A guide to the planets: Mars

Mars is the most visited planet beyond Earth, but there is still a lot we don't know about the Red Planet.

Viking image of Mars

What is Mars like?

Mars is a cold and dead world, but it wasn’t always this way. We know water once flowed on Mars: orbital images reveal its valleys were formed by rivers, while surface experiments have found minerals that require liquid water to form.


Today, however, Mars’s rivers are no more.

It’s thought that the Red Planet’s thin atmosphere is to blame as the low pressure caused most of the oceans to boil away, while temperatures mean any remaining water is frozen.

What isn’t known is how long there was water on the surface.

It could be that ancient Mars was warm enough to hold permanent oceans, but it could also be the case that the water spent most of its time frozen, and only thawed when a volcano eruption or meteor impact heated the planet enough to create a flash flood.

Such eruptions were common in Mars’s early history.

With low gravity and air pressure, the volcanoes on Mars could grow to enormous size and the planet is home to the Solar System’s largest known volcano: Olympus Mons, a huge shield volcano 624km wide and 22km high.

However, the planet’s interior solidified around a billion years after its formation, freezing the planet in this early stage of formation.

Water on Mars
Though dry and arid today, Mars once had huge liquid oceans.
NASA’s Goddard Space Flight Center

Mars facts

  • Diameter: 6792km (0.53 times Earth)
  • Mass: 642 billion trillion kg (0.11 times Earth)
  • Distance from the Sun: 228 million km (2.45 AU)
  • Length of day: 24 hours, 37 minutes
  • Length of year: 687 days (1.9 years)
  • Number of moons: 2
  • Temperature: -143ºC to 35ºC
  • No of spacecraft visitors: 25+
  • Number of moons: 2
  • Type of planet: Rocky

How can I observe Mars?

Mars is best viewed around the time of its closest approach to Earth, its opposition, when it appears largest on the sky.

To the naked eye, Mars is bright and has a slightly reddish hue, while through a telescope you should be able to make out the features on the lunar surface.

It is possible to see the planet the rest of the year provided it is up in the night sky, but it will be much smaller and dimmer.

From where does Mars get its name?

Mars is named after the god of war, whose Greek name is Ares, as the planet’s red colour is reminiscent of blood.

Mars’s two diminutive moons are called Phobos and Deimos (meaning fear and dread) after the god’s twin sons that are sometimes depicted as the horses pulling Mars’s chariot.

Curiosity on Mars
Curiosity is one of four rovers to have visited Mars, though another three are due to make the trip in 2020.

How have we explored Mars?

Mars is probably the most thoroughly explored planet after our own, though it has a reputation for being cursed. Around half of all missions that attempt to travel to the planet have failed.

Despite this, the planet has seen several orbiter missions and a great many landing missions.

These have looked at all aspects of Mars’s geology, particularly focusing in on the history of water on the planet in an effort to predict whether life has ever existed on the planet.

To date, NASA has sent four rovers to the planet: Sojourner, Spirit, Opportunity and Curiosity.

However in 2020, another three rovers are due to travel to the planet: NASA’s Mars 2020 rover, ESA’s Rosalind Franklin and a Chinese rover. These should arrive in 2021.


Explored by: Mariner programme (1965–71, NASA); Viking (1976, NASA); Mars Global Surveyor (1997, NASA); Pathfinder and Sojourner (1997, NASA); Mars Odyssey (2001); Mars Exploration Rovers, Spirit and Opportunity (2004, NASA); Phoenix (2008); Curiosity (2012, NASA); ExoMars Trace Gas Orbiter (2016); InSight (2018)