Ottawa 2011
Technical Program

SS24:  Planetary Geology & Geophysics: A Canadian Perspective 
Sponsored by / Parrainé par:  GAC® Planetary Sciences Division; GEDEX / Division des sciences planetaires de l'AGC®; GEDEX
Organizers / Organisateurs:  Marie-Claude Williamson (GSC-Ottawa), Catherine Johnson (U. of British Columbia), Paul Sylvester (MUN)
Room / Salle:  Morisset MRT218

Date:  26/05/2011
Time:  9:20 AM
Presenter:  Sean C. Solomon

Keynote (40 min):  The exploration of Mercury by the MESSENGER spacecraft

Solomon, S.C., Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA,

NASA’s MESSENGER spacecraft flew by Mercury three times in 2008-09 en route to insertion into orbit about the planet in March 2011.  The flybys confirmed that the planet’s internal magnetic field is dominantly dipolar, with a vector moment closely aligned with the spin axis.  MESSENGER detected Mg and Ca+ in Mercury’s exosphere, demonstrated that Mercury’s anti-sunward neutral tail contains multiple species, and revealed that exospheric Na, Ca, and Mg vary differently with space and time, signatures of multiple source processes.  MESSENGER’s laser altimeter showed that the equatorial topographic relief of Mercury exceeds 5 km, revealed an equatorial ellipticity aligned with the ellipticity in Mercury’s gravitational potential, and profiled numerous impact craters and fault scarps.  MESSENGER images provided evidence for widespread volcanism, and candidate sites for volcanic centers were identified.  Newly imaged lobate scarps and other tectonic landforms support the hypothesis that Mercury contracted globally in response to interior cooling.  The ~1500-km-diameter Caloris basin was a focus for volcanic centers, some with evidence of pyroclastic deposits, and widespread contractional and extensional deformation; smooth plains interior and exterior to the basin postdate the basin-forming event.  The interior plains of the ~290-km-diameter Rachmaninoff basin are among the youngest volcanic material on the planet.  Mercury surface units are distinguishable by color and composition; smooth plains occupy ~40% of the surface area, and low-reflectance material occupies ~15% of the surface area and is primarily seen in deposits excavated by impact.  Reflectance spectra show no evidence for FeO in surface silicates, and reflectance and color imaging observations support the view that Mercury’s surface material consists dominantly of Fe-poor, Ca-Mg silicates with an admixture of spectrally neutral opaque minerals.  In support of the hypothesis that those opaque minerals are Fe-Ti oxides, thermal neutron measurements indicate that the surface abundance of Fe plus Ti is comparable to that of some lunar mare regions.  MESSENGER’s flybys revealed that Mercury’s magnetosphere is more dynamic and responsive to imposed solar wind conditions than that of any other solar system body, and they showed that the planet often experiences conditions favorable to direct impact of solar wind plasma onto the dayside surface, an important contributor to Mercury’s exosphere and space weathering of surface materials.  MESSENGER’s continuous operation for one Earth year in a near-polar orbit has enabled global observations of Mercury and its environment at higher spatial and temporal resolution than was possible during the flybys.