Many years ago, Mars' landscape was very different to how it looks now; it wasn't the cold, arid planet it is today.

Images from the European Space Agency (ESA) taken from the Mars Express Orbiter show a system of dried-up riverbeds.

Though it's still a mystery where all of the water came from, it's clear Mars used to have a far warmer and more humid past.

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Billions of years ago, Mars wasn't the cold, dry wasteland we picture when we think of the Red Planet today; it was much warmer and far more humid.

It's still very much unclear whether lifeforms existed there — or whether they could even still be present today — yet there's increasing evidence that the planet once fulfilled the necessary conditions for life to exist there.

The dried out river valley network on Mars. ESA / DLR / FU Berlin

Images from the European Space Agency (ESA), taken by high-resolution cameras aboard the Mars Express Orbiter, show a system of dried-up riverbeds.

According to the ESA, the way the valley system branches out and forms little tree-like branch patterns — or tributaries — stemming from a central "trunk" is described as "dendritic".

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The term for this pattern of river system is derived from the Greek for tree "dendron".

The Mars river valley network in 3D. ESA / DLR / FU Berlin

The system lies in the southern highlands of Mars, east of the Huygens crater and north of Hellas crater, the largest impact crater on Mars.

This heavily cratered area is one of the oldest surfaces on Mars and is dated to an age of about four billion years.

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The images show that the water probably flowed from the higher layers on the right to lower layers on the left.

What remains of the river valleys has been heavily eroded, which is evident from the smooth and undefined valley edges, which extend from the East to the West.

Artist’s impression showing Schiaparelli separating from the Trace Gas Orbiter and heading for Mars. ESA/D. Ducros

According to the press release, the little branching tributaries were probably carved out by heavy rainfall coupled with surface water runoff from a once-strong river flow.

The current is thought to have cut through existing terrain, forging new "branches" and constructing a completely new landscape.

Read more: NASA may have accidentally burned evidence of life on Mars around 50 years ago

Though it still remains a mystery where all of this water originated from, it's clear that Mars previously had a far warmer and more humid past compared with the vast dry, red landscape that we associate with the planet now.

It's now clear that Mars previously had a far warmer and more humid past. ESA

The search for life on Mars continues

Exo Mars, the joint mission of ESA and the Russian Space Agency, will send a rover to Mars next year to drill and search for traces of life beneath the surface of Mars. This will make Exo Mars the first mission to do so.

Read more: NASA captured images of a 'blue' sand dune on Mars

The ExoMars Trace Gas Orbiter, which has been orbiting the planet since the end of 2016, is currently investigating the composition of the planet's atmosphere.

As well as revealing more details about Mars' past, its findings could potentially uncover indications of past — or present — life in the planet's atmosphere.

With various plans — both ongoing and planned — committed to investigating Mars' past, it looks like water will be key in uncovering the red planet's mysteries.