Radioactive Elements on Mercury’s Surface from MESSENGER: Implications for the Planet’s Formation and Evolution

Abstract

The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

“…Alkalis (Na and K), manganese and phosphorous are first considered as minor elements in these MgO-rich compositions (see later for the potential effect of sodium). For example, MESSENGER's gamma-ray spectrometer measured an average of 0.2870.05 wt% K 2 O in surface materials (Peplowski et al, 2011). This initial estimate considers only the silicate component of the XRS measurements.…”

“…The first measurements of surface elemental abundance ratios by MESSENGER's X-Ray Spectrometer (XRS; Nittler et al, 2011) and the Gamma-Ray Spectrometer (GRS; Peplowski et al, 2011) produced surprises based on expectations and understanding of Mercury prior to the satellite's arrival. Some volatile elements (K and S) are not depleted and the low FeO content of the surface points to highly reduced conditions (Wadhwa, 2008; Zolotov, 2011;McCubbin et al, 2012).…”

Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma‐Ray Spectrometer

“…Application of the principles of radioactive decay, in conjunction with the known half-life of 40 K (t 1/2 = 1.248 Â 10 9 yr), its isotopic abundance (0.0117%), and its probability for emitting a 1461-keV gamma ray (10.66%), yields a flux at the surface of 19.6 cnts min À1 cm À2 per wt% K [Peplowski et al, 2011b]. The surface flux was propagated to the spacecraft for each 60-s GRS integration period in the data set to determine the flux per wt% at the spacecraft altitude.…”

“…Analysis of the 1779-keV Si, 1461-keV K, and 2615-keV Th gammaray photopeaks are complicated by the presence of overlapping interference peaks, requiring spectral fitting of these complex regions to isolate the peak areas of interest. Spectral fitting is carried out using exponentially modified Gaussian (EMG) functions [Felinger, 1994] with polynomial backgrounds, a procedure that was outlined by Peplowski et al [2011b] and is briefly overviewed here. These fits were carried out with the nonlinear least squares fitting routine MPFIT [Markwardt, 2009], which is based on the LevenbergMarquardt fitting algorithm [Moré, 1978].…”

“…Initial results from the GRS include relative measurements of the abundances of stable elements, such as Fe, Al, Ca, S, and Na (L. G. Evans et al, Major-element abundances on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer, submitted to Journal of Geophysical Research, 2012, and P. N. Peplowski et al, Aluminum abundance on the surface of Mercury: Application of a new background reduction technique for the analysis of gamma-ray spectroscopy data, submitted to Journal of Geophysical Research, 2012), as well as the radioactive elements K, Th, and U [Peplowski et al, 2011b]. Those results are average abundances over the northern hemisphere and contain no information about variations in the elemental composition over the surface.…”