Why is Mars red?

The master science writers, Marcus Chown and Govert Shilling, give the lowdown on the red planet… in tweets

Mars, because of striking red colour, was named after Roman god of war. It orbits the Sun once every 1.88 years.

Mars, at 228m km from Sun, orbits outside Earth’s orbit. When Earth overtakes it (every 26 months), Mars is visible all night long.

Through a telescope, Mars appears similar to Earth: dark surface markings, polar caps, tilted rotational axis, day of 24.6 hours.

Main difference: Mars is much smaller (6794 km). Surface gravity only 38% of Earth’s. Can hang onto only very thin atmosphere, mostly CO2.

While Mercury and Venus have no natural satellites and Earth has one, Mars has two: Phobos (27 km) and Deimos (15 km), discovered in 1877.

In 1972, Mariner 9 mapped Mars from orbit, discovering huge canyons, giant volcanoes, dry river beds, outflow channels and dune fields.

Valles Marineris (4000 km long, 6 km deep) is largest canyon in Solar System. Olympus Mons (500 km across, 25 km high) is tallest mountain.