Radioactive elements on Mercury's surface from MESSENGER: implications for the planet's formation and evolution

Mercury's surface composition remains poorly understood due to its proximity to the Sun and limited data from prior missions. Prior research has shown that Mercury's high density suggests a large iron core, but the role of volatile elements in its formation is unclear. No prior work had resolved whether Mercury formed from volatile-poor materials or contained significant volatile components. This gap motivated the use of MESSENGER's instruments to measure radioactive elements. The MESSENGER mission provided the first detailed measurements of K, Th, and U abundances on Mercury's surface. These elements are important for understanding planetary formation and thermal evolution. Their abundances can indicate whether Mercury's materials were heated to extreme levels or retained volatiles. Prior assumptions about Mercury's formation often excluded volatile elements. This study aimed to clarify Mercury's geochemical history by analyzing these key radioactive elements.

This study aimed to determine the abundances of potassium, thorium, and uranium on Mercury's surface using data from the MESSENGER Gamma-Ray Spectrometer. These elements are crucial for understanding Mercury's formation and thermal evolution. The researchers focused on the northern hemisphere, where data collection was most effective. The goal was to test whether Mercury's composition aligns with models involving extreme heating or with chondritic materials. The study sought to address uncertainties about Mercury's volatile content and formation history. The researchers wanted to compare Mercury's surface composition to that of other planetary bodies. By measuring radioactive elements, they could infer Mercury's thermal history and volcanic activity. This work aimed to provide insights into Mercury's internal structure and evolution.

The study used data from the MESSENGER Gamma-Ray Spectrometer, which detected gamma rays emitted by radioactive elements. The spectrometer measured the abundances of potassium, thorium, and uranium in Mercury's northern hemisphere. The researchers analyzed gamma-ray spectra collected during the spacecraft's orbit around Mercury. They compared the measured abundances to expected values from different planetary formation models. The team applied statistical methods to estimate uncertainties in the measurements. The data were averaged over large surface areas to reduce noise. The researchers also considered the effects of solar wind and other external factors. The study relied on established techniques in gamma-ray spectroscopy and planetary geochemistry.

The study found potassium at 1150 ± 220 ppm, thorium at 220 ± 60 ppb, and uranium at 90 ± 20 ppb on Mercury's surface. These values are higher than expected for a planet formed under extreme heating. The potassium-to-thorium ratio suggests Mercury retained moderate volatile elements. The data support formation from materials similar to chondritic meteorites. The abundances indicate that Mercury's internal heat production has declined over time. The findings suggest widespread volcanism occurred after the late heavy bombardment 3.8 billion years ago. Limited volcanic activity has occurred since that period. These results challenge models that assume Mercury formed from highly volatile-poor material.

The authors propose that Mercury's surface composition supports formation from volatile-containing materials rather than extreme heating. The abundances of potassium, thorium, and uranium suggest Mercury retained some volatile elements. The data are inconsistent with models requiring complete loss of volatiles during formation. The study supports the idea that Mercury's materials resemble chondritic meteorites. The decline in internal heat production implies limited recent volcanic activity. The findings align with widespread volcanism after the late heavy bombardment. The researchers suggest Mercury's thermal evolution has been dominated by early volcanic processes. These conclusions challenge prior assumptions about Mercury's formation and evolution.