Mars once had vast oceans on its surface – then its magnetic field weakened, its atmosphere thinned, and its water vanished. But the numbers don’t add up: for the Red Planet to have transformed from a watery world into the red dust bowl we know today, all that water had to end up somewhere. So where did it go?
A new study published in the National Science Review may finally have an answer, as a research team from China, Australia and Italy have identified a vast reservoir of water that may lie deep beneath the planet’s surface. What’s more, unlike possible icy reservoirs found on other parts of Mars, this water is thought to still be in its liquid form, making it one of the best bets yet in the search for extraterrestrial life.
Around four billion years ago, much of the Martian surface was covered in liquid water. If spread evenly across the planet, this water would have formed a global ocean 1,500m (4,920ft) deep – a figure known as the global equivalent layer (GEL). In total, that’s about the same amount of water as Earth’s Indian Ocean holds today.
While there’s still debate over the exact amounts, this estimate leaves a glaring discrepancy.
“The estimated volumes of liquid water lost to space and through crustal hydration were 10–200m (33–656ft) GEL and 550m (1,800ft) GEL, respectively,” Weijia Sun, a professor of geophysics at the Chinese Academy of Sciences and first author of the new study, told BBC Science Focus.
“The present-day water inventory in Mars’s atmosphere and as ice in its polar regions or subsurface ice totals 20–40m (66–131ft) GEL.”
That leaves between 710 and 920m (2,330–3,020ft) GEL unaccounted for: the “missing water” of Mars, as Sun and his colleagues put it.
Marsquakes and meteorites
When NASA’s InSight lander touched down on the surface of Mars on 26 November 2018, it opened a new window into the subsurface. On board was a small, round dome containing a seismometer – the same devices that measure earthquakes on Earth – designed, in NASA’s words, to take the planet’s ‘pulse’.
In the new study, the team measured seismic waves travelling through the planet from two meteorite impacts and one ‘marsquake’. Speaking to BBC Science Focus, study co-author Prof Hrvoje Tkalčić likened this technique to using ultrasounds to see inside a patient’s body.
“In a nutshell, seismic waves spreading through the planet’s interior from distant events enter the crustal layers beneath the receiver,” Tkalčić said. “And we infer the thicknesses of these layers and the depth of the boundaries from mapping their reverberations.”
Seismic waves travel faster through dry rock than through rock saturated with water. Therefore, by measuring the wave speeds from the impacts and quakes, scientists can detect water deep underground without needing to drill into it.
This state-of-the-art technique – known as ‘receiver functions’ – allowed the team to detect a layer around 5.4–8km (3.4–5 miles) below the Martian surface where seismic waves are slowing down in just the right way to signal the presence of water.
At such a depth, the temperature is high enough for the water to exist as a liquid. And, crucially, the amount of water down there was estimated by the researchers to be between 520 and 780m (1,700–2,560ft) GEL – coinciding neatly with the 710–920m (2,330–3,020ft) GEL of ‘missing water’.
Life on Mars?
If a giant aquifer does exist beneath the surface of Mars, it would make for a good place to hunt for extraterrestrial life. After all, water is the key ingredient for life on Earth, and deep subterranean life (especially bacteria and similar microorganisms known as archaea) is so significant on our planet that it accounts for around 15 per cent of Earth’s total biomass.
We’re unlikely to find flora or fauna at such depths on Mars – but microbial life? That remains a possibility.
“The presence of liquid water is regarded as one of the most critical factors in this endeavour, as it is a fundamental prerequisite for life as we know it,” Tkalčić said. “Consequently, identifying locations with liquid water on Mars is a key strategy in the search for potential life.”
Moreover, as we look to establish human settlements on Mars, water will be nothing short of essential. Drilling kilometres beneath the surface would be a formidable engineering feat, but – when it comes to building a home on another planet – such challenges are expected.
But before you start saving for your ticket to Mars, Sun and Tkalčić emphasise that the presence of the aquifer isn’t certain just yet. Working with such a limited dataset, they say more data is needed before conclusions can be drawn.
Indeed, while liquid water is the most likely explanation based on available data, there are other plausible explanations for the seismic waves slowing down, such as a layer of sediment.
On Earth, seismological measurements are carried out using scores of seismometers across the globe, with multiple data points helping to verify one another. On Mars, it’s a different story.
“We should always keep in mind that we are dealing with the data from a single seismometer on a distant planet, that is a very prohibitive observational environment, and we are doing the best we can given the quality and quantity of data we have,” said Tkalčić.
The researchers are hopeful that future Mars missions equipped with more seismometers will allow us to probe a larger portion of the planet in greater detail. In time, we may even be able to drill into the crust itself – analysing its chemistry for direct evidence of water, and perhaps even signs of life.
For now, the study is a promising glimpse of what future missions might uncover. As Sun put it: “These findings provide critical insights into the Martian water cycle and the evolution of its habitable environments, laying a crucial foundation for future research on the search for Martian life and the planet’s climate history.”
Read more:
- Massive star-forming cloud found strangely close to Earth
- Something strange is happening to Earth’s rotation. Now we know why
- How did Mars lose its atmosphere?
About our experts
Weijia Sun is a professor of geophysics in the Key Laboratory of Earth and Planetary Physics at the Chinese Academy of Sciences. His research has been published in journals including Nature, Geophysical Research Letters and the Journal of Geophysical Research.
Hrvoje Tkalčić is the head of geophysics and the director of the Warramunga Seismic & Infrasound Facility at the Australian National University. His research, focusing on observational seismology with a special interest in deep Earth structure and dynamics, has been published in journals including Science, Geophysical Research Letters and the Journal of Geophysical Research.