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Planetary News: Mercury (2008)

New Details in Images of Mercury, Tethys, and Dione Require New Names

When spacecraft return photos of previously unexplored places in the solar system, one of the first tasks that falls to scientists is to name the newly revealed features.  Recently, the International Astronomical Union (IAU) has approved dozens of new names for features on the face of Mercury not seen by Mariner 10, and in regions of Saturn's moons Tethys and Dione where Cassini's camera system has improved greatly on the maps of areas first seen by the Voyagers.

On Mercury, the enigmatic "spider" feature can now be properly referred to as Pantheon fossae, with the crater Apollodorus sitting atop it.  On Tethys, the 300-kilometer (180-mile) impact basin at the southern end of the massive Ithaca system of canyons now has the name Telemus; and on Dione, an even larger (360-kilometer or 220-mile) southern hemisphere impact basin is now named Evander.

Click to enlarge > New names for features on Mercury On April 9, 2008, the International Astronomical Union approved twelve new crater names (Apollodorus, Atget, Cunningham, Eminescu, Kertész, Neruda, Raditladi, Sander, Sveinsdóttir, Xiao Zhao) as well as the names of Beagle Rupes and Pantheon Fossae. The newly named features are on the face of Mercury first imaged by MESSENGER. On Mercury, craters are named for significant deceased artists, musicians, painters, and authors; rupes (scarps or cliffs) for ships of discovery; and fossae (long, narrow depressions) for significant works of architecture. Names in italics were assigned previously, based on Mariner 10 images. Credit: NASA / JHUAPL / CIW

The Naming of Things Is a Difficult Matter

Gone are the days when the captain of the ship had the privilege of assigning whatever names he wished to newly discovered territories.  In space, the naming of features on the solid surfaces of planets and moons is governed by the IAU's Working Group for Planetary System Nomenclature.  Within this Working Group are six Task Groups overseeing the naming of different parts of the solar system, one each for the Moon, Mercury, Venus, Mars, the outer solar system, and "Small Bodies" (which includes asteroids, comets, and trans-Neptunian objects). 

When the first images of each place were returned by exploratory missions, like Mariner 10 for Mercury and the Voyager mission for the Saturn system, the Task Groups chose themes for naming features on each body and named a few of the most prominent features.  For example, Mercury's craters may be named for "Deceased artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field and have been recognized as historically significant figures for more than 50 years," while Mercury's rupes (scarps or cliffs) are named for ships of discovery.  On the smaller worlds Tethys and Dione, features of any type are being given names of people and places from Homer's Odyssey and Virgil's Aeneid, respectively.

Click to enlarge > Feature names on Dione On March 17, 2008, the International Astronomical Union approved a number of new names for features across Dione. Features on Dione are named for people and places in Virgil's Aeneid. Credit: NASA / JPL / SSI / mosaic by Steve Albers / map by Jason Perry

Click to enlarge > Feature names on Tethys On April 28, 2008, the International Astronomical Union approved a number of new names for features across Tethys. Features on Tethys are named for people and places in Homer's Odyssey. Credit: NASA / JPL / SSI / mosaic by Steve Albers / map by Jason Perry

Click to enlarge > Eminescu crater, Mercury The 125-kilometer-diameter crater Eminescu on Mercury has terraced walls and a peak ring. The peaks in the center of the crater sit in the middle of distinctive bright deposits, themselves surrounded by a halo of dark-toned material. The rest of the crater floor is flat. These features all suggest that the crater has been modified by some geologic activity, possibly volcanic, since it formed. The crater is surrounded by sprays of smaller craters called secondaries, formed when the ejecta from the Eminescu impact slammed back to ground. A bright fan of material entering the image from the right (east) side is bright ejecta from the smaller, fresher crater Xiao Zhao. The crater is named for Mihail Eminescu, a Romanian poet who lived from 1850 to 1889. Credit: NASA / JHUAPL / CIW

Click to enlarge > Highest-resolution global mosaic of Tethys Tethys as seen by Cassini on approach to its September 24, 2005 flyby. Ithaca Chasma stretches from the central peak Telemachus crater in the north -- which was named based on Voyager images -- down to the south pole, where it wraps around a large impact basin that has been so battered by subsequent craters as to be nearly invisible.  That basin has now been named Telemus.  A square crater near the center of the image now bears the name Salmoneus, for a son of Aeolus, master of the winds, and the crater immediately to Salmoneus' right is named for the hero Achilles. Credit: NASA / JPL / SSI

The IAU doesn't approve names for every last crater or crack on a planet; rather, the IAU's rules dictate that "features should be named only when they have special scientific interest, and when the naming of such features is useful to the scientific and cartographic communities at large."  The newly named craters on Mercury include such scientifically interesting features as fresh craters with large bright ejecta deposits, craters observed to have unusual, bright central peak complexes, and odd dark-rimmed craters within the Caloris basin.

When a scientist determines that it would be useful for a newly observed feature to be named, the scientist may not have the privilege of choosing the name.  A scientist may submit a request for a feature to be named, identifying the feature's location and description, and explaining why a name is justified.  The scientist may also recommend a name for the feature, provided it conforms to the thematic system already identified by the IAU for the body.  However, the IAU may disregard the recommendation.  The name request must first be approved by the appropriate Task Group, and is then reviewed by the Working Group for Planetary System Nomenclature.  Once it has been approved by the members of the Working Group, the names can be used on maps and in publications.

The scheme is providing plenty of names for Mercury, Tethys, and Dione.  However, the system is not perfect.  When the Voyagers passed through the Saturn system, their cameras were unable to pick out many surface details on the two-faced moon Iapetus, because neither spacecraft passed very close to the moon, and because the high contrast between the dark-stained Cassini regio and the bright, icy trailing hemisphere made it difficult to discern surface features.  At the time, the IAU picked "people and places from Sayers' translation of the Song of Roland" as a source for Iapetian feature names.  Now that Cassini has revealed the heavily cratered and fascinating surface of Iapetus in detail, the IAU has run into a problem: the Song of Roland simply does not contain enough proper names to supply the quantity needed for the surface of Iapetus, so before Iapetus can be mapped, a new scheme must be identified, one that will either supplant or live alongside the already-approved feature names.

Following are all the names approved for Mercury, Tethys, and Dione.  This information is copied from the United States Geological Survey's Planetary Nomenclature Gazetteer website, which maintains detailed, up-to-date databases of feature names for solid surfaces in the solar system, as well as information on feature types and the naming schemes for all these worlds.