shell makes photosynthesis highly unlikely on Europa. Organisms would not have rapid or easy access to the surface. If organisms inside Europa can survive without sunlight, then the thickness of the shell is of only secondary importance. After all, organisms do quite well on the bottom of Earth?s oceans quite well without sunlight, surviving on chemical energy. This could be true on Europa if it is possible for living organisms to originate in this environment in the first place.
Then too, Europa’s ice shell could have been much thinner in the distant past, or perhaps it didn’t exist at some point and the ocean was exposed naked to space. If that were true, then a variety of organisms could evolve, depending on chemistry and time. If the ocean began to freeze over, the surviving organisms could then evolve to whatever environments allowed them to survive, such as volcanoes on the ocean floor (if volcanoes form at all).
Can we explore for life on Europa if the ice shell is thick?
If the crust is indeed this thick, then drilling or melting through the ice with tethered robots would be impractical! Nonetheless, we can search for organic ocean chemistry or life in other locations. The challenge will be for us to devise a clever strategy for exploring Europa that won?t contaminate what is there yet find it nonetheless. The prospect of a thick ice shell limits the number of likely sites where we might find exposed oceanic material. Most likely, ocean material will have to be embedded as small bubbles or pockets or as layers within ice that has been brought to the surface by other geologic means. Three geologic processes could do this:
1. Impact craters excavate crustal material from depth and eject it out onto the surface, where we might pick it up (50 years ago we could pick up iron meteorite fragments on the flanks of Meteor Crater in Arizona, but most have been found by now). Unfortunately, the largest known crater on Europa, Tyre, excavated material from only 3 kilometers deep, not deep enough to get near the ocean (due to geometry and mechanics, craters excavate from the upper part of the crater, not the lower). If a pocket or layer of ocean material were frozen into the crust at shallow depth, it might be sampled by an impact crater. Indeed, the floor of Tyre has a color that is slightly more orange than the original crust. However, roughly half of Europa was well seen by Galileo, so a larger crater might be present on the poorly seen side. We will have to go back to find out.
2. There is strong evidence that Europa?s icy shell is somewhat unstable and has been (or is) convecting. This means that blobs of deep crustal material rise upward toward the surface where they are sometimes exposed as domes several kilometers wide (think Lava Lamp, except that the blobs are soft solid material like Silly Putty). Any ocean material imbedded within the lower crust could then be exposed to the surface. This process could take thousands of years, and the exposure to Jupiter?s lethal radiation would be unfriendly to say the least! But at least we could investigate and sample what remains behind.
3. Resurfacing of wide areas of Europa?s surface where the icy shell has literally torn through and split apart. These areas are not empty but have been filled with new material from below. These areas do not appear to have been flooded by ocean material, but rather by soft warm ice from the bottom of the crust. Despite this it is very possible that oceanic material could be found within this new crustal material.
Our understanding of Europa’s surface and history is still very limited. Unknown processes could occur that bring ocean material to the surface, but only a return to Europa will tell.
What next for Europa?
With the recent cancellation of a proposed Europa Orbiter due to cost overruns, this is a good time to reexamine our strategy for exploring Europa?s ocean. Tethered submarines and deep drilling probes are rather impractical in such a deep crust, but surface landers could be very important nonetheless. Before we send a lander to the surface, we should send a reconnaissance mission, in either Jupiter or Europa orbit, to search for exposures of ocean material and thin spots in the crust, and to scout out the best landing sites. Such a mission would make use of vastly improved infrared mapping capabilities for mineral identification (after all, the Galileo instruments are nearly 25 years old). Stereo and laser instruments would be used for topographic mapping. Together with gravity studies, these data could be used to search for relatively thin regions of the icy crust. Finally, Galileo observed less than half of Europa at resolutions sufficient for mapping, including impact craters. Craters on this poorly seen hemisphere, for example, could indicate whether Europa?s ice shell was thinner in the past.
A Lander for Europa?
A lander with a seismometer could listen for europa-quakes generated by the daily tidal forces exerted by Jupiter and Io. Seismic waves can be used to precisely map the depth to the bottom of the ice shell, and possibly the bottom of the ocean as well. Onboard chemical analyzers would then search for organic molecules or other biologic tracers and potentially determine ocean chemistry, one of the fundamental indicators of Europa?s prospects as an ?inhabited? planet. Such a lander would probably need to drill several meters to get through the zone of radiation damage at the surface. Only after these missions are under way can we then begin the true exploration of this tantalizing planet-sized moon. To paraphrase Monty Python, ?It?s not dead yet!?
Original Source: USRA News Release
Potential Discovery of Water Ice on Mars?
Space agency watchdog Keith Cowing is reporting that NASA is due to announce the discovery of large amounts of water ice on the surface of Mars. The speculation is that data from one or several instruments on board the Mars Odyssey spacecraft have confirmed the presence of underground ice, and that NASA will announce the findings at a press conference on Thursday, May 30. If true, the discovery of this much water ice will have tremendous implications on the search for life on the Red Planet. It’s all very preliminary right now, so stay tuned for some actual confirmation by NASA.