The chain of logic connecting ‘habitable world’ to ‘magnetic field’ typically proceeds as follows: life requires liquid water, which requires atmosphere, which requires magnetic fields.
The connections are reasonable. All of the millions of species of life on Earth require liquid water. There must be sufficient pressure for water to exist as a liquid. And, until recently, it was widely accepted that a global magnetic field prevents an atmosphere from escaping to space.
However, each link in this chain also contains implicit assumptions. The first link assumes that all life is Earth-like, but other liquids such as ammonia or methane have been discussed as viable alternatives to water if life formed elsewhere [e.g. Benner et al., 2004]. The second link assumes that life only exists at a planet’s surface, though subsurface life could (and on Earth, does) survive without any significant planetary atmosphere. The third link assumes that magnetic fields protect atmospheres from being stripped to space. This last assumption has been debated in recent years based on comparisons of in situ observations of atmospheric escape made at Venus, Earth, and Mars.
Current State of the Field
In broad terms, any particle near the top of an atmosphere (where collisions are rare) can escape a planet’s gravity if it is given enough energy and is directed “upward.” For neutral particles this energy can come from heating by solar photons (only important today for hydrogen at Venus, Earth, and Mars), from chemical reactions (important today mostly for oxygen at Mars), or from collisions (not thought to be important at any of the three planets today). Because ions are charged particles, however, they have access to additional important sources of energy through interactions with electric fields and other charged particles. Ion escape can therefore be especially effective for heavier species that require substantial energy for escape. Magnetic fields influence the motion of charged particles, and therefore a strong planetary magnetic field will influence ion escape processes.
The early U.S. and Soviet missions to Mars and Venus suggested that the two planets lacked significant global dipole fields, a fact confirmed by more recent satellite missions to these planets. The atmospheres of Venus and Mars are enriched in heavy isotopes such as deuterium – a signature of the removal of particles from the top of the atmosphere. This enrichment is greater than for Earth, suggesting that the two un-magnetized worlds have lost more atmosphere than magnetized Earth. Most recently, observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission currently orbiting Mars confirm the loss of substantial amounts of atmosphere over the history of the planet. This is consistent with the idea that unmagnetized worlds easily lose their atmospheres, as depicted in the artist’s rendition of an earlier time on Mars when the planet may have had water.
The idea that planetary scale magnetic fields lead to reduced ion escape rates has been recently questioned. The reported ion escape rates at Venus, Earth, and Mars agree to within about an order of magnitude. This suggests that the efficiency of atmospheric escape is roughly the same for magnetized and unmagnetized worlds. It has also been shown that ion outflow in Earth’s magnetic cusps is proportional to the Poynting flux in the solar wind, suggesting a direct link between the solar wind energy and the energy of escape. In the artist’s concept above, the solar wind interacts with Mars’ upper atmosphere, but is deflected past Earth by a global magnetic field (image courtesy NASA/GSFC).
A planet with an intrinsic dipole field encounters more power from the solar wind than an unmagnetized planet due to the large size of the magnetosphere relative to the planet. The solar wind energy may be transferred along magnetospheric field lines to the top of the atmosphere in the cusp regions, possibly resulting in more ion escape in the absence of a global field. But the basic premise that magnetic fields prevent the solar wind from accessing upper atmospheres is also difficult to dismiss.
Modelling work will play an essential role in quantifying the effect of a global field on ion loss from a planet. Physics-based global magnetospheric models provide a powerful means of computing ion escape rates from planets. Unlike spacecraft data, models ‘see’ the entire system at any given moment, and can be used to probe the physics responsible for energizing and removing ions. Models can also vary a single parameter (such as the intensity of the planetary dipole moment) to determine its influence on the system. In order to do this, we require a model that includes proper physical processes and is validated by observations. This model should be able to handle both unmagnetized and magnetized planets with varying dipole field strength. To date, such a model does not exist. The development of a proper model is the ambitious main goal of the modeling work of the MACH center in Phase I. The biggest challenge of the work will be to identify relevant physical processes not currently captured in existing models and then determine “where” and “how” to include these processes. The Modeling tasks thus depend on the Theory tasks.
|Y. Ma||Multi-species single fluid MHD (BATSRUS)||Ma et al. [2004, 2007, 2014a, 2014b, 2015, 2017, 2018a, 2018b]|
|Y. Ma||Multi-fluid MHD
|Najib et al., ;
Dong et al., 
|K. Seki||Multi-species MHD||Terada et al., ;
Sakai et al. 
|H. Egan||Hybrid (RHybrid)||Egan et al., |
Our current modeling team has access to modify and run at least four global models, including: two multi-species MHD models, one multi-fluid MHD model and one hybrid model. The highlights and references of each model are listed in the table. These models can be used to help the theory team identify the relative importance of different physical processes by turning on and off individual physical process, represented by different terms in the physical equations iterated by the models. Though we will not extend the hybrid model to quasi-Earth dipole field strengths due to current computational limitations, the advantage of having a hybrid model included in Phase I is to better understand some of the ion kinetic processes included in the model that are thought to operate at planets.
The MACH Center’s work makes use of primary data from a number of scientific missions, as well as data and analysis from previously accomplished studies. For information on the missions that have been used to look into magnetic fields, ion escape and planetary atmosphere see their respective websites.
NASA mission studying the upper atmosphere of Mars.
ESA mission characterizing Martian water and volatiles.
ESA mission to study the atmosphere of Venus.
ESA mission to study Earth’s magnetic fields and its interaction with the Sun.
Former aurora research probe from NASA.
NASA mission that collects and analyzes material of solar, interplanetary, interstellar and galactic origins.