Mercury may have shrunk up to 30 percent more than scientists previously estimated, as billions of years of impact debris likely concealed geological signs of the planet's contraction, according to a new study.
The research, published Thursday in the journal Geophysical Research Letters, suggests the radius of the solar system's smallest planet may have contracted by about 7.2 miles (11.6 kilometers). Previous studies placed the change in radius at between 0.6 and 1.2 miles (1 to 2 kilometers), and up to 4.3 miles (7 kilometers).
Mercury, which orbits the Sun every 88 days at an average distance of 36 million miles (58 million kilometers) according to NASA, formed about 4.5 billion years ago from swirling gas and dust. Like other newly formed planets, it was extremely hot, and the energy from violent collisions in the early solar system generated even more heat. As that heat escaped over time, the rocky world contracted, leaving its surface scarred with ridges and cliffs that scientists call shortening structures. Some of these cliffs reach up to 1.6 kilometers in height and stretch for hundreds of kilometers.
However, the planet was also bombarded by asteroids and comets, leaving it heavily cratered like Earth's Moon. Gaku Nishiyama, a planetary scientist at the German Aerospace Center's Space Research Institute in Berlin and the study's lead author, said that as Mercury shrank, cracks likely appeared consistently across its surface, but craters and debris covered many of these fissures.
Nishiyama and his team analyzed data from NASA's MESSENGER mission to create a global map of Mercury's surface roughness. They discovered a trend where the roughest parts of the planet had fewer visible wrinkles than the smoother regions. Nishiyama said this led the team to conclude that a process, such as debris coverage, was obscuring the shortening structures. The researchers then calculated how many wrinkles would be visible if they were not hidden, leading to the revised shrinkage estimate.
Clues to Mercury's core
Understanding the rate of Mercury's shrinkage could clarify how the planet evolved and reveal secrets about its mysterious interior. Nishiyama said a greater contraction suggests the planet could have a larger metallic core, fewer light elements like silicon mixed into it, or a higher initial temperature. Mercury is the second densest planet in the solar system after Earth, primarily due to its large metallic core.
Hannes Bernhardt, an assistant researcher in planetary sciences at the University of Maryland, College Park, said Mercury likely shrank more than any other planet because its core is proportionally much larger than those of other rocky worlds. Bernhardt, who was not involved in the new study, wrote in an email that lessons about the formation and evolution of large rocky bodies like Earth can be learned on Mercury better than anywhere else.
While Bernhardt called the study's methodology convincing and solid, he noted that such a drastic reduction in size should have caused visible large-scale deformation. He added that his own team is working on determining the planet's shrinkage, but said orbital data alone might not be enough. Bernhardt said that of all the things government money can buy, supporting the best engineers and scientists to put robotic boots in the scorching dust of Mercury might be one of the most inspiring.
Paul Byrne, an associate professor at Washington University in St. Louis who also did not participate in the research, said it was plausible that previous studies, including his own, missed the geological cracks because they are difficult to observe in rugged terrain or did not form due to the irregular crust. Accurate measurements of Mercury's contraction would allow for better estimates of its internal layers, the size and composition of its core, its tectonic and volcanic history, and how its magnetic field is generated, according to Byrne, who directs the Geoscience Node of NASA's Planetary Data System.
BepiColombo mission
Mercury's proximity to the Sun makes it a challenging environment to explore. Only two spacecraft have visited the planet in recent decades: NASA's Mariner 10, which revealed the planet's wrinkles during flybys in 1974, and the MESSENGER orbital mission in 2011.
An unprecedented mission is set to provide new answers when it launches two orbital probes around Mercury in November. The joint European and Japanese BepiColombo mission, which recently began its arrival process after an eight-year journey, will deliver new topographic data. Nishiyama, who is a member of the BepiColombo science team, said this could allow scientists to estimate the shrinkage more accurately and show the planet's cliffs, craters and ridges like never before.
Byrne said BepiColombo will make the first comprehensive global measurements of Mercury's topographic properties, which MESSENGER could not do. Byrne said the new data will be essential to test whether the latest findings are correct, adding that he expects they will be proven right and that there is still much to learn about the tiny planet.
Kelsey Crane, a geologist at Seres Engineering and Services who previously analyzed MESSENGER data, said the new orbiters will measure topography and surface roughness across Mercury's southern hemisphere, whereas MESSENGER collected its high-resolution data in the north. Crane, who was not part of the new study, said BepiColombo's instruments will detect gravity anomalies, improve crustal thickness and internal structure estimates, and map surface elemental and mineral composition, helping scientists tell a more complete story of planetary evolution and stimulating questions for future missions.
