NASA’s Hubble Space Telescope and the Mars Atmosphere and Volatile Evolution (MAVEN) mission have worked together to unravel one of the biggest mysteries surrounding Mars: what happened to its water? Mars, once a planet rich in surface water, has gradually lost most of it over the past 3 billion years.
The Process Behind Mars’ Water Loss
The study reveals that water molecules in the Martian atmosphere are broken down by sunlight into their atomic components—hydrogen and oxygen. Of particular interest to researchers is hydrogen and its heavier isotope, deuterium. Deuterium is hydrogen with an extra neutron in its nucleus, making it heavier and less likely to escape into space compared to regular hydrogen. Over time, as Mars lost hydrogen at a faster rate than deuterium, the ratio between these two isotopes increased, providing scientists with a method to estimate how much water Mars used to have during its wetter periods.
"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," explained John Clarke, lead researcher from Boston University’s Center for Space Physics. By using data from Hubble and MAVEN, Clarke and his team were able to measure the current escape rate of hydrogen atoms and extrapolate that information to understand the long-term history of water on Mars. This process helps scientists trace the fate of Mars' water over billions of years and offers new clues about the Red Planet’s ancient climate.
Hubble and MAVEN Reveal a Dynamic Martian Atmosphere
One of the most striking discoveries made by the Hubble and MAVEN missions is that the Martian atmosphere is much more dynamic than previously thought. Mars’ elliptical orbit brings it closer to the Sun during certain parts of its year, causing rapid changes in the atmosphere. When Mars is near its closest point to the Sun, known as perihelion, the planet’s atmosphere heats up, and water molecules rise through it more quickly. These molecules are broken apart at higher altitudes, releasing hydrogen and oxygen atoms into space at a faster rate.
"Scientists have found that Mars has an annual cycle that is much more dynamic than people expected 10 or 15 years ago," Clarke explained. "The whole atmosphere is very turbulent, heating up and cooling down on short timescales, even down to hours." The discovery that atmospheric conditions on Mars can change so rapidly, expanding and contracting based on the planet’s position relative to the Sun, adds a new layer of complexity to understanding how Mars has lost its water over time.
Hubble’s far-ultraviolet imaging, combined with MAVEN’s atmospheric data, has allowed scientists to map these changes in unprecedented detail. When Mars is farthest from the Sun, or at aphelion, hydrogen escape slows down, but at perihelion, the rate increases significantly. These findings overturn earlier assumptions that hydrogen atoms slowly diffused upwards through the atmosphere. Instead, the water molecules are pushed to higher altitudes rapidly when Mars is closest to the Sun, accelerating the process of water loss.
The Role of Solar Wind and Chemical Reactions
The study also revealed that additional energy sources are required to explain how hydrogen and deuterium atoms reach escape velocity. At the temperatures found in Mars’ upper atmosphere, only a small fraction of hydrogen atoms would have the necessary speed to escape Mars' gravity. To account for this, scientists identified two key factors that provide the extra "kick" needed for these atoms to escape: solar wind collisions and sunlight-driven chemical reactions in the upper atmosphere.
Solar wind particles, which continuously stream from the Sun, collide with atmospheric particles, transferring energy and boosting the speed of hydrogen atoms. At the same time, solar radiation triggers chemical reactions that produce super-thermal hydrogen atoms—atoms moving fast enough to escape Mars’ gravitational pull. These mechanisms have contributed to the accelerated loss of Mars’ atmosphere, particularly during periods of high solar activity. The interaction between the solar wind and Mars' atmosphere further emphasizes how the planet's distance from the Sun affects its ability to retain water.
Understanding Mars as a Proxy for Distant Exoplanets
Beyond solving the mystery of Mars’ water loss, these findings have broader implications for understanding the evolution of planets both inside and outside our solar system. Mars, Earth, and Venus all reside within or near the habitable zone of the Sun, the region where conditions could potentially support liquid water. However, the present-day environments of these planets are drastically different. While Earth remains rich in water, Venus has undergone a runaway greenhouse effect, and Mars has lost much of its atmosphere and water over time.
"Studying the history of water on Mars is fundamental not only to understanding planets in our own solar system but also the evolution of Earth-size planets around other stars," Clarke pointed out. Astronomers are finding more exoplanets within the habitable zones of distant stars, but it is difficult to study them in detail. Mars serves as a valuable proxy for these distant worlds, offering clues about how planets lose their atmospheres and water over billions of years.
The collaboration between Hubble and MAVEN provided the first holistic view of hydrogen atoms escaping Mars, helping scientists piece together the planet’s water history and offering a framework for studying other rocky planets in similar orbits around distant stars.
Looking Forward: The Future of Mars Exploration
As the MAVEN mission prepares to celebrate its 10th year at Mars in September 2024, scientists continue to gather data that will enhance our understanding of the Red Planet. The mission, which is managed by NASA’s Goddard Space Flight Center, has played a crucial role in explaining how the Martian atmosphere behaves and how water escapes into space. Meanwhile, the Hubble Space Telescope, which has been in operation for more than three decades, continues to provide key observations that help solve long-standing questions about the universe, including planetary evolution and atmospheric processes.
Together, these missions are providing a clearer picture of Mars’ past and present, offering insights into the planet's potential to host life billions of years ago. With further research, scientists hope to unlock more secrets about the planet's geological history and its capacity to support life. As John Clarke summarized, "To understand how much water there was and what happened to it, we need to understand how the atoms escape into space." This ongoing research will undoubtedly shape future Mars exploration missions and enhance our understanding of the solar system’s most enigmatic planet.