The planet Mercury is dotted with holes that appear to be unlike any other landform yet seen in the solar system, new pictures show.

High-resolution photographs from NASA's MESSENGER spacecraft revealed the shallow, rimless, irregularly shaped depressions—similar to the holes in Swiss cheese—in impact craters all over Mercury.

The features are "widespread both in latitude and longitude," said study co-author David Blewett, of the Johns Hopkins University Applied Physics Laboratory in Maryland.

Dubbed hollows, the odd landforms can be tens of meters to a few kilometers wide, whereas the impact craters that contain them are tens of kilometers wide or bigger.

The hollows are often seen in clusters on the walls, floors, and peaks of the craters. Many hollows have smooth, flat bottoms and feature highly reflective material.

While Mercury had previously been thought of as a geologically dead planet, with few changes to its surface over the past billion years, "these [hollows] just look fresh," Blewett added.

"I think there's a distinct possibility that they're active today."

Solar Wind Zapping Mercury's Minerals?

The researchers considered the possibility that the hollows were formed during Mercury's volcanic past. On other planets, volcanism can form rimless depressions such as calderas and vents.

The team notes, however, that the hollows are much smaller than known volcanic pits, and the holes appear in places on Mercury that aren't likely to have experienced volcanic activity.

What's more, the hollows look distinctly fresh, because they haven't been reshaped by later impact events.

The shapes of Mercury's hollows bear some similarities to holes seen in the polar ice caps on Mars, in regions known to astronomers as Swiss cheese terrain, Blewett said.

The Martian depressions form as carbon-dioxide ice sublimates—turns directly from a solid to a gas—during seasonal temperature changes, hinting that some type of sublimation may be happening on Mercury.

"But on Mercury it's happening in solid rock, not in ice, so it's sort of a unique expression of geological processes that happen elsewhere, but maybe not as vigorously," he said.

The researchers theorize that the hollows could form when volatile materials such as sulfur on the surface are exposed to the harsh solar wind—actually a stream of charged particles from the sun. (Related: "Sun's Mysterious Waves Found; May Be Solar Wind Source.")

Since the tiny planet has no atmosphere, these particles can hit the surface directly, vaporizing volatile minerals. Or the close sun's intense heat could "boil" the minerals away.

"Say there was a sulfide mineral that gets zapped by a solar wind particle," Blewett said. "That sulfur could be lost, and the rock these minerals are in is basically undermined and would crumble away."

Mercury Not a "Burned-Out Cinder"

Researchers aren't yet sure what Mercury's rocks are made of, although one goal of MESSENGER's mission is to map the planet's surface composition.

In another paper also released this week, for instance, scientists using MESSENGER's x-ray spectrometer found that Mercury's surface has much more sulfur than that of any other rocky planet in the solar system.

While many unknowns remain, the new photos of Mercury overturn one longstanding theory about the tiny planet.

"The old thinking was, Oh, Mercury, it's an old burned-out cinder and not so interesting," Blewett said. Now "here's this jaw-dropping thing that nobody ever predicted."