One of the James Webb Space Telescope’s (scheduled for launch on October 21) first tasks will be to learn why Mars lost so much water over its 4.5-billion-year history turning the once blue planet into a frigid desert world. Today, Mars has a carbon dioxide atmosphere 100-times thinner than Earth’s that makes up more than 95 percent of the planet’s atmosphere, which has a surface pressure of only 0.6 percent that of Earth.
One potential explanation for Mars’ much thinner atmosphere is that at some point in its history the planet had a protective magnetic field which switched off. A strong magnetic fields protects and preserves a planet’s oceans; as Mars’ atmosphere thinned, most of the ocean, it’s long been thought, was lost to space.
An Asteroid the Size of Pluto?
NASA scientists have speculated that an impact basin in Valles Marineris deep enough to swallow Mount Everest highlights what might be the result of an ancient asteroid collision that switched off Mars magnetic field, bathing the planet in deadly radiation, and eroding its atmosphere by particles streaming from solar winds.
Evidence suggests that in the early history of our solar system, Mars hosted an ocean as deep as the Mediterranean, with huge rivers cutting gullies and channels on the eerie desolate landscape we see today, unchanged for millions of years. Scientists have long known that water was abundant on ancient Mars, but its fate, a mystery.
Massive Ice Deposit and Hidden Lakes at Mars South Pole
In January 2020, a Caltech team probed a mysterious feature at the south pole of Mars –a massive deposit of CO2 ice and water ice in alternating strata, like the layers of a cake, that extend to a depth of one kilometer, with a thin frosting of CO2 ice at the top. The layer-cake deposit contains as much CO2 as in the entire Martian atmosphere today highlighting a prediction of a theory developed by two Caltech scientists —with enormous implications for climate change on Mars—that its atmospheric pressure would swing in value as the planet wobbles on its axis during its orbit around the sun, exposing the poles to more or less sunlight.
This strange feature was preceded in 2018 by discovery of evidence suggesting that far beneath the deeply frozen ice cap at Mars’s south pole lies a lake of liquid water—the first found on the Red Planet. Detected from orbit using ice-penetrating radar of Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), the finding resembles the interconnected bodies of water buried under several kilometers of ice in Greenland and Antarctica, where a network of 400 lakes have been detected.
“Huge Trapped Reservoirs”–New Seminal Research
The “lost to space’ hypothesis dominated until now: according to new research from Caltech and JPL, a significant portion of Mars’s water–between 30 and 99 percent–is trapped within minerals in the planet’s crust, challenging the current theory that the Red Planet’s water escaped into space.
The Caltech/JPL team reconfirmed that around four billion years ago, Mars was home to enough water to have covered the whole planet in an ocean about 100 to 1,500 meters deep; a volume roughly equivalent to half of Earth’s Atlantic Ocean.
But, they report, a billion years later, the planet was as dry as what we see today.. Previously, scientists seeking to explain what happened to the flowing water on Mars had suggested that it escaped into space, victim of Mars’s low gravity. Though some water did indeed leave Mars this way, it now appears that such an escape cannot account for most of the water loss.
“Atmospheric escape doesn’t fully explain the data that we have for how much water actually once existed on Mars,” says Caltech PhD candidate Eva Scheller, lead author of a paper on the research that was published by the journal Science on March 16 and presented the same day at the Lunar and Planetary Science Conference (LPSC).
The team studied the quantity of water on Mars over time in all its forms (vapor, liquid, and ice) and the chemical composition of the planet’s current atmosphere and crust through the analysis of meteorites as well as using data provided by Mars rovers and orbiters, looking in particular at the ratio of deuterium to hydrogen.
Water is made up of hydrogen and oxygen: H2O. Not all hydrogen atoms are created equal, however. There are two stable isotopes of hydrogen. The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction (about 0.02 percent) exist as deuterium, or so-called “heavy” hydrogen, which has a proton and a neutron in the nucleus.
A Telltale Signature –“An Outsized Portion of Deuterium”
The lighter-weight hydrogen (also known as protium) has an easier time escaping the planet’s gravity into space than its heavier counterpart. Because of this, the escape of a planet’s water via the upper atmosphere would leave a telltale signature on the ratio of deuterium to hydrogen in the planet’s atmosphere: there would be an outsized portion of deuterium left behind.
However, the loss of water solely through the atmosphere cannot explain both the observed deuterium to hydrogen signal in the Martian atmosphere and large amounts of water in the past. Instead, the study proposes that a combination of two mechanisms–the trapping of water in minerals in the planet’s crust and the loss of water to the atmosphere–can explain the observed deuterium-to-hydrogen signal within the Martian atmosphere.
When water interacts with rock, chemical weathering forms clays and other hydrous minerals that contain water as part of their mineral structure. This process occurs on Earth as well as on Mars. Because Earth is tectonically active, old crust continually melts into the mantle and forms new crust at plate boundaries, recycling water and other molecules back into the atmosphere through volcanism. Mars, however, is mostly tectonically inactive, and so the “drying” of the surface, once it occurs, is permanent.
“Atmospheric escape clearly had a role in water loss, but findings from the last decade of Mars missions have pointed to the fact that there was this huge reservoir of ancient hydrated minerals whose formation certainly decreased water availability over time,” says co-author Bethany Ehlmann, professor of planetary science and associate director for the Keck Institute for Space Studies..
“All of this water was sequestered fairly early on, and then never cycled back out,” Scheller says. The research, which relied on data from meteorites, telescopes, satellite observations, and samples analyzed by rovers on Mars, illustrates the importance of having multiple ways of probing the Red Planet.
“Mars once was wet and fertile. It’s now bone dry,” said Cosmos host, Neil deGrasse Tyson who was not involved in the JPL study. “Something bad happened on Mars, I want to know what happened on Mars so that we may prevent it from happening here on Earth.”
Source: “Long-term Drying of Mars Caused by Sequestration of Ocean-scale Volumes of Water in the Crust,” published in Science on 16 March 2021