Is Martian water the secret to the Red Planet’s colour?

Mars's distinctive red rust may have been formed by water in the planet's ancient past, scientists believe.

Geologists say the dusty red surface that inspired astronomers to name the planet after a Roman god of war was covered in water billions of years ago. They believe they have found a substance with a "watery signature" that explains Mars's dramatic colour.

Scientists "were trying to create a replica Martian dust in the laboratory using different types of iron oxide," explained Adomas Valantinas, a planetary geologist at Brown University in the US. The one they found to be the "best fit" is known as ferrihydrite, a chemical compound containing iron and water.

The substance is found in volcanic areas on Earth but on Mars it "could only have formed when water was still present on the surface", Mr Valantinas said, suggesting Mars rusted earlier than had previously been thought. "Mars is still the Red Planet," he said. "It’s just that our understanding of why Mars is red has been transformed."

Scientists have long known that rusting iron is the key to Mars's colour, with winds believed to have spread the dust across the planet's surface after it was broken down over billions of years. However, the absence of water today led to suggestions that a dry compound, such as hematite, must have been responsible.

Question of life

But scientists have pointed the finger at ferrihydrite after grinding it down to a 100th of the size of human hair to replicate fine Martian dust, and using observations from spacecraft and "novel laboratory techniques", according to the European Space Agency.

It said the exact chemistry of Martian rust has been "intensely debated", because it provides a window into the planet's history and is "closely linked" to the question of whether Mars has ever been habitable.

"We eagerly await the results from upcoming missions like ESA’s Rosalind Franklin rover and the NASA-ESA Mars Sample Return, which will allow us to probe deeper into what makes Mars red," said Colin Wilson, a project scientist at the agency. He said samples of Martian dust had already been collecting and were awaiting return to Earth.

"Once we get these precious samples into the lab, we’ll be able to measure exactly how much ferrihydrite the dust contains, and what this means for our understanding of the history of water – and the possibility for life – on Mars," he said.