The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet,^[f] with a diameter about one-quarter that of Earth (comparable to the width of Australia).^[16] The Moon is a planetary-mass object with a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term and larger than all known dwarf planets of the Solar System.^[17] It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's at 0.1654 g, with Jupiter's moon Io being the only satellite in the Solar System known to have a higher surface gravity and density.