After years of inspecting the craters and smooth plains of Mercury’s surface, NC State researchers have pieced together more clues to the small planet’s history. Data suggests effusive volcanic activity, meaning the widespread, steady lava flow responsible for the formation of Mercury’s outermost layer, stopped about 3.7 billion years ago. Effusive lava flow, at least on other planets, is generally more protracted.

“Sure, it’s not as active on Venus or the moon or Mars anymore, but there’s still a pretty long period of time when this stuff was oozing out,” said Paul Byrne, an assistant professor of planetary geology and co-author of an article about flood volcanism on Mercury.

However, this didn’t seem to be the case on Mercury.

“[On Mercury,] most volcanic material is manifested as something we call ‘smooth plains,’” Byrne said. “They look relatively smooth, but not entirely. They usually correspond to huge, really aerially massive expansive areas of land that got poured out … places where volcanoes just vomited land in a really short period of time geologically.”

Mercury’s smooth plains all seem to be close in age, leading scientists to believe that the flow began and ended in one relatively short interval, presumably in the first 20 percent of the planet’s life.

There are several explanations for this phenomenon, rooted in the planet’s particular geologic characteristics. When determining a planet’s fundamental processes, scientists tend to look at two main driving forces: tectonic forces and volcanic activity.

Most people are familiar with Earth’s tectonic behavior: seven major plates that make up the crust move by convective forces, pushing and pulling, forming major landforms and enabling volcanic activity. Mercury, however, is a one-plate planet. Consequently, tectonic activity is far different than what is conventionally studied. Uncertainty surrounding Mercury’s composition means room for speculation in core processes that drive tectonic activity.

Scientists assume the planet began to cool early on, leading to contraction of Mercury’s inner planet. Planetary cooling isn’t unusual — in fact, it stands that smaller planets tend to start a gradual cooling process sooner than larger bodies. Mercury falls under the former category; it’s only a bit bigger than Earth’s moon.

Byrne and other scientists had conducted research concerning Mercury’s tectonic cooling and contraction a few years ago. Finding out how much the planet had shrunk meant learning more about its composition. In turn, this meant learning more about its cooling history.

“Turns out that most things that govern the planet’s evolution are driven by loss of heat,” Byrne said. “The sun plays a role but … it’s running out of heat, stuff gets sluggish. We wanted to know what that meant for Mercury’s tectonics.”

Presumably, the tectonic contractions suppressed magma generation, which would explain the premature ending of effusive volcanism: an early waning supply of magma meant less volcanic activity later in Mercury’s history. Scientists also noticed a period where frequency of impacts seemed to taper off, around 3.8 or 3.9 billion years ago. These impacts were also somewhat responsible for stimulating volcanic activity, so this sharp reduction could’ve also contributed to less effusive lava flow.

To explore this, researchers focused on analyzing ages of Mercury’s surface. This proves tricky when scientists don’t have actual samples of Mercury. Instead, they used relative dating based on a simple principle. The more craters an area has, the older it is — having been exposed for a longer period of time, it’s more susceptible to impacts. 

However, these estimations leave room for a lot of error.

“There are some caveats,” said Byrne. “When a crater forms because an impactor hits it, there’s a giant explosion. These things are hypervelocity impacts — there’s an impact flash, it’s like a nuke. It blows a hemispherical gouge in the ground. It blows debris up, and debris can fall down and be deposited in a number of ways.”

When debris hit the surface, they create new craters. These secondary (and sometimes tertiary) craters are misleading. If not accounted for, they throw off age estimates, making regions seem apparently older. There does tend to be a pattern here, though, where they fall in a chain-like formation. Scientists take these kinds of clues into account, but there can be a lot of remaining guesswork.

Ages can also be determined by contrasting samples to predetermined data. Scientists already know the ages of two planetary bodies very well: Earth and the moon. Advanced dating practices and imaging technology help calculate the rate of which craters form on these sites. Comparisons can calibrate a close idea of what to expect. From there, scientists essentially make a series of well-informed assumptions.

“Then you could be very clever — and be in maths, or a physicist or something — and … you can extrapolate,” Byrne said. “You can use dynamical models to suggest what the impactor flux at Mars is: as a function of how far it is from the sun, how big the volume of space it occupies, how near to the asteroid belt it is — but you can take some sensible estimates. And you can extrapolate that data to Mercury … but by the time you find real world values for Mercury, you’re extrapolating on extrapolations that were extrapolated from other extrapolations.”

Essentially, nothing’s quite absolute in these calculations.

“They’re not wrong, you just don’t want to push it too much,” Byrne said. “It’s an imperfect system … Sometimes people say, ‘this is 3.76 billion years old.’ That’s bullshit. Because it’s probably older than three, probably younger than four billion years old. One decimal place off, and we use adjectives and modifiers like ‘about’ and ‘around,’ because we don’t really know. We know it’s around that ballpark. The model data tells you 3.76 billion, but the model tells you what you want. It just isn’t realistic.”

Having actual samples from Mercury would clear up a lot of uncertainty, but getting these samples isn’t logistically feasible. A mission to collect samples from Mercury — either in person or through robotics — would be wildly expensive. It’s also probable that bits of Mercury have hit Earth, but distinguishing them would be difficult. 

“Because we don’t know what Mercury would look like, people collect meteorites and they just go into collections,” Byrne said. “Sometimes you’re like, ‘Oh. It … It’s a rock.’ You don’t know where it’s from, no one knows quite what to look for.”

Expecting to possess samples in the future, though, is reasonable.

“That will confirm whether we’ve been right all along or if we’ve been completely bananas,” Byrne said. “But it’s like a lot of things in planetary — we do our best, but it’s hard to really know for sure.”

Information from the study also helps scientists understand more about other planets.

“There are certain things about other planets that don’t exist on Earth and there are things on Earth that don’t exist on other planets,” said Karl Wegmann, associate professor of geology. “But we can use the absence of one property to inform us.”

In particular, when considering the amount of erosion Earth experiences, Mercury’s landscape demonstrates a completely different environment. The information related to its preserved surface is a valuable key to geologists looking to understand more about Earth.

“For example, how do faults operate and deform the surface of [other planets]?” said Wegmann. “Here they may deform the surface of the planet, but after tens of thousands of years or less, that starts to be affected by erosion. We can learn something about how fault networks work on Earth, even when the evidence has been obliterated.”

After researchers’ extensive crater-comparison, the study data further clarifies Mercury’s life, piecing together an explanation for its volcanic activity. On a larger scale, identifying why this happened, and the mechanics behind it, gives insight into the timescale of planetary lifespans and other planets’ processes.