Text S1
Discussion of Uncertainties:
The uncertainties reported in this work are based on measurement uncertainties from the
GRS instrument. Uncertainties vary for each GRS 5°x5° pixel due to a variety of factors. These
factors include the specific element measured, topography, and latitude [see Boynton et al., 2007
and Karunatillake et al., 2007 for detailed discussion on GRS measurement uncertainties]. On
average, the per pixel elemental concentrations used for this study have uncertainties ranging
from 3-10%.
GRS derived K concentrations have average corresponding sigmas of ±100ppm.
Likewise, GRS derived Th concentrations have corresponding sigmas of ±0.06ppm. Uranium
abundances (235U and 238U) were calculated by assigning a Th/U ratio of 3.8. This is a canonical
cosmochemical value thought to be representative of most planetary bodies and that also agrees
with analyses of most martian meteorites [McLennan, 2001; Meyer, 2003; Taylor and
McLennan, 2009]. As such, uncertainties for U are determined from Th uncertainties. Note
however, should future studies show an unexpectedly different Th/U ratio, heat production and
heat flow values would need to be adjusted accordingly.
The Neumann et al., [2004] crustal thickness model presents a reasonable average crustal
thickness of 45 km. The heat flow estimates presented here would need to be scaled
appropriately for future models suggesting a different average crustal thickness.
Taylor et al., [2006a,b] argued that if surface concentrations measured by GRS were
representative of the entire crust, as much as 50% of the total planetary budget of heat producing
elements could have been sequestered into the crust. This estimate is based on the
cosmochemical models of Dreibus and Wanke, [1985] and Wanke and Dreibus, [1988] for a
bulk chemical composition of Mars. Taylor et al., [2006a,b] does not quantify the uncertainty for
this estimate. Also in this paper, some values are reported as “approximate” if the uncertainties
are not easily quantified – for example, estimates for heat flow at the time of formation for
certain regions or locations where the age of the feature is poorly constrained.
References
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