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Fw: {MPML} Ron Balke on The Fireballs of February and on The Many Moods of Titan

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  • Bruce Kamiat
    A list for asteroid and comet researcher ... From: mpml@yahoogroups.com To: mpml@yahoogroups.com Sent: Friday, February 24, 2012 4:30 AM Subject: {MPML} Digest
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      A list for asteroid and comet researcher

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      Sent: Friday, February 24, 2012 4:30 AM
      Subject: {MPML} Digest Number 4061

      A list for asteroid and comet researcher
      Messages In This Digest (2 Messages)
      1. The Fireballs of February From: Ron Baalke
      2. The Many Moods of Titan From: Ron Baalke
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      1. The Fireballs of February
      Posted by: "Ron Baalke" baalke@...
      Thu Feb 23, 2012 1:15 pm (PST)


      The Fireballs of February

      Feb. 22, 2012: In the middle of the night on February 13th, something
      disturbed the animal population of rural Portal, Georgia. Cows started
      mooing anxiously and local dogs howled at the sky. The cause of the
      commotion was a rock from space.

      "At 1:43 AM Eastern, I witnessed an amazing fireball," reports Portal
      resident Henry Strickland. "It was very large and lit up half the sky as
      it fragmented. The event set dogs barking and upset cattle, which began
      to make excited sounds. I regret I didn't have a camera; it lasted
      nearly 6 seconds."

      Strickland witnessed one of the unusual "Fireballs of February."

      "This month, some big space rocks have been hitting Earth's atmosphere,"
      says Bill Cooke of NASA's Meteoroid Environment Office. "There have been
      five or six notable fireballs that might have dropped meteorites around
      the United States."

      It's not the number of fireballs that has researchers puzzled. So far,
      fireball counts in February 2012 are about normal. Instead, it's the
      appearance and trajectory of the fireballs that sets them apart.

      "These fireballs are particularly slow and penetrating," explains meteor
      expert Peter Brown, a physics professor at the University of Western
      Ontario. "They hit the top of the atmosphere moving slower than 15 km/s,
      decelerate rapidly, and make it to within 50 km of Earth’s surface."

      The action began on the evening of February 1st when a fireball over
      central Texas wowed thousands of onlookers in the Dallas-Fort Worth area.

      "It was brighter and long-lasting than anything I've seen before,"
      reports eye-witness Daryn Morran. "The fireball took about 8 seconds to
      cross the sky. I could see the fireball start to slow down; then it
      exploded like a firecracker artillery shell into several pieces,
      flickered a few more times and then slowly burned out." Another observer
      in Coppell, Texas, reported a loud double boom as "the object broke into
      two major chunks with many smaller pieces."

      The fireball was bright enough to be seen on NASA cameras located in New
      Mexico more than 500 miles away. "It was about as bright as the full
      Moon," says Cooke. Based on the NASA imagery and other observations,
      Cooke estimates that the object was 1 to 2 meters in diameter.

      So far in February, NASA's All-Sky Fireball Network has photographed
      about a half a dozen bright meteors that belong to this oddball
      category. They range in size from basketballs to buses, and all share
      the same slow entry speed and deep atmospheric penetration. Cooke has
      analyzed their orbits and come to a surprising conclusion:

      February Fireballs (meteorcam, 200px) <http://fireballs.ndc.nasa.gov/>
      This camera is part of NASA's All-Sky Fireball Network. [more

      "They all hail from the asteroid belt - but not from a single location in
      the asteroid belt," he says. "There is no common source for these
      fireballs, which is puzzling."

      This isn't the first time sky watchers have noticed odd fireballs in
      February. In fact, the "Fireballs of February" are a bit of a legend in
      meteor circles.

      Brown explains: "Back in the 1960s and 70s, amateur astronomers noticed
      an increase in the number of bright, sound-producing deep-penetrating
      fireballs during the month of February. The numbers seemed significant,
      especially when you consider that there are few people outside at night
      in winter. Follow-up studies in the late 1980s suggested no big increase
      in the rate of February fireballs. Nevertheless, we've always wondered
      if something was going on."

      Indeed, a 1990 study by astronomer Ian Holliday suggests that the
      'February Fireballs' are real. He analyzed photographic records of about
      a thousand fireballs from the 1970s and 80s and found evidence for a
      fireball stream intersecting Earth's orbit in February. He also found
      signs of fireball streams in late summer and fall. The results are
      controversial, however. Even Halliday recognized some big statistical
      uncertainties in his results.

      NASA's growing All-Sky Fireball Network could end up solving the
      mystery. Cooke and colleagues are adding cameras all the time, spreading
      the network's coverage across North America for a dense, uninterrupted
      sampling of the night sky.

      "The beauty of our smart multi-camera system," notes Cooke, "is that it
      measures orbits almost instantly. We know right away when a fireball
      flurry is underway—and we can tell where the meteoroids came from." This
      kind of instant data is almost unprecedented in meteor science, and
      promises new insights into the origin of February’s fireballs.

      Meanwhile, the month isn't over yet. "If the cows and dogs start raising
      a ruckus tonight," advises Cooke, "go out and take a look."

      Author:Dr. Tony Phillips
      Production editor: Dr. Tony Phillips
      Credit: Science@NASA

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      2. The Many Moods of Titan
      Posted by: "Ron Baalke" baalke@...
      Thu Feb 23, 2012 1:19 pm (PST)


      The Many Moods of Titan
      Jet Propulsion Laboratory
      February 23, 2012

      A set of recent papers, many of which draw on data from NASA's Cassini
      spacecraft, reveal new details in the emerging picture of how Saturn's
      moon Titan shifts with the seasons and even throughout the day. The
      papers, published in the journal Planetary and Space Science in a
      special issue titled "Titan through Time", show how this largest moon of
      Saturn is a cousin - though a very peculiar cousin - of Earth.

      "As a whole, these papers give us some new pieces in the jigsaw puzzle
      that is Titan," said Conor Nixon, a Cassini team scientist at the NASA
      Goddard Space Flight Center, Greenbelt, Md., who co-edited the special
      issue with Ralph Lorenz, a Cassini team scientist based at the Johns
      Hopkins University Applied Physics Laboratory, Laurel, Md. "They show us
      in detail how Titan's atmosphere and surface behave like Earth's - with
      clouds, rainfall, river valleys and lakes. They show us that the seasons
      change, too, on Titan, although in unexpected ways."

      A paper led by Stephane Le Mouelic, a Cassini team associate at the
      French National Center for Scientific Research (CNRS) at the University
      of Nantes, highlights the kind of seasonal changes that occur at Titan
      with a set of the best looks yet at the vast north polar cloud.

      A newly published selection of images - made from data collected by
      Cassini's visual and infrared mapping spectrometer over five years -
      shows how the cloud thinned out and retreated as winter turned to spring
      in the northern hemisphere.

      Cassini first detected the cloud, which scientists think is composed of
      ethane, shortly after its arrival in the Saturn system in 2004. The
      first really good opportunity for the spectrometer to observe the
      half-lit north pole occurred on December 2006. At that time, the cloud
      appeared to cover the north pole completely down to about 55 degrees
      north latitude. But in the 2009 images, the cloud cover had so many
      gaps it unveiled to Cassini's view the hydrocarbon sea known as Kraken
      Mare and surrounding lakes.

      "Snapshot by snapshot, these images give Cassini scientists concrete
      evidence that Titan's atmosphere changes with the seasons," said Le
      Mouelic. "We can't wait to see more of the surface, in particular in the
      northern land of lakes and seas."

      In data gathered by Cassini's composite infrared mapping spectrometer to
      analyze temperatures on Titan's surface, not only did scientists see
      seasonal change on Titan, but they also saw day-to-night surface
      temperature changes for the first time. The paper, led by Valeria
      Cottini, a Cassini associate based at Goddard, used data collected at a
      wavelength that penetrated through Titan's thick haze to see the moon's
      surface. Like Earth, the surface temperature of Titan, which is usually
      in the chilly mid-90 kelvins (around minus 288 degrees Fahrenheit), was
      significantly warmer in the late afternoon than around dawn.

      "While the temperature difference - 1.5 kelvins - is smaller than what
      we're used to on Earth, the finding still shows that Titan's surface
      behaves in ways familiar to us earthlings," Cottini said. "We now see
      how the long Titan day (about 16 Earth days) reveals itself through the

      A third paper by Dominic Fortes, an outside researcher based at
      University College London, England, addresses the long-standing mystery
      of the structure of Titan's interior and its relationship to the
      strikingly Earth-like range of geologic features seen on the surface.
      Fortes constructed an array of models of Titan's interior and compared
      these with newly acquired data from Cassini's radio science experiment.

      The work shows the moon's interior is partly or possibly even fully
      differentiated. This means that the core is denser than outer parts of
      the moon, although less dense than expected. This may be because the
      core still contains a large amount of ice or because the rocks have
      reacted with water to form low-density minerals.

      Earth and other terrestrial planets are fully differentiated and have a
      dense iron core. Fortes' model, however, rules out a metallic core
      inside Titan and agrees with Cassini magnetometer data that suggests a
      relatively cool and wet rocky interior. The new model also highlights
      the difficulty in explaining the presence of important gases in Titan's
      atmosphere, such as methane and argon-40, since they do not appear to be
      able to escape from the core.

      The Cassini-Huygens mission is a cooperative project of NASA, the
      European Space Agency and the Italian Space Agency. NASA's Jet
      Propulsion Laboratory manages the mission for NASA's Science Mission
      Directorate, Washington, D.C. The visual and infrared mapping
      spectrometer team is based at the University of Arizona, Tucson. The
      composite infrared spectrometer team is based at NASA's Goddard Space
      Flight Center in Greenbelt, Md., where the instrument was built. The
      radio science subsystem has been jointly developed by NASA and the
      Italian Space Agency.

      Jia-Rui Cook 818-354-0850
      Jet Propulsion Laboratory, Pasadena, Calif.

      Elizabeth Zubritsky 301-614-5438
      Goddard Space Flight Center, Greenbelt, Md.
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