Billions of years in the past, Mars was a water world, covered in lakes, rivers, and seas. Over time, this liquid water was lost, creating the frigid desert world we see in the modern age. Today, Mars holds onto its scarce reserves of surface water in the form of frozen ice hiding under its ruddy crust. The mystery of how Mars lost its liquid water could now be answered by studying powerful dust storms, capable of covering the planet.
These massive dust storms are capable of creating powerful forces, lifting water vapor from typical heights of 20 kilometers (12 miles) above the surface of the planet to altitudes of around 80 kilometers (50 miles) new research reveals. Once there, water in the atmosphere can interact with ultraviolet light from the Sun, turning into hydrogen and oxygen, which subsequently escapes to space.
“When you bring water to higher parts of the atmosphere, it gets blown away so much easier,” explained Geronimo Villanueva, a researcher at NASA Goddard exploring the mysteries of Martian water.
You Can Never be too cold or too Thin
The atmosphere of Mars is thin and rarefied, like the air on top of the highest mountains of Earth, holding on to thin, wispy clouds of ice. Recent observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter studying Mars, as well as data form the Hubble Space Telescope (HST), show hydrogen escaping the atmosphere of the Red Planet. This outgassing increases during dust storms, as well as during summer in the southern hemisphere, suggesting a “heat pump” is driving water up to great altitudes of the atmosphere.
This idea could help explain why liquid water, which covered much of Mars billions of years ago, has been lost to space. On Earth, water in the atmosphere condenses, falling back to the ground in the form of rain or snow, feeding waterways and starting the process anew. On Mars, water high in the atmosphere, converted into oxygen and hydrogen, is lost forever to the vast expanse of space.
“The sun is operating as a pump, which ‘activates’ in the daytime and helps water reach heights of over 60 kilometers above the ground. During a dust storm, the concentration of moisture in the air and airflow speed are higher, and therefore, the ‘pump’ is able to lift water higher up,” Dmitry Shaposhnikov of the Moscow Institute of Physics and Technology (MIPT) explained.
Going the Long Way Around
The orbit of Mars around the Sun is much more elongated than that traversed by Earth. Because of this, Mars is significantly closer to the Sun during the time when southern hemisphere is experiencing summer. When summer comes to the northern hemisphere, Mars is at its most distant point from the Sun (the aphelion of its orbit). These orbital mechanics result in significantly cooler summers in the northern hemisphere of Mars than those experienced in the southern hemisphere.
When summer comes to the southern hemisphere of Mars, ice sublimates, changing directly from solid ice to water vapor, and enters the atmosphere. Below an altitude of 40 km (25 miles) above the surface, this water remains as vapor. But, above this altitude, the water forms clouds of ice.
Heating from the Sun creates a temporary updraft between 20 and 70 degrees southern latitude during local summer, driving water high into the air. Seasonal winds push this water toward the north. During its journey, a fraction of this water is subjected to ultraviolet light from the Sun, breaking apart into hydrogen and oxygen. Most of the material that remains cools and falls near the north pole, forming the northern polar cap.
The Water, my Friend, is Blowing in the Wind
Typical dust storms on Mars cover areas equivalent in size to the United States for a period of a couple days. Global dust storms, like the one that silenced the Opportunity rover after nearly 15 years on the Red Planet, can engulf the entire planet and last for months. The first global dust storm seen on Mars was recorded by the Mariner 9 spacecraft in 1971, Since that name, researchers have witnessed similar events happen twice in 1977, followed by storms in 1982, 1994, 2001, 2007, and 2018. While the most recent dust storm encircled the planet, eight spacecraft — the greatest number ever — were studying the world.
Astronomers once believed the Earth was the only water world in our solar system, but we now know of multiple bodies in our family of planets playing host to significant bodies of water. A few of these may even be capable of supporting primitive life. Outside the solar system, super-Earths orbiting distant stars could be covered in massive oceans, potentially housing a wide variety of alien lifeforms.
As nations and private organizations prepare to send the first humans to Mars, it is vital to learn everything we can about water on the Red Planet and the nature of the dust storms which help drive the climate on that world.