The Science

What is the Exosphere?

The exosphere is the outermost layer of Earth’s atmosphere, a region from approximately 310 to perhaps 119,000 miles above Earth’s surface (500 to 192,000  kilometers). That is halfway to the Moon! Its full extent is unknown, which is a mystery that the Carruthers mission will be able to solve. In the exosphere, the density of gas particles is so low that they do not collide with each other. The gas is composed primarily of neutral hydrogen, the lightest and most abundant element in the Universe. We can observe the exosphere by capturing images of ultraviolet (UV) light emitted by the Sun that is scattered in all directions by the hydrogen. This UV glow of the exosphere is called the geocorona.  

Carruthers will investigate the role Earth’s exosphere plays in: 

Space Weather

Disturbances in the solar wind can impact Earth, and create geomagnetic storms. These storms can be a danger to our technology, including spacecraft, communications passing through our atmosphere, and power grids on the ground. As our reliance on technology grows, we need to better understand and predict space weather storms. A missing piece of the puzzle to understand and predict space weather events on Earth is the exosphere’s response. We have only ever taken four images of the global exosphere, and have otherwise been limited to observations taken from within. With so few observations to date, we do not know how the exosphere responds to incoming solar wind, and solar storms. We also expect that the exosphere helps Earth recover from geomagnetic storms by draining the energy out of the huge electrical currents that they cause in our atmosphere.

Atmospheric escape from planets

Planets can lose their atmospheres over time. High energy radiation (UV, X-rays, Gamma-rays, Cosmic rays) and chemical reactions can break apart molecules in planetary atmospheres. Low mass particles, like hydrogen, diffuse up into the exosphere where they can achieve escape velocity from Earth’s gravity into interplanetary space. Surface water is lost this way: 

  • Liquid water evaporates into the atmosphere
  • H2O breaks up into H and O, 
  • H escapes.

Carruthers will study how this process works on Earth. When we combine this knowledge with studies of the atmospheric loss process on Mars, which does not have a global magnetic field like Earth, it will give us a more complete picture of how planets lose their surface water. Since all life as we know it requires liquid water, this will be important for understanding the long-term habitability of terrestrial worlds in our Solar System and beyond.