According to the leading hypothesis, about 4.5 billion years ago a Mars-sized body called Theia slammed into the young Earth. Debris thrown into orbit then clumped together to form the Moon. A key clue: lunar rocks brought back by Apollo are strikingly similar to rocks from Earth's mantle.
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.