In a study published on Feb. 2 in Communications: Earth & Environment, an international team of researchers reveals how, in 2023, an atypical storm during Mars’ northern summer lifted water into the planet’s upper atmosphere, allowing hydrogen to escape. This discovery, a first of its kind, has big implications for understanding the evolution of Mars’ climate.
“The findings reveal the impact of this type of storm on the planet’s climate evolution and opens a new path for understanding how Mars lost much of its water over time,” Adrián Brines, study co-author and researcher at the Instituto de Astrofísica de Andalucía, said in a press release.
Why isn’t Mars wet?
The atmosphere is too thin and the temperatures too low for liquid water to persist on the martian surface. What water there is on Mars exists as frozen crater ice or subsurface permafrost. Though scientists know from data gathered by the Viking landers, Mars Reconnaissance Orbiter, and Perseverance, among other missions, that the planet once contained a vast amount of liquid water, what has eluded them is where it all went.
Because of its axial tilt, Mars experiences four seasons just like Earth. But due to its elliptical orbit, the seasons in each hemisphere are distinct. Southern summer on the Red Planet means warmer temperatures as the planet swings closer to the Sun, while northern summer brings much cooler temperatures as the planet moves away. These temperature swings also affect the dustiness of each season. Southern summer is particularly dusty as warmer temperatures cause the strong winds that pick up dust, while northern summer is mostly dust-free.
For years, planetary scientists have known that the large-scale global dust storms of the southern summer can eject water from the atmosphere. Because of the warmer temperatures, water vapor cannot condense at lower altitudes during the southern summer, allowing the dust to lift it to altitudes of over 62 miles (100 kilometers). At those heights, the water vapor is broken down into hydrogen and oxygen by a process called photolysis. The hydrogen atoms can then escape the atmosphere’s exobase – the point at which particles can easily escape a planet’s atmosphere, around 87-124 miles (140-200 km) from the surface on Mars.
The rate at which those hydrogen atoms leave the atmosphere is called escape flux, and is measured by the number of hydrogen atoms leaking out of the atmosphere per square centimeter every second. During the dusty southern summer, this flux averages about 109 cm-2s-1. By contrast, the northern summer is typically much calmer, with a baseline flux of only 107 cm-2s-1. Under normal conditions, the southern summer is 100 times more powerful at stripping hydrogen away than its northern counterpart.
Missing water mystery
It would seem then that global dust storms during the southern summer must have driven the historic loss of Mars’ liquid water. However, there’s one small problem: According to martian global climate models, southern summer dust storms couldn’t have removed all the water on their own. As the authors explain in the paper, “a 137 m deep GEL [global equivalent water layer] is expected to have been lost into space throughout recent Martian history.” This means if the escaped water was spread evenly across Mars, it would average a depth of 137 meters (approximately 449 feet). For that much water to escape, it would have to occur at a faster rate than what’s predicted by simulations. Therefore, there must be other mechanisms capable of removing water from the atmosphere.
A possible explanation
That’s where the new study comes into play. The researchers detected a strange spike in the amount of water vapor present in the middle atmosphere during Mars Year (MY) 37’s northern summer (2023 on Earth) — a time when the middle atmosphere is usually quite dry. Since 2018, the Mars Trace Gas Orbiter’s Nadir and Occultation for Mars Discovery (NOMAD) instrument has been recording the vertical distribution of water vapor in the martian atmosphere. By comparing the data from MY 37 to MY 35 and 36, the team observed a significant, unseasonal increase in water vapor in the middle atmosphere during MY 37. NOMAD recorded a large amount of water vapor (70 parts per million [ppm]) at a height of 37 miles (60 km). The vapor remained at this height for around 6 sols (martian days). By contrast, NOMAD data from around the same days in MY 35 recorded less than 4 ppm at that height.
Additionally, the Emirates Mars Ultraviolet Spectrometer, a spectrometer onboard the United Arab Emirates Space Agency’s Hope probe, detected an increase in the amount of hydrogen at the exobase at the same time. Not only was the water vapor reaching strange heights, but it was escaping the atmosphere. The hydrogen escape flux at the time reached 5×108 cm-2s-1, approximately 50 times higher than the baseline for a northern summer. Something was lifting an unusual amount of water vapor into the air and out of the atmosphere. But what?
Researchers believe the extraordinary vertical water transportation and hydrogen escape were the result of a regionalized dust storm that occurred in MY 37’s northern summer. This specific type of event is known as a “rocket dust storm.” These anomalous storms are explosive, localized events driven by a process known as deep convection. On Earth, convection usually happens when moist air rises and releases heat; on Mars, these storms are fueled by dust particles that soak up sunlight and heat the surrounding air. This rapid heating causes the air to rise, carrying dust and water vapor miles into the sky. In the case of the MY 37 storm, this heated air transported the water vapor into the middle atmosphere, where it was then able to escape. While global dust storms in the southern summer occur annually, rocket dust storms are exceedingly rare. The only other rocket storm on record was observed in MY 27 (2004-2005 on Earth).
Implications for the martian climate
While this single event may seem small compared to the global storms of the southern summer, its implications for the martian climate are significant. During the storm, the rate of escaping hydrogen was roughly a factor of two lower than the massive rates seen during the dusty southern season, yet it still represents a massive spike for the otherwise calm northern summer. This proves that water loss is not strictly a seasonal phenomenon; instead, localized events can trigger escape pathways at any time of year, provided the dust activity is intense enough.
This finding helps bridge the gap between observed escape rates and the massive historical water loss suggested by isotopic data. Furthermore, the researchers note that while these storms are infrequent in the present day — occurring only twice in the last several decades of observations — they may have been far more common in Mars’ past. During periods when Mars’ axial tilt was more extreme, more intense surface winds could have made these storms a primary driver of the planet’s transition from a water-rich world to the dusty desert it is today.
