Mercury is hard to study: It exerts little gravity, orbits very elliptically and travels close to the sun.
But scientists turned the hazardous radiation in Mercury’s vicinity to their benefit by equipping an X-ray spectrometer (XRS) – a detector that relied on X-rays – on NASA’s MESSENGER orbiter (Mercury Surface, Space Environment, Geochemistry and Ranging), which circled the planet between 2011 and 2015.
Now, new analysis of data from that mission published in the Journal of Geophysical Research: Planets paints a fuller picture of how the innermost planet evolved.
“We'll probably be arguing about it for a long time, but having the measurements helps us better understand what's going on deep inside the planet,” said lead author Larry Nittler, of ASU’s School of Earth and Space Exploration.
Nittler was able to squeeze more analysis from MESSENGER’s data by looking at periods of solar flare activity, which boosted X-rays enough for XRS to detect more difficult elements like chromium.
“It's during those that we really got the best data,” Nittler said. “For some of the heavier elements, like calcium and iron and chromium, we can only measure those elements during solar flares.”
Chromium is useful because of any way Mercury is unusual. It is what scientists call “chemically reduced.”
“That means that, when it formed, it had less overall oxygen in its mix,” said Nittler, who was also a co-principal investigator on MESSENGER and led X-ray analysis for the mission. “So, that changes the way the elements behave when the planet differentiates and melts.”
Oxidation and reduction are interrelated chemical processes. The former involves losing electrons; the latter, gaining them.
It turns out Earth and Mars are oxidized, but Mercury is reduced. Scientists don’t yet know much about the geochemistry of Venus, which lies between Earth and Mercury.
Because chromium reacts readily to oxygen, it can offer a useful indication of how much oxygen was present during Mercury’s development. Put simply, chromium would bond with oxygen if it could, so a lack of chromium oxides suggest a lack of oxygen.
What’s more, because oxidative and reductive reactions affect which elements bond to each other, they also influence whether elements end up in the core, mantle or crust, and if they are redistributed by volcanism.
Thus, chromium’s chemistry and distribution could fill in important blanks regarding why the innermost planet’s geochemistry is at odds with other rocky worlds like Earth and Mars.
Although the evidence does not yet support a conclusion on that score, it’s a good bet that hotter conditions nearer the sun made water ice scarcer in Mercury’s part of the solar nebula.
“I think that's probably going to be the explanation: that there is essentially a gradient in oxidation in the inner solar system,” Nettler said.
He added that Mercury in a valuable study subject for scientists interested in highly reduced systems.
“We're learning a lot more all the time about how these systems behave,” he said. “Hopefully, our new data will help spur on more of that.”
Although Nittler has more projects in mind for MESSENGER data, he is also looking forward to his next role as co-investigator for the X-ray spectrometers aboard BepiColombo, a joint Mercury mission of the European Space Agency and the Japan Aerospace Exploration Agency due to arrive in late 2025.
