How Magnetic Tornadoes Might Regenerate Mercury’s Atmosphere

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Compared to Earth, Mercury doesn’t have much of an atmosphere.  The smallest rocky planet has weak surface gravity, only 38% that of Earth.  And the scorching-hot daytime surface temperatures of 800 degrees Fahrenheit (approximately 450 degrees Celsius) should have boiled away any trace of Mercury’s atmosphere long ago.  Yet recent flybys of the MESSENGER spacecraft clearly revealed Mercury somehow retains a thin layer of gas near its surface.   Where does this atmosphere come from?

“Mercury’s atmosphere is so thin, it would have vanished long ago unless something was replenishing it,” says Dr. James A. Slavin of NASA’s Goddard Space Flight Center, Greenbelt, Md., a co-investigator on NASA’s MESSENGER mission to Mercury.

The solar wind may well be the culprit.  A thin gas of electrically charged particles called a plasma, the solar wind blows constantly from the surface of the sun at some 250 to 370 miles per second (about 400 to 600 kilometers/second).  According to Slavin, that’s fast enough to blast off the surface of Mercury through a process called “sputtering”, according to Slavin.  Some sputtered atoms stay close enough to the surface to serve as a tenuous yet measurable atmosphere.

But there’s a catch – Mercury’s magnetic field gets in the way. MESSENGER’s first flyby on January 14, 2008, confirmed that the planet has a global magnetic field, as first discovered by the Mariner 10 spacecraft during its flybys of the planet in 1974 and 1975.  Just as on Earth, the magnetic field should deflect charged particles away from the planet’s surface.  However, global magnetic fields are leaky shields and, under the right conditions, they are known to develop holes through which the solar wind can hit the surface.

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury’s magnetic field can be extremely leaky indeed. The spacecraft encountered magnetic “tornadoes” – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 500 miles wide or a third of the radius of the planet.

“These ‘tornadoes’ form when magnetic fields carried by the solar wind connect to Mercury’s magnetic field,” said Slavin. “As the solar wind blows past Mercury’s field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet’s magnetic shield through which the solar wind may enter and directly impact Mercury’s surface.”

Venus, Earth, and even Mars have thick atmospheres compared to Mercury, so the solar wind never makes it to the surface of these planets, even if there is no global magnetic field in the way, as is the case for Venus and Mars. Instead, it hits the upper atmosphere of these worlds, where it has the opposite effect to that on Mercury, gradually stripping away atmospheric gas as it blows by.

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth’s magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury.

“Mercury’s proximity to the sun only accounts for about a third of the reconnection rate we see,” said Slavin. “It will be exciting to see what’s special about Mercury to explain the rest. We’ll get more clues from MESSENGER’s third flyby on September 29, 2009, and when we get into orbit in March 2011.”

Slavin’s MESSENGER research was funded by NASA and is the subject of a paper that appeared in the journal Science on May 1, 2009.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory, Laurel, Md., built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

Source:  NASA

Have Astronomers Discovered A New Type of Supernova?

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A team of astronomers at the University of Warwick think they’ve finally explained what caused the bizarre transient object SCP 06F6.  By comparing the optical spectrum of SCP 06F6 to that of carbon-rich stars in our own galaxy, the team concludes the sudden outburst was not a low-energy local event but a supernova-like explosion within a cool carbon-rich atmosphere some 2 billion light years away.  If they’re right, it means the collapse of carbon-rich stars may lead to supernovae unlike any yet seen.

First observed in 2006 by U.S. researchers on images from the Hubble Space Telescope, SCP 06F6 flashed suddenly then faded from view over some 120 days.  The U.S. team published their findings in September 2008.  But they had no idea what might cause this outburst.  The event was so unusual, if fact, that astronomers had didn’t know whether SCP 06F6 was located in our own galaxy or at the other end of the universe.  Talk about experimental uncertainty!

The Warwick team noticed the optical spectrum of SCP 06F6 looked a lot like light from cool stars with molecular carbon in their atmosphere.  But to get a close spectral match with SCP 06F6, the team had to apply a redshift to the spectra of the carbon stars to correspond to a rapidly receding object some 2 billion light years away.  The large distance and the sudden appearance of SCP 06F6 suggest the object may be related to the sudden collapse of a carbon-rich star.  If so, it’s a brand new type of supernova.

But questions remain.  SCP 06F6 seems to be alone in space… it has no known visible host galaxy.  And the 120-day time scale of the object’s rise and fall in brightness is four times longer than most Type-II supernovae (the kind caused by the core-collapse of a massive star).

What’s more, X-ray observations by the European satellite XMM-Newton show the object blasts out up to 100 times more X-rays energy than expected from a typical Type-II supernova.

The strong X-ray emission may suggest the star was ripped apart by a black hole rather than exploding on its own.  But according to Boris Gansicke, the lead researcher of Warwick team, “The lack of any obvious host galaxy for SCP 06F6 would imply either a very low black hole mass (if black holes do exist at the centres of dwarf irregular galaxies) or that the black hole has somehow been ejected from its host galaxy. While neither is impossible, this does make the case for disruption by a black hole somewhat contrived.”

The findings were published in the June 1, 2009 issue of Astrophysical Journal Letters.

Source:  University of Warwick

Also see the Universe Today article about the discovery of SCP 06F6

Astronomers Observe Formation of Largest Bound Structures in the Universe

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An international team of astronomers has mapped the density and temperature of X-ray-emitting gas in the outskirts of a distant galaxy cluster.   The results, obtained with the orbiting Japanese X-ray telescope Suzaku, give the first complete X-ray view of a galaxy cluster, and provide insight into how such clusters come together.

“These Suzaku observations are exciting because we can finally see how these structures, the largest bound objects in the universe, grow even more massive,” said Matt George, the study’s lead author at the University of California, Berkeley.

The team trained Suzaku’s X-ray telescopes on the massive galaxy cluster PKS 0745-191, which lies 1.3 billion light-years away in the southern constellation Puppis.  Between May 11 and 14, 2007, Suzaku acquired five images of the million-degree gas that permeates the cluster.

The X-ray images of the cluster helped astronomers measure the temperature and density of the gas.  These provide clues about the gas pressure and cluster’s total mass.  The hottest, densest gas lies near the cluster’s center, while gas temperature and density steadily decline away from the center.

Astronomers believe the gas in the inner part of a galaxy cluster has settled into an ordered “relaxed” state in equilibrium with the cluster’s gravity.  But in the outer regions, where galaxies first begin a billion-year plunge towards the cluster’s center, the gas remains in a disordered state because it’s still falling inward.

“Clusters are the most massive, relaxed objects in the universe, and they are continuing to form now,” said team member Andy Fabian at the Cambridge Institute of Astronomy in the UK.

For the first time, this study shows X-ray emission and gas density and temperature out to the region where the gas is disordered, and where the cluster continues to assemble.

“It gives us the first complete X-ray view of a cluster of galaxies”, said Fabian.

This Suzaku image shows X-ray emission from hot gas throughout the galaxy cluster PKS 0745-191. Brighter colors indicate greater X-ray emission. The circle is 11.2 million light-years across and marks the region where cold gas is now entering the cluster. Inset: A Hubble optical image of the cluster's central galaxies is shown at the correct scale.
This Suzaku image shows X-ray emission from hot gas throughout the galaxy cluster PKS 0745-191. Brighter colors indicate greater X-ray emission. The circle is 11.2 million light-years across and marks the region where cold gas is now entering the cluster. Inset: A Hubble optical image of the cluster's central galaxies is shown at the correct scale.

In PKS 0745-191, the gas temperature peaks at 164 million degrees Fahrenheit (91 million C) about 1.1 million light-years from the cluster’s center. The temperature declines smoothly with distance, dropping to 45 million F (25 million C) more than 5.6 million light-years from the center.

To accurately measure X-ray emission at the cluster’s edge requires detectors with exceptionally low background noise.  Suzaku has advanced X-ray detectors, and it lies in a low-altitude orbit near the Earth’s magnetic field, which protects the observatory from energetic particles from the sun and beyond.

“With more Suzaku observations in the outskirts of other galaxy clusters, we’ll get a better picture of how these massive structures evolve,” added George.

Suzaku (Japanese for “red bird of the south”) was launched on July 10, 2005. The observatory was developed at the Japanese Institute of Space and Astronautical Science (ISAS), which is part of the Japan Aerospace Exploration Agency (JAXA), in collaboration with NASA and other Japanese and U.S. institutions.

The results were published in the May 11 edition of the Monthly Notices of the Royal Astronomical Society.

Source: NASA