JOIN USWhy magnetic shield matters
It was suggested at a recent workshop of the Planetary Science Division of NASA by Jim Green, Director of Planetary Science, NASA, and others that a giant magnetic dipole shield, positioned at Mars L1 (Lagrangian point), be deployed to form a protective magnetic shield around Mars to shelter its atmosphere from the solar wind.
This could presumably make the red planet habitable for possible future generations of colonisers from Earth. There are already plans to launch manned flights to Mars within a decade.
The solar wind is a continuous stream of high energy particles (mainly protons) spewed out by the Sun’s turbulent atmosphere. On a planet without a magnetic field like Mars, these charged particles from the solar wind can slowly strip the atmosphere. It would also break up any water vapour present into constituent atoms, besides causing damage to biological tissues including the DNA. As a result, this reduces the possibility of life evolving in the planet. The Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft has recently confirmed that the solar wind continues to strip the gas into space and this includes heavier gases like argon. On Earth, argon constitutes about one per cent of the atmosphere (this is actually 30 times more than the carbon dioxide present). Thus, the Martian atmosphere was supposedly much thicker a few billion years ago and could have supported life then.
At that epoch, liquid water could have been stable on the Martian surface and there is evidence for this from features that look like dry riverbeds and minerals that only form in the presence of liquid water. At present, the Martian atmosphere is too thin and cold to support surface liquid water. Again, Mars being a much lighter planet (with one tenth of Earth’s mass) would have in any case lost its atmosphere sooner or later, but the absence of any magnetic field accelerates the process with the solar wind driving out even the heavier constituents away into space. The Moon also has no magnetic field because of which it is constantly bombarded by high energy particles from cosmic rays as also the solar wind, apart from hazards from meteorites which do not burn up.
Protection from radiation
Among the terrestrial planets, Earth fortunately has a substantial magnetic field. Though it is not strong (less than a tenth of a millitesla), it is enough to deflect high energy particles from the solar wind and galactic cosmic rays away from the surface. These deflected particles form radiation belts around Earth at the height of several tens of thousands of
kilometres.
The ozone layer protects Earth from Sun’s ultraviolet radiation, while the terrestrial magnetic field prevents the bulk of high-energy charged particles from doing any damage to the atmosphere and biosphere. The water vapour from the oceans survives without being broken up. An astronaut on Mars would have to be shielded from these high energy cosmic rays and solar wind protons which would pepper the surface in the absence of a magnetic field. There could be long-term effects due to this radiation exposure.
This is why the suggestion has been made to create an artificial giant magnetic shield around Mars, in the interests of future colonising manned missions. It has been proposed that large inflatable structures can generate a magnetic
dipole field of about one tesla (ten thousand gauss).
Such a structure is placed at the Mars Lagrangian point (this is the region where the gravitational fields from Mars and the Sun on a spacecraft are comparable). This Lagrangian point is located a million kilometres from Mars. It can be calculated that one tesla field generated at this point could provide about one gauss strength magnetic field at Mars. So, the positioning of such a magnetic shield could ensure that the regions where most of the Martian atmosphere is lost would be protected. This artificial magnetosphere would be subject to simulated testing.
This brings us to the question of planetary magnetic fields in general. The Earth has a partially liquid iron core with a fairly fast rotation velocity of one kilometre a second and this could generate a magnetic field. As Mars and Moon have a much lower density, they do not have such massive iron cores. Jupiter and Saturn are fast rotators and have larger magnetic fields than Earth. So they have extensive magnetospheres, and some of Jupiter’s moons like Io are immersed in this field leading to a whole range of interesting phenomena.
We know much less about Earth’s magnetic field than Sun’s field. We know that the Sun’s magnetic field reverses its polarity and its association with solar activity every 22 years. The Earth’s magnetic field is also known to reverse but the time scale is not known. There is evidence from paleomagnetism in ancient rocks that the reversal time scale could be very long, perhaps millions of years.
It is not clear whether the transition period between reversals could have any effect on living systems. The presence of a magnetic field and an ozone layer makes the environment more conducive for life on Earth. This was also the reason why a large ozone hole in the Antarctic was viewed with much concern. Apart from protecting terrestrial life from hostile particle radiation, Earth’s magnetic field has also been useful to various species in other ways — like its role in the accurate circumnavigation of migratory birds.
Magnetic fields in exoplanets?
The recent discovery of several exoplanets around low luminosity red dwarfs, some of them with atmosphere, has again raised the question of magnetic fields. Although atmospheres have been spotted around some of the exoplanets, whether they have magnetic fields is yet to be answered definitely.
Many of these red dwarfs are known to be flare stars, and their stellar flares could be hundreds or even thousands of times stronger than our solar flares. So, their effects on the planets orbiting close to these stars would be significant. Moreover, these stars have the so called convective atmospheres and spots having large magnetic fields (larger than sunspots).
The Sun, fortunately, does not vary much in brightness (intensity varies about 0.1%). However, these low mass red dwarfs could have very intense flares and larger variations in luminosity. They are the most common types of stars in our galaxy (constituting about 90%). To sustain life, and to retain their atmosphere and water for long, these planets should have a substantial magnetic field. At least in the case of Earth, our planet’s magnetic field that sustained for over billions of years seems to have contributed to the evolution of life.