The red color of Mars prompted it to be named after the Roman God of war. For centuries, people have conjectured about life on Mars and the idea of Martians coming to Earth was staple for science fiction from H.G. Wells to Olaf Stapledon to the pulp comics and B movies. In 1976, NASA’s Viking mission sent back the first set of pictures from the surface of Mars. Many fantasies were crushed when these photographs revealed a cold and barren desert.
Some water was present in the frozen polar ice caps. But Mars was too cold to support liquid water.
On the Earth, the heat from the sun gets reflected from the surface and then doesn’t escape back into space, but gets trapped by the thick atmosphere containing argon, water vapor and carbon dioxide. This is the greenhouse effect (which is currently going into overdrive as more CO2 is pumped into the air). On Mars, the atmosphere is too thin to trap enough heat to melt water.
But close examination revealed some strange features. Many Martian features looked suspiciously like dried river beds. Further, there were minerals called hydrates which form in the presence of liquid water. All this led scientists to believe that billions of years ago, Mars had flowing liquid water. This meant it must have also had a thick atmosphere that could trap enough heat to raise the temperature beyond the melting point of water.
Ice at a Martian pole
So what happened?
NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has recently shown that the solar wind could have been one of the main responsible forces. The so called solar wind consists mainly of protons and electrons traveling at about a million miles per hour. These are ejected by the sun from the upper layers of its atmosphere.
Now if you remember your high school physics, you know that protons and electrons carry an electric charge. A moving charge means a current. A current means a magnetic field and a changing magnetic field means an electric field. That is what happened around Mars where the solar wind created a massive electric field. This field gave energy to the charged gas molecules in the Martian atmosphere till they gathered enough energy to escape the effect of Mars’ gravity.
MAVEN measurements indicate that the solar wind continues to strip the Martian atmosphere at a rate of 100gms/second. But this increases radically during solar storms.
But why hasn’t this solar wind affected Earth’s atmosphere? After all, being closer to the Sun, the Earth receives a much stronger dose of the solar wind. The reason is that the Earth produces a powerful magnetic field of its own. This drives away most of the solar wind particles coming from the Sun. This is the same magnetic field we observe when a compass needle points north. The root cause of this field are the powerful flows of magnetic material in the Earth’s core.
Earth’s Magnetic field deflects the solar wind
In contrast, Mars has a very peculiar magnetic field. For starters, it is 40 times weaker than Earth’s. For another, note in the picture below how the magnetic field seems concentrated in the southern hemispheric region, being relatively weak in the northern hemisphere.
Magnetic Map of Mars
There are many ideas about what could have caused this. Some scientists have speculated that at some point in its history, Mars would have been struck a glancing blow by something the size of Pluto. This caused its internal magnetic field producing dynamo to shut down and produce this strange magnetic map.
With its magnetic field stopped, there was nothing to stop the solar wind from stripping away the atmosphere from its northern hemisphere.