“This means that Mars has been dry for a long time,” said Eva Scheller, a Caltech PhD 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 researchers have long marveled at ancient evidence of flowing water carved into the surface of Mars – gigantic canyons, tendrils of winding river channels, and deltas where the rivers ejected sediments into lakes. NASA’s newest robotic Mars explorer, Perseverance, who landed in Jezero Crater last month, will head to a river delta on the river’s edge in hopes of finding signs of past life.

Without a time machine, there is no way to directly observe how much water was on a younger Mars more than three billion years ago. But the hydrogen atoms floating in the Martian atmosphere today retain a ghostly breath of the ancient ocean.

On Earth, about one in 5,000 hydrogen atoms is a version known as deuterium, which is twice as heavy because its nucleus contains both a neutron and a proton. (The nucleus of a normal type of hydrogen atom has only one proton, no neutrons.)

However, on Mars, the concentration of deuterium is significantly higher, about one in 700. Scientists at NASA’s Goddard Space Flight Center, who reported the finding in 2015, said this could be used to calculate the amount of water Mars once had. Mars probably started out with a similar ratio of deuterium to hydrogen as Earth, but the amount of deuterium increased over time as the water evaporated and hydrogen was lost to space, as the heavier deuterium is less likely to escape the atmosphere.

The problem with this story, said Renyu Hu, a scientist at NASA’s Jet Propulsion Laboratory and another author on the current science paper, is that Mars was not losing hydrogen fast enough. Measurements with NASA’s Mars Atmosphere and Volatile Evolution Orbiter (MAVEN) have shown that the current rate extrapolated over four billion years “can only account for a small part of the water loss,” said Dr. Hu. “This is not enough to explain the great drying of Mars.”