One idea for a Mars missions is a library sent to the surface of that planet.[38] The Phoenix lander, which landed on the North polar surface of Mars in 2008, included a DVD library that was heralded as the first library on Mars.[38] The Phoenix library DVD would be taken by future explorers who could access the content on the disk.[38] The disc, both a memorial to the past and a message to the future, took 15 years to produce.[38] The content on the disc includes Visions of Mars.[38] One idea for exploration is knowledge arks for space, a sort of back-up of knowledge in case something happens to Earth.[39]