Despite decades of exploration and research, Mars still holds a fair share of secrets. In particular, scientists are still trying to find out what happened to the water that once flowed on the surface of Mars. Unfortunately, the Martian atmosphere was carried away by solar wind billions of years ago, which over time also resulted in the loss of its surface water – although it was not entirely clear where it was going and what mechanisms were involved.
To address this, a team of scientists recently used data from three orbiter missions to study the Martian atmosphere. They found evidence that the smaller regional dust storms that occur almost annually on Mars make the planet drier over time. These results suggest that storms are a major driving force behind the evolution of the Martian atmosphere and its transition to the icy and arid place we know today.
Dust storms are a regular occurrence on Mars and occur whenever the lower atmosphere warms, causing air currents to pick up dust and circulate it around the planet. This can occur when Mars is closest to the Sun (perihelion) and can also be made worse by temperature differences between the hemispheres – if either of them experiences summer, atmospheric circulation can increase dramatically.
These dust storms heat the upper areas of the barren Martian atmosphere and prevent the water molecules from freezing as usual, forcing them to climb even higher. In these highest areas of the Martian atmosphere, water molecules are susceptible to ultraviolet radiation, which causes them to chemically dissociate and break down into their constituent parts – hydrogen and oxygen.
As the oxygen (the heavier element) either escapes into space or settles back on the surface, the hydrogen is easily lost into 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 of the study. As he said in a recent NASA press release:
“In order to lose water permanently, you only have to lose one hydrogen atom, because then hydrogen and oxygen cannot recombine to form water. So if you’ve lost a hydrogen atom, you’ve definitely lost a water molecule. “
Scientists have long suspected that Mars lost most of its water to dust storms, but they failed to realize the importance of regional storms. These occur almost every summer in the planet’s southern hemisphere, while the larger storms (which can span the entire planet) typically occur every three to four Martian years – the equivalent of roughly five and a half to seven and a half Earth years.
Image captured by the MRO on November 30, 2010 showing a “spire” (bottom center), a concentrated cloud of dust that can rise dozens of miles above the surface. The blue and white plumes are clouds of water vapor. Credits: NASA / JPL-Caltech / MSSS
In the past, these massive storms and the hot summer months in the southern hemisphere (when Mars is closer to the sun) were considered the main causes. However, after consulting the data obtained from the three Mars orbiters, Chaffin and his colleagues found that Mars loses about twice as much water during a regional storm than it does in the southern hemisphere in summer without regional storms.
These data come from the 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 co-author of the paper), was also a member of the Trace Gas Orbiter science team. As he explained:
“This paper helps us practically go back in time and say, ‘Okay, now we have another way of losing water that helps us, this little water we have on Mars today with the huge amount of water that we’ve had in the past … Instruments should all tell the same story, and they do. ”
Since water is one of the most important ingredients in life as we know it, scientists are keen to find out where it went and how long it existed on the surface of Mars. Essentially, they want to know if it has existed long enough to allow basic life forms such as unicellular microbes to emerge. Knowledge of the mechanisms of water loss is also crucial for future manned missions to Mars, where water sources must be secured on site.
Artist’s impression of the arrival of the MAVEN mission around Mars. Photo credit: NASA
While scientists had many theories about what would happen to water on Mars today, they lacked the measurements necessary to get a complete picture. Then Chaffin and his colleagues took the opportunity when a rare convergence of spacecraft orbits occurred during a regional dust storm (which lasted from January to February 2019), allowing scientists to make unprecedented observations.
Each orbiter performed a different type of scientific operation. While NASA’s MRO measures temperature, dust, and water-cice concentrations from the surface to 100 km (62 miles) above it, ESA’s TGO measures the concentration of water vapor and ice in the same altitude range. Meanwhile, NASA’s MAVEN spacecraft recorded the amount of hydrogen gas at altitudes over 1000 km (620 miles) above the surface.
In total, four instruments from the three spaceships collected data on the regional dust storm to determine its role in the water leakage from Mars. This included the TGO’s spectrometers, which detected the water vapor in the lower atmosphere before the dust storm began. It was also observed how water vapor rose to the middle atmosphere at the beginning of the storm and eventually reached concentrations ten times higher than before the storm erupted.
This coincided with data from the MRO’s Mars Climate Sounder (MCS), which recorded rising temperatures in the atmosphere as dust was raised high above the planet. As expected, water and ice clouds also disappeared, as there was no more ice in the warmer lower atmosphere. Meanwhile, MAVEN’s Imaging Ultraviolet Spectrometer (IUS) showed that before the storm hit, ice was visible over the massive volcanoes in the Tharsis region of Mars.
True color image of a storm front near Utopia Planitia, near the northern polar ice cap of Mars. Credit: Credits: ESA / DLR / FU Berlin
The same clouds disappeared when the storm started and reappeared as soon as the sky cleared. Seeing this unfold in their eyes confirmed what Chaffin and his colleagues had suspected all along. While some modeling and indirect evidence suggests that there is a link between dust activity and water loss on Mars, this is the first study able to differentiate between seasonal water loss and dust-powered forcing.
Although some modeling and indirect observations suggest that dust activity may explain the seasonal trend, no previous study has been able to clearly distinguish seasonal from dust-powered forcing. These findings not only provide new insights into the dynamics that drive the Martian environment, they could also be of great importance in planning manned missions to Mars.
After all, understanding Mars’ limited hydrological cycle might be important in order to actually find sources for it!
Further reading: NASA, nature
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