For decades, we’ve said that Mars lost all its water to space. It turns out, though, that we were wrong.

In a new paper published this week in Science and presented at the Lunar and Planetary Science Conference, researchers have announced that between 30 and 99 percent of Mars’ ancient water is not lost in space but rather locked up in minerals on the red planet.

Let’s go back to the beginning. Billions of years ago, Mars had enough water to be covered in an ocean somewhere between 100 to 1,500 meters deep, about half of our Atlantic Ocean in volume. That water was only on the surface for a billion years, though, and then Mars looked as dry then as it does now. The previous hypothesis was that the water evaporated and escaped into space due to Mars’ low gravity. And some water molecules definitely did that. However, the scientists evaluated the amount of water on Mars in all three forms – ice, liquid, and vapor – and analyzed the current atmosphere and crust.

So what were they looking for? Here’s where a little chemistry comes in. From the press release: Water is made up of hydrogen and oxygen: H2O. Not all hydrogen atoms are created equal, however. There are two stable isotopes of hydrogen. The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction (about 0.02 percent) exists as deuterium, or so-called “heavy” hydrogen, which has a proton and a neutron in the nucleus.

That lighter weight hydrogen would escape more easily into space than the heavier deuterium, and the difference in escape rates should leave a signature called the deuterium to hydrogen (D/H) ratio. If all or even most of the hydrogen escaped, the Martian atmosphere’s D/H ratio should lean heavily toward the deuterium, but the measured value doesn’t match the expected value.

Okay then. Let’s look at the rocks. Water erodes rocks but it also causes chemical weathering that creates clays and various hydrous minerals. We can find evidence of these processes here on Earth, so it happened on Mars as well. The difference is that on Earth, we have plate tectonics and volcanoes that recycle all that locked up water back into the mantle and then into our atmosphere during eruptions. Mars seems to be mostly inactive when it comes to volcanoes, though. So the water that gets locked up in minerals, stays locked up.

Co-author Bethany Ehlmann sums up the work: Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time.

One of the best parts of this research was how it used data from all kinds of sources: meteorites, telescopes, satellite observations, and samples that our army of rovers analyzed. Now we just need Perseverance to find us some evidence of past life!

More Information

Caltech press release

NASA press release

NASA JPL press release

“Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust,” E. L. Scheller et al., 2021 March 16, Science