No other planet has been studied as closely as Mars - but what do we know about the Red Planet? And could humans ever live there?
Planet Mars facts
Position in Solar System: fourth planet from the Sun
Size: 6794 km in diameter
Distance from Sun: 228 million km (142 million miles) on average
What does Mars look like - and can you see it from the Earth?
From the Earth, Mars is easily seen in the night sky as a red star-like object that moves through the sky with a period of just over two years.
Seen through a telescope Mars appears as a small reddish disk on whose surface dark markings can (with difficulty) be seen. One or its polar caps may also be visible.
However, even with large telescopes it is very difficult to see detail, and many experienced observers were deceived into thinking that they had glimpsed features, such as the infamous canals, that in fact were not there.
Almost all that we know about the surface of Mars and its atmosphere has been discovered by the various space probes that have orbited the planet and landed on its surface.
Why is Mars red?
Mars is frequently referred to as the Red Planet. Its uique colour is due to iron minerals in the Martian surface. The iron oxidises or rusts, causing the planet to appear red.
What is the atmosphere of Mars?
We know that the atmosphere of Mars has a pressure less than one hundredth of the Earth's, and that it is composed mainly of carbon dioxide with a little nitrogen and argon. There is almost no oxygen.
Is there water on Mars?
There is a small amount of water vapour which condenses in some places to produce thin clouds and fog. There are polar caps at each pole which grow in the Martian winters with the addition of deposits of solid carbon dioxide, and decrease in the Martian summers to leave what is thought to be a small residue of ordinary water ice.
The surface of Mars shows impact craters, like the Moon, mountains, rift valleys, ridges, hills plains and extinct volcanoes. There are signs that water existed on the surface at some earlier stage of the planet, while NASA's Mars Reconnaisance Orbiter has uncovered what could be large amounts of water ice beneath the surface of Mars.
What is the temperature on Mars?
The surface temperatures on Mars can rise to about 0°C in the summer but fall to near -100°C before sunrise. Winds can be very severe and are responsible for extensive weathering of the rocks on the surface. Sometimes the winds blow up enormous sandstorms that obscure the view of the planet's surface.
How long is a day on Mars?
The planet Mars rotates in 24 hours and 37 minutes about an axis tilted by 24° to its orbital plane. That makes a day on Mars only slightly longer than a day on Earth. However, a year on Mars is much longer than a year on Earth: a Martian year lasts 687 Earth days.
How many moons does Mars have?
The planet Mars has two moons, Phobos and Deimos. They are both very small, being less than 30 km across. It is likely that they were both asteroids that were captured by Mars.
Phobos is very close to Mars and its orbital period is less than the rotational period of Mars. It thus would be seen to rise in the west and set in the east. Deimos is further away from Mars and would be seen to behave more conventionally.
How long would it take to get to Mars?
On average Mars is 225 million km (140 million miles) from Earth, although that distance is constantly changing as both the Earth and Mars orbit the Sun.
The moment of 'Close Approach' is when Mars and Earth come nearest to each other during their orbits. The last time this happened in 2018 Mars was just 57.6 million km (35.8 million miles) away from Earth. The next Mars Close Approach will be on 6 October 2020.
The time it takes to get to Mars from Earth depends upon where each planet is in the Solar System and what the spacecraft aims to do when it gets there. The Mars Reconnaisance Orbiter in 2005 for example took 210 days to reach Mars, while Mariner 9, the first spacecraft to orbit Mars, took 168 days in 1971.
Nasa's Mars 2020 mission, a robotic rover set to launch in 2020, is due to launch between 17 July and 5 August and land on Mars on 18 February 2021, suggesting a flight time of between 197 and 216 days.
Sunlight meanwhile takes 13 minutes to travel from the Sun to Mars.
Is there life on Mars?
NASA has stated that it does not expect to find current living organisms on Mars. However, the various missions to the Martian surface have attempted to deduce whether the planet ever could have supported life - and whether it could in the future.
Can't stop. Won't stop.
I've been exploring #Mars for seven years, traveled 13 miles (21 km), climbed 1,207 feet (368 m), found conditions on ancient Mars were favorable for life as we know it, and I'm not done yet.
Here's what's new (plus a 360 view): https://t.co/KW5uyV2Uum pic.twitter.com/ljnGp75cu6
— Curiosity Rover (@MarsCuriosity) 5 August 2019
The Mars Curiosity Rover has been exploring the planet since 6 August 2012, while one of the stated goals of the Mars 2020 mission is to research how the planet could sustain future human missions to Mars. This will include testing a method for producing oxygen from the Martian atmosphere.