The Blue Marble a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It show Africa, Antarctica and the Arabian Peninsula. Image: NASA, caption: Wikipedia.
The origin of life on planet Earth remains one of the most fundamental topics in science. An important aspect of this problem relates to how the Earth acquired its water and the other nutrients that are required by living organisms. It is widely believed by scientists that the inner solar system was a dry and barren environment shortly after the formation of the terrestrial planets. So where did Earth get all its water? One possibility is that comets from the outer solar system delivered water and other essential molecules early in Earth history. This idea is attractive, but requires further work to test its viability.