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| Titel |
Uranium-Thorium evolution of extrasolar silicate worlds |
| VerfasserIn |
Elizabeth A. Frank, Stephen J. Mojzsis |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074880
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| Zusammenfassung |
| As the suite of known exoplanets expands with the discovery of thousands of worlds around
other stars, the need arises to further constrain the range of possible compositions of these
objects. This is of particular importance for geophysical modeling of terrestrial exoplanets
that may be capable of supporting life. A poorly constrained parameter in these models is
radiogenic heating from the long-lived, heat-producing isotopes that are key to keeping
planetary interiors warm. In Earth’s mantle, the decay of 40K, 232Th, 235U, and
238U provides the majority of the heat output that helps sustain plate tectonics.
Given that plate tectonics plays an integral role in keeping Earth’s surface habitable,
radiogenic heat production is an important factor to consider in modeling terrestrial
exoplanets. In published models, radiogenic heat production is scaled from Earth’s or
chondritic values. Given that Earth’s own heat production has decreased at least
fivefold over solar system history, it is unreasonable to assume that exoplanets with
different ages and geochemical histories should have comparable values. Age is an
important factor for heating in that not only will old stars form with a lower metallicity
than young stars, but their isotopes will have had a longer period of time to decay.
Here we present a model in which we make predictions for the heat generated by
232Th, 235U, and 238U in a solar system within the (galactic) solar cylinder as a
function of age. The primary constraints in our model are (i) the ages of our solar
system and galaxy, (ii) production ratios of the isotopes, (iii) concentrations of
232Th, 235U, and 238U at the time our solar system formed, and (iv) half-lives.
We assumed a hybrid model of the production of these nuclides in our galaxy by
taking into account both the burst of nuclides created by massive stars at galaxy
formation and those generated in supernovae over galactic history. We numerically
solved for the relative contributions of nuclides generated by each process and
then used the resulting proportions to constrain the heat they generate in a solar
system given its age. Sensitivity tests indicate that of the constraints, the 235U/238U
and 232Th/238U production ratios have the most significant effect on the results.
By comparison, our predictions are relatively insensitive to the age of the galaxy.
The 235U/238U production ratio is particularly fickle due to the relatively short
half-life of 235U compared to 238U. For a galaxy age of 12.5 Gyr, the 235U/238U
production ratio can only provide a minimum production ratio, found to be 1.42,
whereas the allowable spectrum of the 232Th/238U production ratio is found to be
1.02 to 1.64, the higher values of which are consistent with published ratios. To
ground-truth our model, we compared predicted Th/U ratios to published observations
of ancient, metal-poor halo stars. Despite the wide error bars for the calculated
ages of these stars due to observational limitations, our model predicts Th/U ratios
consistent with observations. As such, in addition to making predictions for heat
production in extrasolar systems, our model may also be used to constrain stellar ages. |
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