Space probes are uncrewed spacecraft that carry scientific tools to study planets and other bodies in space, as they can complete missions too dangerous or expensive for humans. Key probes include Luna 9, the first to soft land on the moon; Venera 9, the first on Venus which found rocks similar to Earth's and intense heat; and the Viking missions, which searched for life on Mars and found it was once warmer. Galileo and Cassini were the first probes to the outer planets, with Galileo discovering Jupiter's moons may have liquid water and magnetic fields.