Billions of years ago, liquid water carved Mars' landscapes under a thick atmosphere. Where did it all go? According to a new study by researchers at NASA's Jet Propulsion Laboratory (JPL) and Caltech, a significant portion likely remains hidden deep underground.
Around four billion years ago, Mars resembled a wetter Earth, with evidence of a northern ocean holding about half the volume of Earth's Atlantic Ocean. Fast-forward a billion years, and it became the cold, arid desert we observe today. Rovers and orbiters continue to uncover remnants of this watery past.
The prevailing theory attributes the water's disappearance to escape into space.
Mars' lower mass results in weaker gravity, allowing lighter atmospheric gases like nitrogen to escape early on. The planet cooled rapidly, solidifying its molten core and eliminating its protective magnetic field.
Without this shield, solar winds stripped away the atmosphere. Declining volcanism further thinned it, dropping pressure and making liquid water unsustainable. Much of the water vaporized and fled to space.
However, JPL and Caltech researchers argue this doesn't account for all the water. Their models indicate much of it became chemically bound in the planet's crust minerals.
The team analyzed water inventories across Mars' history, factoring in atmospheric and crustal chemistry.
They scrutinized the deuterium-to-hydrogen (D/H) ratio. Deuterium, a heavier hydrogen isotope, escapes space less readily, so prolonged atmospheric loss should enrich it over time.
Observations don't match expectations from space loss alone. The current D/H ratio suggests 30-99% of Mars' original water infiltrated minerals via chemical weathering.
"Whenever a rock interacts with water, complex reactions form hydrated minerals," explains co-author Eva Scheller. This process occurs on Earth too, as in clay formation.
Earth recycles such water through tectonics and volcanism, but Mars lacks these. Trapped water likely endures today.
Details appear in the journal Science.
