This is a real image of a perfectly spherical rock on the martian surface.

Back in 2004. The opportunity rover lands on Mars.

Soon after, it found something really weird & unexpected— millions of tiny metallic balls spread across the surface.

Scientists now call them blueberries. 🫐

What’s really interesting is that they’ve almost confirmed the presence of liquid water on Mars billions of years ago.

If Mars had water, it also had mud.

This mud was absurdly rich in iron. (Remember why Mars is red?)

There’s a limit to how much iron can be dissolved in water. It’s called saturation point. If it is crossed, water starts vomiting iron out of it.

But instead of floating here and there, this iron sticks to a grain of sand, a fragment, a piece of organic matter, or whatever it gets, to form a lump.

The lump gets larger as more iron sticks on it, taking a roughly spherical shape. A hard thing. It has no gaps or pores, but layers of iron, like an onion. 🧅

Then the mud dries, becomes a rock. Millions of years of erosion break it to sand. But the lumps of iron, being older and harder, survive.