Alex McColgan

So, Percy took a closer look, and it saw something amazing.

The rock, also known as Chiava Falls, was rich in organic compounds like carbon, phosphorus and iron arranged into rings.

Affectionately named leopard spots and poppy seeds, the tiny spots span 200 micrometres to 1 millimetre in diameter, but it was enough to raise the blood pressure of astrobiologists everywhere.

The light inner part of the leopard spot is chemically similar to the surrounding rock, but the dark outer rim is enriched with iron and phosphorus.

It seems to be evidence of localised iron reduction.

Percy also detected organic, carbon-based compounds in the rock, and based on its texture and geochemical composition, we strongly suspect this rock was once in contact with water.

Usually when we see such a combination of organics, water and iron reduction on Earth is interpreted as a sign of microbial life.

Suddenly, NASA had something very unique on its hands.

Could this mudstone rock hold the first alien biosignature ever found?

At the centre of this story are two very special minerals, vivianite and greigite.

Vivianite is an iron phosphate.

On Earth it forms near metal ores and river sediments, where microbes like geobacter metabolise iron instead of oxygen.

They take in iron oxide and release iron as a waste product.

The energy given off by this reaction then powers their metabolism, a process known as chemosynthesis.

When the expelled iron-2 reacts with the phosphate and water in the environment, it forms vivianite.

Greigite follows a similar story.

Sulfate-reducing microorganisms on Earth break down sulfate into sulfide, which reacts with iron to make greigite.

But let's be sceptics for a moment, and rule out microbes for now.

What else could have caused these reduction reactions?

Well, one explanation could be very high temperatures.