Do Habitable Worlds Require Magnetic Fields?

In recent years the idea that a planet's magnetic field shields its atmosphere from being removed to space through interaction with a stellar wind has been increasingly questioned, based largely on observations of atmospheric escape from Venus, Earth, and Mars. This has motivated new thinking about the role that a magnetic field plays in determining atmospheric escape rates — whether it shields the atmosphere by deflecting incident charged particles from the planet's host star, or whether it collects more energy from the stellar wind to drive escape by increasing the cross-sectional area of the planet. Since an atmosphere is required to keep water stable in liquid form at a planet's surface, this question is relevant to our understanding of the habitability of planets.

Various approaches are currently being used to make progress on this question. Basic theoretical arguments have been advanced, but require observations to confirm them. Computer simulations of atmospheric escape are being pursued with promising results, but are difficult to validate. Observations offer opportunity for ground truth, but comparisons between planets in our own solar system are complicated by the fact that planets differ in many ways, so that isolating the influence of a global magnetic field is non-trivial.

Mars may offer an opportunity for a planetary-scale control experiment on the influence of magnetic fields on atmospheric escape. This is because Mars possesses regions of strongly magnetized crust, as well as regions of non-magnetized crust. Thus it is possible to study atmospheric escape from different regions of Mars while controlling for many of the other variables that could influence escape. The MAVEN spacecraft currently orbiting Mars is making measurements that allow us to address this question.

In this presentation we will summarize the current understanding about the role of magnetic fields in atmospheric retention, and present an analysis of MAVEN measurements of charged particle escape from the atmosphere from magnetized and unmagnetized regions. The analysis consists of both global statistics of the spacecraft observations and detailed case studies above individual regions.