Comparative Magnetospheres: Mars and Comets

Mars and comets have much in common. Mars is a small planet, and due to its low mass it has an exosphere that extends far out into space. A comet has a really low mass, and could be seen as an extreme version of Mars.

However, comets move through the solar system on highly elliptical orbits, and when they are far away from the Sun they are cold frozen lumps without atmospheres.

As they move in closer to the Sun, their surfaces heat up and gases are emitted forming an atmosphere called the coma.The gases that leave the nucleus of a comet are ionised by ultraviolet light from the Sun, and that also happens at Mars, and indeed at all planets that have atmospheres, forming an ionosphere.

In both the planet and comet cases shielding boundaries form in the plasma: a bow shock and an induced magnetosphere boundary at Mars, and at a comet there is a bow shock, a solar wind ion cavity and a diamagnetic cavity. For Mars theses boundaries undergo some changes with the seasons and with the changing solar wind parameters.

Comets, on the other hand, are completely transformed every time they go around their orbit through the solar system. Far away from the Sun, none of these boundaries exist. As the comet moves closer to the Sun we can see how they start to develop.

For example, the first boundary to develop is the infant bow shock, a highly asymmetric structure that later develop into a large bow shock that is similar to planetary bow shocks.

The infant bow shock was discovered by our team at comet 67P/Churyumov-Gerasimenko, using data from the Rosetta Spacecraft.A comet can be seen as a laboratory that can be used to study the magnetospheric physics of unmagnetised planets such as Mars, because comets go through a range of different regimes that are not accessible at planets.

In this way comets can tell us how a system reacts to changes in solar UV radiation, solar wind parameters, and outgassing rates.

Regimes that are not accessible at Mars today, can nevertheless be important when considering planetary-solar wind interaction throughout the history of the solar system or exoplanets in solar systems other than ours.

We will use NASA´s MAVEN spacecraft together with ESA´s Mars Express spacecraft to explore asymmetries in the bow shock of Mars in the direction of the solar wind electric field.

This is the direction in which the infant bow shock at comets has been found to be asymmetric.At comet 67P it was found that the electric field at large heliocentric distances could be described as a sum of three contributions: the solar wind convective electric field; the ambipolar electric field which is caused by the electrons leaving the region near the nucleus faster than the ions; and the polarisation electric field, which is caused by the electrons drifting in a direction different than the ions.

We will use the data obtained by ESA´s Rosetta spacecraft at comet 67P to find out in what parameter regimes the polarisation electric field is important.

We will use MAVEN data to answer the same question for the ion plume of Mars.What we find in observations of both the electric fields and the bow shocks will be compared to simulation results.

We will use both Particle In Cell simulations that include both electrons and ions and hybrid simulations, where the electrons are treated as a fluid.

This will help us interpreting the results, and also allow us to make predictions relevant to for example ESA:s upcoming Comet Interceptor mission.This research is significant for atmospheric escape, since the position of the boundaries around planets influence how the solar wind interacts with the atmosphere, and the electric fields are responsible for accelerating the ions that escape.

Atmospheric escape is, in turn, important in answering the question whether a planet can retain its atmosphere, and this has implication for the habitability of planets, both in our solar system and beyond it.