This means that Mars has been dry for quite a long time, said Eva Scheller, a Caltech graduate student who was the lead author of the Science paper.
Today, there is still water equivalent to a global ocean 65 to 130 feet deep, but that is mostly frozen in the polar ice caps.
Planetary scientists have long marveled at ancient evidence of flowing water carved in the Martian surface gigantic canyons, tendrils of winding river channels and deltas where the rivers disgorged sediments into lakes. NASAs latest robotic Mars explorer, Perseverance, which landed last month in the Jezero crater, will be headed to a river delta at its edge in hopes of finding signs of past life.
Without a time machine, there is no way to observe directly how much water was on a younger Mars more than three billion years ago. But the hydrogen atoms floating today in the atmosphere of Mars preserve a ghostly hint of the ancient ocean.
On Earth, about one in 5,000 hydrogen atoms is a version known as deuterium that is twice as heavy because its nucleus contains both a neutron and a proton. (The nucleus of a common-variety hydrogen atom has only a proton, no neutrons.)
But on Mars, the concentration of deuterium is markedly higher, about one in 700. Scientists at the NASA Goddard Space Flight Center who reported this finding in 2015 said this could be used to calculate the amount of water Mars once had. Mars probably started with a similar ratio of deuterium to hydrogen as Earth, but the fraction of deuterium increased over time as the water evaporated and hydrogen was lost to space, because the heavier deuterium is less likely to escape the atmosphere.
The problem with that story, said Renyu Hu, a scientist at NASAs Jet Propulsion Laboratory and another author of the current Science paper, is that Mars has not been losing hydrogen fast enough. Measurements by NASAs Mars Atmosphere and Volatile Evolution orbiter, or MAVEN, have showed that the current rate, extrapolated over four billion years, can only account for a small fraction of the water loss, Dr. Hu said. This is not enough to explain the great drying of Mars.