Terrestrial planet
Mercury
The swift planet
Mercury is the smallest planet in our solar system and the closest to the Sun. It races around the Sun every 88 days, faster than any other planet, which is why it was named after the swift Roman messenger god. Despite being closest to the Sun, Mercury is not the hottest planet; its lack of a thick atmosphere means heat escapes quickly at night, producing the most extreme day-night temperature swing of any planet. Its heavily cratered surface looks much like our Moon.
Quick facts
- Distance from Sun
- 58 million km (0.39 AU)
- Diameter
- 4,879 km (0.38x Earth)
- Day length
- 59 Earth days (176 days sunrise to sunrise)
- Year length
- 88 Earth days
- Moons
- 0
- Avg. temperature
- -173 C at night to 427 C in daylight
What's it like there?
Standing on Mercury, you would see a gray, cratered landscape under a black sky, because there is almost no atmosphere to scatter sunlight. The Sun would appear more than three times larger than it does from Earth. Daytime temperatures soar hot enough to melt lead, while at night, without an insulating atmosphere, temperatures plunge nearly 600 degrees. Ice may hide in permanently shadowed craters at the poles.
Fun facts
A year on Mercury (88 days) is shorter than one Mercury day-night cycle (176 days).
Mercury is shrinking: as its iron core cools, the planet has contracted by about 7 km in radius.
Despite scorching days, water ice exists in shadowed polar craters.
Mercury has the largest iron core relative to its size of any planet.
Mercury's Caloris Basin, an impact crater, is about 1,550 km wide.
Exploration history
Mariner 10 1974-75
First spacecraft to fly by Mercury, mapping about 45% of its surface.
MESSENGER 2011-15
First to orbit Mercury; mapped the entire planet and found polar ice evidence.
BepiColombo 2025+
ESA/JAXA joint mission en route to study Mercury in detail.
Mercury vs Earth
| Property | Earth | Mercury |
|---|---|---|
| Diameter | 12,742 km | 4,879 km |
| Year length | 365 days | 88 days |
| Moons | 1 | 0 |
| Average temperature | 15 C | 167 C (mean) |
| Gravity | 1 g | 0.38 g |
Scientific Sources & Provenance
All data is verified against official space agency records (NASA, ESA, and IAU).
- NASA Science / Mission Datahttps://science.nasa.gov/mercury/
- NASA Science / Mission Datahttps://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html
- ESA Datahttps://www.esa.int/Science_Exploration/Space_Science/BepiColombo