About four billion years ago, Mars had liquid water, much like Earth. Mars’s rock record shows the remnants of rivers, lakes, and vast seas that might have supported life. But something triggered the loss of most of the planet’s water, and today Mars is a dry wasteland, apart from its polar ice caps and some possible underground lakes.
Exactly how and why Mars lost its water remains unclear. Data from NASA’s MAVEN spacecraft, which orbits Mars to analyse its upper atmosphere, suggested that water was lost into space as the planet’s atmosphere was stripped away by the Sun’s radiation. In our solar system, high energy photons emitted by the Sun travel through space and enter the atmospheres of the planets. When one of these photons collides with a molecule in the atmosphere, it knocks loose an electron turning the molecule into an ion. As the Sun’s magnetic field, carried by the solar wind, sweeps past a planet, some ions are carried off into space. Other ions collide with molecules in the atmosphere causing them to “sputter” into space, resulting in atmospheric loss. When Mars lost its protective magnetic field, solar radiation stripped away its atmosphere, and took water with it.
However, the current rate of atmospheric loss is not sufficient to explain why so much of Mars’s water disappeared. An alternative explanation is needed.
A new study, published in the journal Science and presented at the Lunar and Planetary Science Conference, concludes that a large proportion of Mars’s water became trapped within minerals in the crust, where it remains today. Caltech PhD candidate Eva Scheller and her colleagues are the team of geologists and atmospheric scientists who study Martian meteorites and data sent back by rovers and orbiting spacecraft about ancient rocks and the current Martian atmosphere. This allows them to estimate how much water was on the planet’s surface throughout its history. Their model estimates that between 30 and 99 percent of the planet’s water was incorporated into minerals — the rest escaping into space.
The uncertainty in their estimate stems from uncertainty in past rates of atmospheric loss. At present-day rates of loss, the amount of crustal hydration is likely closer to 99 percent. But if the rate of atmospheric loss was greater in the past, less water will have been incorporated into crustal minerals.
The timing of water loss on Mars is crucial in understanding whether life could have existed there. The researchers estimate that between 40 and 95 percent of Mars’s water became trapped in minerals during the planet’s Noachian period, between 4.1 and 3.7 billion years ago. Bursts of volcanic activity may have released some of the trapped water later on, potentially boosting Mars’s habitability to microbial life. Further research into how long volcanic activity continued to release water will help assess how long life could have persisted on the planet.
This study has shown that early in its history Mars was a much wetter planet than it is today, with surface oceans of between 100 and 1500 metres deep. Mars was most habitable at this time, and any microbial life would have spread through the vast body of water.
With the landing of NASA’s Perseverance rover in February this year, scientists can take more measurements of Mars’s rocks to determine how extensive hydrated minerals are in Jezero Crater, the rover’s landing site. Perseverance will also collect samples that will be returned to Earth in the next decade for detailed analysis, further improving our understanding of our planetary neighbour.
