Mercury is the unloved planet of our Solar System, a barren rock too small and too near the Sun to be interesting. At least, that’s what we thought until we took a closer look and discovered everything about this plain little planet breaks our initial models for how it formed.
Mercury is a hard planet to observe. It’s so close to the Sun that Earth-based telescopes are easily blinded by the star, and locked in orbital resonance that leaves us peering at the same patch of rocks over and over. Even sending spacecraft to investigate is a problem, with probes speeding up as they fall down the gravitational well towards the Sun. It took looping around Venus for Mariner 10 to slow down and redirect into Mercury flybys in the mid-1970s, and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) probe needed to make an even more elaborate dance to slip into orbit in 2011.
The challenges of observing Mercury paired with its apparent simplicity to leave it frequently overlooked in favor of more dynamic planets. It’s slightly larger than Earth’s Moon, so scientists assumed it was another cold, dead, and dull cratered world without much to offer to our understanding of the story of planetary formation.
Mercury’s distinguishing characteristic is that it is unusually dense for its size, so dense that it must have a massive metallic core. The core is dramatically disproportional to those found within other terrestrial planets, over half of the planet’s volume while Earth’s is less than 10%. Scientists theorized that it originally formed as a larger planet similar to the Earth and Venus, and either had a major collision strip it of much of its crust (akin to the theory of how Earth gained its Moon), or that it’s close proximity to a younger, hotter Sun boiled the crust and vaporized lighter elements.
During its flybys, Mariner 10 mapped a series of long, tall cliffs called scarps and a series of wrinkled folds. Paired with the knowledge that Mercury has an unusually large core, scientists theorized that the planet’s surface crumpled as the core cooled and condensed over billions of years.
But the more we learn, the more those glib stories unravel.
Mercury has an active, modern magnetic field, the only terrestrial planet besides Earth that protects its surface from the solar wind. During daring buzzes close to the surface of the planet during its final year of operation, MESSENGER detected lingering shadows of the magnetic field in the rocks, confirming Mercury’s magnetic field has lasted billions of years. The most likely explanation is that it has a liquid core, hot metal moving fast to generate a magnetic field. This idea is reinforced by Mercury’s libration, a dramatic over-rotation at the apex of its orbit that could be enabled by a liquid layer decoupling a solid inner core from the rigid surface.
The next complication to the puzzle is that the large, global scarps and wrinkles first spotted by Mariner 10 were also spotted by MESSENGER, along with much smaller version if the same structures. The scarps first identified in 1974 and 1975 are hundreds of kilometers long and several kilometers tall, enormous cliffs towering over the landscape. The latest observations supplement those with scarps a few kilometers long and only tens of meters tall, and more importantly, geologically young. The larger scarps are ancient and softened by the passage of time, but the smaller ones are crisp and fresh, likely formed within the last tens of millions of years. This suggests that instead of being cold and dead, Mercury is still geologically active today as a hot core cools, the planet shrinks, and the surface crumples.
The story of Mercury gets stranger still.
When mapping the geology of Mercury’s surface, MESSENGER found distinct regions of unique geochemistry, a mix of terrains instead of a single roughly-similar surface. The spacecraft also spotted delicate volatiles like potassium and uranium, chemicals that should’ve been destroyed by the enormous heat of a crust-shearing collision or the scorching heat of a crust-boiling Sun. By their very presence, these chemicals argued against either theory for Mercury’s oddly-large core, leaving scientists scrambling for a new theory.
More data poured in from MESSENGER, with scientists identifying pyroclastic flows that could only be the result of explosive volcanism of lava mixed with volatile gases like water and carbon dioxide, more chemicals scientists didn’t expect in abundance on a planet so near the Sun. Soon after, the spacecraft’s radar spotted water ice lurking in the shadows of polar craters, reminiscent to the ice hiding in polar craters of our Moon.
The MESSENGER mission came to an end in 2015, the spacecraft running out of fuel and deliberately crashing into the planet. Now, scientists are left analyzing and reanalyzing the data from its 4,105 orbits to try to piece together a new explanation for how Mercury formed that pull all these pieces together. But in all the mysteries lays a tantalizing new possibility for investigating this deceptively-simple planet: quakes.
If Mercury truly does have a hot, liquid core and is geologically active today, every time the crust moves to create a new ridge or wrinkle, a quake releases seismic waves propagating through the planet. On Earth, we detect that seismic energy with seismometers, observing the speed and distribution of the waves to map the planet’s interior. Astronauts did the same thing on a smaller scale during the Apollo missions, observing moonquakes to understand what was going on below the surface. Scientist are planning to apply the same trick on Mars, sending a seismometer on NASA’s delayed InSight lander to peer into the interior or Mars. This same idea could work on Mercury, with observations of seismic waves pinning down the true size and structure of the core, and detecting if, like the Earth, it has a warm liquid outer core around a solid interior.
And if we can do that? We’ll be one step closer to understanding how Mercury formed, why it is the way it is today, and what to expect of other rocky planets around distant stars.