Mercury is, by far, the tiniest planet in our solar system — about 29 times smaller than Jupiter and just one-third the width of Earth. One day, it might be even smaller.
When Mercury formed about 4.5 billion years ago in the raucous collisions of the early solar system, it was a much hotter planet. Over many millenniums, however, its metallic core has cooled and contracted, causing the planet’s surface to warp and wrinkle.
Previous estimates suggested that Mercury had shrunk by just a few miles since its formation. Now, scientists say it is shrinking up to 30 percent faster than expected. It might have even shrunk by as much as 14.5 miles, which is nearly one percent of its current diameter of 1,516 miles.
This new estimate, published Thursday in the journal Geophysical Research Letters, gives astronomers a new understanding of how the planet — and others like it — formed and evolved.
“Many bodies, like the moon and Mars, are shrinking nowadays,” said Gaku Nishiyama, a planetary scientist at the German Aerospace Center Institute of Space Research who led the study. “But in the case of Mercury, it can shrink more because of its large metal core.”
To arrive at their conclusion, Dr. Nishiyama and his team used images from the 2004 flyby of NASA’s Messenger spacecraft and more recent digital models to analyze the planet’s ridges and valleys — the surface’s “wrinkles” that form as the planet shrinks. They also looked at features that might obscure them, like lava flows and impact debris, that could make it difficult for researchers roughly 97 million miles away to see the full picture.
“The reason why the estimation remains difficult is that we have to rely on topographies that are visible on the surface of the planet,” Dr. Nishiyama said.
More data and imagery of the planet’s surface will be needed to nail down how much Mercury has shrunk over the past 4.5 billion years, but the study’s range of 10 to 30 percent faster than previous estimates unlocks another piece of the puzzle of planet evolution, experts say.
“Every planet is an odd little duck that has a story to tell,” said Sam Birch, a planetary geoscientist at Brown University who was not involved in the study. “If you can understand how fast Mercury is shrinking, you can understand how fast it’s cooling, what size it started at and what it’s made out of.”
The study authors hope to apply what they’ve learned about planets’ hidden wrinkles to other rocky worlds, such as Mars and the moon.
Fortunately, new data might be on the horizon: Later this year, the European Space Agency and Japan Aerospace Exploration Agency will launch their BepiColombo mission, which will become the second to ever enter Mercury’s orbit, providing an up-close look at this rarely seen planet.
“We could test this pretty soon, which is nice,” Dr. Birch said. “Whether it is exactly 30 percent or 10 percent is the question.”
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