Mars is a source of deep fascination for us here on Earth. The two planets are similar in size and terrain, and so the thought that the red planet could also hold life has captured the human imagination for ages.

Life on Earth depends on water, so for more than a century scientists have searched for signs of water on Mars. Today, several missions are exploring the terrain, targetting evidence of water among other goals. So far, extensive mapping shows that there is none, and that the arid planet is dry, dusty and cold. But some believe there is still water beneath the surface, and even if not, there is reason to believe the planet once boasted H₂O.

So, where did the water on Mars go? Recent research suggests that its disappearance is, like so many things in science, complicated.

Was There Once Liquid Water on Mars?

A watery history for Mars does seem to exist. Space surveys performed by orbiting spacecraft and surface rovers show that certain Martian features were probably formed by liquid water flowing across the surface. There’s even evidence that some of it remains as a deep subglacial lake of saltwater under the Martian South Pole. Deep surface penetration scanning showed the salty deposits, which have also been identified in several other areas. However, Nature notes that these will probably not exist as liquid water on Mars and are more likely to be slush or sludge due to the extreme cold.

Topographical analysis and comparisons with similar formations on Earth have also pointed to there being oceans in Mars’s distant past. Nova notes that researchers found a 30-kilometer difference in height between the northern and southern hemispheres of Mars. Their explanation was that this step is evidence of an ocean covering some of the planet, since a similar step appears on Earth.

Not Always Too Cold for Liquid Water on Mars

Our planet is very much a blue planet, contrasting with the dry and arid environment on Mars. One reason for this is that Earth is warmed by the sun, so our water exists in liquid form. Inverse notes that Mars is just too cold for liquid water, but this may not have always been so.

The National Oceanic and Atmospheric Administration weather forecast for Mars gives the average daily temperatures there as -81 degrees F (-63 degrees C). A paper published in the Proceedings of the National Academy of Sciences suggests that clouds could have contributed to planetary warming and thus supported pools of surface water. Adapting terrestrial computer modelling, the scientists determined (through a cloud greenhouse hypothesis) that a warm arid climate might have existed on Mars, which could have raised temperatures despite the planet only receiving about 30% of the sun power hitting us here on Earth.

Where Did the Water on Mars Go?

Despite evidence of a watery past for the planet, Mars today is dry and dusty. Any water it contained in the past is now only suggested through landscape features. Caltech describes how an orbital survey by NASA’s Odyssey mission observed large salt deposits on the Martian surface, showing that water existed in large surface ponds more than a billion years longer than previously believed.

There are various theories to explain why Mars today is dry and contains no or very little water in liquid form. One theory, described by NPR, is that the planet is simply too small. Since gravity is a factor of mass and density, the size of Mars does not create enough gravity to keep hold of its water. The significance of this for space exploration is in exoplanet research, as hunting for Earth-like planets that might support life must factor in size and gravity in assessing the likelihood of life being present.

However, water evaporating into space doesn’t account for all of the water lost on Mars. Some scientists posit that’s because some of it is still there. Caltech describes how water could remain on the surface of Mars, just not as a liquid. Scientific American describes this as “crustal hydration,” in which water is effectively incorporated into minerals. When Mars inexplicably lost its magnetic field and most of its atmosphere, the surface became more exposed to the sun. Water simply became trapped within minerals in the Martian crust and may remain there to this day.

While the red Mars of today is arid and dusty, it likely once held expansive oceans. More research into the ancient waters of Mars and how the red planet lost its oceans will likely lead to even more profound insights in the years to come.

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