In 1950, over lunch at Los Alamos, physicist Enrico Fermi wondered aloud: with billions of stars far older than the Sun, some civilisation should have had plenty of time to spread across the galaxy. So why no sign of anyone? The Fermi paradox remains unsolved. Rare life, short-lived civilisations, vast distances: theories abound, evidence doesn't.
A large share of Earth's water molecules formed in the interstellar cloud that existed before the Sun was even born, more than 4.6 billion years ago. You are literally drinking a cosmic fossil.
By day, the Martian sky looks butterscotch because of fine dust hanging in the air. But at dusk, those same grains scatter blue light forward, creating a bluish glow around the Sun. Rovers have photographed these blue sunsets: the exact reverse of what we see on Earth.