For many years, scientists viewed Mercury as a world that no longer had an active geological history. The planet appears to be almost completely uniform, dry, barren, and mostly unchanged for the past 10 billion years. However, new evidence from Dr. Valentin Bickel and his colleagues suggests that this view of Mercury may be inadequate and that Mercury is actually continuing to release materials from its interior today.
An international collaboration led by Dr. Valentin Bickel at the Center for Space and Habitability, University of Bern, together with participants from the National Centre of Competence in Research PlanetS and the Astronomical Observatory of Padua, Italy, has produced the first complete survey of slope streaks on the planet Mercury, also known as “lineae.”
This study revealed that these bright streaks, which appear as very narrow, elongated stripes running down steep slopes within impact craters, are the result of the escape of volatiles from beneath the surface of Mercury. The findings demonstrate that the planet is still more active than previously thought.
To conduct a worldwide census of slope streaks using machine learning, the research team analyzed approximately 100,000 high-resolution photographs of Mercury taken by the MESSENGER spacecraft between 2011 and 2015. The team utilized a deep learning system to detect bright, linear features that fell by the force of gravity down the planet’s steepest slopes. The dataset was processed by a deep learning algorithm that compared the two-dimensional Canny edge detection method against the actual steep slope images.
Using expert validation and filtering of duplicate findings, the research group produced an approximate count of 400 separate unique slope lineae for use in the upcoming Global Map of Mercury.
“Lineae on the planet Mercury have not previously been systematically mapped or studied. Only a handful of such streaks had been identified,” Dr. Bickel explained. By using image analysis, the researchers were able to make an inventory of slope streaks located on Mercury, creating what they believe to be the first census of slope streaks.
The slope streaks are not uniformly distributed over the planet’s surface. Rather, they tend to occur in clusters in several different locations on the planet. Clusters have been identified in three distinct regions: Budh Planitia, Sobkou Planitia, and areas surrounding large impact craters, such as the region near Degas crater.
Almost all of the slope streaks identified by image analysis are located in the northern hemisphere. This is most likely due to the fact that this area received the most detailed images from the MESSENGER spacecraft.
How and where slope lineae form is of considerable scientific interest. Most of the slope lineae are found on the sunlight-facing walls of young impact craters. Many of these young craters are located on top of volcanic plains, and it is believed that many extend through older layers that may have contained volatiles.
Most of the slope lineae begin at the tops of steep crater walls. Approximately 87% of the slope streaks begin either directly from bright depressions called hollows or from very small features resembling hollows. Hollows are thought to form when volatile materials escape into space from the surface, leaving the remaining material unstable and prone to collapse into shallow pits.
“The volatile materials would have risen to the surface through a series of cracks in the rock that formed from the impact of the crater,” Bickel said. “So it is logical to think that the greatest percentage of the slope lineae would have originated from a bright hollow.”
In addition to examining the relationship between hollows and slope lineae, the study team analyzed each streak’s color, temperature, and shape. They also evaluated the overall slope of the crater wall on which each streak formed.
The length of the streaks can vary from a few hundred meters to multiple kilometers. The width of the streaks is likely only a few meters, and they do not cast any visible shadows in even the clearest images taken thus far.
The spectral data add another piece to the puzzle. In the Degas crater region, where suitable imaging exists for comparison, the streaks display a blue color similar to that of the hollows immediately surrounding them. This close correspondence indicates that the same material or formation mechanism is responsible for forming both the streaks and the hollows.
Temperature also appears to influence the streaks themselves. The streaks typically occur on slopes that reach slightly higher temperatures than their surroundings. These slopes receive intense solar radiation, particularly near Mercury’s equator.
Even though the temperature differences are relatively small, they support the hypothesis that thermal outgassing of volatiles could be responsible for the formation of the streaks.
Elemental sulfur is a prime candidate for the material being lost via thermal outgassing. Due to the extreme heat on Mercury’s surface, temperatures can become high enough for sulfur to vaporize under certain conditions. Other lighter elements may also have contributed to the formation of the streaks.
“Thus, with our investigation, we would call upon the evidence and propose that the slope lineae form as a result of the outgassing of volatiles, such as sulfur or other light elements, from the interior of the planet,” Bickel added.
The research group also considered multiple alternative explanations for the formation of the streaks. The streaks do not consistently align with tectonic faults or volcanic vents. The formation of new impact craters during the MESSENGER mission did not create new streaks.
Over the past several years, continuous resampling of the same selected sites has shown no apparent differences between image sets. These data indicate that the geological process on Mercury is slow compared to short-lived activity that may have occurred in the distant past.
Another possibility is that the process occurs through short bursts of activity separated by long periods of inactivity.
“The data that we have obtained provide a radically different view of Mercury, which was thought to be a dead and boring planet,” Bickel said.
The researchers also closely collaborated with the SIMBIO-SYS imaging team at the Astronomical Observatory in Padua to build new and improved elevation models based on MESSENGER data.
The acquisition of improved elevation data for regions containing known slope lineae will result from comparisons between the MESSENGER and BepiColombo datasets. By comparing observations from both missions, the researchers hope to determine whether slope lineae observed by MESSENGER are also present in BepiColombo data.
“Through these investigations, we hope to better understand the formation mechanisms and temporal development of these structures and better understand the role of volatiles in driving Mercury’s geological activity,” Bickel said.
Mercury’s continuing loss of volatiles alters how scientists understand rocky planets. The slope lineae may be used to quantify the remaining volatiles on Mercury and estimate the rate at which these materials are lost to space. These measurements will help researchers reconstruct Mercury’s internal structure and thermal history.
The findings may also benefit researchers studying similar processes on other airless bodies in the solar system, such as the Moon and asteroids. On these bodies, similar volatile-driven processes are believed to occur.
Understanding how volatiles behave on planets without atmospheres provides important insight into planetary evolution across the solar system.
Future space missions may now have definitive locations for monitoring active geology on a planetary body once believed to be inactive. If BepiColombo confirms ongoing geological activity, Mercury may stand as a rare example of present-day geological processes on a small, rocky planet.
Research findings are available online in the journal Communications Earth & Environment.
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