What it was
Air leaks from Mars one particle at a time. The planet still has a sky, but its pressure is tiny beside Earth's. To find air that thin here, the chapter sends you above a stack of 4 Everests.
Old Martian rocks keep traces of a magnetic field that once wrapped around the planet. Such a field bends the solar wind, a stream of charged particles from the Sun. It cannot seal a sky shut. Earth has a field and loses air too.
When and where
Mars grew among the inner planets. In the Grand Tack model, young Jupiter swept through nearby rubble and left Mars less stuff to grow from. That could explain why Mars stayed small. It is one proposed history, with other paths still in the running.
How big
Mars is about half Earth's width and roughly a tenth its mass. A small world has more surface for the warmth it holds, so heat escapes more easily. Its weaker gravity also gives gas less to fight against when atoms reach the top of the sky.
How we know
Spacecraft measure the escaping gas and map magnetism left in old crust. Argon, a gas that hardly reacts with rock, helps show how much air has gone. Bits knocked off Mars also land on Earth. Scientists can pick up a meteorite here and test a piece of another planet.
Still a mystery
Scientists still argue about when Mars's field switched off; younger magnetic rocks make a simple timeline hard to draw. No Martian meteorite has proved life lived there. Some shapes once linked to microbes can form when water reacts with rock. The rock is Martian; life remains unconfirmed.