Streaks on Mercury show: Mercury is not a "dead planet"

Image of the streaks or ‘lineae’ on the slopes of a crater wall on Mercury and the bright hollows from which the streaks originate. The image was taken by MESSENGER on April 10, 2014.
Image Credit: © NASA/JHUAPL/Carnegie Institution of Washington

Scientific Frontline: "At a Glance" Summary

• Main Discovery: A systematic analysis has identified approximately 400 bright slope streaks, or "lineae," on Mercury, indicating the planet is currently geologically active through the outgassing of subsurface volatiles.
• Methodology: Researchers employed a deep learning algorithm to automatically screen and analyze over 100,000 high-resolution images captured by NASA's MESSENGER spacecraft during its 2011–2015 orbital mission.
• Key Data: The study produced the first comprehensive census of roughly 400 streaks—compared to only a handful previously known—revealing a distinct accumulation on the sun-facing slopes of young impact craters.
• Significance: These findings overturn the prevailing assumption that Mercury is a "dead" and static world, suggesting a continuous, solar-driven release of elements like sulfur into space.
• Future Application: This inventory will serve as a baseline for the ESA/JAXA BepiColombo mission to re-image these regions, allowing scientists to detect new streak formation and quantify the planet's volatile budget.
• Branch of Science: Planetary Geology and Remote Sensing.
• Additional Detail: The formation of these streaks is attributed to solar radiation mobilizing volatiles through crack networks created by impact events, often originating from bright, shallow depressions known as hollows.

Image of the streaks or ‘lineae’ on the slopes of a crater wall on Mercury and the bright hollows from which the streaks originate. The image was taken by MESSENGER on August 1, 2012.
Image Credit: © NASA/JHUAPL/Carnegie Institution of Washington

Although Mercury was geologically active in its early days, today its surface appears almost completely static. This is why it is often perceived as a dead and dry planet. A new study led by Dr. Valentin Bickel from the Center for Space and Habitability at the University of Bern and from the NCCR PlanetS together with researchers from the Astronomical Observatory of Padua (INAF) has now carried out the first systematic analysis of so-called slope streaks, or "lineae". The abundance and distribution of lineae provide new insights into the geological activity of the innermost and smallest planet in our solar system. 

In their study, the research group first used machine learning to create a systematic inventory of the lineae on Mercury. With a geostatistical analysis of this inventory, the researchers were able to show that these bright, linear streaks, which can be seen on slopes across the planet, were probably driven by the outgassing of subsurface volatile material from the subsurface. This indicates that there is a continuous release of volatiles from Mercury into space – even today. The study has just been published in the journal Nature Communications Earth & Environment. 

Inventory of the streaks on Mercury thanks to machine learning 

The research team used a deep learning approach to analyze around 100,000 high-resolution images from NASA's MESSENGER space probe, which explored the planet Mercury from 2011 to 2015. They mapped the overall distribution and morphological properties of around 400 bright streaks on Mercury. "Until now, lineae on Mercury had not been systematically mapped and studied; only a small handful of streaks were known. With the image analysis, we were able to create the first census, i.e. a systematic inventory, of slope streaks on Mercury," explains first author Valentin Bickel. 

Image of the streaks or ‘lineae’ on the slopes of a crater wall on Mercury and the bright hollows from which the streaks originate. The image was taken by MESSENGER on October 19, 2013.
Image Credit: © NASA/JHUAPL/Carnegie Institution of Washington

Geological activity on a supposedly dead planet 

The inventory shows that the bright streaks mainly occur on the sun-facing slopes of young impact craters that penetrated volcanic material and into potentially volatile-rich bedrock underneath. The fact that the lineae accumulate in these particularly exposed and temporarily warm regions indicates that solar radiation plays an important role in the activation of lineae formation. "Volatile material could reach the surface from deeper layers through networks of cracks in the rock caused by the preceding impact," explains Bickel. "Most of the streaks appear to originate from bright depressions, so-called 'hollows'. These hollows are probably also formed by the outgassing of volatile material and are usually located in the shallow interior or along the edges of large impact craters," Bickel continues. 

"With our analysis, we were therefore able to propose that slope lineae are probably formed by the outgassing of volatiles such as sulfur or other light elements, that are sourced from the interior of the planet," Bickel continues. The new study indicates that the planet Mercury is more geologically active than previously thought. "Our findings paint a completely different, dynamic picture of the supposedly dead, dry and boring planet Mercury," says Bickel. 

Important findings for future missions to Mercury 

The role of lineae as potential indicators for the outgassing of volatiles on Mercury provides important insights into the geological dynamics and composition of the planet. Bickel says: "Our results suggest that Mercury not only has a turbulent past but is still subject to modification today." The results obtained are particularly important for future missions to Mercury. "As the streaks on Mercury are presumably caused by the outgassing of volatile material, they could be a promising indicator of Mercury's 'volatile budget', i.e. how much volatile material the planet is continuously losing," explains Bickel. The researchers now hope that they will be able to clearly prove their assumption about the activity of lineae with new images of Mercury, which the BepiColombo mission of the European Space Agency ESA and the Japan Aerospace Exploration Agency JAXA should provide. 

The BepiColombo mission is currently on its way to Mercury. The University of Bern is involved in multiple instruments on this mission: The laser altimeter BELA (BepiColombo Laser Altimeter), the mass spectrometer STROFIO and the plasma instrument ENA (Energetic Neutrals Analyzer). BELA was designed and built at the Physics Institute of the University of Bern, among others, and will use laser pulses to measure the distance to Mercury's surface with an accuracy of around 10 cm from an orbital altitude of around 1,000 km. This data will be used to create a detailed 3D model of Mercury's topography. In addition, existing models of tectonic deformation and the surface composition of the planet will be refined so that the geological processes on Mercury can be reconstructed even more accurately. STROFIO is a NASA instrument for which the University of Bern designed, calculated and built the so-called ion-optical system. STROFIO will record Mercury's very thin atmosphere and analyze its chemical composition. ENA is an imaging plasma instrument of the Swedish Institute of Space Physics (IRF), to which the University of Bern has also contributed ion-optical components for the targeted guidance and bundling of charged particles. Valentin Bickel is also closely collaborating with the SIMBIO-SYS principal investigator and science team at the Astronomical Observatory of Padua (INAF). SIMBIO-SYS is an integrated imaging and spectroscopic instrument which is also on its way to Mercury with the BepiColombo mission. Among other things, it will provide high-resolution images and 3D images of Mercury's surface. 

The research team plans to use the inventory created as part of the current study to re-photograph and examine certain regions with lineae using the BepiColombo mission. The aim is to determine whether and how many new streaks have emerged between the observations from the MESSENGER space probe and the future images from BepiColombo. "With these investigations, we want to better understand the formation mechanisms and the temporal development of these structures and thus gain further indications of the role of volatiles in driving geological activity on Mercury," concludes Bickel. 

Published in journal: Nature Communications Earth & Environment

Title: Slope lineae as potential indicators of recent volatile loss on Mercury

Authors: V. T. Bickel, G. Munaretto, S. Bertoli, G. Cremonese, P. Cambianica, and N. A. Vergara Sassarini

Source/Credit: University of Bern

Reference Number: ps012726_01

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