Bright streaks reveal Mercury still geologically active

by Robert Schreiber
Berlin, Germany (SPX) Jan 29, 2026

A new analysis of Mercury has uncovered hundreds of bright linear streaks on crater slopes that point to ongoing loss of volatile material from the planet's interior, challenging the view of Mercury as a geologically dead and dry world. The work, led by researchers at the Center for Space and Habitability (CSH) at the University of Bern together with colleagues at the Astronomical Observatory of Padua (INAF), shows that these features, known as slope streaks or lineae, are widespread and linked to recently active geological processes.

The team carried out the first systematic inventory of Mercury's slope lineae by applying deep learning techniques to about 100,000 high resolution images taken by NASA's MESSENGER spacecraft during its orbital mission from 2011 to 2015. Using this automated approach, they mapped the global distribution and shapes of roughly 400 bright streaks that had previously escaped comprehensive cataloging, extending earlier work that had identified only a small number of such features.

Geostatistical analysis of the new inventory shows that the lineae occur preferentially on sun-facing slopes inside relatively young impact craters that cut through thick volcanic deposits into underlying bedrock rich in volatile elements. The concentration of streaks in these thermally stressed environments indicates that solar heating is an important trigger for the processes that generate the features, enhancing volatile escape from near surface layers.

Many of the streaks originate in small, bright depressions called hollows that dot crater floors and walls. These hollows have long been interpreted as products of volatile loss, and their close association with the lineae supports the view that both structures form when volatile components such as sulfur or other light elements escape from the subsurface, leaving behind bright, altered material and sharp, fresh looking textures.

According to the team, fracture networks created by the original impact events likely provide pathways that allow volatile rich material from deeper levels to reach the surface. As solar radiation warms these exposed zones, volatiles can escape into space, driving the development or modification of the bright streaks down slope. The new results imply that this outgassing is not only a relic of early Mercury but continues into the present epoch.

The study therefore paints a more dynamic picture of Mercury than the traditional view of a small, airless body that has remained largely unchanged for billions of years. The inferred ongoing loss of volatiles has direct implications for how scientists reconstruct the planet's interior composition, thermal evolution and the history of its tenuous exosphere, which is continually replenished by material escaping from the surface.

The researchers emphasize that the slope lineae could provide a valuable handle on Mercury's volatile budget, effectively recording how much volatile material the planet is still losing over time. By comparing the distribution and properties of streaks with models of volatile content and transport, it should be possible to refine estimates of the total inventory of such materials in the crust and mantle.

These findings arrive as the joint ESA and JAXA BepiColombo mission is en route to Mercury, carrying an advanced payload that includes several key contributions from the University of Bern. The BepiColombo Laser Altimeter (BELA), designed and built in part at the Physics Institute of the University of Bern, will use laser pulses from an orbit roughly 1,000 kilometers above the surface to measure elevations with about 10 centimeter precision, enabling a detailed three dimensional reconstruction of Mercury's topography.

BELA's data will help refine models of tectonic deformation and surface composition, providing a tighter framework for interpreting the locations and morphologies of lineae and hollows. The Bern team also contributed the ion optical system for STROFIO, a NASA mass spectrometer on BepiColombo that will measure the composition of Mercury's extremely thin atmosphere, connecting present day volatile escape at the surface to the surrounding exosphere.

In addition, the University of Bern has supplied ion optical components to the Energetic Neutrals Analyzer (ENA), an imaging plasma instrument led by the Swedish Institute of Space Physics. ENA will map interactions between Mercury's surface, its exosphere and the solar wind, complementing the geological and atmospheric measurements and helping to close the loop between interior volatiles, surface features and space environment processes.

Lead author Valentin Bickel is working closely with the SIMBIO SYS principal investigator and science team at INAF in Padua. SIMBIO SYS, an integrated imaging and spectroscopic system flying on BepiColombo, will deliver high resolution images and stereo data of Mercury's surface, allowing scientists to resolve lineae and hollows in far greater detail than was possible with MESSENGER.

The research team plans to use the current inventory of slope streaks as a baseline for future comparisons once BepiColombo begins returning data. By imaging key regions again, they aim to determine whether new streaks have formed or existing ones have changed since the MESSENGER era. Any such changes would provide strong evidence that volatile driven processes are still reshaping Mercury's surface on human timescales.

Through these follow up observations, the scientists hope to better constrain the mechanisms that generate slope lineae, quantify their rates of formation and evolution, and clarify how volatiles contribute to ongoing geological activity on the smallest and innermost planet in the solar system.

Research Report:Slope Lineae as Potential Indicators of Recent Volatile Loss on Mercury