January 17, 2008 | David F. Coppedge

Messenger Sends Postcards from Mercury

Images downloaded from MESSENGER’s first flyby of Mercury on January 14 are starting to be published.  The Science Images page of the MESSENGER website (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) posted the first image January 16, with more being added from time to time.  Launched in 2004 (07/27/2004, bullet 3), the spacecraft has unveiled surface features like flattened craters, crater chains, bright rayed craters, concentric craters, fault scarps, mountains, depressions, wrinkle ridges, ejecta blankets and possible flow features.
    This was the spacecraft’s first encounter with Mercury of three flybys before orbit insertion in 2011.  The photos end a 30 year data gap since Mariner 10 imaged one side of the planet in the 1970s.  MESSENGER’s new images reveal the unseen side of the innermost planet for the first time.  Some images overlap with the earlier set, now made with sharper optics than the vidicon camera aboard Mariner 10.
    Interested readers might want to follow the dialogue among serious planet enthusiasts about the MESSENGER pictures going on at Unmanned Spaceflight.  They usually get pretty excited at historic events like this.  The Planetary Society blog has some initial impressions.  Emily was particularly struck by the flatness of most crater floors – an observation that could imply geological activity, though slumping, rebound and impact-related effects would have to be ruled out in light of Mercury’s bulk composition and gravity.
Update 01/22/2008:  Pictures released a few days after the flyby reveal additional geological complexity on Mercury.  The Jan. 20 image shows ghost craters, explained in the caption: “Ghostly remnants of a few craters are seen on the right side of this image, possibly indicating that once-pristine, bowl-shaped craters (like those on the large crater’s floor) have been subsequently flooded by volcanism or some other plains-forming process.”  The conundrum of secondary craters was described in the Jan. 18 image: “With their large size and production of abundant secondary craters, these flat-floored craters both illuminate and confound the study of the geological history of Mercury.”  The process of inferring the timing and sequence of geological events was discussed in the caption of the Jan. 17 image.  The first-ever color image of Mercury was also released.

The captions show that some inferences can be made about the sequence of events: a crater, then an ejecta blanket, then a scarp and perhaps an uplift, then another crater.  The timing between them is far more speculative.  Mercury has a number of characteristics that challenge standard theories about planetary origins: its global magnetic field (02/12/2004, 05/04/2007), its iron abundance, and its surface features that might indicate geological processes continuing long after the crust and mantle should have solidified.  Most of the processing is undoubtedly impact-related; if, however, tectonic or volcanic processes are the best explanation, then it would raise questions about the age of the planet.
    It is also interesting to note that many exoplanets (planets around other stars) are Jupiter-sized yet with orbits closer to their host star than Mercury is to the sun (07/15/2005, 05/07/2004; cf. 07/21/2007).  This puzzle challenges traditional theories of planet formation.  The new MESSENGER images should be studied by unbiased observers not constrained to preserve traditional theories with their obligate billions of years.  Mercury belongs to all observers.  Join the discovery team.

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Categories: Solar System

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