Mars Lost Its Water Rapidly
A new mechanism for desiccation on Mars could have dried up an ocean much faster than previously thought.
Bedrock at site of putative lakebed. Bedrock at this site added to a puzzle about ancient Mars by indicating that a lake was present, but that little carbon dioxide was in the air to help keep a lake unfrozen. Credits: NASA/JPL-Caltech/MSSS, Feb 6, 2017
Credits: NASA/JPL-Caltech/MSSS, Feb 6, 2017
Planetary scientists have long inferred vast quantities of water in Mars’s past based on canyons, channels, sediments and erosion patterns. There is no water today. Why? The common answer has been that the solar wind and ultraviolet light from the sun gradually reduced the Martian atmosphere to the point where there was insufficient pressure to keep water in the liquid state. The drying process, thought to be “a trickle,” would have been accelerated as the red planet’s magnetic field decayed. Today, there are only patches of magnetic field remaining.
Now, Science has reported another process that could have accelerated the loss of Mars water: dust storms. Paul Voosen writes about findings from the MAVEN mission in his article, “Dust storms on Mars propel water’s escape to space.” These “churning dust storms” which mysteriously blanket the entire globe every few years, act like water pumps, propelling water from the ground out into space. This mechanism had been suggested before.
During the 2018 storm, however, Shane Stone, a graduate student at the University of Arizona, was looking at data from MAVEN, which has studied the planet’s upper atmosphere since 2014. One MAVEN instrument directly samples the gossamer atmosphere as the probe dips to its lowest orbital altitude of 150 kilometers, and Stone and his colleagues couldn’t believe what it was reporting: While the dust swirled at lower altitudes, a deluge of water was reaching the edge of space. “This was really a smack in the face,” Stone says. “The global dust storm stands out in the data like nothing else.”
Other scientists realized that the process creates a feedback loop that actually increases the draw of water up high into the atmosphere.
Fedorova and colleagues believe dust drives a positive feedback that pumps water into the upper atmosphere. Sun-warmed dust particles radiate heat into the lower atmosphere, driving winds that kick up more dust, which in turn leads to more heat and more dust. A warmer atmosphere can hold more water vapor, and the dust itself likely drags water along with it as it swirls into the upper atmosphere.
The TGO observations showed water levels gradually increase through the southern summer, but the MAVEN data suggest regional and global dust storms drive the largest spikes. Over the course of 2 days near the start of the 2018 storm, water abundance in the upper atmosphere, normally about 3 parts per million, more than doubled; by summer’s peak, the storm and the overall warming ultimately combined to push those levels to 60 parts per million. “It’s just a huge influx of water,” Stone says.
On top of that, carbon dioxide in the solar wind is now thought to be more efficient at breaking up water molecules. High in the atmosphere, CO2 “vigorously splits the water molecules apart,” the article says. Then, the ions are swept out into space, never to return.
A “lake” of briny water is inferred to exist under Mars’ north polar cap.
To estimate how quickly Mars water would be depleted would require knowing the initial quantity of water and the frequency of dust storms. Being moyboys, the planetary scientists assume that the dust-pump process continued over billions of years.
The MAVEN team has calculated that if the observed loss rates persisted throughout martian history, the planet would have lost a global ocean more than 25 meters deep. But how long this dust-driven destruction has operated on Mars is anyone’s guess, Stone adds. It could be recent, or it may have been key to drying out the planet billions of years ago. Researchers believe the planet once had a protective magnetic field that failed early in its history, allowing the solar wind to penetrate deeper in the atmosphere, where global dust storms were putting water in harm’s way. That could have caused water loss to surge, Chaffin says. “Maybe you fall off a cliff and get this seasonal and dust-driven loss,” he says. What MAVEN is witnessing now, he adds, could be the end of a “planetary death spiral.”
If the global ocean had been shallower than that, and the magnetic field had not started out strong, the desiccation of Mars could have been more rapid.
Figuring out how rapid could be an interesting research project for someone looking at the data with different assumptions about age. What is telling about this article is that the secular planetary scientists were surprised about the rapidity of the process. Maybe the “planetary death spiral” occurred much more recently than billions of years ago.