Why do some scientist think mars once had an extensive ocean?

Answer 1

because it has ice caps at the top and bottom

Answer 2

Some scientists believe that Mars once had an extensive ocean because of geological features on the planet's surface, such as ancient shorelines and dried-up river valleys, that suggest the presence of water in the past. Additionally, the detection of minerals like hematite and clay, which typically form in the presence of water, supports the idea of a wetter Martian climate in the past.