How can a rock from Mars reach Earth?
A large asteroid impact on Mars can blast surface rocks fast enough to escape the planet's gravity, about 5 kilometres per second. Those fragments then orbit the Sun, sometimes for millions of years, until a few happen to cross Earth's path.
It sounds impossible that a rock could travel from one planet to another, yet a few hundred Martian meteorites are known. The journey begins with an impact. When an asteroid strikes Mars at high speed, it creates a crater and throws out enormous amounts of material. Most falls back, but rocks near the surface, at the edge of the impact zone, can be flung out at speeds above Mars's escape velocity, about 5 kilometres per second. Mars helps by having lower gravity than Earth and a thin atmosphere that offers little resistance.
How do rocks survive such violence? Modelling and laboratory experiments suggest that a zone near the surface experiences less compression than rock deeper down, so some fragments are accelerated hard without being melted. Martian meteorites do show the scars: many contain shock features and glass, and the plagioclase feldspar in many has been turned into a glassy form called maskelynite by the intense pressure.
Once free of Mars, the fragments orbit the Sun independently. Gravitational tugs from the planets slowly change their orbits, and a tiny fraction eventually meet Earth. We can measure how long they travelled using their cosmic-ray exposure ages: while a small rock drifts in space, cosmic rays create particular isotopes in it, which build up like a clock. For Martian meteorites these ages range from under a million years to around 20 million years, and they cluster into groups, suggesting that only a handful of impacts launched the meteorites we hold.
Rocks travel the other way too, and Earth rocks have almost certainly reached Mars and the Moon. The same process explains lunar meteorites, which are even easier to launch because the Moon's escape velocity is only about 2.4 kilometres per second.