Mercury (planet)

• 1. Introduction

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  Mercury is the first planet from the Sun and the smallest in the Solar System, with an equatorial radius of 2439.7 km. It is so close to the Sun that it never strays far from the glare, appearing only as a morning star or an evening star in Earth's twilight. For many years astronomers were convinced it kept one face turned toward the Sun forever, locked in place like the Moon is to Earth. Then radar caught it spinning, and a much stranger truth emerged. Here is a world where a single solar day, sunrise to sunrise, lasts about 176 Earth days. Why does its core fill more than half its volume? Why did one observed wobble in its orbit help confirm Einstein's theory of gravity? And why is reaching this small, scorched neighbor harder than escaping the Solar System entirely?

• 2. A Day Longer Than A Year

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  Radar observations in 1965 overturned a long-held belief about Mercury's spin. Italian astronomer Giuseppe Colombo noticed the planet's rotation value was about two-thirds of its orbital period. He proposed a 3:2 spin-orbit resonance, meaning Mercury rotates three times for every two trips around the Sun. Data from Mariner 10 later confirmed this view.

  Mercury's sidereal year of 88.0 Earth days and its sidereal day of 58.65 Earth days sit in that 3:2 ratio. The strange consequence is that one solar day stretches to about 176 Earth days, twice the length of the planet's year. One side bakes in sunlight for a full 88 Earth days, then plunges into darkness for the next 88 before the Sun rises again.

  The eccentricity of Mercury's orbit makes this resonance stable. At perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky. The tidal force has a maximum at perihelion, which locks the planet so it points its axis of least inertia roughly at the Sun during closest approach. Accurate modeling shows Mercury was captured into this 3:2 state within 20 million years of its formation, more likely within 10 million.

• 3. When The Sun Walks Backward

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  Approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity. At that moment the Sun's apparent motion in the sky ceases. Closer to perihelion the orbital velocity exceeds the rotational velocity, so to an observer on Mercury the Sun appears to move in a retrograde direction, backward across the sky. Four Earth days after perihelion, the Sun's normal motion resumes.

  Two points on Mercury's equator, 180 degrees apart in longitude, experience this most dramatically. Around perihelion the Sun passes overhead, reverses and passes overhead again, then reverses once more and passes overhead a third time. The whole sequence takes about 16 Earth-days. For two or three weeks the Sun hangs almost stationary overhead, at its most brilliant because Mercury is closest to the Sun.

  This prolonged exposure makes those two points the hottest places on Mercury. Maximum temperature comes when the Sun sits about 25 degrees past noon, due to diurnal temperature lag. The longitude convention for Mercury places the zero of longitude at one of these hottest points. When Mariner 10 first arrived, that zero meridian was in darkness, so a small crater further west, called Hun Kal, was chosen to fix the reference at the 20 degree west meridian.

• 4. An Oversized Iron Heart

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  Mercury's density is the second highest in the Solar System at 5.427 grams per cubic centimeter, only slightly less than Earth's 5.515. Strip away gravitational compression and the truth is starker. Mercury's uncompressed density of 5.3 outranks Earth's 4.4, because Mercury is much smaller and its interior is not as squeezed. For it to be so dense, its core must be large and rich in iron.

  The radius of that core is estimated at 2020 kilometers, give or take 30, based on a moment of inertia factor of 0.346. The core occupies about 57 percent of the planet's volume. For Earth the figure is just 17 percent. Research published in 2007 suggests Mercury has a molten core. Above it sits a mantle-crust layer 420 kilometers thick, with the crust itself perhaps 35 kilometers deep by Mariner 10 and MESSENGER data.

  Mercury holds more iron in its core than any other planet, and rival theories compete to explain it. One proposes Mercury began with about 2.25 times its current mass, then a planetesimal several thousand kilometers across struck it and stripped away crust and mantle. A second imagines surface rock vaporized at temperatures possibly reaching 10,000 K near the young Sun. A third blames drag in the solar nebula sweeping away lighter particles. MESSENGER found higher-than-expected potassium and sulfur on the surface, elements the violent giant impact and vaporization would have driven off.

• 5. A Surface Of Scars And Wrinkles

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  Caloris Planitia, the largest known crater, spans 1550 kilometers, about one-third the planet's diameter. The impact that carved it was powerful enough to trigger lava eruptions and raise a concentric mountainous ring roughly 2 kilometers tall. At its antipode, exactly 180 degrees away, lies a region of chaotic hills called the Weird Terrain, possibly fractured by shock waves that traveled around the planet and converged.

  Mercury was heavily bombarded by comets and asteroids during and shortly after its formation, with a later episode, the Late Heavy Bombardment, ending 3.8 billion years ago. With no atmosphere to slow impactors, the whole surface took hits. Among the strangest features is Apollodorus, nicknamed the Spider, with radiating troughs spreading out from its center. In all, 46 impact basins have been identified, including the 400 kilometer Tolstoj Basin and the 625 kilometer Beethoven Basin.

  Numerous compression folds, called rupes, crisscross the plains as the planet's interior cooled and contracted. These scarps can reach 1000 kilometers in length and 3 kilometers in height. Most major thrust activity ended about 3.6 to 3.7 billion years ago. Yet small scarps tens of meters high appear less than 50 million years old, a sign that Mercury is still shrinking and still geologically alive at the surface.

• 6. Ice In A World Of Fire

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  Equatorial temperatures on Mercury swing from minus 170 C at night to 420 C in sunlight, driven by the planet's high orbital eccentricity and lack of atmosphere. The intensity of sunlight ranges between 4.59 and 10.61 times the solar constant of 1,370 watts per square meter. The subsolar point at perihelion can climb to about 700 K. And yet, observations strongly suggest frozen water exists here.

  Mercury's axial tilt is almost zero, measured as low as 0.027 degrees, so the floors of deep polar craters never see direct sunlight. Temperatures there stay below 102 K, creating a cold trap where ice can gather. Water ice strongly reflects radar, and in the early 1990s the 70-meter Goldstone Solar System Radar and the VLA found patches of high radar reflection near the poles. MESSENGER images of north pole craters later confirmed the ice.

  The icy crater regions hold an estimated 100 trillion to a quadrillion kilograms of ice, perhaps shielded by a layer of regolith that slows sublimation. Its origin remains unknown, with outgassing from the interior and comet impacts the leading candidates. Above all this drifts a tenuous exosphere, not a true atmosphere, with a surface pressure under about 0.5 nanopascals. Atoms of hydrogen, helium, sodium, calcium and more are continuously lost and replenished, some sodium emissions clustering near the magnetic poles.

• 7. A Magnetic Field That Leaks

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  Despite a slow 59-day rotation, Mercury carries a global magnetic field about 1.1 percent the strength of Earth's, as measured by Mariner 10. At the equator its strength is about 300 nanotesla. The field is dipolar and nearly aligned with the spin axis, tilted 10 degrees compared to Earth's 11. When Mariner 10 first detected it, planetary geologists were stunned, since the slow rotation seemed too sluggish to drive a dynamo.

  The field is likely generated by circulation in the iron-rich liquid core. Strong tidal heating from the high orbital eccentricity helps keep part of that core molten, sustaining the dynamo. The field deflects the solar wind into a magnetosphere small enough to fit within Earth, yet strong enough to trap solar wind plasma.

  During its second flyby on the 6th of October 2008, MESSENGER found the field could be extremely leaky. The spacecraft passed through magnetic tornadoes, twisted bundles up to 800 kilometers wide, a third of the planet's radius. These flux transfer events open windows in the magnetic shield, letting the solar wind strike the surface directly. MESSENGER measured a reconnection rate ten times higher than at Earth, only a third of which proximity to the Sun can explain.

• 8. The Hardest Planet To Reach

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  Mercury requires the highest delta-v of any planet for travel from Earth. A Mercury-bound spacecraft must fall over 91 million kilometers down the Sun's gravitational potential well. Mercury's orbital speed is 47.4 kilometers per second against Earth's 29.8, so a probe must shed enormous velocity to catch it and enter orbit. A trip to Mercury demands more rocket fuel than escaping the Solar System completely. Only three space probes have ever visited.

  Mariner 10 came first, in 1974 and 1975, the first craft to use a gravitational slingshot, borrowing Venus's gravity to reach Mercury. It came within 327 kilometers of the surface, but the same face was lit at each close approach, mapping less than 45 percent of the planet. On the 24th of March 1975, eight days after its final pass, it ran out of fuel and controllers shut it down.

  MESSENGER launched on the 3rd of August 2004, entered orbit on the 18th of March 2011, and crashed into the surface at 3:26:01 p.m. EDT on the 30th of April 2015, gouging a crater about 16 meters across. Long before any probe, in 1859, Urbain Le Verrier found Mercury's perihelion precession could not be explained by Newtonian mechanics, prompting a hunt for a hidden planet named Vulcan that was never found. Einstein's general relativity finally accounted for the excess, a tiny 42.980 arcseconds per century. The joint European and Japanese mission BepiColombo, launched on the 20th of October 2018, completed its sixth flyby on the 9th of January 2025, and will enter Mercury's orbit in 2026.