What are by far the largest abundant biological entities on Earth and outnumber all the others put together ?
Surface liquid , while abundant on Earth ( the largest body of surface liquid being the World Ocean ) is rare elsewhere , a notable exception being Titan which has the largest known hydrocarbon lake system ( Lakes of Titan ) while surface water , abundant on Earth and essential to all known forms of life is thought only to exist as Seasonal flows on warm Martian slopes and in the habitable zones of other planetary systems .