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How Mars’s Magnetic Field Let Its Atmosphere Slip Away

Accepted submission by hubie at 2020-04-03 18:24:55 from the Slip-Slidin'-Away dept.
Science

One of the primary drivers for the distinct lack of a Martian atmosphere is believed to be the loss of atmospheric molecules that are stripped away by the passing solar wind. A strong planetary magnetic field would divert the solar wind around the planet and protect the Martian atmosphere. The MAVEN [nasa.gov] spacecraft confirmed [soylentnews.org] this is happening [soylentnews.org], and some interesting ideas have been floated [soylentnews.org] to mitigate the effect.

A recent paper published in JGR: Space Physics [wiley.com] used Magnetohydrodynamic (MHD) models to investigate the magnitude of the effect on atmospheric retention that a magnetic field would have on Mars and found a very interesting result [eos.org]. Their models confirmed that the rate of atmospheric loss was six times higher for a planet with no magnetic field compared to a planet with a strong magnetic field; however, they found the highest rate of atmospheric loss was actually when there was a weak magnetic field.

But the highest rate of atmospheric ion loss was with a weak magnetic field—6 times faster than with no magnetic field at all. The team found the reason was the magnetic field lines, which guide the motion of charged particles, were easily blown back by the solar wind, creating a path for these ions to escape into space above Mars’s nightside. This means that instead of providing a small measure of protection, Mars’s remnant magnetic field could actually have sped the planet’s transformation into the cold, barren world it is today.

Research Paper: Sakata, et al., Effects of an Intrinsic Magnetic Field on Ion Loss From Ancient Mars Based on Multispecies MHD Simulations, Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA026945 [doi.org], 2020

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Author's Plain Language Summary:

It has been suggested that ancient Mars had an atmosphere thick enough to sustain liquid water on its surface, while present Mars only has a thin atmosphere. Ion loss to space is one of the important processes for the removal of the atmosphere because a younger Mars would have been exposed to much stronger solar activity. Over 4 Ga, Mars had an intrinsic magnetic field like that of the Earth. The existence of an intrinsic magnetic field changes the electromagnetic environment around the planet and affects the ion loss. We investigate the ion loss from Mars at approximately 4.5 Ga, assuming both the strong solar conditions and the existence of an intrinsic magnetic field using numerical simulations. The results show that the existence of the weak dipole field increases the loss of molecular ions such as O2+ and CO2+. Contrary to the weak intrinsic magnetic field, however, a strong intrinsic magnetic field substantially decreases the loss of molecular ions. The ion loss processes are also affected by the intrinsic magnetic field. These effects of the intrinsic magnetic field are less pronounced for O+ loss because of the extended O+ corona.


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