September 30, 2011 | David F. Coppedge

Mercury Messenger: Surprise!

As boring as the moon?  Just a burned-out cinder?  Not Mercury.  True to tradition for planetary exploration, the MESSENGER spacecraft has served up a plate of surprises about the innermost planet.  In orbit since March, the ship is sending theorists back to the drawing board to figure out a number of puzzling phenomena, some unique to Mercury.  Commentators fall into two categories: those that are flabbergasted, and those who say all is well.

Science magazine published the first seven papers this week since the orbital tour began.  Here were the headlines that resulted on various news outlets:

  • Science Daily: “Mercury Not Like Other Planets, MESSENGER Finds.”
  • PhysOrg: “Epic volcanic activity flooded Mercury’s north polar region”
  • BBC News: “‘Hollows’ mark Mercury’s surface.”  The article begins, “Hands up who thought Mercury was just a dull rock circling close to the Sun?  The latest data returned by Nasa’s Messenger probe shows that view couldn't be further from the truth.”
  • National Geographic: “Mercury ‘Hollows’ Found—Pits May Be Solar System First.
  • New Scientist: “Bright ‘hollows’ on Mercury are unique in solar system.”
  • Space.com: “Planet Mercury Full of Strange Surprises

By contrast, Richard Kerr’s summary article in Science was titled, calmly, “Mercury Looking Less Exotic, More a Member of the Family" (30 September 2011: Vol. 333 no. 6051 p. 1812, DOI: 10.1126/science.333.6051.1812).  Judging from abstracts and reports, the following discoveries seem the most interesting:

  1. Hollows:  The hollows spoken of are unique structures found within some craters.  Irregular in shape and up to miles across, these depressions with sharp rims, often found in clusters and found across Mercury, appear to be collapse pits – as if volatile substances escaped from underground and caused surfaces to fall.  The closest analogues are on Mars, where similarly shaped features result from sublimation of ice at the poles; but here on Mercury there is no ice.  Science Daily called them “an unexpected class of landform on Mercury and suggest that a previously unrecognized geological process is responsible for its formation.”  New Scientist said of them, “They may have been formed by processes still active today, and change our view of the small rocky planet’s history.”  Science Daily quoted a scientist who believes they are actively forming today – further evidence that “Mercury is radically different from the Moon in just about every way we can measure.”  National Geographic quoted David Blewett (Johns Hopkins): “"The old thinking was, Oh, Mercury, it’s an old burned-out cinder and not so interestinghere’s this jaw-dropping thing that nobody ever predicted.
  2. SulfurSpace.com introduced this surprise: “Mercury is not just hellishly hot but apparently covered in brimstone. A vast part of the planet is covered with dried lava  – enough to bury the state of Texas under 4 miles of the stuff, scientists say.”  Richard Kerr in Science said “Surprisingly, it has 10 times the sulfur of Earth’s rock.
  3. Reducing conditions:  Mercury doesn’t fit another expectation.  Richard Kerr explained, “The combination of high sulfur and low iron in Mercury's rock must have come from minerals that could have existed only if Mercury formed under chemically reducing conditions. That sounds bizarre, because all the other rocky planets formed under the opposite conditions: oxidizing ones.”  He was quick to find a scientist who “showed that probably only water-free, organics-rich, comet-dust–like stuff would have survived near the sun to make Mercury. With no oxygen atoms from water around, reducing conditions would have prevailed.”  Still, it makes Mercury a special case compared to nearby Venus and Earth.
  4. PotassiumScience Daily explained why elevated potassium levels seen on the surface is a challenge to explain: “Measurements of Mercury’s surface by MESSENGER's X-Ray and Gamma-Ray Spectrometers also reveal substantially higher abundances of sulfur and potassium than previously predicted. Both elements vaporize at relatively low temperatures, and their abundances thus rule out several popular scenarios in which Mercury experienced extreme high-temperature events early in its history.”
  5. Lava flows:  Evidence of volcanism had been observed on the previous three flybys, but the extent of lava plains exceeded expectations – some five million cubic kilometers.  The BBC News had a comparison some angry voters might like: “This is enough lava to cover the City of Washington DC to a depth of over 26,000 km, which is about 72 times higher than the orbit of the International Space Station.” Seen primarily in previously-unseen northern regions, the lava is thought to have oozed out of fissures, rather than coming from eruptive centers that produce familiar cone-shaped mountains.  Space.com explained, “Based on the way this lava apparently eroded the underlying surface, the researchers suggest it rushed out rapidly.”  Lead scientist James Head (Brown U) thinks the flows date from billions of years ago, but remarked, “We can’t say if it took 2.7 days or 15 years or any exact time from orbit, but it wasn’t hundreds of millions of years.” Why extensive volcanism would turn on like that, last a few years, and then stop – only to remain unchanged for billions of years – seems odd.
  6. Magnetic field:  Of the rocky planets, only Earth and Mercury have global magnetic fields.  Unlike other magnetic fields, Mercury has one that is only 3% offset from its polar axis, but is inexplicably displaced some 300 miles northward from the center of the planet.  Mercury’s is also much weaker than Earth’s – too weak to provide protection from the solar wind.  See Science Daily for details.

Science Daily began its coverage with this summary that emphasized the theoretical challenges:

Only six months into its Mercury orbit, the tiny MESSENGER spacecraft has shown scientists that Mercury doesn’t conform to theory. Its surface material composition differs in important ways from both those of the other terrestrial planets and expectations prior to the MESSENGER mission, calling into question current theories for Mercury’s formation. Its magnetic field is unlike any other in the Solar System, and there are huge expanses of volcanic plains surrounding the north polar region of the planet and cover more than 6% of Mercury’s surface…. Theorists need to go back to the drawing board on Mercury's formation,” remarked the lead author of one of the papers, Carnegie’s Larry Nittler. “Most previous ideas about Mercury’s chemistry are inconsistent with what we have actually measured on the planet’s surface.”

Science Daily ended with a quote by Sean Solomon: “Mercury is not the planet described in the textbooks. Although a true sibling of Venus, Mars, and Earth, the innermost planet has had a much more exciting life than anyone predicted.

It could be argued that planetary scientists need surprises to justify their jobs.  Would the public continue to support space exploration if everything was as boring as predicted?  Reporters, too, need to fan the hype with exciting headlines, or else advertisers might not get their return on investment.  That doesn't seem to be the case here.  Long before this mission was conceived, planetary scientists had their theories based on the 1970s data from Mariner 10.  And it's not like they were all waiting with nothing to do before MESSENGER got there.  Many of these things seem like genuine surprises that are important. 

While we join the revelry of new discoveries and applaud the many designers, engineers, scientists and assorted workers for a job well done getting this ship into orbit and delivering the data, we think much of the hand-wringing about textbooks being blown out of the water is a result of thoughtless adherence to the old Moyboy* Myth, that old Law of the Misdeeds and Perversions that Cannot be Altered.  To keep the data in the ASS (age of the solar system, 4.5 billion years), believers have to tell fantastic tales: lava erupted quickly and suddenly all over the north, then shut off for billions of years, while depressions are being hollowed out in a process that could still be ongoing today.  Inside a planet smaller than Saturn's moon Titan, they have to keep an iron core liquid long enough so that a global magnetic field can survive.  They have to make a planet out of volatile elements that were believed not possible to exist so close to the sun, but then keep vast deposits of it intact after billions of years of solar heat and bombardment.

MESSENGER is just the messenger.  The sender, Mercury, is saying, "Think outside the moyboy* box."

*Millions of Years, Billions of Years

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