[meteorite-list] Dawn Mission Discovers Hydrogen on Giant Asteroid Vesta

[Ron Baalke]

NEWS RELEASE FROM THE PLANETARY SCIENCE INSTITUTE


FROM:
Alan Fischer
Public Information Officer
Planetary Science Institute
520-382-0411
520-622-6300
fischer at psi.edu

Dawn mission discovers hydrogen on giant asteroid Vesta  

Sept. 20, 2012, Tucson, Ariz. -- The first measurements of the elemental 
composition of the surface of the giant asteroid Vesta indicate that 
hydrogen was brought to the body by impactors, research by a team led by 
Planetary Science Institute Senior Scientist Thomas H. Prettyman has shown.

Using data collected by the NASA Dawn mission's spacecraft's gamma ray and neutron 
detector instrument - GRaND - as it circled the giant asteroid, researchers 
also confirmed the elemental composition of the surface of Vesta matches 
meteorites found on Earth believed to have originated from Vesta.

The hydrogen content of Vesta's regolith, or rocky surface materials, is 
consistent with delivery of hydrogen-bearing, carbonaceous chondrite materials 
to Vesta, Prettyman and co-authors report in a paper titled "Elemental 
Mapping by Dawn Reveals Exogenic H in Vesta's Regolith" that appears 
in Science.

The highest concentrations of hydrogen were found in equatorial 
regions, where water ice is not stable.  The lowest amounts were found 
within the giant, south-polar Rheasilvia impact basin.  The amounts of 
hydrogen and its association globally with broad, low albedo  -  low 
reflective -  regions on Vesta indicate the infall of carbonaceous chondrites 
bearing hydrated minerals as the likely origin.  A companion paper by 
Brett Denevi of Johns Hopkins University Applied Physics Laboratory shows 
pitted regions within young craters that formed due to high velocity impacts 
into volatile-rich material.

"Where did the hydrogen from Vesta's surface come from? We ruled out the wind 
of charged hydrogen particles streaming off the sun as a source, as well as 
water ice within the top few feet of Vesta's surface," Prettyman said. "The 
hydrated minerals appear to be delivered by carbon-rich space rocks that 
collided with Vesta at speeds slow enough to preserve their volatile content." 

GRaND's first elemental measurements of Vesta's surface showed the composition 
matches howardite, eucrite and diogenite (HED) meteorites believed to 
have been blasted off of Vesta, Prettyman said.

"The Vesta-HED connection was established long ago by comparing the spectrum of 
reflected light for the HED meteorites with telescopic measurements of Vesta," 
he said. GRaND confirms this by showing that the abundances of iron and silicon 
are consistent with HEDs.

Dawn orbited within 210 kilometers of Vesta, allowing GRaND a unique opportunity 
to analyze the asteroid. "From a broader perspective, we are reporting the first 
direct measurements of Vesta's elemental composition, which can only be 
accomplished by getting very close to Vesta," Prettyman said. "It is unlikely 
these measurements will be repeated any time soon."

Dawn left Vesta's orbit Sept. 4 and is making its way to a rendezvous with the 
dwarf planet Ceres in early 2015.

The data acquired by GRaND are available to the public through the NASA 
Planetary Data System.

Prettyman is lead author on the Science paper, which includes PSI co-authors 
Naoyuki Yamashita, William C. Feldman, Pasquale Tricarico, Robert C. Reedy and 
Jian-Yang Li. 

GRaND is operated and managed by the Planetary Science institute under the 
leadership of Prettyman, who is also the lead for Geochemistry on Dawn.  This 
work is supported by NASA under a subcontract from the Jet Propulsion Laboratory 
to the Planetary Science Institute.  GRaND was built by Los Alamos National 
Laboratory under Prettyman’s direction and supervision.

The Dawn mission to Vesta and Ceres is managed by the Jet Propulsion Laboratory, 
for NASA's Science Mission Directorate, Washington. It is a project of the Discovery 
Program managed by NASA's Marshall Space Flight Center, Huntsville, Ala. 
UCLA is responsible for overall mission science. Orbital Sciences Corporation 
of Dulles, Va., designed and built the Dawn spacecraft.


CONTACT:
Thomas H. Prettyman
Senior Scientist
505 690-5128
prettyman at psi.edu


PSI INFORMATION:
Mark V. Sykes
Director
520-622-6300
sykes at psi.edu