JUPITER'S RINGS FORMED BY DUST BLASTED OFF SMALL MOONS, SCIENTISTS SAY
Jupiter's intricate, swirling ring system is made up of dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons, said scientists studying data from NASA's Galileo spacecraft. Images sent by Galileo also reveal that the outermost ring is actually composed of two rings, one embedded within the other.
"We now know the source of Jupiter's ring system and how it works," said Cornell University astronomer Joseph Burns, who reported on the first detailed analysis of a planet's ring system, along with Maureen Ockert-Bell and Joseph Veverka of Cornell, and Michael Belton of the National Optical Astronomy Observatories.
"Rings are important dynamical laboratories to look at the processes that probably went on billions of years ago when the solar system was forming from a flattened disk of dust and gas," Burns explained. "I expect we will see similar processes at Saturn and the other giant planets."
In the late 1970s, NASA's two Voyager spacecraft first revealed the structure of Jupiter's rings: a flattened main ring and an inner, cloudlike ring called the halo, both composed of small, dark particles. One Voyager image seemed to indicate a third, faint outer ring. New Galileo data reveal that this third ring, known as the gossamer ring because of its transparency, consists of two rings. Both rings are composed of microscopic debris from two small moons, Amalthea and Thebe.
"The structure of the gossamer rings was totally unexpected," Belton added. "These images provide one of the most significant discoveries of the entire Galileo imaging experiment."
Galileo took three dozen images of the rings and small moons during three orbits of Jupiter in 1996 and 1997. The four moons display "bizarre surfaces of undetermined composition that appear very dark, red, and heavily cratered from meteoroid impacts," Veverka said. The rings contain very tiny particles resembling dark, reddish soot. Unlike Saturn's rings, there are no signs of ice in Jupiter's rings.
Scientists believe that dust is kicked off the small moons when they are struck by interplanetary meteoroids, or fragments of comets and asteroids, at speeds greatly magnified by Jupiter's huge gravitational field. The small moons are particularly vulnerable targets because of their relative proximity to the giant planet.
"In these impacts, the meteoroid is going so fast it buries itself deep in the moon, then vaporizes and explodes, causing debris to be thrown off at such high velocity that it escapes the satellite's gravitational field," Burns said. If the moon is too big, dust particles will not have enough velocity to escape the moon's gravitational field. With a diameter of just 5 miles (8 kilometers) and an orbit that lies just at the periphery of the main ring, tiny Adrastea is "most perfectly suited for the job."
As dust particles are blasted off the moons, they enter orbits much like those of their source satellites, both in their size and in their slight tilt relative to Jupiter's equatorial plane. A tilted orbit wobbles around a planet's equator, much like a hula hoop twirling around a person's waist. This close to Jupiter, orbits wobble back and forth in only a few months.
Additional images and information are available at the Galileo website.
PATHFINDER DATA POINTS TO LONG DRY PERIOD ON MARS
A year after the landing of Mars Pathfinder, mission scientists say that data from the spacecraft paint two strikingly different pictures of the role of water on the Red Planet, and yield surprising conclusions about the composition of rocks at the landing site.
"Many of the things that we said last summer during the excitement after the landing have held up well," said Matthew Golombek, Pathfinder project scientist at NASA's Jet Propulsion Laboratory. "But we have now had more time to study the data and are coming up with some new conclusions."
Similar to ongoing science results from NASA's Mars Global Surveyor spacecraft currently in orbit around Mars, Pathfinder data suggest that the planet may have been awash in water 3-4.5 billion years ago. The immediate vicinity of the Pathfinder landing site, however, appears to have been dry and unchanged for the past 2 billion years.
Several clues from Pathfinder data point to a wet and warm early history on Mars, according to Golombek. Magnetized dust particles and the possible presence of rocks that are conglomerates of smaller rocks, pebbles, and soil suggest copious water in the distant past. In addition, the bulk of the landing site appears to have been deposited by large volumes of water, and the hills on the horizon known as Twin Peaks appear to be streamlined islands shaped by water.
But Pathfinder images also suggest that the landing site is essentially unchanged since catastrophic flooding sent rocks tumbling across the plain 2 billion years ago. "Since then this locale has been dry and static," Golombek said.
While the area appears to have been untouched by water for eons, wind appears to have been steadily eroding rocks at the landing site. Analysis of Pathfinder images shows that about 1-2 inches (3-5 centimeters) of material has been stripped away from the surface by wind, Golombek noted.
"Overall, this site has experienced a net erosion in recent times," said Golombek. "There are other places on Mars that are net `sinks,' or places where dust ends up being deposited. Amazonis Planitia, for example, probably has about 3-6 feet (1-
2 meters) of fine, powdery dust that you would sink into if you stepped on it."
Chemical analysis of a number of rocks by the alpha proton X-ray spectrometer (APXS) instrument on Pathfinder's mobile Sojourner rover, meanwhile, reveals an unexpected composition that scientists are still trying to explain.
The current assessment of data from this instrument suggests that all the rocks studied by the rover resemble a type of volcanic rock with a high silicon content known on Earth as andesite, covered with a fine layer of dust. All the rocks appear to be far different chemically from meteorites discovered on Earth that are believed to have come from Mars.
"The APXS tells us that all of these rocks are the same thing with different amounts of dust on them," said Golombek. "But images suggest that there are different types of rocks. We don't yet know how to reconcile this."
When molten magma oozes up from a planet's mantle onto the surface of the outer crust, it usually freezes into igneous rock of a type that geologists call a basalt. This is typical on the floors of Earth's oceans, as well as on the maria or "seas" of the Moon and in many regions of Mercury and Venus. By contrast, andesites typically form on Earth in tectonically active regions when magma rises into pockets within the crust, where some of its iron- and magnesium-rich components are removed, leaving rock with a higher silicon content. "We don't believe that Mars has had plate tectonics, so these andesites must have formed by a different mechanism," Golombek said.
The rocks studied by Pathfinder most closely resemble andesites found in Iceland and the Galapagos Islands, tectonic spreading centers where plates are being pushed apart, said Joy Crisp of JPL. Andesites from these areas have a different chemical signature from andesites formed at subduction zones (areas where one edge of a crustal plate descends below another), mostly because wet ocean sediments carry more water down into the mantle at the subduction zones. "On Mars, where the water content is probably lower and there is no evidence of subduction, we would expect a closer chemical similarity to Iceland andesites," said Crisp.
"In any event, the presence of andesites on Mars is a surprise, if it is borne out as we study the data further," said Crisp. "Most rocks on Mars are expected to be basalts lower in silicon. If these are in fact andesites, they are probably not very abundant."
MARTIAN MOON PHOBOS HIP-DEEP IN POWDER
New temperature data and close-up images of the martian moon Phobos gathered by NASA's Mars Global Surveyor indicate the surface of this small body has been pounded into powder by eons of meteoroid impacts, some of which started landslides that left dark trails marking the steep slopes of giant craters.
New temperature measurements show the surface must be composed largely of finely ground powder at least 3 feet (about 1 meter) thick, according to scientists studying infrared data from the Thermal Emission Spectrometer instrument on the spacecraft. Measurements of the day and night sides of Phobos show such extreme temperature variations that the sunlit side of the moon rivals a pleasant winter day in Chicago, while only a few kilometers away, on the dark side of the moon, the climate is more harsh than a night in Antarctica. High temperatures for Phobos were measured at 25°F (-4°C) and lows at -170°F (-112°C).
The extremely fast heat loss from day to night as Phobos turns in its seven-hour rotation can be explained if hip-deep dust covers its surface, said Philip Christensen of Arizona State University, Tempe, principal investigator for the experiment on the Mars Global Surveyor spacecraft.
"The infrared data tells us that Phobos, which does not have an atmosphere to hold heat in during the night, probably has a surface composed of very small particles that lose their heat rapidly once the Sun has set," Christensen said. "This has to be an incredibly fine powder formed from impacts over millions of years, and it looks like the whole surface is made up of fine dust."
New images from the spacecraft's Mars Orbiter Camera show many never-before-seen features on Phobos, the innermost and larger of the planet's two moons, and are among the highest-resolution pictures ever obtained of the rocky martian satellites. A 6-mile (10-kilometer)-diameter crater called Stickney, which is almost half the size of Phobos itself, shows light and dark streaks trailing down the slopes of the bowl, illustrating that even with a gravity field only about 1/1000 that of the Earth's, debris still tumbles downhill. Large boulders appear to be partly buried in the surface material.
Additional images and measurements are available at http://photojournal.jpl.nasa.gov/.
NEPTUNE'S LARGEST MOON MAY BE WARMING UP
Observations obtained by NASA's Hubble Space Telescope and groundbased instruments reveal that Neptune's largest moon, Triton, seems to have heated up significantly since the Voyager spacecraft visited it in 1989. "Since 1989, at least, Triton has been undergoing a period of global warming percentage-wise, it's a very large increase," said James L. Elliot, an astronomer at the Massachusetts Institute of Technology.
The warming trend is causing part of Triton's frozen nitrogen surface to turn into gas, thus making its thin atmosphere denser. Elliot and his colleagues from MIT, Lowell Observatory, and Williams College published their findings in the June 25 issue of Nature.
Even with the warming, no one is likely to plan a summer vacation on Triton, which is a bit smaller than Earth's moon. The 5% increase means that Triton's temperature has risen from about 37° K (-392°F) to about 39°K (-389°F). If Earth experienced a similar change in global temperature over a comparable period, it could lead to significant climatic changes.
Triton, however, is a very different and simpler world than Earth, with a much thinner atmosphere, no oceans, and a surface of frozen nitrogen. But the two share some contributing factors to global warming, such as changes to the Sun's heat output, how much sunlight is absorbed and reflected by their surfaces, and the amount of methane and carbon monoxide (greenhouse gases) in the atmosphere.
"With Triton, we can more easily study environmental changes because of its simple, thin atmosphere," Elliot explained. By studying these changes on Triton, the scientists hope to gain new insight into Earth's more complicated environment.
Elliot and his colleagues explain that Triton's warming trend may be driven by seasonal changes in its polar ice caps. Triton is approaching an extreme southern summer, a season that occurs every few hundred years. During this special time, the moon's southern hemisphere receives more direct sunlight, which heats the polar ice caps. "For a northern summer on Earth, it would be like the Sun being directly overhead at noon north of Lake Superior," Elliot said.
The scientists are basing a rise in Triton's surface temperature on the Hubble telescope's detection of an increase in the moon's atmospheric pressure, which has at least doubled in bulk since the time of the Voyager encounter. Any nitrogen ice on Triton that warms up a little results in a considerable leap in atmospheric pressure as the vaporized nitrogen gas joins the atmosphere. Because of the unusually strong link between Triton's surface ice temperature and its atmospheric pressure, Elliot says scientists can infer a temperature rise of 3°F over nine years.
Elliot and his colleagues list two other possible explanations for Triton's warmer weather. Because the frost pattern on Triton's surface may have changed over the years, it may be absorbing a little more of the Sun's warmth. Alternatively, changes in reflectivity of Triton's ice may have caused it to absorb more heat.