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Marsbugs: The Electronic Astrobiology Newsletter, Volume 12, Number 11, 30 March 2005
Marsbugs: The Electronic Astrobiology Newsletter
Volume 12, Number 11, 30 March 2005
Editor/Publisher: David J. Thomas, Ph.D., Science Division, Lyon College, Batesville, Arkansas 72503-2317, USA.
Marsbugs is published on a weekly to monthly basis as warranted by the number of articles and announcements. Copyright of this compilation exists with the editor, but individual authors retain the copyright of specific articles. Opinions expressed in this newsletter are those of the authors, and are not necessarily endorsed by the editor or by Lyon College. E-mail subscriptions are free, and may be obtained by contacting the editor. Information concerning the scope of this newsletter, subscription formats and availability of back-issues is available at http://www.lyon.edu/projects/marsbugs. The editor does not condone "spamming" of subscribers. Readers would appreciate it if others would not send unsolicited e-mail using the Marsbugs mailing lists. Persons who have information that may be of interest to subscribers of Marsbugs should send that information to the editor.
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Marsbugs: The Electronic Astrobiology Newsletter, Volume 12, Number 11, 30 March 2005
Articles and News
Page 1 COMPARING THE TRIAD OF GREAT MOONS
By Jonathan Lunine
Page 2 NASA'S SPITZER MARKS BEGINNING OF NEW AGE OF PLANETARY SCIENCE
NASA/JPL release 2005-050
Page 3 PUSHING THE PLANETARY ENVELOPE (INTERVIEW WITH NEIL DEGRASSE TYSON, PART 5)
By Leslie Mullen
Page 5 MYSTERY MINERALS FORMED IN FIREBALL FROM COLLIDING ASTEROID THAT DESTROYED THE DINOSAURS
University of Chicago release
Page 6 NEW FRONTIER OPENS IN THE SEARCH FOR LIFE ON OTHER PLANETS
NASA/GSFC release
Page 7 CENSORSHIP OF IMAX FILMS THREATENS INTEGRITY OF SCIENCE, LEADER SAYS
From LiveScience
Page 7 CROSSING THE TREELINE
By Chris McKay
Page 7 CLIMATE MODELS REVEAL INEVITABILITY OF GLOBAL WARMING
By Sarah Graham
Page 7 ON AMMONIA AND ASTROBIOLOGY
By Jonathan Lunine
Mission Reports
Page 8 CASSINI SIGNIFICANT EVENTS FOR 10-16 MARCH 2005
NASA/JPL release
Page 10 DEEP IMPACT MISSION STATUS REPORT
NASA release 05-086
Page 11 MARS EXPRESS: THE MEDUSA FOSSAE FORMATION ON MARS
ESA release
Page 12 MARS GLOBAL SURVEYOR IMAGES
NASA/JPL/MSSS release
Page 12 MARS ODYSSEY THEMIS IMAGES
NASA/JPL/ASU release
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Marsbugs: The Electronic Astrobiology Newsletter, Volume 12, Number 11, 30 March 2005
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Marsbugs: The Electronic Astrobiology Newsletter, Volume 12, Number 11, 30 March 2005
COMPARING THE TRIAD OF GREAT MOONS
By Jonathan Lunine
From Astrobiology Magazine
Jonathan Lunine, a professor of planetary science and physics at the University of Arizona's Lunar and Planetary Laboratory in Tucson, Arizona, is also an interdisciplinary scientist on the Cassini/Huygens mission. Lunine presented a lecture entitled "Titan: A Personal View after Cassini's first six months in Saturn orbit" at a NASA Director's Seminar on January 24, 2005. This edited transcript of the Director's Seminar is Part 3 of a 4-part series.
Saturn's moon Titan is one of a triad of giant moons, the other two being Jupiter's moons Ganymede and Callisto. Interestingly, these moons all have about the same density, and therefore about the same mass and radius. The density for these three objects is between 1.8 and 1.9 grams per cubic centimeter, and the radius is between 2500 and 2600 kilometers. This makes them the three largest moons in the solar system—much larger than our own moon and larger than the planet Mercury. Because they are the same density and size, they are also composed of the same materials.
Left: High resolution close-up of Io's volcanic surface. Only a third the size of Earth and five times as far from the Sun, Io generates twice our total terrestrial heat bill. Image credit: NASA/Galileo. Right: Titan continues to puzzle scientists who speculate about the hydrocarbon rich composition. Image credit: NASA/JPL.
Presumably, there must be some process that truncates the formation or growth of satellites at about the size of Ganymede, Callisto, and Titan. It's not clear what that process is, but one clue can be found in the density—you're making these bodies out of rock and ice. When you get to the size of Titan, the energy that's released during formation, just due to the infall of material, is about equal to the latent heat per gram of the ice-component of the material. In other words, the energy released toward the end of accretion is equal to the heat needed to vaporize water ice. That means that if water ice is the stuff you're accreting, you're vaporizing it and it's going away, so accretion becomes less efficient. Maybe that's a natural truncation process for these rocky and icy worlds around the giant planets.
This image shows a possible caldera from which liquid water or slush may have once flowed on Ganymede. Image credit: NASA/JPL.
What sets Titan apart from Ganymede and Callisto, however, is the presence of an atmosphere. This atmosphere was discovered in 1943, when Gerard Kuiper detected Titan's methane using an Earth-based telescope. Voyager 1 discovered molecular nitrogen as the dominant constituent of Titan's atmosphere in 1980. That was done indirectly, by using an ultraviolet spectrometer to get the density of the atmosphere, and then putting that information together with data from an infrared spectrometer. From those instruments together, it was clear that nitrogen was the dominant constituent of the atmosphere. We don't have to worry about that indirectness anymore, because the Cassini orbiter mass spectrometer and the Huygens gas chromatograph mass spectrometer have both directly detected—or tasted, if you want to think of it that way—nitrogen as the dominant constituent of Titan's atmosphere. They also directly detected methane. So that era of spectroscopy and inference has now drawn to a close because of Cassini.
Because methane is abundant in Titan's atmosphere, it becomes a very interesting place from the point of view of organic chemistry. Its atmosphere is between 2 and 4 percent methane—going from the middle, coldest part of the atmosphere, the tropopause, down to the surface. And that means there is abundant organic chemistry going on, powered primarily in the upper atmosphere by ultraviolet light from the sun. There may be additional chemistry occurring on the surface, powered by other energy sources.
The temperature at the surface of Titan is 95 degrees Kelvin, and the temperature drops off with altitude to a temperature of 70 Kelvin at about 40 kilometers. Titan has a much more distended atmosphere than the Earth's because of the lower gravity. The surface pressure on Titan is one and a half bars, so the density of Titan's atmosphere at the surface is four times denser than the air at sea level on the Earth. This is the second densest atmosphere on the four solid bodies with substantial atmospheres in the solar system, second only to Venus. Earth then is third, and Mars is fourth.
So why does Titan have an atmosphere, when Ganymede and Callisto do not? I think the answer is becoming abundantly clear from the Cassini data. Titan has accreted, or acquired, large amounts of ammonia in addition to the water. Large, meaning maybe a few percent, but that's enough.
We know a fair amount about the interiors of Ganymede and Callisto from the Galileo orbiter that made multiple flybys of the moons of Jupiter from 1995 onward to 2003. Ganymede is highly differentiated—the silicates and the metal are not simply mixed together, it appears that they're actually separated out. That implies fairly highly temperatures during accretion. Ganymede has a silicate mantle around a metal core, then an ice mantle as well, with high-pressure ice phases. Above about 2 kilobars, water ice assumes different structures that are denser than liquid water.
Now, that's important, because on a moon that has some melting, the liquid water layer is going to be sandwiched between ice layers of different pressures. It doesn't appear that there's such a liquid layer within Ganymede and Callisto, although there may have been some softening of the ice.
But if you add ammonia to any of these objects—Ganymede, Callisto or Titan—there would be an ice mantle with a liquid layer within it, due to the ammonia lowering the melting point for the water ice. That liquid layer is sandwiched between the lower density ice and the higher density, high pressure ice phases. Yet the environment where Ganymede and Callisto formed was simply too warm for substantial amounts of ammonia to bond with the water ice. You need a certain temperature to get ammonia hydrates forming in these planetisimals, and I think it was just too warm at Jupiter.
If ammonia is present on Titan, it could be the source of the nitrogen in the atmosphere, since ammonia is the bearer of nitrogen. With a liquid layer, the ammonia also would allow for what's called cryovolcanism, and therefore bleed some material onto the surface. Having a moon that's volatile-rich in this way leads naturally to the occurrence of an atmosphere.
Read the original article at http://www.astrobio.net/news/article1493.html.
NASA'S SPITZER MARKS BEGINNING OF NEW AGE OF PLANETARY SCIENCE
NASA/JPL release 2005-050
22 March 2005
NASA's Spitzer Space Telescope has for the first time captured the light from two known planets orbiting stars other than our Sun. The findings mark the beginning of a new age of planetary science, in which "extrasolar" planets can be directly measured and compared.
This graph of data from NASA's Spitzer Space telescope shows changes in the infrared light output of two star-planet systems (one above, one below) located hundreds of light-years away. The data were taken while the planets, called HD 209458b and TrES-1, disappeared behind their stars in what is called a "secondary eclipse". The dip seen in the center of each graph represents the time when the planets were eclipsed, and tells astronomers exactly how much light they emit. Why a secondary eclipse? When a planet transits, or passes in front of, its star, it partially blocks the light of the star. When the planet swings around behind the star, the star completely blocks its light. This drop in total light can be measured to determine the amount of light coming from just the planet. Image credits: NASA/JPL-Caltech/D. Charbonneau (Harvard-Smithsonian CfA) and NASA/JPL-Caltech/D. Deming (Goddard Space Flight Center).
"Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres and orbits of planets hundreds of light-years from Earth," said Dr. Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, MD, lead author of a new study on one of the planets.
"It's fantastic," said Dr. David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, lead author of a separate study on a different planet. "We've been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered." The Deming paper appears today in Nature's online publication; the Charbonneau paper will be published in an upcoming issue of the Astrophysical Journal.
So far, all confirmed extrasolar planets, including the two recently observed by Spitzer, have been discovered indirectly, mainly by the "wobble" technique and more recently, the "transit" technique. In the first method, a planet is detected by the gravitational tug it exerts on its parent star, which makes the star wobble. In the second, a planet's presence is inferred when it passes in front of its star, causing the star to dim, or blink. Both strategies use visible-light telescopes and indirectly reveal
In the new studies, Spitzer has directly observed the warm infrared glows of two previously detected "hot Jupiter" planets, designated HD 209458b and TrES-1. Hot Jupiters are extrasolar gas giants that zip closely around their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths.
This artist's concept shows what a fiery hot star and its close-knit planetary companion might look close up if viewed in visible (left) and infrared light. In visible light, a star shines brilliantly, overwhelming the little light that is reflected by its planet. In infrared, a star is less blinding, and its planet perks up with a fiery glow. In this figure, the colors represent real differences between the visible and infrared views of the system. The visible panel shows what our eyes would see if we could witness the system close up. The hot star is yellow because, like our Sun, it is brightest in yellow wavelengths. The warm planet, on the other hand, is brightest in infrared light, which we can't see. Instead, we would see the glimmer of star light that the planet reflects. In the infrared panel, the colors reflect what our eyes might see if we could retune them to the invisible, infrared portion of the light spectrum. The hot star is less bright in infrared light than in visible and appears fainter. The warm planet peaks in infrared light, so is shown brighter. Their hues represent relative differences in temperature. Because the star is hotter than the planet, and because hotter objects give off more blue light than red, the star is depicted in blue, and the planet, red. The overall look of the planet is inspired by theoretical models of hot, gas giant planets. These "hot Jupiters" are similar to Jupiter in composition and mass, but are expected to look quite different at such high temperatures. The models are courtesy of Drs. Curtis Cooper and Adam Showman of the University of Arizona, Tucson. Image credit: NASA/JPL-Caltech/R. Hurt (SSC).
To distinguish this planet glow from that of the fiery hot stars, the astronomers used a simple trick. First, they used Spitzer to collect the total infrared light from both the stars and planets. Then, when the planets dipped behind the stars as part of their regular orbit, the astronomers measured the infrared light coming from just the stars. This pinpointed exactly how much infrared light belonged to the planets. "In visible light, the glare of the star completely overwhelms the glimmer of light reflec star-planet contrast is more favorable because the planet emits its own light."
The Spitzer data told the astronomers that both planets are at least a steaming 1,000 Kelvin (727 degrees Celsius, 1340 Fahrenheit). These measurements confirm that hot Jupiters are indeed hot. Upcoming Spitzer observations using a range of infrared wavelengths are expected to provide more information about the planets' winds and atmospheric compositions.