Mercury’s surface is a battered, Sun-scorched landscape of impact craters, vast volcanic plains, towering cliffs, and strange shallow pits found nowhere else in the solar system. At first glance it looks a lot like our Moon, but the resemblance is misleading. Mercury has geological features that tell a very different story: a planet that shrank as it cooled, erupted lava across a quarter of its surface, and somehow holds water ice at its poles despite daytime temperatures hot enough to melt lead. Nearly everything we know about this surface comes from two spacecraft, Mariner 10 in the 1970s and MESSENGER in the 2010s, with a third mission now on its way.
A Landscape Dominated by Craters
Mercury has no meaningful atmosphere to slow incoming debris, so asteroids and comets have been slamming into it at full speed for over four billion years. The result is a surface pockmarked at every scale, from tiny pits to enormous basins. The largest confirmed impact structure is the Caloris basin, roughly 1,550 kilometers across. It is one of the biggest impact features in the entire solar system, and its formation was violent enough to send shockwaves through the planet’s interior and out the other side. On the opposite face of Mercury, directly antipodal to Caloris, lies a region of broken, jumbled terrain. Researchers examining this chaotic terrain found evidence of multi-kilometer drops in surface elevation and widespread landform disruption, consistent with the gradual collapse of a volatile-rich layer beneath the surface rather than a single catastrophic event.
1PubMed Central. The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar SystemThe cratering record serves as Mercury’s primary geological clock. Because the planet lacks plate tectonics, wind, or liquid water to erase old surfaces, craters accumulate over time, and counting them lets researchers estimate the ages of different regions. Using improved data on asteroid populations and crater scaling, geologists have built a time-stratigraphic system for Mercury, dating features like the Tolstoj and Caloris basins and dividing the planet’s history into distinct periods, much as geologists use rock layers on Earth.
2Planetary and Space Science. Geologic evolution and cratering history of MercuryBetween the large basins, Mercury’s older terrain is densely cratered and heavily textured, with overlapping rims and ridges. These intercrater plains are some of the oldest exposed surfaces on the planet, predating the major volcanic episodes that resurfaced so much of Mercury later.
Volcanic Plains That Resurfaced a Quarter of the Planet
One of MESSENGER’s most striking discoveries was just how much of Mercury’s surface was created by volcanism. About 27% of the planet is covered by smooth plains, and the majority of that area, over 65%, is volcanic in origin rather than the product of impact melt or ejecta.
3Journal of Geophysical Research: Planets. The distribution and origin of smooth plains on Mercury These are not small patches. The smooth plains around and inside the Caloris basin alone rival the size of entire countries, and similar deposits extend across much of Mercury’s northern hemisphere and beyond.
The timing of this volcanism is remarkable. A detailed global inventory of both large and small smooth plains found that the overwhelming majority were emplaced within a concentrated window of about 200 million years, centered around 3.7 billion years ago. During that period, at least roughly 25% of the entire global surface was resurfaced by lava. The small smooth plains created by effusive volcanism tend to cluster around the Caloris, Rembrandt, and Beethoven basins and in areas where the crust is thinner, suggesting that the formation of large impact basins may have helped trigger the volcanism by fracturing the crust and allowing magma to reach the surface.
4Geophysical Research Letters. Short‐Term and Global‐Wide Effusive Volcanism on Mercury Around 3.7 GaThe composition of most smooth plains resembles a magnesian alkali basalt, based on their spectral properties. But a smaller fraction of volcanic plains shows a lower reflectance and shallower spectral slope, pointing to a more ultramafic makeup. That implies the magma source regions were hotter and experienced higher degrees of melting, and that these extreme conditions persisted through much of the volcanic era.
3Journal of Geophysical Research: Planets. The distribution and origin of smooth plains on MercuryMercury also shows evidence of explosive volcanism, not just the gentle flooding of plains with lava. Bright deposits called faculae surround vent-like features scattered across the planet. These pyroclastic deposits are the remnants of volcanic eruptions that blasted material outward, much like a volcanic explosion on Earth, though the mechanism differs because Mercury has no atmosphere to shape the eruption column.
5Journal of Geophysical Research: Planets. Spectral Properties and Physical Extent of Pyroclastic Deposits on Mercury: Variability Within Selected Deposits and Implications for Explosive VolcanismLobate Scarps and a Shrinking Planet
One of the most distinctive features on Mercury’s surface is a global network of long, curving cliffs called lobate scarps. Some of these scarps stretch for hundreds of kilometers and rise more than a kilometer above the surrounding terrain. They are the surface expression of thrust faults, places where one block of crust has been shoved up and over another. The driving force behind them is straightforward: as Mercury’s large iron core slowly cooled over billions of years, the entire planet contracted, and the crust had to accommodate the shrinkage by crumpling and faulting.
6PubMed Central. Graphite lubricates Mercury’s global contractionThe total contraction may have reduced Mercury’s radius by several kilometers. That might sound modest for a planet almost 2,440 kilometers across, but it was enough to create a global system of fold-and-thrust belts visible from orbit. These scarps cut across craters and plains of all ages, meaning the contraction has been an ongoing process rather than something confined to one period. Some scarps are among the youngest geological features on Mercury’s surface, suggesting the planet may still be shrinking today.
A recent study proposed that a thin layer of graphite in Mercury’s crust may have acted as a lubricant along these faults, reducing friction and allowing the thrust sheets to slide more easily during contraction. If confirmed, that would connect Mercury’s unusual crustal composition directly to how its surface deformed over time.
6PubMed Central. Graphite lubricates Mercury’s global contractionHollows, Mercury’s Most Puzzling Features
Among the strangest things MESSENGER found are features called hollows: shallow, rimless, irregularly shaped depressions that appear in clusters, often on the floors, walls, and rims of impact craters. They range from tens of meters to a few kilometers across, and they look remarkably fresh. Many have bright, high-reflectance interiors and halos, and they lack the superimposed small craters you would expect if they were ancient.
7PubMed. Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activityTheir freshness matters because it suggests the process creating hollows is geologically recent and may even be ongoing. The leading explanation is that they form when volatile materials near the surface are lost, through some combination of sublimation and space weathering. Spectral analyses favor carbon as a candidate volatile compound, though pinning this down has proved difficult because the volatiles are lost primarily from the steep walls of the hollows, where direct spectral observations are hard to get. Based on the geometry of all visible hollows on Mercury, researchers estimate a minimum volume of lost volatiles of about 1,266 cubic kilometers.
8Journal of Geophysical Research: Planets. Lost Volatiles During the Formation of Hollows on MercuryHollows appear across much of the planet but are not randomly distributed. A comprehensive spatial analysis found they are strongly associated with impact craters and certain structural features, reinforcing the idea that impacts expose volatile-bearing material that was buried in the crust and allow it to escape.
9Earth and Space Science. Hollows on Mercury: A Comprehensive Analysis of Spatial Patterns and Their Relationship to Craters and Structures Nothing quite like hollows exists on the Moon or Mars, making them a feature unique to Mercury’s particular combination of composition, temperature extremes, and intense solar exposure.
Ice at the Poles
It sounds paradoxical: a planet where surface temperatures can exceed 430°C harboring water ice. But Mercury’s axis has virtually no tilt, so the floors of deep craters near the poles never see sunlight. In those permanently shadowed spots, temperatures plunge far below freezing. The first hint came in 1991, when full-disk radar mapping of Mercury at 3.5-centimeter wavelength revealed unusually bright radar returns from the polar regions, consistent with ice.
10PubMed. Mercury radar imaging: evidence for polar iceMESSENGER confirmed the connection. At both the north and south poles, radar-bright deposits align strongly with regions of permanent shadow, consistent with water ice being the dominant component.
11PubMed Central. Investigating Mercury’s South Polar Deposits: Arecibo Radar Observations and High-resolution Determination of Illumination Conditions Some of the ice appears to be exposed right at the surface, while in other craters it sits beneath a thin, dark insulating layer, possibly organic-rich material delivered by comets. Laser altimeter data identified surface ice not just in large craters but also in small-scale cold traps less than five kilometers across, within rough patches and intercrater terrain. The findings suggest that a substantial amount of Mercury’s water ice is distributed in microcold traps, not confined to a handful of large crater floors.
12Geophysical Research Letters. New evidence for surface water ice in small‐scale cold traps and in three large craters at the north polar region of Mercury from the Mercury Laser AltimeterWhere the ice came from remains an open question. Comet and asteroid impacts are the most commonly cited delivery mechanism, but some researchers have proposed that solar wind protons interacting with oxygen-bearing minerals in Mercury’s soil could slowly generate water molecules that migrate poleward. Whatever the source, the ice is geologically young by necessity; even in permanent shadow, it would gradually be lost over billions of years without being replenished.
What Mercury’s Surface Is Made Of
Mercury’s surface chemistry surprised nearly everyone. Before MESSENGER, models predicted a relatively iron-rich surface given the planet’s enormous iron core (which accounts for roughly 70% of Mercury’s mass). Instead, MESSENGER found that the surface silicates contain very little ferrous iron. What the surface does have in abundance is sulfur, at concentrations far higher than those found on the Moon or Mars. Calcium-sulfur and magnesium-sulfur correlations in the data point to sulfide minerals as an important surface component.
13Journal of Geophysical Research: Planets. The redox state, FeO content, and origin of sulfur‐rich magmas on MercuryThis unusual chemistry tells us that Mercury formed under highly reducing conditions, meaning oxygen was scarce relative to other elements during the planet’s early history. In that environment, sulfur bonded with calcium, magnesium, and other metals instead of being driven off as a gas. The result is a surface quite different from any other rocky planet: iron-poor, sulfur-rich, and spectrally dark.
The darkness of Mercury’s surface has its own explanation. Modeling of Mercury’s magma ocean evolution suggests the planet once had a primary crust made partly of graphite that floated to the surface during crystallization. Spectral reflectance models that reproduce Mercury’s observed surface spectra require roughly 1 to 3 weight percent graphite. Researchers have calculated that this primary graphite crust would have been between 40 and 120 meters thick before it was broken up and mixed into the regolith by billions of years of impacts.
14Nature Communications. Carbon distribution in planet Mercury from magma ocean evolution to graphite crust and core composition So when you look at Mercury through a telescope and see a dim, gray world, you are partly seeing the remnants of an ancient graphite shell.
How Space Weathering Reshapes the Surface
Mercury sits closer to the Sun than any other planet and has no protective magnetic field strong enough to fully deflect the solar wind. The result is relentless bombardment by high-energy protons, helium ions, and micrometeoroids, a process collectively called space weathering. Over time, this bombardment alters the chemistry and optical properties of surface minerals, darkening them and changing how they reflect light.
The high sulfur content of Mercury’s surface makes this process especially interesting. Laboratory experiments simulating solar wind irradiation of sulfide minerals at solar wind energies found that the bombardment modifies the chemical bonds and composition of the surface layer. Because Mercury’s surface is so sulfur-rich, these reactions should contribute sulfur-bearing species to the planet’s thin exosphere, the wispy envelope of atoms and molecules that Mercury maintains above its surface through sputtering, desorption, and micrometeoroid vaporization.
15The Planetary Science Journal. Surface Response of Mercury’s Sulfides under Solar Wind Ion IrradiationMercury’s exosphere is not an atmosphere in any practical sense. It is so thin that individual atoms are more likely to bounce off the surface and escape into space than to collide with each other. But it is intimately connected to the surface: sodium, calcium, magnesium, and other elements are continuously knocked off the ground by solar wind ions, ultraviolet photons, and tiny impacts, then briefly arc above the surface before falling back or being swept away. Modeling these processes requires understanding how tightly different atoms are bound to the surface minerals, which varies with composition and temperature.
16The Planetary Science Journal. The Influence of Surface Binding Energy on Sputtering in Models of the Sodium Exosphere of MercuryThe practical effect for anyone studying Mercury is that the surface you see today is not quite the surface that was originally deposited. Billions of years of space weathering have created a fine-grained regolith, a blanket of pulverized and chemically altered material, that coats the bedrock. The thickness and grain size of this regolith affect how Mercury reflects and emits infrared light, which matters for interpreting remote-sensing data.
What BepiColombo Will Add to the Picture
Much of what we know about Mercury’s surface comes from MESSENGER, which orbited the planet from 2011 to 2015. The European-Japanese mission BepiColombo, which launched in 2018 and is currently approaching Mercury, carries instruments designed to fill in the gaps. Among the most anticipated is MERTIS, a thermal infrared spectrometer and radiometer that will map Mercury’s surface mineralogy and temperature at a resolution of about 500 meters per pixel globally, with even finer resolution over selected areas.
17Space Science Reviews. Studying the Composition and Mineralogy of the Hermean Surface with the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the BepiColombo Mission: An UpdateMERTIS operates in a wavelength range, 7 to 14 micrometers, where rock-forming silicates and sulfides produce diagnostic spectral features. This will allow researchers to identify specific minerals rather than just broad compositional classes, something MESSENGER’s instruments could not fully accomplish. It will also provide spatially resolved data on Mercury’s hollows and pyroclastic deposits, potentially settling debates about what volatile compounds are involved in hollow formation and what the explosive volcanic vents actually erupted.
18Copernicus Publications. Emissivity Spectroscopy (7-14 μm) of Powdered Silicates under Simulated Mercury Daytime Surface Conditions: Supporting MERTIS Payload onboard ESA/JAXA Bepicolombo MissionBepiColombo also carries two orbiters rather than one. The planetary orbiter will focus on surface and interior science, while a magnetospheric orbiter will study Mercury’s magnetic field and its interaction with the solar wind. Together, they should clarify how the surface, exosphere, and magnetic environment function as a connected system. Given that Mercury’s surface is constantly being modified by space weathering and that its exosphere is fed directly by surface material, understanding these links is essential to reading the planet’s geological record accurately. First full science operations are expected to begin in 2026, and the mission could reshape much of what we think we know about the innermost planet’s surface.