Posted on by Fraser Cain

Great Mercury Viewing This Week

Mercury on Feb. 13, 2006. Image credit: Jeffrey Beall. Click to enlarge
It’s not every day you get to see a shrinking planet. Today could be the day.

Step outside this evening at sunset and look west toward the glow of the setting sun. As the sky fades to black, a bright planet will emerge. It’s Mercury, first planet from the sun, also known as the “Incredible Shrinking Planet.”

“This is only the second time in my life I’ve seen Mercury,” says sky watcher Jeffrey Beall who snapped this picture looking west from his balcony in Denver, Colorado:

Mercury is the bright “star” just above the mountain ridge, rivaling the city lights.

Mercury is elusive because it spends most of its time hidden by the glare of the sun. This week is different. From now until about March 1st, Mercury moves out of the glare and into plain view. It’s not that Mercury is genuinely farther from the sun. It just looks that way because of the Earth-sun-Mercury geometry in late February. A picture is worth a thousand words: diagram.

Friday, Feb. 24th, is the best day to look (sky map); that’s the date of greatest elongation or separation from the sun. Other dates of note are Feb 28th (sky map) and March 1st (sky map) when the crescent moon glides by Mercury??bf?very pretty.

When you see Mercury popping out of the evening twilight, you’re looking at a very strange place. “Shrinking” is a good example:

In 1974, NASA’s Mariner 10 spacecraft flew by Mercury and, for the first time, photographed the planet from close range. Cameras revealed a densely cratered world??bf?with wrinkles. Planetary geologists call them “lobate scarps” and, like wrinkles on a raisin, they are thought to be a sign of shrinking. What would make a planet shrink? One possibility: Mercury’s oversized iron core has been cooling for billions of years, and its contraction may be the driving force behind the wrinkles. No one knows for sure.

No one knows because Mercury has hardly been explored. Only one spacecraft has ever been there, and during its oh-so-brief visit Mariner 10 managed to photograph less than half (45%) of Mercury’s surface: image. The majority is terra incognita.

Another puzzle is the mystery-substance at Mercury’s poles. Radio astronomers have pinged Mercury from afar using radars on Earth, and they have found something very bright in Mercury’s polar craters. Again, no one knows what it is, although a favorite possibility is ice. Frozen water is a good reflector of radio waves and would explain the observations nicely.

How could frozen water exist on Mercury? The sun heats the planet’s surface to 400 ??bf?C (750 ??bf?F) or more, too hot for frozen anything. Yet deep down in some polar craters, researchers believe, the sun never shines. In permanent shadow, the temperature drops below -212??bf? C (-350??bf? F). Suppose a piece of an icy comet or meteorite landed in such a crater; some of the ice might survive.

Or it could be something else entirely.

What does the unknown half of Mercury look like? Is the planet really shrinking? Can ice stay frozen in an inferno? Mercury poses many questions: list. A new NASA probe named “MESSENGER” is en route to find some answers, but it will not reach Mercury until 2008.

For now, one can only peer into the twilight and wonder. Give it a try, this evening.

Original Source: NASA News Release

Posted on by Fraser Cain

Earth’s Iron Building Blocks

Artist’s conception shows Romulus and Remus orbiting the asteroid 87 Sylvia. Image credit: ESO Click to enlarge
Iron meteorites are probably the surviving fragments of the long-lost asteroid-like bodies that formed the Earth and other nearby rocky planets, according to researchers from Southwest Research Institute (SwRI) and Observatoire de la Cote d’Azur in Nice, France. Their findings are described in the Feb.16 issue of Nature.

Iron meteorites, which are composed of iron and nickel alloys, represent some of the earliest material formed in the solar system, with most coming from the cores of small asteroids. According to Dr. William Bottke, an SwRI research scientist and leader of the joint U.S.-French team, iron-meteorite parent bodies probably emerged from the same disk of planetary debris that produced the Earth and other inner solar system planets.

“Small bodies that form quickly in the inner solar system end up melting and differentiating from the decay of short-lived radioactive elements,” explains Bottke. “Iron meteorites came from the molten material that sinks to the center of these objects, cools and solidifies.”

For these meteorites to arrive on Earth, they must have been extracted from their parent bodies and kept around for billions of years. The team’s computer simulations found that any asteroids managing to avoid being gobbled up by the planets were quickly demolished by impacts. Each breakup, however, produces millions of fragments, many in the form of iron meteorites. These leftovers were scattered across the solar system by gravitational interactions with protoplanetary bodies, with some reaching the relative safety of the asteroid belt. Over billions of years, a few of the survivors escaped their captivity in the asteroid belt and were delivered to Earth.

“This means that certain iron meteorites may tell us what the precursor material for the primordial Earth was like, while also helping us unlock several fundamental questions about the Earth’s origins,” says Bottke. “There’s also the possibility that larger versions of this material may still be hiding among the asteroids. The hunt for them is on.”

A new way to look at iron meteorites

A potential problem in using meteorites to understand the formation of Earth and other terrestrial planets Mercury, Venus and Mars is that most come from the distant asteroid belt. This population of interplanetary bodies, ranging from tiny pebbles to Texas-sized objects, is located between the orbits of Mars and Jupiter about 140 million miles from Earth.

Most members of the asteroid belt are assumed to have formed there, so the vast majority of meteorite samples tell us about formation events in that region, not those that took place near Earth. Meteorite compositions are so diverse, however, that it is difficult to reconcile that all came from this one, fairly narrow region of space.

“While tens of thousands of stony meteorites have been collected, most can be traced back to perhaps a few tens of parent asteroids,” says Dr. Alessandro Morbidelli of the Observatorie de la Cote d’Azur. “What is strange is that the iron meteorites, despite their smaller numbers, represent almost two-thirds of all of the unique parent asteroids sampled to date.”

To explain this discrepancy, the team tracked the origin and evolution of iron-meteorite parent bodies using several computer models. They found that while many iron meteorites are likely residing in the asteroid belt today, their precursors probably did not form there. Instead, the simulations indicate that the precursors of most iron meteorites formed in the terrestrial planet region.

To investigate this hypothesis, the researchers first examined the constraints provided by the meteorites themselves. Iron meteorites are unusual in that most come from the disrupted cores of small melted (differentiated) asteroids that formed very early in solar system history. These are precisely the kinds of bodies that computer models predict should have formed near Earth.

“It is hard to produce small differentiated bodies in the asteroid belt without also melting lots of large asteroids,” explains Dr. Robert Grimm, assistant director of the SwRI Space Studies Department. “These events would produce a number of telltale signs that would be easily detected by observers.”

Using computer simulations, the team then tracked how a disk of asteroid-like bodies interacting with a host of protoplanetary objects in the terrestrial planet region might evolve. Simulations showed that some of these asteroid-like bodies could have scattered far enough to take up residence in the asteroid belt.

“While the amount of material reaching the asteroid belt was limited, much of it was placed in regions likely to produce meteorites,” says SwRI Research Scientist Dr. David Nesvorn??bf?. En route to the asteroid belt, the parent bodies of the iron meteorites were repeatedly bashed by other bodies, allowing core fragments from numerous bodies to escape.

“This could explain the many differences seen among iron meteorites,” says Dr. David O’Brien of the Observatoire de la Cote d’Azur.

Original Source: NASA Astrobiology

Posted on by Fraser Cain

Stardust’s Samples Under Analysis

Stardust’s aerogel sample. Image credit: NASA Click to enlarge
Scientists at the University of Chicago are among the first ever to analyze cometary dust delivered to Earth via spacecraft.

Scientists routinely examine extraterrestrial material that has fallen to Earth as meteorites, but never before NASA’s Stardust mission have they had access to verified samples of a comet. The leftover debris from the formation of the solar system 4.5 billion years ago, comets consist mostly of ice, dust and rock.

“We think comets make up a huge amount of stuff out in the solar system. We’d like to know the mineral composition of this big component of the solar system that we’ve never seen before for sure,” said Lawrence Grossman, Professor in Geophysical Sciences. “Various particles have been measured that have been inferred to be from comets, but nobody’s sure. This would finally provide some ground truth.”

Grossman and Steven Simon, Senior Research Associate in Geophysical Sciences, are members of the Stardust Preliminary Examination Team (PET). So are Andrew Davis, Senior Scientist in the Enrico Fermi Institute, and his colleagues Michael Pellin and Michael Savina of the U.S. Department of Energy’s Argonne National Laboratory. The role of PET is to describe the samples in a general way so that the scientists can propose more detailed studies based on that information.

Davis also is a member of the Stardust Sample Allocation Committee, which will decide how to distribute the samples for additional research once the preliminary examination period ends in mid-July.

The Stardust mission launched in February 1999, carrying a set of instruments that included one provided by the University of Chicago to monitor the impact of cometary dust. On Jan. 2, 2004, the spacecraft came within 150 miles of the comet and collected thousands of tiny dust particles streaming from its nucleus. The Stardust sample-return canister parachuted onto the desert salt flats of Utah on Jan. 15 following a journey of nearly three million miles.

During the 2004 cometary encounter, the University of Chicago’s Dust Flux Monitor Instrument successfully determined the flow and mass of the particles streaming from the comet’s nucleus. Based on data collected by the instrument, the University of Chicago’s Anthony Tuzzolino and Thanasis Economou estimated that the spacecraft had collected at least 2,300 particles measuring 15 micrometers (one-third the size of a human hair) or larger during the flyby.

“It will take the experts many, many months before they will determine the accurate number, but I am sure that in the end their number will be close to what we have predicted,” said Economou, who was at the Johnson Space Center in Houston when the samples were delivered from Utah. “Stardust was very successful beyond all expectation in all its phases.”

The comet dust is now available for comparison to tiny particles constantly raining down on Earth that scientists suspect come from comets. NASA routinely collects these stratospheric dust particles with high-altitude aircraft and maintains a collection of them, Simon said. Certain types of meteorites might also originate from comets, but without having cometary material to compare, “we don??bf?t know,” Grossman said.

Grossman and Simon received several samples on Feb. 7. The samples partly consist of several thin slices of one dust grain mounted in epoxy and held on a round copper grid covered by a thin film. They also received a bullet-shaped epoxy plug holding the remainder of the grain.

“They can make hundreds of slices of each individual grain,” Simon said. He and Grossman are studying their slices with an electron microprobe and a scanning electron microscope (SEM). The microprobe is capable of revealing the chemical composition of microscopically small patches of material, while the SEM provides highly magnified images.

The Stardust cometary materials now join a collection of charged particles from the sun gathered by NASA’s Genesis mission and returned to Earth in 2004. Davis serves as chair of the Genesis Oversight Committee, which guides the curation and analysis of that mission’s extraterrestrial materials.

“Cosmochemistry is a very exciting field these days,” Davis said, referring to research on the origin of the chemical elements and the chemistry of extraterrestrial materials. “It??bf?s an interesting time to get young people involved in the field.” In 2004, along with colleagues at Argonne and the Field Museum, Davis organized the Chicago Center for Cosmochemistry to promote education and research in cosmochemistry.

The Stardust spacecraft, meanwhile, may someday see further cometary action. “Stardust is still very healthy and has fuel left over,” Economou said. “After dropping the Space Return Canister, the spacecraft was diverted from entering the Earth’s atmosphere and placed in an orbit around the sun that could bring it to another comet in February 2011.”

The Stardust mission is managed by NASA’s Jet Propulsion Laboratory, Pasadena, Calif. Lockheed Martin Space Systems, Denver, developed and operated the spacecraft. For more information, see http://stardust.jpl.nasa.gov/home/index.html and http://cosmochemistry.uchicago.edu/.

Original Source: University of Chicago

Posted on by Fraser Cain

The Milky Way Shines in X-Rays Too

The X-ray background consists of a huge number of faint objects. Image credit: NASA Click to enlarge
Using the most sensitive X-ray map of the Galaxy, obtained combining 10 years of data of Rossi XTE orbital observatory, scientists from the Max Planck Institute for Astrophysics have discovered the origin of the galactic background emission. They show that it consists of emission from a million accreting white dwarf binaries and hundreds of millions of normal stars with active coronas.

Nearly 400 years after Galileo determined that the wispy Milky Way actually comprises a multitude of individual stars, scientists using NASA’s Rossi X-ray Timing Explorer have done the same for the X-ray Milky Way.

The origin of the so-called galactic X-ray background has been a long-standing mystery. Scientists now say that this blanket of X-ray light is not diffuse, as many have thought, but emanates from untold hundreds of millions of individual sources dominated by a type of dead star called a white dwarf.

If confirmed, this new finding would have a profound impact on our understanding of the history of our galaxy, from star-formation and supernova rates to stellar evolution. The results solve major theoretical problems, yet point to a surprising undercounting of stellar objects.

Scientists from the Max Planck Institute for Astrophysics (MPA) in Garching, Germany, and the Space Research Institute of the Russian Academy of Sciences in Moscow discuss these results in two papers published in Astronomy & Astrophysics.

“From an airplane you can see a diffuse glow from a city at night,” said Dr. Mikhail Revnivtsev of MPA, lead author on one of the papers. “To say a city produces light is not enough. Only when you get closer do you see individual sources that make up that glow – the house lights, street lamps and automobile headlights. In this respect, we have identified the individual sources of local X-ray light. What we found will surprise many scientists.”

X-rays are a high-energy form of light, invisible to our eyes and far more energetic than optical and ultraviolet light. Our eyes see individual stars sprinkled in a largely dark sky. In X-ray bandwidths the sky is never dark; there is a pervasive and constant glow.

Previous observations could not reveal enough X-ray sources to account for the “X-ray milky way.” This led to theoretical problems. If the X-ray glow were from hot and diffuse gas, it would ultimately “rise” and escape the confines of the galaxy. Furthermore, all that hot gas would need to have come from millions of past star explosions called supernovae, which would imply that estimates of star formation and star death were way off.

“X-ray telescopes can resolve the emission into discrete sources but can only account for about 30 percent of the emission,” said Dr. Jean Swank, project scientist for the Rossi Explorer at NASA Goddard Space Flight Center in Greenbelt, Maryland, USA. “Many have thought that the lion’s share was truly diffuse, for example, from hot gas between the stars. Now it seems that it can all be accounted for a combination of two types of stars.”

The new study is based on nearly 10 years of data collected by the Rossi Explorer and constitutes the most thorough map of the galaxy in X-ray bandwidths. The science team concluded that the Milky Way galaxy is indeed teeming with X-ray stars, most of them not very bright, and that scientists over the years had underestimated their numbers by perhaps a hundredfold.

Surprisingly, the usual suspects of X-ray emission – black holes and neutron stars – are not implicated here. At higher X-ray energies, the X-ray glow arises almost entirely from sources called cataclysmic variables.

A cataclysmic variable is a binary star system containing a relatively normal star and a white dwarf, which is a stellar ember of a star like our sun that has run out of fuel. On its own, a white dwarf is dim. In a binary, it can pull away matter from its companion star to heat itself in a process called accretion. The accreted gas is very hot, a source of considerable X-rays.

At slightly lower X-ray energies, the glow is a mix of about one-third cataclysmic variables and two-thirds active stellar coronas. Most of the stellar corona activity also takes place in binaries, where a nearby companion effectively stirs up the outer parts of the star. This energizes the stellar analogue to produce solar flares, which emits X-rays. The science team says there are upwards of a million cataclysmic variables in our galaxy and close to a billion active stars. Both of these numbers reflect a major undercounting in previous estimates.

“Like a medical x-ray, the chart of the galactic X-ray background reveals details of the Milky Way’s structure,” said Revnivtsev. “We can see through the whole galaxy and count X-ray sources. This is very important to astronomers who calculate the lives of stars.”

NASA Goddard Space Flight Center in Greenbelt, Maryland, USA manages the Rossi Explorer, which was launched in December 1995.

Original Source: Max Planck Society

Posted on by Fraser Cain

Saturn’s Feathery Northern Clouds

Saturn’s northern hemisphere. Image credit: NASA/JPL/SSI Click to enlarge
After a year and a half in orbit, the Cassini spacecraft has begun to image Saturn’s northern hemisphere in detail. The northern latitudes currently are experiencing winter, and atmospheric scientists are interested in determining whether the winter hemisphere is systematically different in appearance than the sunnier southern hemisphere.

This scene contains a great deal of bright, whorl-shaped cloud activity.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 6, 2006, at a distance of approximately 2.9 million kilometers (1.8 million miles) from Saturn. The image scale is 17 kilometers (11 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

The Sky is Full of Black Holes

X-ray image of the Chandra Deep Field-North. Image credit: NASA/PSU Click to enlarge
Data from X-ray observatory surveys show that black holes are much more numerous and evolved differently than researchers would have expected, according to a Penn State astronomer.

“We wanted a census of all the black holes and we wanted to know what they are like,” said Niel Brandt, professor of astronomy and astrophysics. “We also wanted to measure how black holes have grown over the history of the Universe.”

Brandt and other researchers have done just that by looking at a patch of sky in the Northern hemisphere called the Chandra Deep Field-North, using NASA’s Chandra X-ray Observatory and a similar patch in the Southern hemisphere called the Extended Chandra Deep Field-South. Surveys also are being carried out in other parts of the sky using both Chandra and the European Space Agency’s X-ray Multi-Mirror Mission-Newton.

The researchers looked at X-ray emissions because areas around black holes emit X-rays as well as visible light. The penetrating nature of X-rays provides a direct way to identify the black holes. Using X-rays also enables astronomers to pinpoint the black holes at the centers of galaxies without their signal being washed out by the visible light coming from a galaxy’s stars, Brandt told attendees at the annual meeting of the American Association for the Advancement of Science in St. Louis, Mo. Feb. 17. The black holes they studied were those that reside at the centers of galaxies and are actively emitting X-rays, therefore they are called active galactic nuclei.

“We find active super massive black holes at the centers of massive galaxies,” said Brandt. “Our galaxy also has its own black hole at its center measuring 2.6 million solar masses. Our black hole is not active today, but we presume it was active in the past.”

These deep, extragalactic X-ray surveys looked at carefully chosen patches of sky, that are largely free of anything that might interfere with obtaining the X-ray data. Chandra looked at the Chandra Deep Field-North — an area of sky two thirds the size of the full Moon — for the time span of 23 days over a two-year period. The researchers detected about 600 X-ray sources. After comparing the X-ray images with optical images of exactly the same slice of sky taken by the Hubble Space Telescope, nearly all 600 point sources corresponded to optical galaxies, suggesting that the black holes that were sources for the X-ray signature were in the centers of galaxies.

“X-ray astronomers are doing better than anyone else by about a factor of ten, in identifying these active galactic nuclei” said Brandt. “With more time we could do even better, going even deeper.”

What the researchers found was that super massive black holes are more numerous than we might have expected. They also found that black holes evolved differently than astronomers expected prior to the Chandra work. Extrapolating from the 600 black holes found by Chandra, Brandt suggests that there are about 300 million super massive black holes in the whole sky.

The existence of so many black holes, confirmed that what was once thought to be a truly diffuse cosmic X-ray background radiation, actually comes from point sources.

In the 1960s, astronomers discovered quasars, very distant, highly luminous black holes, in galactic centers. Quasars, initially called quasi-stellar radio sources, were studied intensely. Researchers soon realized that only some of these objects were radio emitters and that they formed early in the history of the Universe.

“While quasars are spectacular, they are not representative of typical active galactic nuclei,” said Brandt. “Now, using Chandra and other X-ray observatories, we can find and study the moderate-luminosity, typical active galactic nuclei in the distant, high-redshift Universe.”

Quasars and moderate-luminosity active galactic nuclei also evolved differently. Quasars are a phenomenon of young galaxies, while moderate-luminosity, active galactic nuclei peaked later in cosmic time.

“We would like to know if active galactic nuclei change over cosmic time,” said Brandt. “Do black holes feed and grow in the same way over the history of the Universe?”

Researchers looked at the relative amount of power coming out in X-rays compared to other wavelengths and found that this ratio does not change over 13 billion years of time. They looked at the X-ray spectra and found that these also did not change through time.

“Despite the enormous changes in the space density of back holes, the individual engines powering active galactic nuclei are remarkably stable,” Brandt said.

Brandt believes that Chandra could observe the Chandra Deep Field-North for a longer period of time and obtain more sensitive, deeper data. This would bring to light galaxies that are currently obscured. It would also gather more X-rays allowing better X-ray spectral and variability analyses. With more sensitive probing, the researchers are also detecting an increasing number of non-active galaxies like our own.

“Chandra has worked well for six years now,” said Brandt. “There is no reason why Chandra and Newton cannot continue to observe for another 10 or more years.”

Original Source: PSU News Release

Posted on by Fraser Cain

The Sombrero Galaxy by Adam Block and Morris Wade

The Sombrero Galaxy by Adam Block/Morris Wade/NOAO/AURA/NSF
Located 28 million light years from our planet and rushing away at over 700 miles a second, the Sombrero Galaxy has some impressive statistics: it spans over 50,000 light years, it contains the mass of 800 billion suns and is surrounded by over 2 thousand globular clusters – nearly ten times as many as our own Milky Way. The glowing central region is home to a monster producing a tremendous amount of X-rays. Most astronomers believe it’s an enormous black hole over a billion times more massive than our sun.

The prominent dust lane that cuts horizontally across the edge coupled with the bulge near the center gives the galaxy a hat-like appearance, thus the common name of Sombrero. Slightly too faint to be seen by naked eye, the Sombrero, which is located in the constellation of Virgo, appears to be approximately 1/5 as large as the Moon from our vantage point on Earth.

This picture was taken by Adam Block and Morris Wade using a 20 inch, f/8 RCOS Ritchey-Chretien telescope and a three mega-pixel SBIG astronomical camera at the Kitt Peak National Observatory Visitor Center outside of Tucson, Arizona. The observatory operates an Advanced Observing Program most nights of the year where interested visitors regardless of previous experience can pre-arrange to take amazing astronomical photos like this. This image was produced after two and one half hours of total exposure. Post production processing included two iterations of deconvolution, which increases sharpness, and use of DDP, a digital enhancement technique which helps display both the very faint and very bright parts of the image simultaneously.

Do you have photos you’d like to share? Post them to the Universe Today astrophotography forum or email them, and we might feature one in Universe Today.

Written by R. Jay GaBany

Posted on by Fraser Cain

The Shadow of Phobos

Black and white view of Phobos’s shadow. Image credit: ESA Click to enlarge
This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express spacecraft, shows the fast-moving shadow of the moon Phobos as it moved across the Martian surface.

The HRSC obtained this unique image during orbit 2345 on 10 November 2005. These observations would not have been possible without the close co-operation between the camera team at the Institute of Planetary Research at DLR and the ESA teams, in particular the mission engineers at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany.

They confirm the model of the moon’s orbit around Mars, as it was determined earlier in 2004 also on the basis of HRSC images. They also show that with accurate planning even moving objects can be captured exactly at their predicted position.

The basis for such observations is the accurate knowledge of the spacecraft position in its orbit as well as of the targeted location on Mars to within a few hundred metres.

Phobos is the larger of the two Martian moons, 27 kilometres by 22 kilometres in size, and travels around Mars in an almost circular orbit at an altitude of about 6000 kilometres. Phobos takes slightly more than 7.5 hours to complete a full revolution around the planet.

When it is between the Sun and Mars, Phobos casts a small and diffuse shadow onto the Martian surface. To an observer on Mars, this would appear as a very quick eclipse of the Sun. This is similar to an eclipse on Earth, when the Moon covers the solar disk but much slower.

The shadow of Phobos has an elliptical shape on the Martian surface, because the shadow’s cone hits the surface at an oblique angle. This shadow appears to be distorted even more because of the imaging technique of the HRSC.

The shadow moves across the surface with a speed of roughly 7200 kilometres per hour from west to east. The spacecraft travels with a higher speed of about 12 600 kilometres per hour on its almost polar orbit from south to north.

Since HRSC scans the surface synchronised with the flight velocity of Mars Express, it takes some time to cover the shadow in its full dimension. Within this short time, however, the moon moves on, and therefore the shape of its shadow is ‘smeared’ in the HRSC image.

Another phenomenon, that the shadow is darker at its centre than the edges, can be explained by again imagining the observer on Mars. With its small size, Phobos would only cover some 20% of the solar disk.

Even if the observer stood in the centre of the shadow, they would still be illuminated by the uncovered part of the Sun’s disk, in a partial solar eclipse instead of a total eclipse.

Members of the HRSC Science Team recalculated the orbit of Phobos on the basis of images taken in 2004. With the help of the improved orbit determination ? the moon has advanced about 12 kilometres with respect to its previously predicted position along its orbit ? it was possible to calculate those precise times when the shadow observations could be made. In turn, it was possible to verify the accuracy of the improved orbit determination by the shadow’s position in the new images.

Original Source: ESA Portal

Posted on by Fraser Cain

Tethys and Titan

The two moons Titan and Tethys with its great crater Odysseus. Image credit: NASA/JPL/SSI Click to enlarge
Cassini looks toward Tethys and its great crater Odysseus, while at the same time capturing veiled Titan in the distance (at left).

Titan (5,150 kilometers, or 3,200 miles across) is shrouded in a thick, smog-like atmosphere in which many small, potential impactors burn up before hitting the moon’s surface. Crater-pocked Tethys (1,071 kilometers, or 665 miles across) has no such protective layer, although even a thick blanket of atmosphere would have done little good against the impactor that created Odysseus.

The eastern limb of Tethys is overexposed in this view.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 6, 2006, at a distance of approximately 4 million kilometers (2.5 million miles) from Titan and 2.7 million kilometers (1.7 million miles) from Tethys. The image scale is 25 kilometers (16 miles) per pixel on Titan and 16 kilometers (10 miles) per pixel on Tethys.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

Mars Express Finds Auroras on Mars

An artist’s illustration of aurorae on night-side of Mars. Image credit: M. Holmstrom (IRF) Click to enlarge
ESA’s Mars Express spacecraft has seen more evidence that aurorae occur over the night side of Mars, especially over areas of the surface where variations in the magnetic properties of the crust have been detected.

Observations from the ASPERA instrument on board ESA’s Mars Express spacecraft show structures (inverted-V features) of accelerated electrons and ions above the night side of Mars that are almost identical to those that occur above aurorae on Earth.

Aurorae are spectacular displays often seen at the highest latitudes on Earth. On our planet, as well as on the giant planets Jupiter, Saturn, Uranus and Neptune, they occur at the foot of the planetary magnetic field lines near the poles, and are produced by charged particles ? electrons, protons or ions ? precipitating along these lines.

“Aurorae are created when energetic charged particles collide with the upper atmosphere,” says Rickard Lundin, Principal Investigator for ASPERA, from the Swedish Institute of Space Physics Physics (IRF), Kiruna, Sweden.

“When they are decelerated, energy is released that causes emissions of light – aurorae. During strong aurorae the precipitating particles are accelerated and gain energy, leading to more intense light,” said Lundin.

The scientists have found that the energy flux of the precipitating particles is large enough that it would lead to aurorae comparable to those of weak or medium intensity at Earth.

“Mars lacks a strong intrinsic magnetic or dipole field, and therefore we have not had reason to believe that aurorae occur there,” said Lundin.

A few years ago it was suggested that auroral phenomena could exist on Mars too. This hypothesis was reinforced by the Mars Global Surveyor discovery of ‘crustal magnetic anomalies’, most likely the remnants of an old planetary magnetic field.

This discovery started speculation that auroras could also occur at Mars. In 2004, the SPICAM instrument on board Mars Express observed emissions of light during a magnetic anomalies investigation – emissions that could be due to precipitating energetic particles.

The ASPERA scientists have now found that the structures of accelerated particles are indeed associated with the ‘crustal magnetic anomalies’ at Mars, but that strong acceleration mainly occurs in a region close to local midnight.

The precise emissions of light that occur remain to be studied since the composition of the upper atmosphere on the night side is not well known. On the basis of atmospheric models, the scientists speculate that the classical ‘green’ emission line of oxygen might be present.

“But, as we see Mars as always sunlit, the aurorae on the night side of Mars cannot be observed from Earth,” added Lundin.

Original Source: ESA Portal