Mercury Is in Retrograde, but What's Weirder Is How Much This Planet Stumps Scientists

Updated | Until April 15, here's a convenient excuse for any troubles in your life: It's all the fault of a quirk of the planetary dance that constantly unfolds across our solar system. Right now, Mercury is in retrograde, and astrology buffs claim that the planet's apparent loop backward in our skies somehow makes life here on Earth more complicated. There's even a handy website you can check to confirm the innermost planet is a valid scapegoat for your troubles.

But scientists who study Mercury aren't looking for excuses. They're churning through a treasure trove of data produced by a recent NASA visit and anticipating the October launch of a new mission headed back to Mercury. All of that effort is directed toward solving the puzzles the planet represents: because not only are scientists not sure how it came into being, but what the last spacecraft to visit Mercury found wasn't at all what they expected.

"Mercury is just so weird and so exotic compared to the moon and other terrestrial planets," Kathleen Vander Kaaden, a planetary scientist at the engineering company Jacobs Engineering Group working on a contract at the NASA Johnson Space Center in Texas, told Newsweek. "It's just a very exciting planet."

To untrained eyes, its weirdness may not be clear. Mercury is a tiny hunk of mostly metal, barely larger than the moon, dressed in a drab gray-brown and covered in scars left by billions of years of impacts. It speeds around the sun, which makes its surface incredibly hot during the day, but without an atmosphere blanketing it, the planet rapidly cools off every night. All that heat from the sun also creates a comet-like tail of sodium gas streaming away from the star.

The last spacecraft to visit Mercury was NASA's Messenger mission, which circled the planet from 2011 to 2015 before purposefully crashing itself into it. Messenger revealed that the planet had once been volcanically extremely active, that its dense core was even larger than scientists had expected, and even that the planet is shrinking.

Those discoveries have complicated the process of trying to piece together Mercury's entire life story. Typically, planets that form near each other have pretty similar interior structures. Venus, Earth and Mars all have a small dense metallic core at their centers surrounded by a thick blanket of rock.

But Mercury's structure looks very different. It's missing most of that rocky sheath, and scientists aren't sure why. Perhaps Mercury formed elsewhere, then migrated in toward the center of our solar system. Perhaps it has always been here but had a catastrophic encounter with something even larger that scraped off its skin of lighter material. Scientists know there must be a secret in Mercury's past to explain why it is mostly metal—they just don't know what it is.

It's too big a mystery to tackle all at once, so instead scientists investigate smaller questions that feed into the bigger picture. Vander Kaaden, for example, is trying to replicate Mercury in miniature in her lab. Using tiny mixes of metal and rock that mimic the planet's layers, she then forces them into low-oxygen, high-pressure conditions like those of the planet's past. Then, she compares the results to measurements Messenger took of Mercury's surface in search of a better understanding of how the planet formed and changed over time.

Some of that change is lost in the distant past, like rock formed by the giant sea of magma that likely once covered the planet's surface, which was later buried by eruptions of lava that stopped about 3.5 billion years ago. Or at least scientists thought they had stopped. But then Messenger spotted magma that had oozed out here and there across the planet's surface and bright reddish splotches that seem to suggest there have been some explosive eruptions as well.

Related: We know almost nothing about Venus, and that could mean trouble when we look for habitable alien worlds

It's those bright patches that have fascinated David Rothery, a geoscientist at the Open University in the U.K., who points out that until Messenger's data, scientists had no idea Mercury might have had volcanic activity on its surface within the past billion years. "What we're realizing now is that's not the end of the story," he said. "We really want more detailed images of these vents and the craters in which they occur."

Rothery and his colleagues don't know yet what's driving those explosions—he's not even willing to assume there are volcanoes involved—but he says it has to be some compound that can turn to gas. And here's another weird thing: Mercury is full of compounds that usually turn to gas and dissipate off into space quite quickly. In particular, there's much more sulfur than scientists had expected to find.

"The rocks on the surface that we found out from Messenger are just fascinating because they don't look like any of the predictions that were made before the spacecraft got there," Nancy Chabot, a planetary scientist at Johns Hopkins University in Maryland, told Newsweek.

Chabot focuses her time on what's tucked within the rock, particularly hidden in craters at the planet's poles, where the sun doesn't shine: water ice. Chabot and others studying Mercury don't know yet if the water has always been there or was brought there by the comets that bombard its surface. And right now, she and her colleagues know much more about the North Pole than the South Pole because of Messenger's path around the planet.

Mercury is full of puzzles. Right now, Mercury is in retrograde, and astrology buffs claim that the planet’s apparent loop backward in our skies somehow makes life here on Earth more complicated. NASA

But the next spacecraft to visit Mercury will offer a much better picture of the South Pole—and it's coming soon. Just three years after the Messenger mission ended, Mercury scientists are already preparing its successor, a joint European and Japanese mission called BepiColombo that will launch in October. BepiColombo will take seven years to reach its destination, but once it does, it will separate into two individual orbiters. Until then, scientists studying Mercury will occupy themselves with what they have.

"There's still plenty of fruit on the Messenger tree," Rothery said. "We'll be happily working with Messenger data until BepiColombo gets there for sure."

Sean Solomon, a planetary scientist at Columbia University in New York who was one of the leading scientists behind Messenger, is confident that its successor will bring back as many questions as answers, thanks to its new instruments and flight plan. "That's true with every planet in our solar system," he said. "We have never been so wise as to fully anticipate what our neighboring planets would look like."

Even when Mercury appears to move backward in our sky, it moves forward in science.

This story has been updated to correct Kathleen Vander Kaaden's affiliation.​