Mercury is still shrinking; new study reveals the planet has contracted by up to 11 kilometers
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Mercury is still shrinking; new study reveals the planet has contracted by up to 11 kilometers since formation
New research refines the understanding of Mercury’s contraction since its formation, estimating a radius decrease of 2.7 to 5.6 kilometers. Scientists analyzed thrust faults, using a novel method focusing on the largest fault in datasets to extrapolate planetary contraction. This approach offers a valuable tool for studying tectonics on other rocky planets like Mars, where vast fault systems also mark the surface. Mercury is shrinking because its interior has been cooling since the planet first formed about 4.5 billion years ago. When a planet loses heat, its volume contracts, much like metal shrinking as it cools. Mercury, with its unusually large iron core making up most of its volume, sheds heat more quickly than larger, rockier planets like Earth.
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A new way of measuring shrinkage
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Why is mercury shrinking in the first place?
What it means beyond mercury
Mercury, the smallest planet in the solar system, is still slowly contracting as it cools. Now, new research offers the clearest picture yet of how much the planet has shrunk since its fiery birth 4.5 billion years ago.According to findings published in AGU Advances, Mercury’s radius has decreased by 2.7 to 5.6 kilometers over its lifetime, narrowing down earlier estimates that ranged from 1 to 7 kilometers.Like a baked good that shrinks as it cools, Mercury’s rocky shell has been forced to adjust as the planet’s interior lost heat. This process created massive thrust faults, cliff-like scarps where sections of the crust have been pushed upward.Scientists studied the fault system and then measured the degree of contraction the planet has undergone. However, earlier approaches that calculated shrinkage based on the length and height of landforms often produced inconsistent results.To solve this, researchers Stephan R. Loveless and Christian Klimczak applied a fresh method. Instead of tallying up every fault, they focused on how much the largest fault in each dataset could account for contraction and then extrapolated that across the planet.They tested this approach on three different datasets: nearly 6,000 faults, another with 653, and a smaller set of 100. Remarkably, all pointed to the same result, about 2 to 3.5 kilometers of shrinkage from faulting alone. When combined with additional cooling-driven processes, the total contraction reaches up to 5.6 kilometers. Mercury is shrinking because its interior has been cooling since the planet first formed about 4.5 billion years ago. When a planet loses heat, its volume contracts, much like metal shrinking as it cools. Mercury, with its unusually large iron core making up most of its volume, sheds heat more quickly than larger, rockier planets like Earth. As the core and mantle contract, the planet’s crust is forced to adjust to the smaller volume beneath it.Altogether, this process has reduced Mercury’s radius by about 2.7 to 5.6 kilometers, meaning its diameter has shrunk by as much as 11 kilometers since its formation.The study not only sharpens our understanding of Mercury’s thermal history but also highlights a new tool for planetary science. The same methodology could help investigate tectonics on other rocky worlds, including Mars, where vast fault systems also mark the surface.
How Much Has Mercury Shrunk?
Mercury formed 4.5 billion years ago and has since contracted as it has lost heat. Thrust faults cut through the planet’s rocky surface to accommodate the ongoing shrinking. Researchers used an alternative method for estimating shrinkage caused by faulting on Mercury. They found that no matter which dataset was used, their method estimated about 2 to 3.5 kilometers of shrinkage. The new estimates could help deepen the understanding of the long-term thermal history of Mercury. Meanwhile, the same methodology could be used to investigate the tectonics of other planetary bodies, like Mars, that feature faults.
What do many baked goods and the planet Mercury have in common? They shrink as they cool.
Evidence suggests that since it formed about 4.5 billion years ago, Mercury has continuously contracted as it has lost heat. And somewhat like a fresh-baked cookie or cheesecake, Mercury also cracks as it cools: Thrust faults cut through the planet’s rocky surface to accommodate the ongoing shrinking.
By observing how faults have uplifted parts of Mercury’s surface, researchers can begin to estimate how much Mercury has contracted since it formed. However, prior estimates have varied widely, suggesting that thanks to faulting resulting from cooling, Mercury’s radius has shrunk by anywhere from about 1 to 7 kilometers.
To resolve this discrepancy, Loveless and Klimczak employed an alternative method for estimating shrinkage caused by cooling-induced faulting on Mercury.
Prior estimates all relied on a method that incorporates the length and vertical relief of uplifted landforms, but that produces different shrinkage estimates depending on the number of faults included in the dataset. In contrast, the new method’s calculations are not reliant upon the number of faults. Rather, it measures how much the largest fault in the dataset accommodates shrinkage, then scales that effect to estimate the total shrinkage.
The researchers used the new approach to analyze three different fault datasets: one including 5,934 faults, one including 653 faults, and one including just 100 faults. They found that no matter which dataset was used, their method estimated about 2 to 3.5 kilometers of shrinkage. Combining their results with prior estimates of additional shrinkage that may have been caused by cooling-induced processes other than faulting, the researchers concluded that since Mercury’s formation, the planet’s radius may have shrunk by a total of 2.7 to 5.6 kilometers.
The new estimates could help deepen the understanding of the long-term thermal history of Mercury. Meanwhile, the authors suggest, the same methodology could be used to investigate the tectonics of other planetary bodies, like Mars, that feature faults.
This article originally appeared on EOS, the science news magazine published by the American Geophysical Union (AGU).