What happened to the water that once covered Mars? Scientists know some of it goes deep underground, but where is the rest? Evidence suggests that some water molecules split into atoms that escape through the Martian atmosphere and into space. The team combined data from Hubble and MAVEN to measure the number of hydrogen atoms and their current escape velocity.

They found that the escape velocity of hydrogen and “heavy hydrogen” called deuterium changed rapidly as Mars approached the Sun. This overturned the classical picture that scientists had previously held, that these atoms slowly diffused upward through the atmosphere to a height where they could escape. Estimating the escape velocity back in time helped the team understand the history of water on the Red Planet.

Mars was once a very wet planet, as evidenced by the geology of its surface. Scientists know that at least some of the water has seeped underground over the last 3 billion years, but what happened to the rest? Now NASA’s Hubble Space Telescope and MAVEN (Mars Atmosphere and Volatile Evolution) missions are helping to solve this mystery.

“There are only two places that water can go. It can freeze to the ground, or the water molecule can split into atoms and the atoms can fly through the upper atmosphere into space,” explained study leader John Clark of the Center for Space Physics at Boston University in Massachusetts. “We need to understand how the atoms fly into space to understand how much water is there and what happens to it.”

Clark and his team combined data from Hubble and MAVEN to measure the number of hydrogen atoms escaping into space and the current escape velocity. This information allowed them to extrapolate escape rates back in time to understand the history of water on the Red Planet.

Hydrogen escape and “heavy hydrogen”

Under the influence of sunlight, water molecules in the Martian atmosphere split into hydrogen and oxygen atoms. Specifically, the team measured hydrogen and deuterium, a hydrogen atom with a neutron in its nucleus. This neutron yields deuterium, which is twice the mass of hydrogen. Because of its greater mass, deuterium escapes into space much more slowly than ordinary hydrogen.

Over time, as more hydrogen than deuterium was lost, the ratio of deuterium to hydrogen in the atmosphere increased. Measuring this ratio today gives scientists a clue about how much water was present on Mars during the warm, wet period. By studying how these atoms are escaping now, they can understand the processes that determined the rate of escape over the past four billion years and thus extrapolate back in time.

Interplanetary comparisons and predictions

Although most of the data from the MAVEN spacecraft comes from Mars, MAVEN is not sensitive enough to see deuterium emissions at any given time of the Martian year. Unlike Earth, Mars swings away from the Sun in an elliptical orbit during the long Martian winter, and deuterium emissions become weaker. Clark and his team needed the Hubble data to “fill in the gaps” and complete the three Martian years (687 Earth days each). Hubble also provided additional data from 1991 until MAVEN arrived at Mars in 2014.

Combining data from these missions provided the first consistent picture of how hydrogen atoms escape from Mars into space.

Dynamic and stormy Martian atmosphere

“In recent years, scientists have discovered that Mars has a much more dynamic annual cycle than people expected 10 or 15 years ago,” Clark explained. “The entire atmosphere is very turbulent, heating up and cooling down in a short period of time, even a few hours. The atmosphere expands and contracts as the brightness of the Sun on Mars varies by 40 percent during the Martian year.”

The team found that the rate of hydrogen and deuterium escaping changed rapidly as Mars got closer to the Sun. The traditional picture scientists had of these atoms slowly diffusing upwards through the atmosphere to a height where they could escape.

But that picture no longer tells the whole story, because scientists now know that atmospheric conditions change very quickly. As Mars approaches the Sun, water molecules, which are the source of hydrogen and deuterium, are quickly ejected into the atmosphere, releasing atoms at higher altitudes.

The second conclusion is that the changes in hydrogen and deuterium are so rapid that the atom’s output requires additional energy to account for this. At the temperature of the upper atmosphere, only a small fraction of the atoms have enough speed to escape Mars’ gravity. Faster (superthermal) atoms are formed when something gives the atom extra energy. These events include collisions of protons from the solar wind entering the atmosphere or sunlight triggering chemical reactions in the upper atmosphere.

Performs proxy functions

Studying the history of water on Mars is fundamental not only to understanding planets in our own solar system, but also to the evolution of Earth-sized planets around other stars. Astronomers are finding more and more such planets, but they are difficult to study in detail. Mars, Earth, and Venus are all in or near our solar system’s habitable zone—the region around a star where liquid water could accumulate on a rocky planet—but all three planets have radically different modern conditions. Together with its sister planets, Mars could help scientists understand the nature of distant worlds in our galaxy.

These results appear in this issue Science Developments Published July 26 by the American Association for the Advancement of Science.