It is now widely believed that Mars holds a reasonably large volume of water. However, due to the cold surface, this water exists only as ice.
Scientists do believe that it is essential for the planet to possess liquid water. Ever since technology has enabled mankind to gaze at Mars in detail, humans have been looking for indications that there was water on the red planet.
Back in 1971, the Mariner 9 mission revealed clues of water erosion in river beds and canyons. Missions which later followed from the Viking orbiters first launched in 1975, made a new breakthrough and revealed yet more details about how water flowed on the surface and carved valleys. However, only in 2000, the first proof of liquid water on Mars was discovered.
It was claimed the gullies seen on the surface of Mars had to have been formed by flowing water.
NASA's Phoenix Mars lander confirmed the presence of water ice on Mars on July 31, 2008. After testing the ice water, it was confirmed that it contains the same elements as the water we have on Earth.
Mars features several ancient dried out valleys and river channels that have long pointed towards the possibility of liquid water.
NASA's Perseverance is currently trekking across Mars to explore Jezero Crater, which is believed to have been a lake filled with water 3.5 billion years ago.
However, the running theory has been that the majority of the water had escaped from the atmosphere due to Mars' low gravity – until now.
A recent study, funded by NASA, suggests that between 30 to 99% of the "missing" water is lurking within minerals inside the Red Planet's crust.
Researchers developed a computer simulation that showed how liquid was lost from the planet over time, using data from satellites orbiting and rovers on the Martian world.
According to several research materials and scientists, Mars' current rocky landscape formed three to four billion years ago from volcanic activity. Unlike Earth, which recycles its crust, releasing trapped water, the Red Planet's rocks are old enough to hold on to a vast amount of water. On Earth, old crust continually melts into the mantle, forming a new crust at plate boundaries. This recycles water and other molecules back into the atmosphere through volcanism. On Mars, there are no tectonic plates, so the "drying" of the surface is permanent.
The lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington is Michael Meyer. He said: "The hydrated materials on our own planet are being continually recycled through plate tectonics. Because we have measurements from multiple spacecraft, we can see that Mars doesn't recycle, and so water is now locked up in the crust or been lost to space".
Caltech PhD candidate, Eva Scheller, said: "Atmospheric escape doesn't fully explain the data that we have for how much water actually once existed on Mars".
Along with data from rovers on Mars and satellites orbiting the planet, scientists have finally solved the missing water mystery by analyzing data archived in NASA's Planetary Data System.
The team also collected previous work conducted with meteorites that travelled through space and landed on Earth.
The research further studied the water on the Red Planet in all forms (vapour, liquid, and ice) and the chemical composition of the planet's current atmosphere and crust. They looked specifically at the ratio of deuterium to hydrogen (D/H).
NASA revealed in a statement that: "While water is made up of hydrogen and oxygen, not all hydrogen atoms are created equal".
The vast majority of hydrogen atoms have just one proton within the atomic nucleus, while a tiny fraction, about 0.02%, exists as deuterium. The latter is slightly heavier than hydrogen – it includes an extra neutron in the nucleus. And deuterium is also less likely to escape into space due to its density. If it did escape, the Red Planet's upper atmosphere would contain a larger amount of deuterium.
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.
The new study concluded that a majority of Mars' water was trapped in the crust, and some was lost in the atmosphere.
When water interacts with rock, chemical weathering forms clays and other hydrous minerals that contain water as part of their mineral structure.