The HiRISE (High-Resolution Imaging Science Experiment) camera on the Mars Reconnaissance Orbiter has captured another beauty. This time the image shows water ice peeking out from a cliffside on Mars. A layer of sediment obscures most of the ice, but fingers of it are visible.

Mars likely had ancient oceans, and the remnants of all that water are hidden as ice. It’s mostly buried in the planet’s crust. In this image, it’s under a thick layer of sediment. The image is from Mars’ Milankovic Crater, a prominent impact crater that sits alone to the north of Olympus Mons, Mars’ tallest volcano, and the tallest volcano in the Solar System.

Mars’ ancient oceans were turned to ice when Mars lost its atmosphere between about 3.7 billion to 4.2 billion years ago. The water now exists mostly as subsurface ice. A 2018 study found evidence of a complex of liquid saltwater lakes under the south polar region, which generated a lot of excitement. In 2019, researchers proposed that magma activity in the preceding one million years created enough heat to maintain that water in liquid form. Then in 2021, another study pointed out that the discovery of the subglacial polar lakes could be explained by other phenomena.

The existence of subglacial lakes of water on Mars will likely remain controversial for a long time. But the existence of subsurface water ice isn’t controversial. We’ve seen it.

Mars’ water exists as ice, locked into the planet’s crust at varying depths, except for the possibility of liquid water heated by magma existing under the polar region. Scientists think that there are at least 5 million cubic kilometres of ice underground, with even more at depths beyond the capabilities of our current remote sensing instruments. Some of that ice is visible in the HiRISE image, peeking out from under a layer of sediment.

The leading HiRISE image above is an infrared-red-blue image that highlights the presence of the ice. The RGB image below is more representative of what human eyes would see.

We’ve improved our understanding of Martian water dramatically in recent years. A 2021 study showed that between 30% and 90% of Mars’ original water may be frozen under the surface, with large deposits in the Arcadia Planitia region. In 2019 NASA made a map of Martian water across the planet’s surface. NASA said that some of the water is only 30 cm (12 inches) deep, making it easily accessible to future explorers.

It’s clear that humanity is reaching out to Mars. With our orbiters, landers, and rovers, we’re piecing together the planet’s history. Mars was once wet and warm and may have harboured life. Future sample-return missions might confirm the presence of microbial fossils. That would be a huge discovery, worthy of all the fanfare it would no doubt generate.

But we want to set foot on Mars, sometime, somehow. And when that happens, our explorers will know where to find water.

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