Mercury’s Strange Hollows
published during a waning crescent moon.

Strange Hollows

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Before its timely demise, the Mercury spacecraft, MESSENGER, sent back a steady stream of vacation photos. Sunlit freckles here, a periwinkle crater there.

But the best photos came at the end of its trip, right before MESSENGER’s long, long mission sputtered to a stop. They were of Mercury’s most photogenic feature: hollows, an unusual landform that looks like a crisp, but irregular, crater.


NASA’s MESSENGER spacecraft discovered strange hollows on the surface of Mercury. Images taken from orbit reveal thousands of peculiar depressions at a variety of longitudes and latitudes, ranging in size from 60 feet to over a mile across and 60 to 120 feet deep. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

What makes these holes so hallowed is that they don’t appear in our solar system or any other world we’ve observed. MESSENGER was the little probe that could, and did, putter well past its designed lifetime, giving scientists an intimate look at Mercury’s barren ground. Hollows showed up as vivid little patches, and no one quite knew what to make of them.



Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

That’s what makes them irresistible, Emily Lakdawalla of the Planetary Society writes. They’re a totally new landform to explain that are unique to our small, innermost planet. When the Mariner 10 probe first showed us the planet’s surface, the craters appeared as smudges–like tea leaves too difficult to read. Were they craters? Volcanic pits?

Turns out, hollows aren’t either, but rather shallow, rimless depressions with flat floors, as a 2014 study describes. Craters show the distinctive aftermath of an impact, and volcanic pits are deeper with uneven floors. Hollows are smaller and brighter — and they can be found in craters, whether it’s their bottom, or their wall, or even their rims.

Researchers are still digging into how hollows even coagulate. They’re not evenly distributed on Mercury, nor are they a prominent feature, covering just 0.08% of the surface. Their small size — about half a mile — suggest that they’re young, too, because incoming craters haven’t quite eroded them yet.

That means that activity on Mercury’s brutally hot surface may still be ongoing and that the hollows’ formation most likely has to do with a process called sublimation–when a solid turns to gas. The gap that then forms can give in creating a pit.