Until now, Earth was the only planet known to have vast reservoirs of water in its interior.
But now scientists from the Carnegie Institution for Science and the University of New Mexico have found that Mars has a similar amount of water as our own planet tucked away inside.
The revelation came from analysing the water content of two Martian meteorites originating from inside the Red Planet – and further raises the possibility that Mars could have sustained life.
Water revelation: Mars's interior is more similar to Earth's than we thought
The scientists estimated that the Martian mantle source — the layer under the crust — from which the rocks were derived contained between 70 and 300 parts per million (ppm) water.
For comparison, the upper mantle on Earth contains approximately 50-300ppm water.
Carnegie’s Erik Hauri said: “We analysed two meteorites that had very different processing histories. One had undergone considerable mixing with other elements during its formation, while the other had not.
“We analysed the water content of the mineral apatite and found there was little difference between the two even though the chemistry of trace elements was markedly different.
“The results suggest that water was incorporated during the formation of Mars and that the planet was able to store water in its interior during the planet's differentiation."
He added: “There has been substantial evidence for the presence of liquid water at the Martian surface for some time.
"So it's been puzzling why previous estimates for the planet's interior have been so dry. This new research makes sense and suggests that volcanoes may have been the primary vehicle for getting water to the surface."
The scientists analysed what are called shergottite meteorites.
These are fairly young meteorites that originated by partial melting of the Martian mantle and crystallised in the shallow subsurface and on the surface.
They came to Earth when ejected from Mars approximately 2.5m years ago.
Mars lies on average around 140 million miles from Earth - and is considerably smaller, and colder.
Its diameter, at 4,222 miles, is just over half that of Earth's, while its average temperature is a distinctly chilly -85F (-65C).
The environment differs greatly in another way, too, as the Red Planet's atmosphere is 95 per cent carbon dioxide and less than one per cent of the pressure of Earth's.
The most dramatic aspect of our cosmic neighbour's terrain, meanwhile, is Olympus Mons - the biggest mountain on any planet. It's an inactive volcano that stands 88,000-feet high.
Mars's main similarity to Earth is in its rotation - a Martian day is just 37 minutes longer.
Results of the study are published in Geology.