[meteorite-list] Vesta's Potassium-to-Thorium Ratio Reveals Hot Origins

[Ron Baalke]

https://www.psi.edu/news/vestak-th

Vesta's Potassium-to-Thorium Ratio Reveals Hot Origins
Planetary Science Institute
July 22, 2015
 
Tucson, Ariz. -- Studies of materials on the surface of Vesta offer new 
evidence that the giant asteroid is the source of howardite, eucrite and 
diogenite (HED) basaltic meteorites, supporting current models of solar 
system evolution and terrestrial planet formation, a new paper by Planetary 
Science Institute researcher Tom Prettyman says.
 
Prettyman, a senior scientist at the Planetary Science Institute, and 
co-authors determined the globally averaged concentrations of radioactive 
elements potassium (K) and thorium (Th) on Vesta's surface using data 
from the Gamma Ray and Neutron Detector (GRaND) instrument aboard NASA's 
Dawn spacecraft.
 
PSI Postdoctoral Research Scientist Yuki Yamashita and Senior Scientist 
Bob Reedy were coauthors on the paper "Concentrations of Potassium and 
Thorium within Vesta's Regolith" that appears in the Icarus Special 
Issue on Vesta's Composition.  
 
"The K and Th content is important because together these elements provide 
constraints on the composition of materials from which Vesta was made 
and conditions in the early solar system," Prettyman said. The K/Th 
ratio of Vesta is very similar to that of the HED meteorites and distinct 
from other basaltic meteorites, which strongly supports connection between 
Vesta and the HEDs.  
 
The solar system originated from a molecular cloud that collapsed to form 
the Sun and a rotating disk of gas and dust from which the planets grew. 
Vesta is thought to be a planetary "embryo," a leftover planetary 
building block that survived more or less intact to the present day. Because 
it underwent magmatic processes, similar to the inner planets, Vesta is 
also regarded as "the smallest terrestrial planet."
 
As the gas and dust cooled, elements condensed to form solid compounds. 
 Each element has a representative condensation temperature. Volatile 
elements evaporate/condense at lower temperatures, whereas refractory 
elements condense at higher temperatures.
 
Potassium, a moderately volatile element, would have condensed at relatively 
low temperatures - about 700C - to form potassium feldspar. However, 
thorium condenses at much higher temperatures, 1300C. After accretion 
from the protoplanetary disk, Vesta was heated by the decay of short-lived 
radionuclides to the point of large-scale melting. 
 
K and Th would likely have remained in constant proportions as Vesta cooled 
from its initial melted state to form a layered interior consisting of 
an iron-rich core, a mantle composed of iron- and magnesium-rich minerals, 
and basaltic crust richer in calcium and aluminum. In this process, K 
and Th were almost entirely concentrated in the crust. Thus, the K/Th 
ratio of the primordial material that made Vesta is likely very similar 
to that of Vesta's crust and regolith as viewed by GRaND.
 
All of the inner planets are depleted in moderately volatile elements, 
such as K, and have low K/Th ratios in comparison to the solar photosphere, 
which is thought to be representative of the composition of the solar 
nebula. Measurements of Vesta's K/Th ratio by GRaND show that this inner 
main belt asteroid is also depleted in K relative to Th. In fact, of the 
inner solar system bodies for which K/Th has been measured, Vesta is second 
only to the moon in K-depletion.   
 
"Vesta was probably made early from material that condensed at high 
temperature, which limited the accumulation of K," Prettyman said. "However, 
the mechanisms for depletion of moderately volatile elements are still 
not fully understood."  
 
Alternatives for depletion of K on Vesta include degassing of more volatile 
elements from high-temperature magmas and disruption and re-accretion 
of Vesta following a giant impact, similar to the formation of the Earth-Moon 
system. For various reasons, neither of these scenarios is favored.
 
The dearth of moderately volatile elements throughout the inner solar 
system remains one of the long-standing, unresolved problems of cosmochemistry. 
This paper provides another piece of the puzzle in this complex story 
and further evidence that the HED meteorites hail from Vesta. 
 
The project was funded by a contract from the NASA Jet Propulsion Laboratory, 
which is operated by the California Institute of Technology in Pasadena, 
California. The Dawn mission is led by the University of California, Los 
Angeles under the auspices of the NASA Discovery Program. GRaND is managed 
by the Planetary Science Institute.