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March 3 -- March 16
Friday, March 16
Scientists continue to try to understand what created the gullies on Mars. Some think it may not be water after all. Courtesy NASA/JPL/Malin Space Science Systems, Inc. Click on image for larger view. Since the announcement of gullies on Mars last summer by Michael C. Malin and Kenneth Edgett (Malin Space Science Systems), planetary geologists have been trying to determine what could possibly create the features. Malin and Edgett believe the most likely cause was water seeping from between layers of rock exposed on cliff faces and crater rims - even though water is unstable there due to the Mars's low atmospheric pressure and cold temperatures. At this year's Lunar and Planetary Science Conference in Houston, Texas, some alternatives were offered to explain how the gullies may have formed. Pascal Lee (NASA/Ames Research Center) believes the water comes from outside, not inside, the rock layers. He draws an analogy to Devon Island in arctic Canada where generations of gullies reside on valley walls, and the slopes strongly resemble the Mars Global Surveyor images. On Devon Island, the snow and ice that accumulates in these crevices during winter are the last to melt in summer, trickling down the slope and gradually enlarging the gullies. Moreover, the late-melting ice packs are commonly seen on slopes facing away from the Sun, matching the orientation of most of the Martian gullies. Another conference presentation argued that the gullies are the work of frozen carbon dioxide ("dry ice") rather than water. According to Nick Hoffman (La Trobe University), when a winter veneer of CO2 "snow" warms in the spring, it begins to vaporize at its base. This cushion of gas acts as a lubricant, allowing the dry-ice slab to cascade down the slope in a tumble of fluidized rock and gas resembling a mudflow. This theory also allows for present-day gully formation in the polar regions, when CO2 frost accumulates each winter. However, Edgett notes that many of the gullies appear to begin at a specific layer below the surface, implying that the source comes from within. Even so, he admits that the notion of water seeping from the rock walls is hardly the ideal solution. "I wish somebody would find a way to do these things dry," he says. - David Tytell and J. Kelly Beatty Friday, March 16
The brightest star shining near Mars and Antares is Delta Scorpii, now in its eighth month of an unexpected flareup. Photographs deemphasize the brightness differences between stars, but to the naked eye Delta is now clearly the brightest star in the nearly vertical row of three forming the head of Scorpius. Jimmy Westlake shot this picture from the Colorado Rockies on the morning of March 4th using a 35-mm lens at f/3.3 for a 5-minute guided exposure on ISO 400 film. Observers this week will find Mars a little farther left. Click on image for larger view. As Mars brightens and swings toward Earth it has been attracting skywatchers out under the predawn sky - and many of them have noticed an added attraction in the vicinity. The 2nd-magnitude star Delta Scorpii is now plainly the brightest star in the area after Antares. Normally magnitude 2.3, Delta slowly flared up last July and fluctuated last fall. It is currently shining at about magnitude 1.8, more than half again its normal brightness, noticeably changing the look of the head of Scorpius. Delta is a hot giant star of spectral type B0. It is apparently undergoing a long-term, Gamma-Cassiopeiae-type eruption and mass ejection. Compare it with Beta Scorpii, magnitude 2.6, and Antares, magnitude 1.1. If it stays bright for a few more months it will bring an altered Scorpius into the skies of summer evenings. - Alan MacRobert Friday, March 16
By combining the light from the two 10-meter Keck telescopes atop Mauna Kea, Hawaii, astronomers hope to achieve 0.001-arcsecond optical resolution. Courtesy W. M. Keck Observatory. Click on image for larger view. On March 13th, astronomers successfully combined light from the two largest telescopes in the world, the 10-meter Keck telescopes on Mauna Kea, Hawaii. The event, called "first fringe," celebrated the moment when starlight, captured by two telescopes 85 meters apart, was combined into a single image such that the detected wavelengths of light were perfectly aligned. This process, known as stellar interferometry, is nothing new, but it has never been attempted on such a large scale with visible light. It is now possible to achieve a resolution of 2 to 5 milliarcseconds, an order of magnitude better than a single Keck telescope could reach even using its adaptive-optics system. "We have now entered the era of extremely high resolution astronomy," says James W. Beletic, deputy director of Keck. But the technical demonstration of Keck's prowess is just beginning. Astronomers intend to build four 1.8-meter telescopes (perhaps six in the future), called outriggers, which will give the Kecks a second dimension of resolution. Right now the two Kecks provide only one baseline, so astronomers can resolve just one dimension in the sky. According to Keck director Frederic H. Chaffee, "We're probably looking at three years from now before the full Keck array with two Keck telescopes and four to six outrigger telescopes are actually combined." The success of first fringe is good news for NASA too, which is currently planning the Space Interferometer Mission and Terrestrial Planet Finder - both interferometers that will operate in space. In fact, NASA became a partner in Keck five years ago specifically to develop the techniques needed to execute these missions. But despite the success, the Keck interferometer is a long way from resolving planets around stars or making other similar measurements. First fringe is only a milestone, not a conclusion. "Now begins the months of testing and refining to turn this mode from a technical 'stunt' into a productive scientific capability," says Chaffee. - Stephen James O'Meara Thursday, March 15
Did medieval residents of Canterbury, England, really witness the creation of a large crater on the Moon in June 1178? The brightly rayed crater Giordano Bruno at upper left has been suspected to be the result of such an impact, however, recent studies have put this scenario into doubt. Apollo 8 photograph courtesy NASA. Click on image for larger view. Were a small asteroid to hit the Moon, could we see the impact with the naked eye? In his chronicles of medieval life, Gervase of Canterbury described a dramatic event witnessed on the evening of June 18, 1178: Now there was a bright new Moon . . . and suddenly the upper horn split in two. From the midpoint of this division a flaming torch sprang up, spewing out . . . fire, hot coals, and sparks . . . The body of the Moon which was below writhed . . . throbbed like a wounded snake. Afterwards it resumed its proper state. The phenomenon was repeated a dozen times or more. [Finally] the Moon . . . along its whole length took on a blackish appearance. In 1976 geologist Jack B. Hartung (State University of New York) proposed that this passage describes the creation of Giordano Bruno, a relatively young, 22-kilometer-wide crater near the Moon's northeast limb. Hartung reasoned that, seen from Earth, this brightly rayed crater appears near the midpoint of the young crescent Moon. Astronomers were quick to counter that on the date in question the Moon was only 1.3 days past new and thus too near the Sun to be easily visible at all. Also, Gervase's witnesses claimed to have seen the "flaming torch" many times, which sounds a lot more like the ordinary atmospheric distortions often seen near the horizon. Still, Hartung's hypothesis has made its way into many astronomy books and articles. It proved difficult to confirm or refute because data on Giordano Bruno and its surroundings were limited. Now a new analysis demonstrates that a cratering event could not have happened in 1178. Paul Withers (University of Arizona) finds that an impact large enough to create a 22-km crater would likely have showered Earth with 10 million tons of ejected fragments - perhaps a trillion bright meteors in all - during the days that followed. "A meteor storm as impressive as this and lasting for a week would have been considered apocalyptic by all medieval observers," Withers comments. Yet no mention of such displays appears in English, European, Arabic, or Asian chronicles of the era. Laser-ranging experiments during the 1970s revealed that the Moon nods back and forth by a tiny amount ("free libration"), suggesting to Hartung's supporters that the globe was still reverberating from the impact. But Withers notes that a reanalysis of the laser-ranging data later showed that the slight oscillation arises instead from fluid motions deep in the lunar interior. Furthermore, while Giordano Bruno is indeed the youngest crater of its size anywhere on the Moon, multispectral images from the Clementine spacecraft show that this impact site has to be much older than 800 years. Details of Withers's analysis will appear in the April issue of Meteoritics. - J. Kelly Beatty Wednesday, March 14
This fragment of Tagish Lake meteorite was encased in ice when it was recovered from the lake's frozen surface. The extraterrestrial samples continue to puzzle scientists. Courtesy University of Western Ontario and University of Calgary. Click on image for larger view. At last year's Lunar and Planetary Science Conference, held each March in Houston, Texas, meteorite specialists were salivating over the Tagish Lake meteorite, which had dropped as a hail of fragments onto the Yukon's winter wilderness just two months before. Within days of the fall, local outdoorsman Jim Brook carefully collected nearly a kilogram of icy fragments and stashed them in his freezer. Later a team of Canadian geologists and volunteers scoured the lake's frozen surface to collect as much of the fragile interplanetary material as possible before the spring thaw swallowed up the remaining pieces. Remarkable as much for the rapid, textbook recovery effort as for the stones' black, carbon-rich texture, Tagish Lake was hailed as the most important find in some 30 years. A year later, the Tagish Lake fall is still causing a scientific buzz because its unique composition, forged at the very beginning of the solar system, defies easy explanation. For example, some of its dark, crumbly interior is riddled with carbonate minerals created when liquid water percolated through the rock multiple times. Yet adjacent sections bear no carbonates or other traces of water's influence at all. And though chemists would have bet money that the black stones would have teemed with exotic hydrocarbon compounds, analyses turned up a disappointing yield - a thousandth the organic content of Murchison, a similarly carbon-rich meteorite that fell in 1969. "We were hoping to find all these amino acids," laments Iain Gilmour (Open University), "and they're just not there." What Gilmour and others have identified are puzzling clues to the meteorite's origin. Some of the organic components mimic the nitriles and other aromatic species known to exist in molecular clouds. So might Tagish Lake have an interstellar origin? Or, as Takahiro Hiroi (Brown University) speculates, are these pieces of one of the dark, carbon-rich "D-type" asteroids that lurk in Jupiter's vicinity? More than one specialist openly questioned whether this find could represent chunks of a comet's nucleus. "There are no real conclusions yet," says Sandra Pizzarello (Arizona State University). "This meteorite is extremely difficult to study." - J. Kelly Beatty Wednesday, March 14
In December 1999 the Chandra X-ray Observatory recorded this bright X-ray flare from the brown dwarf LP 944-20 in Fornax. Each panel measures 5 arcminutes across. Courtesy NASA/UCB/Caltech/R. Rutledge and others. Click on image for larger view. Thirteen astronomy students using the Very Large Array (VLA) radio telescope in New Mexico got the thrill of their lives when they detected the first-ever radio emission from a brown dwarf. Glowing very dimly red, brown dwarfs are small, cool objects about the size of Jupiter but with 13 to 75 Jupiter masses - too light to become real stars yet too heavy to be called planets. Edo Berger (Caltech) and his team aimed the VLA's dishes at LP 944-20 in the southern constellation Fornax last July. They detected a constant flux of radio waves at wavelengths of 6 and 3.6 centimeters, as well as three brief flare-ups when the emission intensified by a dozen or more times. The team's findings, reported in the March 15th Nature, are forcing theorists to reexamine their ideas about how a brown dwarf works. The university students' project was part of the National Science Foundation's summer science program at the VLA. They chose LP 944-20 as their target because the Chandra X-ray Observatory had detected X-ray flares from it in 1999. Among ordinary stars, radio and X-ray activity tend to go together. Nevertheless, the students assumed that finding radio emission would be a long shot. The flares were a remarkable bonus. "They got very lucky," says VLA astronomer Dale Frail. "Other astronomers had looked for radio emission from brown dwarfs and not found any. This one flared at just the right time." The steady emission was 10,000 times stronger than would be expected from the X-ray behavior of the object. And yet the students deduced that it has a weak magnetic field - weaker than Jupiter's and not much stronger than Earth's. This agrees with conventional brown-dwarf theory. But such a weak field poses a mystery. In the absence of a strong field, how are vast numbers of electrons being accelerated to near the speed of light to create the radio emission? The brown dwarf's flares actually emit more radio energy than the strongest flares on the Sun, which has a much more intense magnetic field. - Edwin L. Aguirre Friday, March 9
The European Southern Observatory's 2.2-meter reflector at La Silla reveals that Comet Hale-Bopp is still active, despite being 2 billion kilometers from the Sun. Courtesy ESO. Click on image for larger view. Now nearly 2 billion kilometers (13 astronomical units) from the Sun, midway between the orbits of Saturn and Uranus, Comet Hale-Bopp (C/1995 O1) continues to surprise astronomers with its longevity. Glowing at magnitude 14.5 in the far-southern constellation Dorado, the comet has remained unusually active - shedding gas and dust to form a prominent, curved jet and an enormous, fan-shaped coma, estimated to be about 2 million km across. The image above is a composite of 14 exposures obtained in red, yellow, and blue light with the European Southern Observatory's 2.2-meter telescope in La Silla, Chile, from February 27th to March 2nd. Hale-Bopp is currently moving away from us at 1 million km a day. Astronomers plan to continue monitoring it as long as possible, perhaps for the next several decades. For details, see the online press release. - Edwin L. Aguirre Friday, March 9
The 2dF Galaxy Redshift Survey has measured more than 150,000 galaxies out of a planned 250,000. The survey is covering two pie-slice wedges of space, measuring 4° by 90° wide on the sky and extending some 2 billion light-years deep (to about redshift 0.25). Courtesy Nature/2dF Galaxy Redshift Survey. Click on image for larger view. According to results published in yesterday's issue of the journal Nature, only about 35 percent of the mass of the universe (±10 percent) is made of matter, either bright or dark. The rest, most likely, is some kind of dark energy driving the expansion of the cosmos. This result comes from the Two-Degree Field (2dF) Galaxy Redshift Survey being conducted with the 4-meter Anglo-Australian Telescope in eastern Australia. (The survey is named for the spectrometers being used, which cover a 2° field of the sky at once.) Based on the redshifts of 141,000 of the galaxies, the team of astronomers, led by John Peacock (University of Edinburgh), used two strategies to obtain their mass measurement. For the first, they analyzed the attraction of galaxies toward very large clusters, as revealed by individual galaxies' motions. For the second, they compared the amount of galaxy clustering to the small temperature variations seen in the cosmic microwave background radiation. Since these variations reflect density fluctuations in the early universe, comparing them to the amount of galaxy clustering observed today reveals the strength of gravity that was required to pull the clusters together - and hence their mass. This finding closely matches the result the 2dF team announced last summer based on only 106,000 galaxies. At that time they announced that matter totals 40 ± 10 percent of all the matter and energy in the universe. Many other studies have come to a similar conclusion. The 2dF Survey should be completed by the end of this year, once 250,000 redshifts have been measured. - David Tytell Wednesday, March 7
The interior of Jupiter's giant satellite Ganymede has completely separated into a metallic core, a rocky mantle, and a thick water "crust," as depicted in this NASA/JPL illustration. Several planetary scientists now suspect that a deep, briny ocean may exist roughly 170 kilometers below the moon's surface. Click on image for larger view. Larger than Mercury, the Jovian moon Ganymede has numerous planetlike attributes, such as an internal magnetic field and a geologically active surface. Now it appears that the big moon (5,270 kilometers across) may be hiding a deep, global ocean beneath its ice-dominated crust. By combining Galileo and Voyager images to yield stereo views, a team of researchers has found low-lying regions on Ganymede that appear to have been flooded with water or slush roughly one billion years ago. Typically 100 to 1,000 meters lower than their surroundings, the smooth areas occur in swaths of bright terrain that have been heavily fractured, note Paul M. Schenk (Lunar and Planetary Institute) and three colleagues in the March 1st issue of Nature. According to their relatively simple model, a wide stripe of bright terrain forms as a trough and is subsequently flooded by a watery fluid that soon freezes. "They're really like rift valleys on Earth," notes team member William B. McKinnon (Washington University). Not all low areas are flooded, however, and other evidence of icy volcanism - such as source vents and flow lobes - has not been found. Even so, other evidence suggests that a subsurface ocean persists to this day. When Galileo swept 800 km from the equatorial region of Ganymede last May 20th, it detected subtleties in the surrounding magnetic field that cannot be explained by the dynamo churning in the moon's core. A bit of Ganymede's magnetic signature wafts back and forth in concert with the much stronger field of Jupiter. According to Margaret V. Kivelson (UCLA), team leader for the magnetometer experiment, these fluctuations are best explained by a weak magnetic field induced in a salty, conducting layer of water some 170 km below the surface. That depth corresponds precisely to where pressures are calculated to be great enough to force ice to melt. "It does look like more than an accident," Kivelson notes. - J. Kelly Beatty
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