Caitlin V. Reidy (COM`24)

BU Astronomers say erosion patterns on the surface of Mars show that our neighboring planet once had a large amount of water flowing on the ground. That early water either remains frozen into the crust or it has escaped the weak gravity of Mars into space.

“It’s clear that with all the erosion channels seen on the surface and other evidence, when Mars was young, it had a lot of water,” said Professor Emeritus of Astronomy John T. Clarke. “Whether it had a global ocean or sporadic lakes or pooling, water was there permanently, or it came and went.”

But what happened to the water? How can we work backward in time to understand the history of water on Mars? Clarke and Majd Mayyasi, a research scientist at the Center for Space Physics, have been working to answer these questions by measuring the escape of atoms of water into space.

Their new paper in Science Advances, “Martian atmospheric hydrogen and deuterium: Seasonal changes and paradigm for escape to space,” summarizes three Mars-years of measurements of escaping hydrogen and deuterium atoms from the top of the martian atmosphere into space.

“There are only two places water can go. It can freeze into the ground, or the water molecule can break into atoms, and the atoms can escape from the top of the atmosphere into space,” Clarke said. “To understand how much water there was and what happened to it, we need to understand how the atoms escape into space.”

Clarke and his team combined data from the Hubble Telescope and MAVEN, a NASA spacecraft orbiting Mars, to measure the rate of the hydrogen atoms escaping into space. They discovered that the escape rates of hydrogen and deuterium, a type of “heavy hydrogen,” change rapidly when Mars is close to the Sun. Deuterium is chemically similar to hydrogen, but is twice the mass, and thus escapes into space at a slower rate.

“When Mars is close to the Sun, the atmosphere is hot and extended, and water from the surface rides thermal currents in the lower atmosphere upward to supply the atoms to escape,” Clarke said. “As water is lost in space, the ratio of deuterium to hydrogen builds up over time. Overall the data point to a very large amount of water on the primordial Mars.”

Mayyasi said this is result that uses the largest continuous data set of hydrogren and deuterium atoms observed in the atmosphere of Mars to date.

“This is an exciting result that helps us put a key piece of the puzzle together for how water escapes from the red planet and can help us better determine how much water the planet once had,” Mayyasi said. “And this is important for determining the potential existence and evolution of life on Mars.”

Clarke said scientists think that Earth, Mars, and Venus started out with similar conditions. But Venus is so hot that water molecules were broken up into hydrogen and oxygen molecules and escaped the gravity of the planet into space. The deuterium to hydrogen ratio on Venus is now approximately 200 times higher than it is on the Earth.

“This is consistent with Venus having lost an ocean of water, and Mars having lost a significant amount of water,” Clarke said. “Venus, although it has a very thick atmosphere, is bone dry. Any water that was there is long gone.”

But Mars presents a more complicated history given flow patterns that you see on the ground. The deuterium to hydrogen ratio on Mars is now about five to 10 times higher than it is in water in the oceans on the Earth. And Clarke said it’s difficult to tell how much water—if any—still exists frozen into Mars’ surface.

Clarke’s team used an special ultraviolet instrument to measure the reflected UV sunlight from hydrogen atoms in the atmosphere of Mars. This instrument was developed and tested in the “rocket lab” in room B-17 of the CAS building. The team then went into the desert in southern New Mexico to test the equipment on short flights into space. The instrument was developed and tested on these sounding rockets, then flown to Mars on the MAVEN satellite mission.

“The next time you walk past CAS Room B17 you will know that this is where the work was done leading to an instrument now at Mars,” Clarke said.