Enigmatic Mercury will remain a puzzle until someone sends a lander

Mercury remains one of our solar system’s most elusive and difficult planets to study. It’s easier to send probes to the icy moons of Saturn or Jupiter, or even to Pluto, than to land on our innermost planet, Mercury.

Ground-based optical observatories risk damaging their optics in the glare of the nearby sun when they turn their sights on Mercury. Thus, of the planets in our inner solar system, Mercury arguably remains both the least explored and most misunderstood.

NASA’s Mariner 10 performed three flybys of Mercury between 1974 and 1975. This was followed by a 2011 orbital insertion by NASA’s MESSENGER mission to Mercury, which successfully orbited the planet for some four years.

But despite the European Space Agency’s and the Japan Aerospace Exploration Agency’s long-anticipated joint BepiColombo mission to Mercury, we aren’t likely to solve this strange little world’s conundrums until someone figures out a way to send a lander that can safely land and function long enough to return data that would give us a much clearer indication of Mercury’s surface composition and its interior.

BepiColombo, which is scheduled to enter orbit late next month, is currently our only chance to acquire new data about the planet. However, it’s not expected to start collecting science data until April 2027.

After BepiColombo, there’s nothing on the drawing board, Paul Byrne, a planetary scientist at Washington University in St. Louis, told me via email. But a lander is the next natural step in the exploration of Mercury, even if it’s a technically challenging and expensive proposition, Byrne told me.

Without a lander mission, Mercury’s main science puzzles will remain.

They include:

The planet’s disproportionately large iron-rich core, which is thought to make up some 85% of the planet’s radius.

We’re beginning to find evidence for much larger, more massive rocky exoplanets that seem to have giant iron cores, says Byrne. So, he wonders if Mercury is a one-off freak or a planetary outcome that’s more common than we might think.

Two major hypotheses have been formulated to explain the presence of a large metallic core.

The first one posits that Mercury accreted from building blocks richer in metal than those of other planets, such as Earth, Anne Pommier, a staff scientist and high-pressure experimentalist at the Carnegie Laboratory in Washington, D.C., told me via email. The second proposes that Mercury was initially larger than it is today, with a crust-and-mantle-to-core ratio closer to that of Earth, Pommier told me.

Pommier, who is a co-investigator of the magnetometer instrument on BepiColombo, told me that large impacts likely stripped off a massive part of Mercury’s rocky mantle, thus leaving it with an anomalously large iron core.

But the presence of a large metallic core remains difficult to explain; Mercury challenges our understanding of planetary formation, says Pommier.

Aside from Earth, Mercury is the only other rocky planet in our solar system that has a global magnetic field.

Compared with Earth’s, Mercury’s magnetic field is weak, and it is thought that the field is generated in the liquid portion of the core, says Pommier. Continuous motion in this liquid creates electric currents, which in turn create a magnetic field dynamo that can be sustained for billions of years, she says.

A global magnetic field

Mercury currently generates a global magnetic field that is predominantly dipolar at the surface and approximately 100 times weaker than Earth’s surface field, the authors of a 2024 paper appearing in the journal Earth and Planetary Science Letters note. Remanent crustal magnetization with an inferred age of 3.7 to 3.9 billion years suggests the dynamo operated in Mercury’s early history, the authors note.

Mercury does have a tenuous exosphere composed of very limited amounts of oxygen, hydrogen, sodium and helium.

But if Mercury has even a weak magnetic field, why can’t it hold on to a real atmosphere?

It’s not enough to have a magnetic field: You need to be big enough to replenish an atmosphere and far enough away from your star to not have your atmosphere blown off, says Byrne. Mercury is relatively close to the sun, which means it takes a pounding from the solar wind, he says.

Mercury itself is shrinking

There is also evidence at the surface that the planet has been shrinking, says Pommier. This process is possibly due to core cooling but is not well understood, she says.

Finally, Mercury has some sort of enigmatic low-reflectance material covering large areas of its surface.

This material possibly contains endogenic carbon, like graphite.

In some craters, it is expected to be hidden under layers of other deposits, so its presence is both at the surface and in the subsurface, says Pommier. One hypothesis regarding its origin is that it formed as part of the crystallization of a magma ocean early in Mercury’s history, she says.

A unique graphite landscape?

This graphite might be the remains of a once-widespread and probably thin carbon-rich flotation crust that likely covered the planet, says Byrne. Over time, with successive impacts churning up and redistributing crustal materials, as well as surface volcanism, most of this graphite-rich crust was buried, reworked or removed into space, he says.

Are there any known Mercury meteorites here on Earth?

There are currently no officially confirmed meteorites from Mercury on Earth, despite several studies suggesting that some meteorites (such as enstatite chondrites) could come from Mercury, says Pommier.

But this is one reason why landing on the planet is so crucial.

Landing on Mercury and carrying out chemical analyses in situ would give us a much better idea of what to look for in the world’s current meteorite collections, says Byrne. It’s certainly possible that we already have unrecognized pieces of Mercury in these collections, he says.

The challenges are the time en route, the energy needed to slow the spacecraft down for a successful orbital insertion and the landing. Dropping deep into the sun’s gravity well requires a massive change in velocity (or delta-V), requiring even more energy than is needed to send a probe to Pluto.

A 2020 NASA planetary mission concept study notes that a lander mission would have a 10-year cruise phase before touching down about 30 hours before sunset on Mercury. The idea is for the lander to be fully operational during Mercury’s night, which spans some 88 Earth days. The lander would use LED lights to photograph Mercury’s surface and might even be aided by a diffuse glow from the planet’s sodium exosphere.

The bottom line?

A lander must first softly touch down on the surface of Mercury.

With no atmosphere, a lander can only rely on retrorockets; we have experience doing that for the moon, but gravity on Mercury is about twice as high, says Byrne. So, the challenge is greater—and would necessarily happen after a much longer cruise phase than to the moon or even Mars, he says.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

Full profile →

Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

Full profile →