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LCROSS strikes Earth’s moon as other moons continue to puzzle: Fourth dispatch from the annual planets meeting

The views expressed are those of the author and are not necessarily those of Scientific American.


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LCROSS moon impactFAJARDO, Puerto Rico—"We could have just stayed in bed" was one comment I overheard this morning from planetary scientists who had woken up early to see NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) crash into the lunar surface. At 7:31 A.M. (Eastern Daylight Time) the spacecraft’s Centaur-class rocket booster slammed (deliberately) into the moon, the idea being to set off a dramatic explosion and send up a plume of material that telescopes could analyze for signs of water in the lunar soil. The spacecraft flew through the plume and, four minutes later, itself crash-landed. To watch the event, planetary scientists at the annual Division for Planetary Sciences meeting here had crowded into an over-air-conditioned ballroom and poured themselves insipid hotel coffee. As the designated time approached, the moon’s surface loomed larger and larger as the spacecraft closed in [photograph at left].

At 7:30 the room went silent as everyone prepared to cheer wildly as the camera showed a bright flash. But the time came and went, and there was no flash, and no cheers. "It’s hard to know what we saw there," admitted Mike Bicay, science director at NASA Ames Research Center in Moffett Field, Calif. Shortly before the spacecraft itself hit, word came through that the infrared camera had indeed seen a thermal signature of the booster’s crater. This comment was barely audible, though, over the bemused laughter as images of the mission control center showed one controller conspicuously failing to respond to another’s high five.

Although the event was something of a letdown, all this will surely be forgotten when the data comes in and LCROSS adds to the string of recent discoveries showing that the moon is not quite the bone-dry world that scientists once assumed.

People often ask me what I think the biggest news of the meeting has been. For me, these meetings—and science generally—aren’t about the "biggest news." Instead, I come to keep tabs on the broader trends, watching the slow and patient accumulation of knowledge. Yesterday, for example, I attended an engrossing session on the formation of planets and their satellites. The field has inched forward since I started to attend these meetings 20 years ago, yet many of the same questions remain. If planet formation is a mystery, satellite formation is a mystery upon a mystery. Starting from a disk of material orbiting a planet, a satellite can coalesce extremely rapidly—in just a few thousand years. That is faster than the disk itself forms, which seems contradictory. Moreover, why are Jupiter’s and Saturn’s satellite systems so different? Jupiter has four large satellites fairly close to the planet (Io, Europa, Ganymede, and Callisto), whereas Saturn has only one giant satellite (the distant Titan). And why does Saturn have such a huge retinue of runty moons?

Bill Ward of the Southwest Research Institute in Boulder, Colo., has been working with his colleague Robin Canup on reconciling satellite and disk formation times. Their idea is that disk and satellite formation are not sequential but concurrent: they occur together as material dribbles in from the broader disk of material orbiting the sun. The inward dribble slows down satellite formation so that it never gets out of whack with the disk. In his talk yesterday, Ward argued that the requisite conditions naturally occur toward the end of planet-building.

Takanori Sasaki of the Tokyo Institute of Technology followed this up with an elegant explanation for the differences between the Jovian and Saturnian systems. Perhaps Saturn did once have multiple large satellites but rudely cannibalized them. Sasaki and his colleagues proposed that Jupiter’s strong magnetic field opened up a hole in the surrounding disk, whereas Saturn’s disk extended all the way down to the planet. Jupiter’s disk thus looked like a doughnut and Saturn’s like a saucer. One by one, satellites coalesced in the outer reaches of the disk and spiraled inward because of drag forces. In Jupiter’s case, satellites stopped migrating when they reached the inner edge of the disk, and they began to queue up there. In Saturn’s case, there was nothing to stop the hapless satellites from falling all the way in. Titan avoided this fate only because migration ceased as the disk dissipated.

As for Saturn’s family of little moons, Sébastien Charnoz of Paris Diderot University gave a talk entitled "Are Saturn’s small moons the children of Saturn’s rings?" Scientists often think of the rings as a moon that broke up or at least failed to form—the rings reside inside the so-called Roche limit, the distance within which a planet’s tidal forces rip apart any moon. Charnoz and his colleagues inverted this thinking. In their model, as the rings spread out, material exits the Roche limit and coalesces into moons. Once a moon forms, its gravitational interaction with the rings causes it to spiral outward, opening up room for another moon, and another. The outer edge of the rings becomes an assembly line of moons. Eventually the moons’ interaction with the rings becomes so strong that it shut downs the assembly line. This idea not only explains the moons, but also Saturn’s funky F ring: it may be a product of material coming off the innermost satellites.

With work like this, scientists are gradually piecing together a comprehensive view of solar system formation. Now if they can only prevail on their conference hotels to serve better coffee.

Previously:
From carbon planets to the lakes of Titan: Dispatch from the annual planets meeting
What caused Saturn to lurch? Second dispatch from the annual planets meeting

Planetary bombardments, past and future: Third dispatch from the annual planets meeting

View of the moon from LCROSS as the probe neared impact: NASA





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  1. 1. pcunix 1:09 pm 10/9/2009

    OK, so they were expecting to see a flash but did not.

    Wouldn’t that mean that either it hit something different than expected or that something went wrong with the blast?

    Or do the people who calculated what the effect should have been need new sliderules?

    Link to this
  2. 2. PetriDishFan 7:48 pm 10/9/2009

    This is interesting, bombing the moon. I do worry about the outcome of it though. What if the "bombing" went wrong? Would it effect us, poor and innocent, earthlings?

    Why would there be a flash in the first place? Would it be from impact on the moon? The scientists will somehow find a reason as to why there was no flash. How could they make that hypothesis though, if they never bombed the moon before?

    Link to this
  3. 3. Michael Hanlon 10:46 pm 10/10/2009

    Mr. Musser,
    Thank you for excellently reporting the goings on down in Puerto Rico. Where to next?

    Link to this
  4. 4. Michael Hanlon 4:24 am 10/12/2009

    Hopefully I’ve dragged jack.123 with me to bear witness to a prophetic piece of literature. Robert Anson Heinlein wrote "The Moon Is A Harsh Mistress". He discusses projectile effects on the Moon in that story. There is no air, ergo, no BOOM. There was no oxegen, ergo, no fireball. BUT, I wouldn’t have wanted to be standing within 10 miles of the impact! First there would have been BBshot at lethal velocities. Next would come the larger, slower boulder size chunks of debris, straight at you. Then the upward ejecta would start to come down on your head. Then the BB’s again ’cause they would have gone to a higher altitude. Then comes the real killer. Honest, you thought that flying stuff was bad? No way! The ensuing quake of the ground would have imparted g forces great enough to kill to any one within that ten miles. Right jack.123? Anyone at NASA who misled the press corps about what to expect should be forced to go read that book 3 times, once for each stupid, stupid, stupid.

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  5. 5. jack.123 7:31 pm 10/12/2009

    I agree,wouldn’t want to be anywhere near X marks the spot,there or here on Earth!

    Link to this
  6. 6. Michael Hanlon 2:17 am 10/17/2009

    Well, it’s been over a week now. No excuses for no flash, I’ll give ‘em that. But, where’s an inkling of what the data reveals?

    Link to this
  7. 7. jack.123 8:05 am 10/17/2009

    I wonder why it takes NASA,with some of the best minds on the planet,to do a study of the data?When a simple spectrum analysis would give the answers.No wonder, it cost so much to put things into space.To many chiefs not enough indians,eh Michael.

    Link to this
  8. 8. Michael Hanlon 8:05 pm 10/17/2009

    Where’s Lt Col. Custer when we don’t need him?

    Link to this
  9. 9. Michael Hanlon 9:02 pm 10/18/2009

    Today’s news, NASA ‘found’ the feed from another on board camera and in the video from that stream of data, a plume is definitely visible. Found? another camera feed? A week to tell us? Still no word on molecular scan results from particle analysis from the second impactor? When I get to four questions about one simple event, conspiracy comes to mind real quick.

    Link to this
  10. 10. Michael Hanlon 1:05 am 10/30/2009

    Three weeks ……!! ?????
    ."Can I get some more data Mom? No, you didn’t say please."
    ."Fagan says no my young man, you may not have more."
    ."The world will end on Jan 1, 2000", "No it’s Jan 1, 2001 for the new millenium."
    ."It’s like Christmas Eve and the Clock breaks at 11:52 so tomorrow never comes."
    ."Did I miss 1984?"
    ."If beans reach wild forage gladly, will grease become after Rigel 7 ?"
    ."Are they ALL drinking Alice’s potion?"
    ."No, said the cheshire cat, only afraid to admit nothing’s there!"
    ."And only a grin was left."
    .
    .
    .
    I want water data. Wah, Wah, Wahta dah dah dat ha

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  11. 11. Michael Hanlon 10:14 pm 11/13/2009

    November 13, 2009, a full five (5) weeks after impact it is announced that 25 gallons of water were estimated to be part of the plume debris. Though they say it was spectral analysis that led to the interpreted data, it was not stated if that analysis was actually performed on the plume or on the debris which is shown to have spread around the impact sight. This question exists because of the ambiguity of the term "plume debris".
    It was stated that they went through all the possible elemental/material combinations which would have given them the data which indicates the H2O presence. This is good news indeed. Whether the pains it took to get it were worth the blemish to NASA’s reputation remains to be analyzed.
    I’m sure it was calculated but was not detailed as to the % constituency of water. Meaning did it take pulverizing 100 tons of regolith to see 25 gallons or was the figure higher or lower? I’m sre the answer will out but why not now?

    Link to this
  12. 12. jack.123 10:56 pm 11/13/2009

    Michael-Hope this doesn’t change the Presidents mind about going to an asteroid.The capture of Apophis would still be one of the greatest achivements in the history of mankind,and would provide all the resourses that our species needs to save ourselves and the rest of the world from the coming of the next ice age,it may not happen for another 5 or 10 thousand years,unless we do something stupid like reducing Co2′s at too rapid a rate.A rapid cool down would be far worse than a slow warm up,and it almost certainly would cause a world war,because people wouldn’t starve to death without a fight.

    Link to this

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