Creating an artificial magnetosphere on Mars

 I've talked before about the need to create an artificial magnetosphere on Mars, and how future colonists on Mars might do it.  This paper in Science Direct, by 5 physicists, considers a number of different options, discussing the engineering and the cost.

WHY DO WE NEED A MAGNETOSPHERE ON MARS?

The Earth's magnetic field that originates within the iron core from a dynamo process, encompasses the planet and extends out into the near space environment (see Fig. 1). The magnetic field helps to reduce the radiation reaching the surface by re-directing and shielding large numbers of energetic solar particles that would otherwise create a radiation hazard to life. Another important benefit of the Earth's magnetic field is that it inhibits the loss of atmospheric molecules from pick-up by the solar wind during large solar superstorms [[1], [2], [3]]. Increasing Mars atmospheric pressure has been proposed as one of the primary requirements in terraforming Mars, along with warming and altering the atmospheric composition (e.g. Refs. [[4], [5], [6], [7], [8], [9], [10], [11]]). The aim is to achieve a stable ecosystem or ‘ecopoiesis’ [[12], [13], [14]]. But recent studies suggest that these efforts would be undone by a combination of processes driven by extreme ultraviolet light and solar wind from the Sun, removing atmospheric gases from the upper atmosphere to space [[15], [16], [17], [18]]. Refs. [[19], [20], [21], [22]] suggest that the presence of a strong intrinsic global magnetic field substantially decreases the loss of molecular ions and alters atmospheric conditions.

Fig. 1. An artistic impression of magnetosphere around Mars formed by a magnetic dipole field from an artificial ring or loop of electrical current circulating around the planet. The approximate point at which the pressures balance is the stand-off distance Rₛ

In contrast smaller, sub-global magnetic fields such might be considered for a small surface colony offer a mixed benefit. The evidence from observations and simulations of the patches of crustal magnetic field [23,24] that naturally occur already on Mars show that the presence of these anomalies can aid ion loss as much as they might hinder at other times depending upon the orientation of the field and interplanetary environment (e.g. Refs. [[25], [26], [27]]).

Mars is about half the size of the Earth and has a much lower atmosphere density. This therefore makes atmospheric losses much more significant. Terraforming activities designed to build up the atmospheric pressure and alter its composition on Mars will not want this effort to be undone by the first significant solar superstorm to reach the planet. One of the first goals of terraforming will be to increase the atmospheric pressure above the Armstrong Limit (6.3 kPa) [=63 mb; Mars's average air pressure is 6 mb;Earth's 1013 mb], a threshold that removes the requirements of having to wear a full-body pressure suit, although oxygen will still be needed [28]. Below the Armstrong atmospheric pressure limit, water in the lungs, eyes and saliva spontaneously boils [29]. Changing the atmospheric pressure can be expected to have wide ranging consequences to many aspects of living and working on Mars including amongst others to weather patterns, dust storms and transportation to name but a few. Primarily though, a global magnetic field generated magnetosphere, Mars could weather the worst of the atmospheric stripping effects of large solar events and help provide protection from radiation particles.

The past several years has seen an increase in the number of serious scientific investigations of many diverse aspects related to manned exploration of Mars and colonization. These include potential missions, interplanetary vehicles, Mars transportation vehicles, habitats but also socioeconomic concerns (e.g. Refs. [[30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]]). This indicates that the technology is becoming closer to achievable and affordable.

In this article we shall consider potential technological approaches to create an artificial magnetic field to protect Mars. We will not discuss the value or likelihood of humanity colonizing Mars, nor consider the relative merits or performance of magnetospheres, whether they are generated by magnetic fields or otherwise. Nor shall we present an analysis of the possible changes to Mars' atmosphere with and without a planetary magnetic field. Such atmospheric modelling requires dedicated articles and will depend on the choice of location of the magnetic field source - for instance below or entirely above the planet's atmosphere.

What will be presented are multiple options for technology approaches and locations for the magnetic field generating infrastructure along with their pros and cons. The assumption is made here that there is a desire to create a magnetic field similar to that of a natural magnetized planet like the Earth and then follow how this could be done from a purely fundamental perspective. This issue, of creating an artificial structure at unprecedented scale, has not been considered in a peer-reviewed journal before. The calculations of power, resources and other relevant parameters are all deliberately made only to first order, as higher precision figures would be meaningless without a comparable level of precision for the engineering. This can be undertaken later. However, before any more detailed engineering design can be proposed there must first be an evaluation of the benefits and limitations of the different approaches and a choice of principle made. The aim here therefore is to discuss and compare the methods and to finally propose a novel solution.

The technological options we will consider include: re-starting the planet's iron core, using solid state permanent magnets in either continuous loop or a series of discrete magnets, the use of solid state superconductors or a plasma current loop similar to a current driven plasma torus of an artificial plasmasphere. We shall also consider some of the factors concerning the source location of these generated magnetic fields. Within this analysis, we shall outline the issues and concerns that define the design such as general mass and electrical current needs. Specific timescales and logistics of installation will not be considered here, as it is anticipated that terraforming Mars will be a worldwide and multi-century endeavor and the potential for paradigm-changing developments would radically alter these. The one exception is the assumption of the development of successful nuclear fusion reactors [41] as an efficient energy generation option. Nuclear fusion is already an extensive international scientific and engineering program that is ever closer to being achieved [41]. Fusion power is a likely necessary enabler for considering substantive colonization and terraforming in general. Fusion based propulsion has been proposed as an important development for human planetary exploration [42], although at this time a successful economic fusion reactor has yet to be developed.

To form an artificial magnetosphere a magnetic field needs to be created artificially. There are several ways this might be done in principle. Fig. 2 shows the options for the different approaches to creating a suitable current loop. The options are:

(A)

dynamo circulation of a molten planetary core,

(B)

a continuous solid superconducting current loop or loop of permanent solid-state magnets,

(C)

a chain of discrete coupled current sources made of a controlled beam of charged particles forming an electrical current, and

(D)

a plasma torus of positively and negatively charged particles with artificial current drive forming a resultant current loop of a solenoid.

Fig. 2. Approaches to creating a magnetic field. The options for the different approaches to creating a current loop are; (A) molten iron core dynamo, (B) solid superconducting current loop or permanent magnets, (C) a chain of discrete coupled current or magnetic sources and (D) a current driven plasma torus.

[....]

SUMMARY AND CONCLUSIONS

If Mars is ever to be a long-term abode for human life, it will possibly need the protection of an artificially created magnetic magnetosphere of planetary dimensions. Earth's magnetosphere helps protect the planet from the potential sterilizing effects of cosmic rays and helps retain the atmosphere from significant stripping during large solar superstorms as they pass over the planet. Here we have shown some simple calculations exploring the basic physics and engineering of what would be needed practically to create a planet sized artificial magnetic field similar to Earth's. Clearly the resources needed would be vast. The purpose here has not been to examine the performance of a magnetic based magnetosphere at Mars, nor to justify the need for magnetic shield. Rather the intention here is to quantifiably explore the practical ways this might be done if humanity chose to do so and to make some estimate of the resources that would be involved. This is done for the first time in a scientific journal. This has deliberately been done to one significant figure precision as each approach presented would need a separate article to detail the level of technology development needed to justify more exacting figures. However, this first brush does allow a comparison of approaches and exploration of ideas.

No individual solution comes without vast technical challenges, many of which go beyond what can be described here. The primary challenge is not the intensity of magnetic field needed but the size of the required spatial dimensions. Evidence from Earth's magnetosphere is that the magnitude of the magnetic field intensity to hold back the solar wind is about ∼100 nT. However, to protect the whole of Mars this would need to be a continuous field over an absolute minimum area of ∼109km2 (the surface area of Mars assuming a 100 km atm). To allow for such a magnetosphere to persist during the interaction with the solar wind under all conditions, this would need to be very much larger.

Of the options considered here it is unlikely that restarting Mars' core will ever be a viable option. The problem is not just the minimum of 1011, 1 Megaton hydrogen bombs needed to be distributed about the iron core to melt it, but the uncertainty that the dynamo would even restart if this was done or how long any circulation would continue, as it is currently uncertain why Mars’ dynamo stopped in the first place - assuming Mars did once have a natural magnetic field arising from its core, like Earth.

Solenoid loops are the next option and there are a variety of potential locations and technologies. In terms of location these range from on the planet's surface, to stable orbits and co-incident with Mars' moons. With an artificial system the magnitude of the magnetic field at the source and the size of the structure creating it are available to be traded. What we have shown is the advantage that a wide radius (R0) solenoid loop provides not only in terms of lower magnetic field requirements at the coil surface (which would be safer to work and live around) but that the reduction in the rate of decrease of magnetic field with distance is less by R03 making it much more efficient at covering a wider area than a small radius loop with higher field. A wide diameter current loop requires a larger physical structure to be built in space, however.

The magnetic field generating structures could be made of superconducting materials or permeant magnets both of which minimize operating power but have the disadvantage of being heavy and made from rare minerals. Alternatively, carbon nanotubes offer a potentially lighter conducting structure but are fragile and have a finite resistivity requiring continuous power and therefore power losses to overcome.

We have shown that the currents required are between ∼0.2–0.5 GAmps in one or many solenoid loops. Whilst the power requirement will depend upon the material used, it can be estimated to be between 0.1 and 100 GW which is between less than one but up to 50 typical 2 GW power stations. While not trivial this is not unimaginably large, especially if controlled nuclear fusion has been successfully developed as an efficient energy source in the future.

One final approach to reduce the mass burden is to use a beamed plasma current rather than any form of solid conductor. In this scenario the current traverses the vacuum of space. To do this, the charged particles making up the current need to be accelerated to velocities where the interaction with the surround plasma environment is not sufficient to disrupt the loop of current. Exceeding the runaway limit would mean the particles would not be prone to ‘pick-up’ from solar storms. Once the magnetic field is established, the plasma current channel would reside in the relative protection of its own magnetosphere. Evidence from natural planetary observations is that even non-current driven, non-runaway plasma torii around planets (like the radiation belts) are not totally eroded by solar wind particle pick-up, though losses are inevitable. It would undoubtably be necessary to direct and replenish the plasma current via a series of aligned space stations. However, depending upon its size and location, such a relativistic particle beam could be a potential radiation hazard to transiting spacecraft. So, an alternative would be to ‘mass load’ the plasma loop by accelerating the particles to a lower non-hazardous velocity but with overwhelming much larger number density. This could be done by evaporating matter from Phobos or Deimos, ionizing it and using electromagnetic current drive techniques to accelerate the resulting charged particles. The closest natural phenomena to this without the current drive, is the plasma torus created in Io's orbit around Jupiter. What Io's example shows is that a high Z plasma loop around a planet can form and persist (although in Io's case the enormous magnetic field of Jupiter amongst other factors will help confinement). We have shown that, for Mars, the mass needed would not substantially erode the moons at approximately ∼15 kg per orbit per loop. Using higher Z ions to form the torus will aid retention.

In conclusion, as anticipated the resources needed to create a planetary sized magnetic field are non-trivial and there is much further research to be done. What has been presented here are some unique solutions for the approaches required to create an artificial planetary sized magnetic field.

Whilst the ideas presented here are at the scale of a planet like Mars, the principles are equally applicable to smaller scale unmagnetized objects like manned spacecraft, space stations or moon bases, creating protective ‘mini-magnetospheres’.

With a new era of space exploration now underway, this is the time to start thinking about these new and bold future concepts. As has been proposed by the recent White Paper for NASA's planetary decadal survey Interdisciplinary Research in Terraforming Mars: State of the Profession and Programmatics [95], there is a need to close strategic knowledge gaps and begin to further these concepts and others, in order to work toward a solution that will make colonizing Mars by humans an eventual reality.

My take from this is that creating an artificial magnetosphere for Mars (though not for space stations, interplanetary rocket ships, etc.) is such a vast and such an expensive task, that it will only be affordable when Mars's population is in the hundreds of millions.  The International Space Station cost $150 billion.  But its cost was spread across hundreds of millions of people in developed countries.  What's more, it doesn't look as if it can be done in stages.  It needs to be all or nothing.

For prolly the first hundred years, settlers on Mars will live in domes or underground.  There'll be plenty of terraformed soil and habitat, but it'll be under the protection of domes made of composite layers, as I discuss here