July 9, 2008 | David F. Coppedge

First Mercury Research Papers in from MESSENGER

Science published a suite of papers analyzing data from the first MESSENGER spacecraft flyby of Mercury.1  The flyby last January was the first since Mariner 10 visited in the 1970s.  Mariner 10 had left many questions that are now being revisited.  Among the dozen papers and articles, here are three that discuss the most significant discoveries touching on Mercury’s origin and history.

  1. Volcanoes:  The volcanoes are officially real.  Mariner had left doubt whether the smooth plains and craters with flattened floors were produced volcanically.  Now, MESSENGER scientists have confirmed numerous cases of intrusive and extrusive volcanism.2  Many impact craters and areas between craters have been flooded with lava.  In addition, small “ghost” craters can be seen in some lava flows.  The volume of lava in some craters is impressive.  Some flooded craters are hundreds of miles wide and 5 km deep.  Grabens and wrinkle ridges subsequent to the eruptions suggest that Mercury’s surface shrank subsequent to the planet’s formation.  Overall, the volcanic evidence on Mercury resembles that of earth’s moon.
  2. Craters:  The interpretation of surface age by crater counts has continued to be controversial, reported Strom et al in another paper.3  Crater-counters will need to take into consideration what they said: “Clusters of secondaries, seen in some higher-resolution Mariner 10 images, were presumed to constitute a minor fraction of Mercury’s smaller craters.  MESSENGER images suggest that secondary cratering is much more important than had been thought, as exemplified by the many distinct chains and clusters of craters radiating away from prominent, large, fresh impact craters and basins.”
        Additional findings cast more doubt on the assumption that craters signify how old things are.  Some heavily-cratered plains appear much younger than others which presumably formed during a hypothetical Late Heavy Bombardment 3.5 billion years ago.  “In any event, the use of small craters for dating of geological units on Mercury must be done with even greater caution than is needed for other bodies.”  Why?  “Whereas an older unit will tend to have more secondaries on it than a younger unit, there cannot be the one-to-one correspondence of crater density with relative or absolute age (as there is for primary craters) because of the temporally and spatially nonuniform production of secondaries.”  If crater counts are to have any age interpretation, it seems that each planet or moon will have to have its own rules.
  3. Magnetic field:  The fact that two rocky planets (Mercury and Earth) have global magnetic fields, and the other two (Venus and Mars) do not remains puzzling.  Mariner 10 “yielded the surprising result that Mercury has a coherent, intrinsic magnetic field,” Anderson et al stated in another paper.4  For a small, ancient, slow-spinning body to maintain a global field, when larger planets do not, is a mystery.  There has not been a statistically significant change in Mercury’s magnetic field strength since 1974.
        The field seems predominately dipolar.  How is it produced?  The leading theory suggests a stagnant layer at the top of the outermost molten core, where convection currents might generate a dynamo.  MESSENGER measurements are “consistent” with this view.  The field produces a sizeable magnetosphere, which was analyzed by Slavin et al.5  Mercury is unique in having a double magnetopause and multiple current sheets: “This double MP signature had not been observed previously at Mercury or any other planetary magnetosphere,” they said.  Perhaps the reason is Mercury’s unusual proximity to the sun and the solar wind.  The magnetosphere is immersed in a cloud of comet-like planetary ions that influence the field’s shape and behavior.
        The MESSENGER press release said, “Researchers have been puzzled by Mercury’s field since its iron core should have cooled long ago and stopped generating magnetism.  Some researchers have thought that the field may have been a relic of the past, frozen in the outer crust.”  The fact that the field remains dipolar rules that out, and “supports the view that we’re seeing a modern dynamo.”  If so, the core is not solid as it should be for a small planet after billions of years.

MESSENGER’s next flyby is October 6.  After another flyby September 29, 2009, the spacecraft reaches orbit around Mercury on March 18, 2011.  The MESSENGER home page contains details on the mission and science goals, along with a gallery and summary of findings (see the July 3 press release).  Popular reports were posted on National Geographic and Space.com.

1.  Solomon et al, “Return to Mercury: A Global Perspective on MESSENGER’s First Mercury Flyby,” Science, 4 July 2008: Vol. 321. no. 5885, pp. 59-62, DOI: 10.1126/science.1159706.
2.  Head et al, “Volcanism on Mercury: Evidence from the First MESSENGER Flyby,” Science, 4 July 2008: Vol. 321. no. 5885, pp. 69-72, DOI: 10.1126/science.1159256.
3.  Strom et al, “Mercury Cratering Record Viewed from MESSENGER’s First Flyby,” Science, 4 July 2008: Vol. 321. no. 5885, pp. 79-81, DOI: 10.1126/science.1159317.
4.  Anderson et al, “The Structure of Mercury’s Magnetic Field from MESSENGER’s First Flyby,” Science, 4 July 2008: Vol. 321. no. 5885, pp. 82-85, DOI: 10.1126/science.1159081.
5.  Slavin et al, “Mercury’s Magnetosphere After MESSENGER’s First Flyby,” Science, 4 July 2008: Vol. 321. no. 5885, pp. 85-89, DOI: 10.1126/science.1159040.

Interpretations are a dime a dozen, but raw data from distant places are worth their weight in gold.  We’ve seen a remarkable paradigm shift about crater count dating (09/25/2007, 10/20/2005).  Space.com delivered the usual assumption: “Scientists will also be able to use variations in crater density across the surface to date the sequence of geological events,” it piped; “‘The longer a surface sits out there, the more cratered’ it becomes, Solomon explained, so more cratered surfaces should be older formations.”  Should be and Are are different words.  If scientists cannot infer absolute or relative dates, they cannot infer ages much at all.  You just saw them admit that secondary cratering went from minor to major concern, and greater caution must be used in interpreting crater ages.  This implies that they were not cautious before when they made bold pronouncements about how old Mercury was.  Their readers should therefore exercise even greater caution.
    Don’t expect observations of remnant volcanism to provide any stronger evidence for hidden epochs of time.  A lot can happen in a short time if the conditions are right.  The presence of ghost craters hints that very little time elapsed from the point eruptions began and craters were emplaced.  The only ones who need an old, old Mercury are the evolutionists.  The solar system is exempt from the requirements of their world view.

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