A Planet’s Magnetic Field Protects Its Water

Disaster movies seem to be a staple for Hollywood. One such disaster movie that I found enjoyable (enough to watch it four times on my way to Japan) was The Core. The movie opens by showing some (unrealistic) consequences of Earth’s magnetic field disappearing (caused by a stoppage of the rotation of Earth’s core). The remainder of the movie details how a team of scientists attempt to restart Earth’s core and save the planet. While the film abounds with scientific inaccuracies and impossibilities, its premise highlights an important characteristic of Earth’s habitability, namely a strong magnetic field.

Venus and Earth are remarkably similar in composition and size. Venus has 81% of Earth’s mass, and its radius and density are only about 5% smaller than Earth’s. Due to their forming in a similar section of the solar system, both Earth and Venus likely started covered with water and had essentially identical atmospheres. While these two “sister planets” began similarly, they could not be more different today in terms of their habitability.

Earth’s atmosphere consists of mainly nitrogen (78%) and oxygen (21%), with trace amounts of other gases like carbon dioxide and helium. Additionally, abundant clouds of water vapor fill the skies. In stark contrast, Venus’ atmosphere is comprised of mostly carbon dioxide (96%) and nitrogen (3%) with dense clouds of sulfuric acid! Earth’s atmosphere causes surface temperatures around 70^oF, Venus’ surface sits around 800^oF. What caused this difference?

Many factors contribute to the disparity between Earth and Venus, but recent results from the Venus Express satellite highlight one of the more important differences. Because Earth rotates once every 24 hours, this motion causes its iron core to generate a strong magnetic field. This magnetic field shields Earth from cosmic rays, in addition to protecting Earth’s atmosphere from the solar wind. Venus rotates only once for every 243 Earth days. Consequently, Venus has no significant magnetic field to shield its atmosphere from the solar wind.

Without a magnetic shield, the solar wind strips away all the water from Venus’ surface. Ultraviolet radiation from the Sun breaks water molecules down into two hydrogen ions and one oxygen ion (atoms with an electric charge due to an extra electron or a deficiency of an electron). The charged particles in the solar wind accelerate these ions and strip them from the atmosphere. However, Earth’s magnetic field deflects the solar wind around the atmosphere so the ions are not stripped off into space. The results published in Nature from the Venus Express mission demonstrate that the ions coming from Venus match the composition expected if water is being stripped from its atmosphere. The water currently being stripped from Venus arises from recent small comet impacts that deposit water in Venus’ atmosphere (similar processes occur on Earth also). However, the solar wind has also stripped away all the water that Venus started with over four billion years ago.

These findings highlight how a habitable planet must not only be similar in size and composition to Earth, but it also must have a strong magnetic field. Thus, one must read optimistic announcements such as this with a bit of caution.