Mars’ water history is fundamental to understanding an evolution of Earth-like planets. Water escapes to space as atoms, and hydrogen (H) atoms escape faster than deuterium (D) — which is a hydrogen atom with a neutron in its nucleus — giving an increase in the residual D/H ratio. The present ratio reflects the total water Mars has lost. These are far-ultraviolet Hubble images of Mars near its farthest point from the Sun, called aphelion,...