Science | April 16, 2024

The Seven Most Amazing Discoveries We’ve Made by Exploring Mercury

Only two robotic missions have made it to the Swift Planet, but they were crucial for upending many false assumptions of that sun-scorched world

This false-colored image of Mercury reflects the varying age and mineral composition of the surface. NASA / JHU Applied Physics Lab / Carnegie Inst. Washington

Shi En Kim

Reporter

It seems like Mercury, sitting so close to the sun, should already be a dead planet. Scientists thought the humble hunk of rock that’s only slightly girthier than Earth’s moon would have been battered to barrenness by the sun’s intense solar radiation. It’s a hostile world: The day and night sides of the planet see a temperature difference nearly 600 degrees Celsius, or 1,100 degrees Fahrenheit.

But Mercury has proved to be a world of contradictions, a dynamic planet that hides more surprises than scientists initially gave it credit for. A thin “atmosphere,” a magnetic field and a stash of volatile compounds still persist on Mercury, features that scientists usually associate with planets that are larger and farther away from the sun.

“We expected this thing to have been beaten up, processed—it’s done for, it’s baked and cooked,” says Deborah Domingue, a senior scientist at the Planetary Science Institute. Now, she says, new observations from the last few decades have given scientists good cause to revise their opinion of Mercury. “There’s all this intriguing evidence that Mercury is not this desiccated piece of stone.” For one, Mercury still has pockets of ice.

So far, only two missions have made it to the Swift Planet. A third one, the Japanese and European space agencies’ BepiColombo, is currently en route and will arrive in late 2025. While these few ground-based observations and on-site missions have barely scratched the surface, they helped clear up many of our initial misunderstandings of Mercury’s mystique. Here are some of the most amazing discoveries about Mercury to date and why they’re important.

Mercury is metal

Mercury's Core — Mercury’s large, mostly iron core explains why the planet is so dense. Dorling Kindersley via Getty Images

Mercury may be small, but it’s hefty. Although its diameter is barely larger than the moon’s, Mercury is over four times as massive as Earth’s natural satellite. In fact, Mercury is the second-densest planet in our solar system, after Earth.

The planet’s extreme density comes from the fact that it has a large iron core that makes up about 60 percent of the planet’s volume. In contrast, Earth’s core by volume is only about 15 percent.

The abnormal internal structure has spawned several theories on Mercury’s birth. Scientists think that perhaps early Mercury’s outer layer was boiled away by the sun or scattered by solar winds. Or perhaps the young planet could have been struck by a large impactor that stripped away most of the softer outer layer, leaving behind its sturdier heart. Since part of the mantle and crust are still present today, this suggests that the collision may not have been a head-on smack but a glancing brush that preserved some of Mercury’s original layers.

Churning insides power a magnetic field

The first mission to Mercury, Mariner 10, launched in 1973 and found that Mercury still sustains a magnetic field. The discovery came as a shock to the scientific community, who had assumed that such a small planet would have rapidly cooled and its insides would have congealed, thereby losing any means of magnetism at the global scale. The presence of a magnetosphere implies that part of Mercury’s core is still churning.

Mercury’s magnetic field is about 100 times weaker than Earth’s on their respective planetary surfaces. The sluggish dynamo means that the planet is at the tail end of its developmental stage, on its way to becoming a dead planet like Mars.

In the 2010s, the second Mercury mission, Messenger, documented that the planet’s magnetic field is off kilter. The magnetic south pole doesn’t sit on the geographic south pole; instead, it is buried almost at the center of the planet.

The magnetic field offers a glimpse of the planet’s interior and a snapshot of its history, hinting at how far the whirling insides have decelerated over billions of years, says Antonio Genova, an aerospace engineer who studies planetary geodesy and geophysics at the Sapienza University of Rome. His team will leverage the current BepiColombo return mission to further interrogate the subterranean secrets of Mercury.

The planet has a wispy “atmosphere”

Mercury's Tail — The glowing tail of Mercury Andrea Alessandrini via Wikimedia Commons

Mercury has such a thin atmosphere that it doesn’t really qualify as an atmosphere. Instead, scientists call this wispy layer of gas an exosphere, where the gas is so rarefied that it hardly registers a pressure reading.

Mercury takes a constant beating from the sun, so its exosphere has to regenerate itself from the surface. Since the 1980s, astronomers have detected in Mercury’s exosphere atomic sodium, potassium and calcium, metals with strong emission signals that can be observed all the way from Earth with telescopes. These metallic elements are not usually thought of as gases. But they find their way into Mercury’s skies by dint of solar particles and meteorites striking the planet’s surface.

Solar winds gouge the resulting exosphere, and the interactions between the gasses and sun-flung particles fashion a 15-million-mile-long glowing tail behind Mercury. This tail lengthens and shrinks seasonally depending on Mercury’s proximity to the sun. If you were standing on Mercury and looking up at the right time of the year, Mercury’s long tail would appear in the sky as a faint orange glow, as if the heavens were illuminated by sodium streetlights.

Ice sits at the poles

Ice on Mercury — Mercury appears to have ice at its poles, which are shaded from the sun Ron Miller / Stocktrek Images Via Getty Images

A planet perched right at the sun’s maw shouldn’t harbor any water, let alone ice—or so researchers thought. But in the 1990s, scientists at Goldstone in California and the Arecibo radio telescope in Puerto Rico directed a stream of radar signals toward Mercury. The teams were astonished to observe two brightly reflective spots at the poles—likely icy deposits.

In 2012, Messenger confirmed that the ice at Mercury’s north pole was frozen water. Laser measurements onboard identified carbon-rich material on the surface that insulates an ice pack underneath.

Mercury has managed to cling onto its water because it has pockets in permanent shadow. Relative to its orbit around the sun, the planet spins perfectly upright, like a soldier at attention facing a commanding officer. That means the impact craters near the poles have insides that never see any daylight. Temperatures within these divots dip below minus 280 degrees Fahrenheit, close to the temperature where nitrogen gas liquefies. “It’s cold enough that ice can be stable over geological time frames,” says Sean Solomon, a planetary scientist at Columbia University and the principal investigator of the Messenger mission.

As on most other rocky planets, the water on Mercury probably came from asteroids that crash-landed onto the surface. This water hides in Mercury’s craters, untampered with since primordial times. On other terrestrial planets in the solar system, geological processes such as weather cycling have scattered the externally delivered water all around the globe. If scientists want to sample ancient ice in its pristine form in the solar system, Solomon says, the best source is probably Mercury’s poles.

Mercury harbors other volatiles, too

Volatiles on Mercury — Mercury’s crust is rich with moderately volatile elements such as potassium and sulfur. NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury defied expectations again when Messenger detected volatiles on the sun-scorched world. Volatiles are chemicals that can jump between solid and gas phases in a short temperature change. Mercury already proved it sheltered water, a volatile, but Messenger found others such as sulfur, potassium and chlorine that easily vaporize at moderately high temperatures. These volatiles are distributed all across the surface of the planet. For its size, Mercury contains as much volatile content as other terrestrial planets in the solar system that are farther from the sun and hence much colder.

Where the volatiles come from and how Mercury has retained them are still matters of scientific debate. Some researchers think that the volatiles came from underground relatively recently, while others think the chemicals have persisted on the surface since an embryonic Mercury condensed from its protoplanetary disk.

The volatiles on Mercury raise questions. For example, if planets that edge up against their stars have volatiles, especially water, could these locales also be habitable? “Mercury shows us: Don't discount [planets] close to the sun,” Domingue says.

Unique geologic features adorn Mercury’s face

Mercury has irregular depressions in the ground called hollows. Mariner 10 first glimpsed them in 1975; then Messenger photographed these shallow tracts at fine resolution. The depressions span from a few dozen feet to over a mile across, and they may be as deep as 120 feet. They look like the soft, airy insides of a sourdough loaf cut open.

The hollows may be the doing of volatiles, scientists theorize. Since the atmosphere-less Mercury has no rain or wind to carve the land, surface features like hollows can only be formed from other processes—such as the ground leaking volatiles into space. Hollows are relatively young formations, about 100,000 years old on average, compared with some four-billion-year-old impact craters on Mercury. Scientists think hollows are still forming today.

And hollows are unique to Mercury. No other celestial body in the solar system seems to present similar pockmarks.

In recent years, scientists have identified other structural formations on Mercury. Chaotic terrains—jumbled ridges that look like a failed game of Jenga—cut up vast swaths of the land. Some researchers think the chaotic terrains are the result of the ground wafting volatiles until it loses structural integrity and collapses. Other scientists think that chaotic terrains form after ripples from an asteroid impact encircle the planet and meet on the opposite side of the globe.

Mercury was once volcanically active

Smooth Plains on Mercury — In this Messenger-snapped composite photo of Mercury’s surface, two large craters—at the top and in the left of the image—seem to have been filled in to form smooth plains. NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury’s topography provides clues that volcanoes once spewed lava on the planet. Mariner 10 first glimpsed and then Messenger later confirmed that glossy plains spread across Mercury’s surface. Each plain suggests pooled lava smoothed over older craters and ridges. Researchers think that magmatic volcanism halted on Mercury anywhere between one billion to 3.5 billion years ago, due to planetary cooling and contraction that plugged up the magma’s escape routes.

Mercury has also shown signs of another kind of volcanism: explosions. If streaming lava is like a runny nose, then explosions are the volcanic equivalent of a sneeze. Irregular pits several miles across and more than two miles deep hint at ancient pyroclastic volcanoes that self-destructed. Scattered around these pits are deposits that researchers think are ejecta from the explosion.

These types of volcanic bursts are probably caused by volatiles underground, experts say. When these buried chemicals rise to the surface, they expand in volume. Eventually, the pressure buildup of gasses causes the volcano to pop, like an overfilled balloon.

BepiColombo scientists are hoping to learn more about where these volatiles come from. Mapping them on a global scale will provide clues about how they got there. “The origin of the volatiles is one of the main themes of space exploration in general,” Genova says.

Editors' Note, April 18, 2024: This article has been updated to show Sean Solomon’s status as a working planetary scientist, to explain the magnetic properties of Mercury, and to reflect the timing of discoveries in the planet’s exosphere.

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.