2020 Volume 48 Issue 4 Pages 6-
This paper reviews results of Solar System explorations and telescope observations of geologically-active icy bodies (Europa, Enceladus, and Ceres) and Mars, where liquid waters are, or were, present. In particular, we focus on the availability of both reductants and oxidants as well as water chemistry on these planetary bodies. This paper also proposes an idea of “planetary redox”—a planetary-scale system that can generated a redox and pH gradient through geohydrological cycles. Based on the understandings of planetary redox on Europa, Enceladus, and Mars, we discuss about bioavailable energy on these bodies and science targets in future missions. Knowledge on the presence or non-presence of life in these habitable worlds would shed light on the conditions where life could emerge on early Earth.