Mercury
Mercury is the nearest planet to the Sun and the smallest planet in the Solar System. Its rocky, cratered surface resembles the Moon at first glance. Mercury, however, has its own magnetic field, an exceptionally large metallic core and deposits of water ice inside polar craters. The planet has no moons or rings.[2]
Size and physical characteristics
Mercury has a mean diameter of about 4,880 km, roughly 38% of Earth's diameter. It is only moderately larger than the Moon, whose diameter is 3,475 km. Mercury is much denser: its mean density is about 5.43 g/cm³, close to that of Earth, whereas the Moon's density is approximately 3.34 g/cm³.[3]
The planet has a mass of approximately 3.30 × 1023 kg, about one eighteenth of Earth's mass. Surface gravity is 3.7 m/s², nearly the same as on Mars. An object that presses on a support with a force of 100 newtons on Earth would exert a force of about 38 newtons on Mercury.[3]
Orbit and rotation
Mercury's distance from the Sun varies from about 46 million km to 69.8 million km. Its orbit is distinctly elongated, and one year lasts only 88 Earth days. The mean orbital speed is 47.4 km/s, or about 170,000 km/h—the highest average speed of any of the eight planets.[1][3]
Mercury's rotation is locked to its orbital motion in a 3:2 resonance: during two revolutions around the Sun, the planet rotates three times relative to the distant stars. This state is known as a spin–orbit resonance.[4]
One rotation relative to the stars takes approximately 58.65 Earth days. From one sunrise to the next, however, about 176 Earth days pass—two Mercurian years. A prolonged day is therefore followed by an equally long night.[1]
Surface temperature
Temperatures on the illuminated surface can reach 430 °C, while nighttime temperatures fall to about −180 °C. The difference exceeds 600 degrees. Mercury has no dense atmosphere capable of efficiently transporting heat between its day and night sides.[2]
Despite its proximity to the Sun, Mercury is not the hottest planet. On Venus, a dense atmosphere and a powerful greenhouse effect maintain an even higher surface temperature of about 464 °C.[5]
Surface and geological activity
One of Mercury's largest impact structures is the Caloris basin, approximately 1,500 km across. It formed when a large celestial body struck the planet. Plains occupy parts of the basin, while rings of highlands and impact ejecta surround it.[6]
Some areas are covered by solidified lava. Extensive smooth plains in the northern hemisphere occupy more than 6% of the planet's surface. In the distant past, volcanic eruptions flooded lowlands and buried older terrain.[7]
MESSENGER images revealed numerous scarps and thrust faults. As Mercury's interior cools, it contracts, causing the crust to shorten and shift along faults. Some small scarps are geologically young; their state of preservation indicates that tectonic movement continued into the comparatively recent past.[8]
Another characteristic landform is the hollow: a shallow, irregular depression with bright margins, often occurring in groups. Hollows are linked to the loss of volatile components from surface rocks.[9]
Internal structure
Mercury's metallic core occupies more than 80% of the planet's radius. The rocky shell above it is comparatively thin. With a diameter of almost 4,900 km, Mercury contains a disproportionately large amount of metal compared with the other terrestrial planets.[10]
Measurements of the planet's rotation and gravity field indicate a liquid outer core surrounding a solid inner core. Motion of electrically conductive material within the liquid region sustains Mercury's intrinsic magnetic field.[11]
Exosphere and magnetic field
Mercury is surrounded by an extremely tenuous exosphere. Its particles are released from surface rocks by micrometeoroid impacts, solar radiation and particles from the solar wind. Some material returns to the surface, while some escapes into space.[12]
Mercury's intrinsic magnetic field is much weaker than Earth's, but it forms a magnetosphere that interacts with the solar wind. During a June 2023 flyby, BepiColombo measured its boundaries and regions of charged particles, including a plasma sheet on the nightside.[12]
Water ice at the poles
In 2012, MESSENGER data confirmed the presence of water ice in permanently shadowed regions near Mercury's poles. The conclusion combines neutron measurements, laser reflectance observations and calculations of surface temperatures.[13]
Mercury's rotation axis is almost perpendicular to its orbital plane. Sunlight therefore never reaches the floors of some deep polar craters. Ice can survive in these cold traps even though illuminated parts of the planet heat to hundreds of degrees Celsius.[13]
Spacecraft exploration
The first spacecraft to reach Mercury was Mariner 10. It completed three flybys in 1974–1975, returned photographs of almost half the surface and detected the planet's magnetic field.[14]
In 2011, MESSENGER became the first spacecraft to enter orbit around Mercury. During four years of orbital operations it produced global maps and investigated the surface composition, interior and magnetic environment. The mission ended on 30 April 2015, when the spacecraft struck the surface after exhausting its propellant.[15]
The next stage of exploration is BepiColombo, a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency. It consists of two orbiters: the European Mercury Planetary Orbiter and the Japanese Mio. As of November 2026, they are travelling toward Mercury, with orbital insertion scheduled for 21 November 2026.[16]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 Mercury Fact Sheet // NASA, National Space Science Data Center.
- ↑ 2.0 2.1 2.2 2.3 Mercury Facts // NASA Science.
- ↑ 3.0 3.1 3.2 Planetary Fact Sheet — Metric // NASA, National Space Science Data Center.
- ↑ Correia A. C. M., Laskar J. Mercury's capture into the 3/2 spin-orbit resonance including the effect of core-mantle friction // Icarus. 2009.
- ↑ Venus Facts // NASA Science.
- ↑ Mercury's Caloris Basin // NASA Science.
- ↑ The Origin, Evolution and Volatile Inventories of Terrestrial Planets // NASA Astrobiology Institute. 2012.
- ↑ The Incredible Shrinking Mercury Is Active After All // NASA. 2016.
- ↑ Mercury's Strange Hollows // NASA Science.
- ↑ MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities // NASA. 2012.
- ↑ A Closer Look at Mercury's Spin and Gravity Reveals the Planet's Inner Solid Core // NASA. 2019.
- ↑ 12.0 12.1 Mercury's magnetic landscape mapped in 30 minutes // European Space Agency. 2024.
- ↑ 13.0 13.1 NASA Spacecraft Finds New Mercury Water Ice Evidence // NASA Jet Propulsion Laboratory. 2012.
- ↑ Mariner 10 // NASA Science.
- ↑ MESSENGER // NASA Science.
- ↑ Latest updates: BepiColombo's arrival at Mercury // European Space Agency. Updated 6 October 2026.