Planet Profile: Mercury
1. Basic Facts
Mercury is the closest planet to the Sun and the smallest planet in the Solar System. It has a diameter of 4,880 km (3,032 miles), which is only slightly larger than Earth’s Moon. Because of its proximity to the Sun, Mercury experiences extreme solar radiation and has a rapid orbital speed. Despite being the closest planet to our star, it is not the hottest—Venus holds that title due to its thick atmosphere. Mercury has no moons or rings, and its surface is covered in craters from constant asteroid impacts. Scientists believe Mercury’s composition is unique, with a very large metallic core making up the majority of its mass. Although ancient civilizations observed Mercury for centuries, its secrets remained largely unknown until the space age, when spacecraft like Mariner 10 and MESSENGER provided detailed images and data about this mysterious world.
2. Orbital & Rotational Characteristics
- Fastest Orbit – Mercury zips around the Sun in just 88 Earth days, completing a full revolution in much less time than any other planet.
- Slow Rotation – A single day (one full rotation on its axis) takes 59 Earth days. Because of this, a single solar day (one sunrise to the next) lasts 176 Earth days.
- Strange Sunrises – Due to Mercury’s unique spin-orbit resonance, the Sun appears to rise, set, and rise again in some parts of its sky before continuing its usual motion.
3. Extreme Temperatures
- Huge Temperature Swings – Mercury has the most extreme temperature variations in the Solar System:
- Daytime: Up to 800°F (430°C) – hot enough to melt lead.
- Nighttime: Drops to -290°F (-180°C) – colder than Pluto in some cases.
- These extremes occur because Mercury has no significant atmosphere to trap heat, causing the planet to lose heat rapidly at night.
4. Surface & Composition
- Covered in Craters – Mercury’s surface resembles the Moon, covered with impact craters from space debris. Some craters remain preserved due to the planet’s lack of weathering processes.
- Largest Crater: Caloris Basin – a massive impact site spanning 1,550 km (960 miles), formed by a giant asteroid.
- Mostly Iron Core – Mercury’s core is disproportionately large, taking up 85% of its volume, making it one of the densest planets in the Solar System.
5. Atmosphere & Magnetic Field
- Thin Exosphere – Mercury doesn’t have a true atmosphere but has an extremely thin exosphere composed of oxygen, sodium, hydrogen, helium, and potassium. This thin layer is constantly stripped away by solar wind.
- Weak but Active Magnetic Field – Unlike Venus and Mars, Mercury has a global magnetic field, though it is only 1% as strong as Earth’s. Scientists believe its weak field is generated by a partially molten outer core.
6. Exploration & Discoveries
Since Mercury is difficult to observe from Earth due to its proximity to the Sun, spacecraft missions have played a crucial role in our understanding of the planet. The first mission to visit Mercury was Mariner 10 (1974-1975), which mapped about 45% of the surface. Later, MESSENGER (2004-2015) became the first spacecraft to orbit Mercury, discovering evidence of water ice in permanently shadowed craters at the planet’s poles. These discoveries were surprising, as Mercury’s extreme daytime heat would suggest it to be a dry, barren world. Currently, the BepiColombo mission, a joint effort between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), is on its way to Mercury and is expected to arrive in 2025. Scientists hope that this mission will provide new insights into Mercury’s geology, magnetic field, and history, deepening our knowledge of the innermost planet of our Solar System.
