By Betty Robinson, bettyrrobinson.ca
The geology on Mars definitely shows evidence that Mars likely supported liquid water in the past. Scientists know that the early Martian atmosphere was thick, which allowed the planet to keep warm enough to allow liquid water. But scientists also know that once Mars lost its atmosphere, the liquid water took a while to disappear. Why? This question is called the Mars climate puzzle. (Of course, the bigger question is, was there life, because water on Earth means life, but that’s for another blog posting if any when any evidence comes in.)
The surface of Mars shows clear evidence of previously flowing water. Shown here are possible clay beds in West Ladon Valles Channels, Mars. Credit: NASA/JPL/University of Arizona
Liquid water on Mars was directly dependent on Mars’s early atmosphere and indirectly dependent on its early, global magnetic field.
About 4 billion years ago, Mars’s atmosphere was much thicker than it is now. And it contained much more carbon dioxide (and other gases). Carbon dioxide is a greenhouse gas, which helps warm the planet. A warm planet can support liquid water (as long as other necessary conditions are also supported).
Scientists theorize that our and Mars’s magnetic fields formed in the same way: Oversimplifying, our own magnetic field is generated as a result of the dynamo effect: the hot, liquid outer core moves around the hot, solid inner core. The movement is opposite in direction to the direction of Earth’s rotation. The movement generates a magnetic field.
Mars is smaller and less dense than Earth, so Mars cooled faster. There is still some question regarding the composition of Mars’s core. But many think that the core has cooled enough that it can no longer generate a magnetic field. So, Mars’s magnetic field disappeared about 4 billion years ago, give or take half a billion years.
Around that same time, the Sun was young, and the solar wind (made up of charged particles) was more intense. Without the magnetic field to deflect the charged particles and protect the planet, the solar wind started stripping the atmospheric particles away, and Mars’s atmosphere started to disappear into space.
So no magnetic field led to no atmosphere. No atmosphere led to no liquid water, eventually.
So, the atmosphere started to thin and disappear, but the water kept flowing and didn’t dry up right away. Why? Why didn’t all the water disappear when the atmosphere disappeared?
According to studies by two independent teams, the clue may be hydrogen. A young Mars was volcanically active, so volcanoes would have spewed a lot of hydrogen into the atmosphere.
One study was in 2018 by a team led by Paul Godin, who at the time was a York University Postdoctoral Fellow. He is currently a senior technologist at the University of Waterloo. Dr. Godin used an instrument at the Canadian Light Source*, Saskatoon, to test a theory based on collision-induced absorption. Molecules have their own absorption properties. Sometimes, two molecules in the atmosphere collide and produce a supermolecule. The supermolecule has its own absorption properties. The theory is that enough of these supermolecules in the thinning Martian atmosphere could have absorbed enough heat to keep the planet warm in order that the liquid water could remain for a while. So maybe the atmosphere didn’t have to be super thick if there were lots of these supermolecules to absorb heat and keep Mars warm.
The team used an instrument called a White cell to bounce light around within a gas, to measure the gas’s absorption properties. They found that supermolecules made of carbon dioxide and hydrogen are weak but could be strong enough to make a difference.
A second team used data obtained by Curiosity in Gale Crater. The full results are reported in a paper in PGR: Planets: Navarro-González et al., published in 2018. In this study, Navarro-González et al. found that rock samples analyzed by Curiosity contained fixed nitrogen**. On Earth, bacteria fix nitrogen. But there are no plants on Mars, so physical processes with a lot of kinetic energy, such as lightning and shock waves from asteroid impacts, likely fixed the nitrogen in the rocks. However, this fixing process can only happen in an atmosphere that is loaded with hydrogen.
In other samples analyzed by Curiosity—younger samples—they found that the amount of nitrogen decreased significantly, suggesting that once the hydrogen was gone, the planet could no longer support liquid water.
So maybe hydrogen is the key to understanding why liquid water remained on Mars after the planet lost its atmosphere.
* Canadian Light Source: The Canadian Light Source is Canada’s only synchrotron. It is located in Saskatoon, Saskatchewan (www.lightsource.ca/). A synchrotron accelerates electrons to near the speed of light. As the electrons accelerate while changing direction (they move in a circle), they emit a bright light. The light is used to study various samples of materials at the molecular level. The facility has other instruments, such as the White cell mentioned in this piece.
** Nitrogen fixation: Nitrogen fixation is the process of combining nitrogen from the air with another element or elements to form a different form of nitrogen, such as ammonia (NH3). Most nitrogen fixation is done by bacteria, but ultraviolet light and other physical processes such as lightning can also fix nitrogen.
The bacteria incorporate nitrogen from the air into compounds that they can use. Credit: Nefronus – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=80370564