Top 10 Mysteries Scientists Are Still Trying to Solve About Mercury

#1: Why is Mercury’s core so massive? (Core radius ~1,100 miles; 85% of planetary radius)

Mercury’s core is astonishingly large for its size, making up roughly 85% of the planet’s radius—nearly 1,100 miles wide. That’s proportionally larger than any other planet in the solar system. Earth’s core, for example, is only about 55% of its radius. This anomaly has baffled scientists since the Mariner 10 mission first hinted at it in the 1970s. One theory suggests that Mercury once had a full rocky mantle like Earth, but a massive impact early in its history may have stripped away much of its outer layers. Another theory posits that Mercury formed in a high-temperature zone of the early solar system, where lighter materials vaporized or never accreted in the first place. The problem is, both scenarios raise new questions. If Mercury was hit by a planet-sized object, where is the evidence of the impact? And if it formed in an ultra-hot region, why didn’t Venus or Earth lose their mantles too? MESSENGER data confirmed the core is not only large, but still partially molten, which deepens the mystery. Understanding the core’s origin could reshape our models of planetary formation across the solar system.

#2: How can such a small planet still have a magnetic field? (Mercury’s magnetic field ~1% as strong as Earth’s)

Mercury is the only other rocky planet besides Earth with a global magnetic field, but it’s surprisingly weak—just about 1% the strength of Earth’s. Still, the fact that it exists at all is perplexing. Traditionally, planetary magnetic fields are thought to be generated by the motion of molten metal in the outer core, driven by convection currents. However, Mercury spins incredibly slowly—one rotation every 58 Earth days—and has a much smaller volume to generate that kind of dynamo effect. MESSENGER found that Mercury’s magnetic field is oddly offset to the north and asymmetrical, which further complicates things. One hypothesis is that iron sulfide in the core may be acting differently under extreme pressure, allowing convection to persist. Another idea is that thermal cooling near the poles helps maintain fluid motion in localized regions. Mercury challenges our textbook understanding of how magnetic fields work, and solving this puzzle could teach us more about Earth’s own magnetic future.

#3: Why is Mercury shrinking? (Planet has contracted by up to 7 miles in radius)

Unlike most other planets, Mercury is actually getting smaller. Geological evidence shows the planet has contracted by as much as 7 miles in radius over the past 3–4 billion years. This discovery came from MESSENGER images that revealed giant thrust faults and cliff-like features called “lobate scarps,” some hundreds of miles long and over a mile high. These features are signs that Mercury’s interior has cooled and solidified over time, causing the crust to buckle and compress. What’s strange is that this shrinkage seems to have continued longer than models predicted, suggesting Mercury’s cooling process is not fully understood. Is it cooling more slowly than expected? Does its large metallic core play a role? Why are similar features not more pronounced on other rocky worlds? These tectonic wrinkles offer physical clues, but they also suggest that Mercury may still be geologically active in subtle ways—something thought impossible not long ago.

#4: Where did Mercury’s volatiles come from? (Surface contains sulfur, potassium, sodium)

Volatile elements like sulfur, potassium, and sodium normally don’t survive close to the Sun, especially on a planet as scorched as Mercury. Yet MESSENGER detected surprising amounts of these substances in the surface regolith. This was unexpected because such elements should have evaporated during Mercury’s formation or been blasted away by solar radiation. Their presence suggests one of two possibilities: either Mercury formed much farther out in the solar system and migrated inward, or it accreted from unusual material rich in volatiles. Both scenarios are scientifically provocative. If Mercury migrated, it might mean the early solar system was much more chaotic than we imagined. If it formed in place, then our assumptions about solar heating and material evaporation need rethinking. The mystery of Mercury’s volatiles forces planetary scientists to revisit basic models of planetary chemistry and origin.

#5: How did Mercury get its dark, carbon-rich surface? (Low albedo ~0.1; similar to charcoal)

Mercury has an unusually dark surface, reflecting only about 10% of the sunlight it receives—similar to the reflectivity of charcoal. Unlike the Moon, which is also cratered and dry but much brighter, Mercury’s darkness doesn’t come from volcanic rock. Instead, scientists believe that a thin layer of carbon may be coating the surface. But where did this carbon come from? One theory suggests it arrived via comet impacts over billions of years. Another hypothesis is that Mercury’s original crust contained graphite, which floated to the surface of a once-molten shell. MESSENGER provided some support for the graphite theory by detecting patches of low-reflectance material that seem ancient. Yet, definitive proof remains elusive. The “charcoal mystery,” as it’s sometimes called, is not just a cosmetic detail—it’s a key to understanding Mercury’s early evolution and surface chemistry.

#6: Why are there water ice deposits on Mercury? (Water ice confirmed in craters at poles)

In a twist worthy of science fiction, scientists have confirmed the existence of water ice on Mercury—yes, the planet closest to the Sun. Using radar and neutron spectrometer data, MESSENGER discovered bright, reflective material inside permanently shadowed craters near Mercury’s north and south poles. Temperatures in these regions never rise above -280°F, cold enough for water ice to remain stable. But how did it get there? One leading theory is that comets or water-rich asteroids deposited the ice over time. Another possibility is that hydrogen from the solar wind combined with oxygen in surface minerals to form water molecules, which then migrated to the cold traps. The presence of organics mixed with the ice deepens the intrigue, raising distant echoes of prebiotic chemistry. Although Mercury is inhospitable today, its ice deposits offer a time capsule of cosmic delivery systems that could shed light on how water—and possibly the ingredients for life—spread through the solar system.

#7: What causes Mercury’s “hollows”? (Hollows range from hundreds of feet to miles wide)

Mercury’s surface features mysterious, irregular depressions called “hollows.” These shallow pits, often found in crater floors or central peaks, are unique in the solar system. They’re bright, rimless, and appear geologically young—suggesting they may still be forming today. Scientists think hollows may be caused by the sublimation of volatile materials, but the exact mechanism is unknown. Are they a result of space weathering? Solar radiation? Subsurface gas pockets? The strange thing is that hollows tend to cluster and lack surrounding ejecta, which would typically form during an impact. MESSENGER’s high-resolution images of these features sparked intense debate, with some proposing they’re signs of Mercury’s still-active surface evolution. The hollows raise tantalizing questions not just about surface chemistry, but about ongoing geological processes on a world once thought dead.

#8: Why is Mercury’s orbit so eccentric and inclined? (Eccentricity ~0.21; inclination ~7°)

Mercury’s orbit is the most eccentric and inclined of all the planets in the solar system. Its path around the Sun is shaped more like an oval than a circle, and it tilts about 7 degrees relative to Earth’s orbital plane. These irregularities make Mercury’s motion complex, even chaotic over long timescales. Einstein’s theory of general relativity was famously confirmed by explaining Mercury’s orbital precession—something Newtonian mechanics couldn’t fully account for. But why does Mercury have such a wild orbit in the first place? Some researchers believe early interactions with Jupiter or Venus may have perturbed its trajectory. Others suggest that Mercury may be the last survivor of a group of inner planets, its orbit shaped by ancient collisions and gravitational nudges. Whatever the cause, Mercury’s orbital dance continues to challenge astrophysicists and inspire refinements in celestial mechanics.

#9: Is Mercury still geologically active? (Potential tectonic movement in last 50 million years)

For a long time, Mercury was thought to be geologically dead—an ancient rock frozen in time. But new findings suggest otherwise. Some lobate scarps and tectonic features on Mercury appear so fresh that they may be less than 50 million years old, and perhaps still forming today. This implies Mercury’s crust is still responding to internal cooling. Earth has active geology driven by plate tectonics and heat circulation, but Mercury doesn’t seem to have similar processes. Instead, its activity may be driven solely by contraction. If Mercury is indeed still active, it would be the smallest planet with a living geology, challenging assumptions about planetary cooling rates and the requirements for geophysical dynamism. Even without volcanoes or earthquakes as we know them, Mercury may still be shifting beneath its cracked and battered surface.

#10: Could Mercury have once supported life? (Evidence of organics and water ice)

At first glance, Mercury is one of the last places you’d expect to find any connection to life. But with the discovery of water ice, organic molecules, and chemically reactive minerals, the question can no longer be dismissed out of hand. If life is defined as a system of chemistry powered by energy, then could there have been a window—however brief—where conditions on or beneath Mercury’s surface allowed for prebiotic chemistry? It’s a long shot, and no evidence of microbial life has ever been found. But the possibility continues to drive curiosity. Some researchers speculate that impacts may have momentarily created warm, wet conditions underground, or that buried ice could harbor chemistry shielded from radiation. While no one expects Mercury to be a cradle of life like Earth, its unexpected ingredients hint at the solar system’s broader chemical richness—and remind us that life’s potential may not be limited to familiar environments.

From Mystery to Mission

Even after centuries of observation and multiple space missions, Mercury remains full of surprises. It defies simple explanation, blending contradictions—hot and icy, small yet magnetic, ancient yet active. These mysteries don’t just make Mercury interesting; they make it essential to understanding the formation and evolution of rocky worlds. As future missions like ESA and JAXA’s BepiColombo prepare to study the planet in greater detail, we may finally start unlocking Mercury’s secrets. But if history is any guide, every discovery will likely lead to more questions. Mercury, the innermost planet, may also be one of the deepest puzzles in our cosmic neighborhood.