MERCURY

A "year" is just the time it takes a planet to complete one full orbit of the Sun — and on Mercury, that happens fast. Mercury orbits the Sun in about 88 Earth days, while Earth takes 365. Mercury is closer to the Sun and moving much faster along a much shorter path, so it laps the Sun roughly 4.15 times in the time it takes Earth to go around once. The diagram below shows both orbits to scale, moving at their true relative speeds.

Watch it long enough and the pattern becomes obvious: Mercury doesn't just orbit faster, it orbits dramatically faster. By the time Earth has made its way a quarter of the way around the Sun, Mercury has already completed a full year of its own — sunrise to sunrise, season to season, start to finish — and is well into its next one.

Mercury is a planet of superlatives and surprises. It's the smallest planet, the closest to the Sun, and the fastest orbiting. It has the most eccentric orbit of any planet and the most extreme daily temperature swings in the solar system. It has almost no atmosphere, no moons, and no rings. By most definitions it should be one of the most straightforward worlds to understand — and yet Mercury has consistently defied our expectations at every turn.

Mercury has the largest day–night temperature swing of any planet in the Solar System — and understanding why tells you something fundamental about atmospheres. On Earth, our atmosphere acts like a blanket, absorbing heat during the day and releasing it slowly at night, keeping temperatures relatively stable. Mercury has almost no atmosphere to speak of — just a wispy exosphere of atoms kicked off the surface by solar wind and meteorite impacts. So when the Sun blazes down, temperatures soar to around 430°C. Then night falls, and with no atmospheric blanket to hold the warmth in, temperatures plummet to around -180°C. That's a swing of over 600 degrees between day and night on the same planet.

Mercury's orbit is also one of the strangest in the solar system. It's highly elliptical — more oval-shaped than any other planet — meaning the distance between Mercury and the Sun changes dramatically throughout the year. At its closest point (perihelion), Mercury is just 46 million kilometres from the Sun. At its furthest (aphelion), it stretches to 70 million kilometres. That means from the surface of Mercury, the Sun would appear to change size noticeably over the course of a single orbit.

At first glance, Mercury's surface looks a lot like our Moon — heavily cratered, ancient, and grey. Both are rocky worlds with surfaces shaped primarily by billions of years of meteorite impacts. But look more closely and Mercury reveals geology that's entirely its own.

The most striking features on Mercury are its lobate scarps — long, winding cliff faces that stretch for hundreds of kilometres across the surface, sometimes cutting right through craters. These formed as Mercury's interior slowly cooled and contracted over billions of years, causing the crust to buckle and fold like the skin of a shrinking apple. Scientists estimate Mercury has shrunk in diameter by as much as 7 kilometres since it formed — significant on a planet just 4,879 kilometres across.

Mercury's interior is one of the great unsolved puzzles in planetary science. The iron core makes up roughly 85% of the planet's radius and about 60–70% of its total mass — the largest core by proportion of any planet in the solar system. If you could strip away Mercury's rocky mantle and crust, you'd be left with a core roughly the same size as our Moon.

Why is Mercury's core so disproportionately large? The leading hypothesis involves a giant impact early in the solar system's history — a massive collision that stripped away much of Mercury's rocky outer layers, leaving the dense iron core dominant. Another possibility is that the intense heat of the early Sun evaporated Mercury's lighter rocky material. Neither explanation is fully settled, and BepiColombo's measurements may finally resolve the debate.

Here's something that seems impossible: water ice on the planet closest to the Sun. And yet, it's there. Mercury's axis is tilted by less than one degree — nearly perfectly upright. The floors of craters near the poles are in permanent shadow, never receiving direct sunlight, and temperatures in those frozen shadows can drop to around -200°C — cold enough to preserve water ice for billions of years.

Radar observations from Earth first detected highly reflective patches at Mercury's poles in the early 1990s. MESSENGER confirmed it: water ice and frozen volatile compounds are preserved in permanently shadowed polar craters, covered in places by a thin layer of dark organic material that acts as insulation. Some of these ice deposits may be hundreds of metres thick. The water was most likely delivered by comets and asteroids over billions of years — the same process thought to have brought water to early Earth.

Getting to Mercury is surprisingly difficult. To reach it from Earth, a spacecraft has to shed an enormous amount of orbital energy to slow down enough to enter orbit rather than fly past. This typically requires multiple gravity-assist flybys of other planets, adding years to the journey. As a result, Mercury remains one of the least visited planets in the solar system despite being our neighbour.

Mercury's position in the solar system makes it a critical reference point for understanding how rocky planets form and evolve. It sits in the innermost orbit, where the young Sun's heat was most intense, and it bears the scars of a violently active early solar system. Its oversized core, shrunken crust, and ancient craters are a record of events that shaped every rocky planet — including Earth.

In about five billion years, the Sun will expand into a red giant and may engulf Mercury entirely. Long before that, Mercury's highly elliptical orbit will continue to evolve under the gravitational influence of Jupiter and the other planets. Simulations suggest there's a small but non-zero chance that over billions of years, Mercury's orbit could become destabilised enough to cause a collision with Venus or even the Sun — a reminder that the architecture of our solar system isn't permanent.