Despite decades of exploration and study, Mars still has its fair share of mysteries. In particular, scientists are still trying to ascertain what happened to the water that once flowed on Mars’ surface. Unfortunately, billions of years ago, the Martian atmosphere began to be stripped away by the solar wind, which also resulted in the loss of its surface water over time – although it was not entirely clear where it went and what mechanisms were involved.
To address this, a team of scientists recently consulted data obtained by three orbiter missions studying the Martian atmosphere. In the process, they found evidence that the smaller regional dust storms that happen almost annually on Mars are making the planet drier over time. These findings suggest that storms are a major driving force behind the evolution of Mars’ atmosphere and its transition to the freezing and desiccated place we know today.
Dust storms are a regular occurrence on Mars and occur whenever the lower atmosphere heats up, which causes air currents to pick up dust and circulate it around the planet. This can occur when Mars is at the closest point in its orbit to the Sun (perihelion) and can also be exacerbated due to variations in temperature between the hemispheres – when one of them is experiencing summer, atmospheric circulation can dramatically increase.
These dust storms have the effect of heating the upper areas of Mars’ sparse atmosphere, preventing water molecules from freezing as they normally would and forcing them to rise even higher. In these highest reaches of the Martian atmosphere, water molecules are vulnerable to ultraviolet radiation, which causes them to undergo chemical disassociation and break into their constituent elements – hydrogen and oxygen.
Whereas the oxygen (the heavier element) will either escape to space or settle back to the surface, the hydrogen is easily lost to space. Michael S. Chaffin, a researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder, was also the lead author on the study. As he said in a recent NASA press release:
“All you have to do to lose water permanently is to lose one hydrogen atom because then the hydrogen and oxygen can’t recombine into water. So when you’ve lost a hydrogen atom, you’ve definitely lost a water molecule.”
Scientists have long suspected that Mars lost most of its water due to dust storms but did not realize the significance of regional storms. These happen nearly every summer in the planet’s southern hemisphere, whereas the larger storms (which can encompass the entire planet) typically happen once every three to four Martian years – the equivalent of about five and a half to seven and a half Earth years.
Previously, these massive storms and the hot summer months in the southern hemisphere (when Mars is closer to the Sun) were thought to be the main drivers. However, after consulting data obtained by the three Mars orbiters, Chaffin and his colleagues found that Mars loses about twice the amount of water during a regional storm as it does during summer in the southern hemisphere without regional storms.
This data comes from NASA’s Mars Reconnaissance Orbiter (MRO), the ESA’s Trace Gas Orbiter (TGO), and the Mars Atmospheric and Volatile EvolutioN (MAVEN) orbiter. Geronimo Villanueva, a Martian water expert at NASA’s Goddard Space Flight Center (and a co-author on the paper), was also a member of the Trace Gas Orbiter’s science team. As he explained:
“This paper helps us virtually go back in time and say, ‘OK, now we have another way to lose water that will help us relate this little water we have on Mars today with the humongous amount of water we had in the past… The instruments should all tell the same story, and they do.”
Since water is one of the key ingredients for life as we know it, scientists are very interested in determining where it went and how long it existed on the surface of Mars. Essentially, they want to know if it existed long enough to allow for the emergence of basic life forms, like single-celled microbes. Knowing the mechanisms for water loss is also crucial for future crewed missions to Mars, which will need to secure sources of water locally.
While scientists had many theories about what was happening to the water on Mars today, they lacked the measurements needed to come up with a complete picture. Then, Chaffin and his colleagues were presented with an opportunity when a rare convergence of spacecraft orbits took place during a regional dust storm (which lasted from January to February 2019) allowed scientists to make unprecedented observations.
Each orbiter carried out a different kind of science operation. While NASA’s MRO measured the temperature, dust, and water-ice concentrations from the surface to 100 km (62 mi) above it, the ESA’s TGO measure the concentration of water vapor and ice in the same altitude range. Meanwhile, NASA’s MAVEN spacecraft recorded the amount of hydrogen gas to altitudes of over 1000 km (620 mi) above the surface.
All told, four instruments on the three spacecraft collected data on the regional dust storm to determine its role in Martian water escape. This included the TGO’s spectrometers, which detected the water vapor in the lower atmosphere before the dust storm began. It also witnessed the water vapor rise into the middle atmosphere as the storm began, eventually reaching concentrations that were ten times greater than before the storm took off.
This coincided with data obtained by the MRO’s Mars Climate Sounder (MCS), which detected rising temperatures in the atmosphere as the dust was raised high above the planet. As expected, it also saw water-ice clouds disappear since ice could no longer be in the warmer lower atmosphere. Meanwhile, the MAVEN’s Imaging Ultraviolet Spectrometer (IUS) showed that before the storm hit, ice could be seen above the massive volcanoes in the Tharsis region of Mars.
These same clouds disappeared when the storm began and reappeared the moment the skies cleared. Seeing this unfold in from of their eyes confirmed what Chaffin and his colleagues had suspected all along. While some modeling and indirect evidence have suggested that there is a relationship between dust activity and water loss on Mars, this is the first study that has been able to distinguish between seasonal water loss and dust-driven forcing.
Although some modeling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. These findings not only provide new insight into the dynamics that drive the Martian environment, they could also be highly significant when it comes to planning crewed missions to Mars.
After all, an understanding of Mars’ limited water cycle could be essential to actually find sources of it!