Skip to comments.Astronomy Picture of the Day 03-09-04
Posted on 03/09/2004 5:24:14 AM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2004 March 9
Explanation: The above picture will be replaced later today (between 9 and 10 am EST) by the newly released Hubble Ultra Deep Field (HUDF). The HUDF is expected to be the deepest image of the universe ever taken in visible light. It is expected to show a sampling of the oldest galaxies ever seen, galaxies that formed just after the dark ages, when the universe was only 5 percent of its present age. The Hubble Space Telescope's NICMOS and new ACS cameras took the image. Staring nearly 3 months at the same spot, the HUDF is reported to be four times more sensitive, in some colors, than the original Hubble Deep Field (HDF), currently pictured above.
|The orbiting Chandra X-ray Observatory observed Saturn for 20 hours in April 2003 to produce this false colour image.
Image credit: NASA/Hamburg University/J.-U. Ness et al
The Chandra X-ray Observatory is a space observatory dedicated to X-ray astronomy. Launched on July 23rd, 1999, this X-ray telescope is particularly dedicated to the observation of high-energy sources in the universe. It has also been used to study the following solar system objects: the Moon, Venus, Mars, Jupiter, and even the Comet C/1999 S4 LINEAR.
Using Chandra, Jan-Uwe Ness and his colleagues detected an unambiguous X-ray emission from the planet Saturn for the first time. A few years ago, a possible such emission was observed using ROSAT, but the present detection is the first certain one.
Ness and his team observed Saturn in April 2003 for about 20 hours. The picture above shows the X-ray image of Saturn that they obtained (in false colour). Each colour (RGB) corresponds to a different energy range of the observed X-rays. Beyond the X-ray detection from Saturn, this Chandra observation allowed the investigators to perform the first in-depth analysis of this emission. The same team also detected X-ray emissions from Saturn using the XMM-Newton Observatory. The observed signal was very similar to what was found with Chandra.
Spectral analysis of the X-rays detected from Saturn with Chandra shows that the signal looks remarkably like that of solar X-ray emissions. This indicates that Saturn's X-ray emission originates from solar X-rays scattering in Saturn's atmosphere, a result that is very surprising, as such a process has already been studied on the Moon and was not expected to be so efficient on Saturn.
In addition to the scattering of solar X-rays, on Earth, X-rays are also produced by the same mechanism that creates the aurorae in the polar regions. Aurorae are caused by solar wind electrons that enter the Earth's atmosphere, thus resulting in these spectacular visible lights. At the same time, X-rays are also produced. The same mechanism has been found to exist on Jupiter: Jovian aurorae have been detected in the UV wavelengths, along with an associated X-ray emission. Saturn was also expected to emit X-rays in the polar region, since spectacular polar aurorae have been observed in the UV wavelengths.
With Chandra, J.-U. Ness and his colleagues were able to study for the very first time how the X-rays are distributed on Saturn's disc, in order to look for polar X-ray emission. They found that the observed X-ray emission appears to be concentrated around Saturn's equator. Surprisingly, no auroral X-ray emission has been observed at all. However, the North Pole was occulted by Saturn's rings and X-ray emission from northern polar regions may have been hidden.
Comparison between Saturn's and Jupiter's X-ray emissions shows that their X-ray production mechanisms are very different. First of all, the spatial distribution differs: on Jupiter, X-rays are concentrated in polar regions, suggesting that magnetic fields play an important role. Such a polar X-ray emission has not been found on Saturn, but cannot be excluded. Secondly, the X-ray flux on Saturn is significantly lower than that seen on Jupiter. Nonetheless, the emission level observed from Saturn is consistent with the X-ray equatorial emission from Jupiter, indicating that both emissions originate from similar processes. Ness and his colleagues point out that no single mechanism can easily account for the observed emissions; therefore, they expect combined mechanisms to be involved.
Thanks to the high-angular resolution of the Chandra Observatory, the new detection of X-ray emissions from Saturn, as well as recent results on the same subject from Jupiter, will allow for important progress in the observations of X-rays on giant planets.
Rovers watch solar eclipses by Martian moons
NASA NEWS RELEASE
Posted: March 8, 2004
NASA's Mars Exploration Rovers have become eclipse watchers.
Though the Viking Landers in the 1970s observed the shadow of one Mars' two moons, Phobos, moving across the landscape, and Mars Pathfinder in 1997 observed Phobos emerge at night from the shadow of Mars, no previous mission has ever directly observed a moon pass in front of the sun from the surface of another world.
The Deimos image was taken at 03:04 Universal Time on March 4. This irregularly shaped moon is only 15 kilometers (9 miles) across in its longest dimension. It appears as just a speck in front of the disc of the Sun. The Phobos image was taken as that moon grazed the edge of the solar disc at 02:46 Universal Time on March 7. Phobos is 27 kilometers (17 miles) in its longest dimension. Its apparent size relative to Deimos is even greater because it orbits much closer to Mars' surface than Deimos does. Credit: NASA/JPL/Cornell
Rover controllers at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., are planning to use the panoramic cameras on both Opportunity and Spirit for several similar events in the next six weeks. Dr. Jim Bell of Cornell University, Ithaca, N.Y., lead scientist for those cameras, expects the most dramatic images may be the one of Phobos planned for March 10.
"Scientifically, we're interested in timing these events to possibly allow refinement of the orbits and orbital evolution of these natural satellites," Bell said. "It's also exciting, historic and just plain cool to be able to observe eclipses on another planet at all," he said.
Depending on the orientation of Phobos as it passes between the sun and the rovers, the images might also add new information about the elongated shape of that moon.
Phobos is about 27 kilometers long by about 18 kilometers across its smallest dimension (17 miles by 11 miles). Deimos' dimensions are about half as much, but the pair's difference in size as they appear from Mars' surface is even greater, because Phobos flies in a much lower orbit.
The rovers' panoramic cameras observe the sun nearly every martian day as a way to gain information about how Mars' atmosphere affects the sunlight. The challenge for the eclipse observations is in the timing. Deimos crosses the sun's disc in only about 50 to 60 seconds. Phobos moves even more quickly, crossing the sun in only 20 to 30 seconds.
Scientists use the term "transit" for an eclipse in which the intervening body covers only a fraction of the more-distant body. For example, from Earth, the planet Venus will be seen to transit the sun on June 8, for the first time since 1882. Transits of the sun by Mercury and transits of Jupiter by Jupiter's moons are more common observations from Earth.
From Earth, our moon and the sun have the appearance of almost identically sized discs in the sky, so the moon almost exactly covers the sun during a total solar eclipse. Because Mars is farther from the sun than Earth is, the sun looks only about two-thirds as wide from Mars as it does from Earth. However, Mars' moons are so small that even Phobos covers only about half of the sun's disc during an eclipse seen from Mars.
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