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Scientists detect lithium in Mercury's exosphere using magnetic wave analysis

Using a cutting-edge magnetic wave detection technique, a new study in has identified lithium in Mercury's exosphere for the first time.

Mercury's exosphere is a fragile environment where gas molecules are sparse and rarely interact with each other. Since the 1970s, missions like the Mariner 10 spacecraft and later the MESSENGER have orbited Mercury, collecting data.

Thanks to information gathered by missions and telescopes on Earth, scientists have found that species such as hydrogen, potassium, sodium, and iron are present.

The discovery of alkali metals like potassium and sodium led scientists to speculate that other alkali metals, such as lithium, should exist based on the current understanding of planetary formation.

Over the years, most efforts have yielded no results, implying that lithium may be present in extremely low concentrations in the exosphere.

The research group led by Daniel Schmid at the Austrian Academy of Sciences approached the search from a fresh angle.

Instead of directly searching for lithium atoms, they used magnetic field measurements to identify an electromagnetic wave signature called "pick-up ion cyclotron waves" (ICWs), indicating the presence of lithium.

"During our survey [of MESSENGER's magnetic field data], we identified signatures of pick-up ion cyclotron waves that could be attributed to freshly ionized lithium," said Schmid to ÌÇÐÄÊÓƵ.

"This discovery suggests that Mercury's surface has been enriched with volatile elements through continuous meteoritic impacts, which also facilitate their release into the exosphere and space."

Detecting signatures

ICWs are formed as a result of multiple physical processes set in motion on Mercury's surface and in its atmosphere.

When neutral lithium atoms travel upward from Mercury's surface into space, they encounter intense solar ultraviolet radiation. This radiation strips away electrons from the lithium atoms, transforming them into charged lithium ions.

These newly ionized particles get swept up by the solar wind—the constant stream of charged particles—flowing from the sun.

When the solar wind "picks up" these fresh lithium ions, it creates an instability in the surrounding plasma. The velocity difference between freshly formed lithium ions and solar wind particles is what gives rise to electromagnetic waves that propagate through space.

They produce a characteristic signal. They oscillate at the lithium ion cyclotron frequency, a characteristic frequency determined entirely by lithium's unique mass-to-charge ratio and the local magnetic field strength.

It's akin to each element having its own electromagnetic fingerprint.

"The pick-up ions produce waves at characteristic frequencies, allowing us to identify their presence through their magnetic signatures," explained Schmid.

"Neither particle detectors onboard Mariner 10 and MESSENGER nor ground-based telescopes could confirm the presence of lithium, despite expectations of the existence of lithium based on the detection of other volatile elements."

The research team analyzed four years of magnetic field data from MESSENGER, identifying 12 independent events where ICWs appeared. Lasting only for tens of minutes, each event gave a brief window into the release of lithium into Mercury's tenuous atmosphere.

Meteoroid bombardment

The sporadic and short-lived nature of these detections provided crucial clues about lithium's origin. The researchers ruled out slow-acting processes, including thermal heating and ongoing solar wind bombardment.

Instead, all signs pointed to explosive events of short duration, such as meteoroid impacts.

When meteoroids strike Mercury's surface at velocities around 110 kilometers per second, they create explosive impacts that vaporize both the incoming rock and Mercury's surface material.

The impacts create vapor clouds heated to 2,500–5,000 Kelvin, high enough to loft lithium atoms into Mercury's exosphere.

"The detection of lithium—and its association with impact events—strongly supports the hypothesis," said Schmid. "It demonstrates that meteoroids not only deliver new material but also vaporize existing surface deposits, releasing volatiles into the exosphere and sustaining a dynamic cycle of supply."

The researchers estimated that the meteoroids responsible for the lithium detections ranged from 13 to 21 centimeters in radius, with masses between 28,000 and 120,000 grams.

Remarkably, these impacts can vaporize approximately 150 times more surface material than the meteoroid's own mass.

Rewriting Mercury's story

These findings challenge traditional views of how Mercury acquired its composition. Early models suggested that Mercury's proximity to the sun should have stripped away volatile elements during the planet's formation, leaving a relatively barren world behind.

"Mercury has an unusually high mass density, with an oversized iron core relative to its rocky mantle," explained Schmid.

"One hypothesis suggests that a massive early collision, combined with the planet's proximity to the sun, stripped away much of the mantle and its volatiles. However, MESSENGER detected significant amounts of volatile elements, contradicting this idea."

Instead, the research suggests a different narrative. Mercury's surface has been continuously enriched over billions of years through meteoroid bombardment. This offers a new perspective on how rocky planets evolve under sustained bombardment.

Beyond its application to Mercury, the approach could help scientists explore thin atmospheres throughout the solar system, especially in locations where direct data collection is difficult.

"They suggest that even airless bodies like the moon, Mars, and asteroids may acquire volatiles after formation through extraterrestrial delivery," noted Schmid.

"In fact, this has already been shown on the moon. This has important implications for understanding surface chemistry and long-term space weathering across the inner solar system."