Like Earth, Mars is surrounded by an ionosphere—the part of its upper atmosphere where radiation from the sun knocks electrons off of atoms and molecules, creating charged particles. The Martian ionosphere is complex and continuously changes over the course of the day, but its role in atmospheric dynamics and radio communication signals means understanding it is key for Mars exploration.

One way to study the Martian ionosphere is with radio occultation, in which a spacecraft orbiting Mars sends a radio signal to a receiver on Earth. When it skims across the Martian ionosphere, the signal bends slightly. Researchers can measure this refraction to learn about Martian ionospheric properties such as electron density and temperature. However, the relative positions of Mars, Earth, and the sun mean conventional radio occultation cannot measure the middle of the Martian day.

Now, in an article published in the Journal of Geophysical Research: Planets, Jacob Parrot and colleagues deepen our understanding of the Martian ionosphere using an approach called mutual radio occultation, in which the radio signal is sent not from an orbiter to Earth but between two Mars orbiters. As one orbiter rises or sets behind Mars from the other’s perspective, the signal passes through the ionosphere and refracts according to the ionosphere’s properties.