How Did All the Liquid Water on Mars Disappear?

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley Ph.D., John Hopkins University

Liquid water on Mars was not at all rare some billions of years ago. Mars used to have a gigantic northern ocean and still has the layered rocks on its surface. There are hydrated minerals, debris and sediments that were moved around by mega-tsunamis, and clear shorelines. The water was too much to just evaporate, but still, there is nothing of it left today. Why?

Extremely detailed and realistic high resolution 3D illustration of a terraformed Mars like planet.
Mars used to have enormous amounts of water, but it was ‘blown away’ by the solar wind and impactors. (Image: Vadim Sadovski/Shutterstock)

Heavy Water on Mars

What happened to the giant ocean covering 36% of Mars’s surface? What happened to all the lakes, waterfalls, and rivers? Some water is concentrated at the poles as ice caps. There might be a big lake under the southern ice cap. Nevertheless, they in no way amount to the past water on Mars.

The ratio of heavy water to ordinary water, also known as the D (deuterium) to H (hydrogen) isotopic ratio is important on Mars. Deuterium is a heavy isotope of hydrogen that contains a neutron in addition to the regular proton contained in hydrogen.

Neutrons are as heavy as a proton but have no electric charge. Thus, a hydrogen atom with an extra neutron weighs twice as much as normal hydrogen. Some water molecules have deuterium and are heavier than the others, but on Mars, there was more deuterium than there should have been. Thus, more deuterium was retained as the hydrogen was lost in the atmosphere, due to the solar wind.

Frozen water on Mars.
There is still water ice on Mars, but evidence shows the heavy water was always too much on the planet. (Image: Flashinmirror/Shutterstock)

Slowly, more hydrogen escaped Mars, and more deuterium remained. Comparing the D-to-H ratio in meteorites over four billion years old shows that Mars lost about of its initial water in the first 400 million years. The amount of water was equivalent to around 40 to 100 meters of a global ocean on Mars. It has lost another ten to 50 meters since 4.1 billion years ago. Mars used to have six times the water it has now, 4.5 billion years ago. Did that not affect the atmosphere?

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

Past Atmosphere of Mars

Mars must have had a thicker atmosphere and high-enough pressure to retain surface liquid water. Today, it has a very thin atmosphere with extremely low surface pressure. Mars is a small planet with a weak gravity, and the high-energy particles of the solar wind could basically blow away its atmosphere.

Where did the thick atmosphere originate from? It’s possible that Mars was able to sustain a thick atmosphere in its early history because it had an important shield: a global-scale magnetic field generated by a planetary dynamo. Some of the older rocks on Mars are still magnetized because they have trapped the magnetic field in their location when they were cooling down as lava. The magnetic field has slowly been destroyed, but was that the only reason that atmosphere faded away?

Large-Scale Impacts

Mars is full of craters from impact events. Utopia Planitia and Hellas Planitia were created by large impacts. Utopia Planitia is inside an even larger basin, covering 40% of the planet. If the 40% basin was also created by an impactor, it could have been big enough to destroy the atmosphere. Some even believe that a large impact might have shut down Mars’s dynamo by generating so much heat that the core is stratified in a way that stops convective motions that Mars needs for a dynamo.

With all the conditions for life, did Mars ever host life?

Learn more about a solar system time machine and meteorites.

Past Life on Mars

An illustration showing a base on Mars.
If astronauts decide to go to Mars, they should be able to breathe, grow food, and stay shielded from the solar particles. (Image: SergeyDV/Shutterstock)

In 1996, the Martian meteorite called Allan Hills 84001 had tubular features that looked like microfossils. However, the tubular features could have formed without the presence of life. This does not wipe out the theory of microfossils, but it sure weakens it.

Another possibility of life was in the saltwater. Saltwater can absorb more oxygen and remain liquid in lower temperatures. The oxygen could have been enough for microbes unless the seasonal flows hypothesized to be evidence for saltwater were sand flows.

The next hint to life is methane. On Earth, the atmospheric methane is a by-product of life. In 2009, methane signatures were seen from Earth, but in 2013, the Mars Curiosity rover found spikes in methane concentrations. Methane could have come from non-biological sources or meteor showers, but also might be from microbes living in the Mars subsurface.

Learn more about what the biggest exoplanets reveal.

Trips to Mars

The nine-month trip to Mars is the simplest step. First, astronauts need shielding from radiation during the trip. Second, they should breathe on Mars and be able to turn CO2 into oxygen with some equipment. Third, growing food in the toxic levels of chlorine compound called perchlorate, less light, more ultraviolet radiation, low temperatures, and less water is a challenge.

The lack of a magnetic field and an ozone layer is also a problem that might keep astronauts underground. Lastly, we might contaminate Mars and threaten life there, or vice versa. The Office of Planetary Protection in NASA tries to take care of this last point, but the overall trip is much harder than depicted in science fiction movies.

Common Questions about Water on Mars

Q: Can there be liquid water on Mars?

There is perhaps a lake of liquid water on Mars, under its southern polar ice cap. There is also seasonal water flow at the edges of some craters, but that water must be very salty not to freeze on the surface.

Q: Was there ever water on Mars?

Yes. Mars used to be covered in vast areas by surface water. There was an ocean in the northern hemisphere, and there are still hydrated rocks that show evidence of past water on Mars.

Q: Was there ever life on Mars?

There were two Martian rocks that contained something like microfossils. According to the evidence of past water on Mars, there might have been life as the planet was warmer and had a higher surface pressure. However, there is no certain evidence yet.

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