The Moon is similar, but has a much smaller iron core. Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. Terrestrial planets can have canyons, craters, mountains, volcanoes, and other surface structures, depending on the presence of water and tectonic activity. Terrestrial planets have "secondary atmospheres", got from volcanism, meteorites, and photosynthesis. In the giant planets, their atmospheres are "primary", captured directly from the original solar nebula.