Apr 10, 2017
Exo-Earths With Global Magnetic Fields Most Likely To Have Life
The most recent results from NASA's MAVEN Mars orbiter is a case study in why global magnetic fields are crucial to preserving planetary atmospheres and protecting such life-giving environments from the ravages of their parent stars'particle outflows. Like raging Piranha, solar wind particles, composed mostly of protons and electrons, apparently can and do strip many rocky planets of most of their atmospheres.
As the MAVEN (Mars Atmosphere and Volatile Evolution) mission recently revealed, by some 3.5 billion years ago, our Sun's solar wind had stripped the Red Planet of as much as 80% to 90% of its atmosphere. Earth was much luckier. But for astrobiologists, the $64,000 question remains: How rare is Earth's own robust global magnetic field? And how well do such magnetic fields hold up, particular when on close orbits around active Red dwarf stars?
Up until some 4 billion years ago, Mars apparently had a robust global magnetic field that coincided with the presence of water on its surface.
"We think loss of a global magnetic field that allowed the solar-wind stripping of the Mars atmosphere beginning around 4.1 billion years ago when the field turned off," Bruce Jakosky, MAVEN's Principal Investigator, told me. "Most of the action would have been completed within a few hundred million years."
The lesson for understanding rocky exo-planets, says Jakosky, appears to be that if there's no magnetic field, you can expect relatively rapid loss of atmosphere with obvious implications for habitability.
However, the amount of such atmospheric stripping would also depend on the intensity of the star and stellar wind over time, the size of the planet, and to a lesser degree the composition of the atmosphere, Dave Brain, a planetary scientist at the University of Colorado in Boulder, told me.
"Some exo-planets are subjected to such harsh stellar environments that scientists have calculated that they should not be able to retain an atmosphere at all – whether or not the planet has a magnetic field," said Brain.