Small, Shrinking Mercury Is a Planetary Space Oddity

Mercury is more than just a dead, roasted chunk of rock. In fact, the solar system’s smallest planet is proving to be surprisingly lively. Well, not in the sense that anything actually lives there, though scientists don’t discount the possibility that some of Mercury’s nooks and crannies could help us understand the origins of life on Earth — especially the permanently shadowed craters at the planet’s north pole, which could harbor organic materials and host a significant amount of water ice despite Mercury’s closeness to the sun.

“Even a planet that is not in the nominal habitable zone of our own star is a witness to the delivery of the ingredients of habitability from the outer solar system to the inner solar system,” said Sean Solomon of Columbia University, at the 46th Lunar and Planetary Science Conference. “In terms of understanding what leads to habitability, Mercury has to be included.”

To be clear, no one actually thinks Mercurial microbes exist. Instead, scientists are mulling over recent evidence that is reshaping how we view the first rock from the sun. For a long time, Mercury was thought to be something of a relic, a stagnant world that hadn’t changed in eons and eons.

“I think that has changed, radically,” said Thomas Watters of Smithsonian’s National Air and Space Museum. “We’re finding that Mercury, like other objects in the solar system including our own moon, is far from dead. There is a lot happening there, both internally and externally.”

Now, it looks as though the planet is active, with ongoing tectonic activity, an extraordinary variety of materials strewn across its surface, and the recent formation of curious surface features.

The new data, presented at the Lunar and Planetary Science Conference, come from NASA’s MESSENGER spacecraft, which has been orbiting Mercury since 2011. If all goes according to plan, the spacecraft’s mission will end in late April when it runs out of fuel and crashes into the planet. For now, though, the spacecraft has been dipping ever lower over Mercury’s surface, collecting high resolution images and other data from as close at 15 kilometers above the cratered terrain.

Among the oddities MESSENGER studied recently are mysterious landforms referred to as hollows. There are thousands of them, ranging in size from a few meters to more than a kilometer across. First appearing as bright spots in early images, the hollows are flat-bottomed depressions that defied easy explanation. Now, scientists’ best guess is that the hollows are formed when volatile materials – probably a sulfur-containing compound – buried beneath Mercury’s surface sublimate, causing the terrain to sink by several tens of meters.

“The evidence suggests that hollows form when some component within the rocks is lost when it’s exposed to the harsh conditions at the surface of the planet,” said David Blewett, a geologist at The Johns Hopkins Applied Physics Laboratory.

That’s not all. Some of the hollows are embedded in otherwise cratered terrain, but appear to have escaped bombardment, providing a clue to their ages.

“Impacts occur randomly all over the planet and accumulate with time,” Blewett said. “The absence of craters inside the hollows means that they must be very young in the geological sense, probably less than a few tens of millions of years, and they may well be actively forming today.”

More evidence for ongoing activity comes from observations of surface features that look like wrinkled ridges, called scarps, and troughs, called graben. These crinkles in Mercury’s surface are formed because the planet is shrinking. As its core cools and contracts, it causes the material on the planet’s surface to buckle and slide; the new MESSENGER data reveal a whole pile of small, young-looking gullies and ridges that weren’t visible before.

“These young scarps really are exciting because they’re showing us that new faults are forming on Mercury as a result of the most recent phase of interior cooling and global contraction,” Watters said. “These faults are so young that they’re probably forming today.”

By measuring the dimensions and number of crinkles, scientists determined that Mercury has shrunk by as much as 10 kilometers over its 4.5 billion-year life.

And it will continue to get smaller as it gets cooler.

But not all the oddities on Mercury’s surface are visible by eye. Some must be spotted by an instrument capable of reading the chemical signatures written into rocks. When MESSENGER recently did this, it discovered a wealth of different elements littering the surface. The low altitude observations “reveal a more extreme range of compositions than we saw before,” said Larry Nittler of the Carnegie Institution of Washington.

The spacecraft also found evidence for what could be an ancient, enormous impact basin. That evidence is in the form of a massive splatter of magnesium, a splotch that could be explained by an impactor excavating portions of the material normally hidden beneath the crust, Nittler said.

Lastly, there are those shadowed craters at the poles. MESSENGER found evidence for water ice in those spots in 2012, and new observations suggested that a dark layer of organic materials might be covering a large swath of ice. Now, MESSENGER is peering into those craters and helping scientists work out what’s in there.

“We’re actually seeing into these regions where the sun never shines on Mercury in higher resolution than has ever been possible before,” said Nancy Chabot of The Johns Hopkins University Applied Physics Lab.

Features inside the craters suggest that a dusting of carbon-rich organic materials sits above the water ice. A sharp boundary separates the darker organics from lighter-colored material, and is also an indication that Mercury is not as placid as it seems.

“Things don’t just stay sharp,” Chabot says. “This material is very young on Mercury – either brought to Mercury very recently or is being refreshed or maintained.”