What makes Mars so hostile to life?

In our solar system, Mars has the next best climate for supporting life, after Earth. Like Earth it's a rocky planet, with polar ice caps and seasons.
Even the days on Mars are similar, lasting 24 hours and 40 minutes.
And, liquid water once flowed on the Martian landscape, according to the latest evidence. So if life flourishes on Earth, why does the Red Planet seem so lifeless?

Too cold for liquid water

 The north polar region of Mars, seen from orbit

Liquid water is essential for life as we know it, but it's a rare commodity in our solar system.
The so-called Goldilocks zone is a narrow region between the orbits of Venus and Mars where the temperature is 'just right' for water to remain liquid - not too hot and not too cold.
On a planet orbiting closer to the Sun water boils away; on a planet orbiting further away water turns to ice.
Our blue planet sits perfectly within this 'just right' zone. Mars sits on the outskirts of this zone, farthest away from the Sun. It gets very cold: minus 60 degrees Celsius on an average day.
These low temperatures, along with Mars' thin atmosphere, make it impossible for liquid water to exist on Mars' surface.

The missing atmosphere

There's evidence that Mars was once covered in oceans of water - at a time when it had an abundant atmosphere.

Blue skies or red

• Scientists believe that Earth and Mars once had similar atmospheres, but they developed very differently
• Mars' atmosphere is very thin, extremely cold and what water remains is frozen or hidden underground
• Most sunlight is able to pass through Earth's atmosphere and warm the planet's surface

An atmosphere is important because it traps some of the heat from the Sun - like the greenhouse gases do on Earth.

But over time, the atmosphere pressure on Mars has reduced to less than 1% of Earth's. This very thin atmosphere can't stop heat from the Sun escaping into space.
Temperatures have plummeted and the surface water has frozen into the polar ice caps we see today.
In fact, if you stood at the equator on Mars, the temperature would be 21C at your feet, where the Sun had warmed the ground, but 0C at the top of your head.
So what happened to the atmosphere on Mars?
Firstly, Mars is smaller than Earth, so it only has a third of the Earth's gravity, making it easier for gases to escape.
The second and most important factor is Mars' missing magnetic field.

The need for a magnetic shield

Magnetic fields of Earth (top) and Mars (bottom)

The Earth has a strong magnetic field, created by the churning of molten iron in the planet's hot core, which rises and sinks as it heats and cools.
This magnetic field shields us from most of the charged particles spewed out from the Sun, known as solar wind. The solar wind is trapped in a giant magnetic bubble and channelled around the Earth back out into space.
Without the magnetic field, life on Earth would not exist. Deadly radiation would bombard the planet's surface, and the solar wind would strip away our atmosphere.
Early Mars did have a magnetic field which may have protected an atmosphere and liquid water. But by looking at the magnetic field lines preserved in Martian rock, we see that Mars' magnetic field suddenly disappeared four billion years ago.

Life on Mars

• Scientists have detected methane clouds that change with the seasons, which may indicate microbial life below the Martian surface
• Mars is the most viable planet for human settlement, but would require extensive atmospheric modification

It's possible the disappearance of the magnetic field was caused by a series of asteroid collisions called the Late Heavy Bombardment.

A massive collision could have heated up Mars' core, permanently disrupting the magnetic field.
However this isn't the only theory - without enough churning in the Martian core, it could have died of its own accord.
Unprotected, the solar wind has slowly stripped away the Martian atmosphere, killing any life that might have existed on the planet's surface.