2010 Annual Research Report

 

 

Feldman's ongoing research includes:

 

1. Volatiles on Mars using the Mars Odyssey Neutron Spectrometer (MONS).

            The primary thrust of this research is to map the present distribution of water-equivalent-hydrogen (WEH) on Mars and to try to understand its molecular associations, its origin, and its evolution.  This past year we have been able to develop a global 2-D map of WEH that includes its burial depth below a relatively desiccated over-layer, D, and its mass fraction within an assumed semi-infinite lower layer, Wdn.  Throughout, the WEH content of the upper layer, Wup, has been assumed equal to 1 percent.  The maps of Wdn and Depth have been made available to the PSI Geographic Information System (GIS).  We will also be working toward developing a map of WEH interpreted as a double layer with Wup determined independently from the MONS data, and as a single, semi-infinite deposit with depth having improved spatial resolution by deconvolving the spatial response function of the MONS.  Theses new data sets will also be made available to the PSI GIS when they are ready.   Although we have made a lot of progress this past year, it will be ongoing this next year.

            A second thrust is to map the seasonal cycle of CO₂ ice deposition onto, and sublimation from, both high latitude terrains during their respective fall, winter, and spring seasons.  We have found that the high-latitude weather patterns are in general quite repetitive and are consistent with the overall patterns cataloged by many previous missions dating back to Viking 1.

 

2. The composition of Mercury using the MESSENGER Neutron Spectrometer.

               The primary thrust of this research to date has been to place constraints on the Fe content of the crust of Mercury.  At the present time, MESSENGER has passed by Venus twice and Mercury three times during its cruise phase, and will be injected into Mercury orbit on March 17, 2011.  During all three flybys of Mercury, counting rates of all three energy ranges (thermal, epithermal, and fast neutrons) measured by the NS were strong and the thermal counting rates during the first flyby were sufficient to determine a very significant neutron absorption rate, which can only result if the crust of Mercury over flown by MESSENGER has a significant amount of either Fe or Ti, or both.  This is a very important discovery because presently acceptable interpretations of Infrared reflectance spectra indicate a general paucity of the FeO content in surface materials.  These results have now been published in Icarus.

              A second thrust is to monitor the flux of fast neutrons produced in solar flares.  We were lucky to be operating during the last solar flare of solar cycle 23 before the Sun went completely dormant.  The signature of this flare in neutrons was well above background, which was the first time solar neutrons in the energy range between 0.5 MeV and 7.5 MeV have been observed in space.  The fact that this flare was of magnitude M2 shows that we have the sensitivity to monitor solar activity during the rise from minimum to maximum activity during the upcoming solar cycle 24, which extends between 2008 and 2012.  These results have been published in JGR.  We are now working on the next event that was observed on August 18, 2010.

 

3. The volatile composition of Vesta and Ceres using the DAWN Neutron and Gamma-ray Spectrometers (GRaND).

              The DAWN spacecraft is presently on its way to Vesta, scheduled to arrive in late July, 2011.  After about a 9-month period in orbit about Vesta, it will be retargeted to intercept Ceres in 2015.  The Gamma-Ray and Neutron Detector (GRaND) is presently operating within specifications and will be ideal for mapping both Vesta and Ceres during the low orbital phases of the Dawn mission.  Both of these asteroids are very different as Vesta is a differentiated basaltic body and Ceres is rich in volatiles. This difference should be readily evident in maps of the gamma-ray and neutron fluxes leaking from their surfaces.  Our first chance at calibrating GRaND came in February, 2009 when DAWN flew by Mars giving us the opportunity to compare both the backgrounds and spectra with those measured by the Neutron and Gamma-ray spectrometers aboard Mars Odyssey.  The results were excellent.

 

Papers:

Lawrence, D.J., Feldman, W.C., Goldsten, J.O., McCoy, T.J., Blewett, D.T., Boynton, W.V., Evans, L.G., Nittler, L.R., Rhodes, E.A., Solomon, S.C., 2010. Identification and measurement of neutron-absorbing elements on Mercury’s surface. Icarus 209, 179–194.

 

David J. Lawrence, Richard C. Elphic, William C. Feldman, Herbert O. Funsten, and Thomas H. Prettyman, Performance of Orbital Neutron Instruments for Spatially-Resolved Hydrogen Measurements of Airless Planetary Bodies, Astrobiology, 10, 183, 2010.

 

William C. Feldman, David J. Lawrence, John O. Goldsten, Robert E. Gold, Daniel N. Baaker, Dennis K. Haggerty, George C. Ho, Sam Krucker, Robert P. Lin, Richard A. Mewaldt, Ronald J. Murphy, Larry R. Nittler, Edgar A. Rhodes, James A. Slavin, Sean C. Solomon, Richard D. Starr, Faith Vilas, Angelos Voulidas, Evidence for extended acceleration of solar flare ions from 1-8 MeV solar neutrons detected with the MESSENGER Neutron Spectrometer, J. Geophys. Res., 115, A01102, doi:10.1029/2009JA014535, 2010.

 

Lawrence, David J., William C. Feldman, John O. Goldsten, Timothy J. McCoy, David T. Blewett, William V. Boynton, Larry G. Evans, Larry R. Nittler, Edgar A. Rhodes, Sean C. Solomon, Identification and Measurement of Neutron-absorbing Elements on Mercury’s Surface (2010) Icarus, 10.1016/j.icarus.2010.04.005, 209, 195 – 209.