Mars is a dry place, and except for a tiny amount of water vapor in the atmosphere, all water exists as ice. But it wasn't always that dry. References to the past wet chapter of the planet are on the surface. Paleolacs like Jezero Crater, soon to be explored by NASA's Perseverance rover, are strong evidence of Mars' ancient past. But what happened to all that water?
It naturally disappeared into space. But if? And how fast?
A new study says Mars lost its atmosphere and water relatively quickly. From a geological point of view, all the water disappeared within a short time, supported by dust storms.
The title of the new study is "Hydrogen Leakage from Mars Is Driven by Seasonal and Dust Storm Transport of Water." The lead author is Shane Stone, a retired laboratory chemist who is now a graduate student at the University of Arizona Lunar and Planetary Laboratory. The paper is published in the journal Science.
This new research focuses on data from NASA's MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. MAVEN's mission is "to study the upper atmosphere and ionosphere of Mars and to study how the solar wind removes volatile compounds from this atmosphere". MAVEN has been orbiting Mars since 2014 and its mission should last until 2030.
"We know that billions of years ago there was liquid water on the surface of Mars," Stone said in a press release. “There must have been a thicker atmosphere, so we know Mars somehow lost most of its atmosphere to space. MAVEN tries to characterize the processes responsible for this loss, and part of that is to understand exactly how Mars lost its water. "
About every 4.5 hours MAVEN dips deeper into the Martian atmosphere and measures charged H2O ions with a spectrometer. These measurements are made at an altitude of about 161 km above the planet's surface. It also performs deeper atmospheric immersions, where it descends to approximately 125 km (77.6 mi) for every 20 orbits, with each immersion lasting five days. At this altitude the atmosphere is much denser.
MAVEN's mission profile means that measurements can be made throughout the upper Martian atmosphere and at different latitudes. With repeated measurements, scientists can calculate the amount of water vapor in the atmosphere.
An artist's impression of NASA's MAVEN spacecraft orbiting Mars. Image: NASA's Goddard Space Flight Center
MAVEN found a surprisingly large amount of water vapor in the upper Martian atmosphere, where it is quickly removed. The water and its destruction are a tantalizing reference to the ancient history of Mars.
Hubble has been observing Martian water with MAVEN, and both missions have determined that Mars' water loss depends on the planet's seasons. When the planet is closest to the sun, the warming will melt more of the planet's water ice. The vapor then rises to the upper atmosphere where it dissolves into space.
In their study, the authors point out that "there is a repeatable seasonal trend in the H2O abundance in the upper atmosphere, which peaks in the southern summer between LS = 250 ° and 270 ° …"
However, seasonal warming is only one factor that contributes to water vapor loss.
"The release of water to the upper atmosphere mediated by seasonal and dust storms could have played an essential role in the evolution of the Martian climate from its warm and humid state billions of years ago to the cold and dry planet we observe today."
From “The hydrogen flight from Mars is seasonally driven
and dust storm transport of water ”by Stone et al., 2020.
The planet's dust cycle also contributes to the loss of water and hydrogen. Stone and the other researchers behind this work found that dust storms – both regional and global, which occur roughly every 10 years – also warm the atmosphere, resulting in faster water loss.
In particular, they found that “during the global dust storm in June 2018, the mean H2O mixture ratio increased by a factor of 2.4 from an average of 3.0 to 7.1 ppm within 2 days. The water frequency then increased in accordance with the seasonal trends and the dust storm trends and reached values> 60 ppm at LS = 204 °. This event included the highest persistent H2O frequencies that we observed, which were tens of ppm for more than 5 months at the end of 2018. "
These results contradict the existing model of Martian water loss. According to this understanding, water ice sublimates into water vapor and is destroyed by the unobstructed radiation of the sun in the lower atmosphere.
As the authors explain in their paper: “Early studies on H production and flight neglected the ionospheric destruction of H2O, as it was assumed that H2O was restricted to low altitudes by a hygropause. It was later found that the height of the hygropause varies with the season, leading to speculation that during dust storms, H2O saturation may not occur at all due to elevated temperatures. "
These new results upset some of this knowledge.
"This is important because we weren't expecting any water in the upper Martian atmosphere," said Stone. “If we compare Mars with Earth, the water on Earth is limited due to the so-called hygropause close to the surface. There is just a layer in the atmosphere that is cold enough to condense (and thus stop) water vapor traveling upwards. "
During normal years, Mars experiences a steady drop of water vapor loss. Photo credit: NASA / Stone et al., 2020.
The hygropause is one of several “pauses” in planetary atmospheres. In essence, they are a region where something is changing. The most famous on earth could be the tropopause between the troposphere and the stratosphere. In the stratosphere, the temperature increases with altitude, but in the troposphere, the temperature decreases with altitude. The tropopause is the region between the two.
The hygropause is similar. Kley et. al. In 1979 it was found that the water vapor mixture ratio on earth increases with higher altitudes through the stratosphere. However, 2 or 3 km above the tropopause there is an absolute minimum of water vapor mixing ratio. They called it the hygropause.
During the southern summer and during regional and global dust storms, water loss into space accelerates. Photo credit: NASA / Stone et al., 2020.
The hygropause is basically a cold region where water vapor condenses and no longer migrates upwards. As Stone says, there should be no water vapor over the hygropause. However, according to the research team, this means that the hygropause of Mars is simply not cold enough to force the vapor to condense.
Since the hygropause of Mars is weak, the vapor travels high enough into the upper atmosphere that the ions break apart very quickly and the resulting byproducts are lost into space.
Mars' water loss has a seasonal element. During the southern summer, the planet warms up and releases more water vapor. Both regional and global dust storms are important drivers of water loss. Photo credit: NASA / Stone et al., 2020.
“The loss of its atmosphere and water to space is a major reason that Mars is cold and dry compared to the warm and wet earth. This new data from MAVEN shows a process by which this loss is still occurring today, ”said Stone.
But how did that develop in Mars' past? Mars was warm and wet once, maybe several times. What does this new knowledge tell us about Mars' past?
The team took their findings and worked backwards a billion years. They found that this water loss mechanism could explain the partial loss of a global ocean if Mars actually did have one.
"If we were to take water and distribute it evenly over the entire surface of Mars, that ocean of water lost to space through the new process we describe would be over 17 inches deep," Stone said. "Another 6.7 inches would be lost due to the effects of global dust storms alone."
An artistic impression of the ancient Martian ocean. According to this new study, the waters of Mars were at least partially lost to space due to global and regional dust storms. Image: ESO / M. Kornmesser, via N. Risinger
Global dust storms play a crucial role in water loss. During one of these mammoth storms, up to 20 times more water can be transported into the upper atmosphere. A press release accompanying the study states that a global dust storm of 45 days can transport as much water vapor into the upper atmosphere as during a regular storm-free Martian year of 687 earth days.
However, this study has limitations. The team couldn't go back more than a billion years since the hygropause is unlikely to be the same long ago. It was likely more powerful, which meant it was more difficult for water vapor to reach the upper atmosphere.
"There must have been a significant atmospheric escape into space before the process we described was operational," Stone said. "We still have to pin down the implications of this process and when it will go live."
Stein has a different goal in mind for similar atmospheric studies. The Saturn moon Titan has its own dynamic atmosphere in which organic chemistry is active. It is also the only body in the solar system other than Earth that has liquid on its surface.
"Titan has an interesting atmosphere in which organic chemistry plays an important role," said Stone. "As a former synthetic organic chemist, I am excited to study these processes."