Redesigned technology used to explore new regions of the Martian atmosphere

Using the newly used device, a team including scientists from Imperial College London measured parts of the Martian atmosphere that had previously been impossible to explore. This includes areas that can block radio signals if not properly accounted for – crucial for future Mars settlement missions.

The results of the first 83 measurements, analyzed by Imperial researchers and European Space Agency (ESA) colleagues from across Europe, are published today in the journal Radio Science.

To achieve this goal, ExoMars’ Trace Gas Orbiter (TGO) teamed up with another ESA spacecraft orbiting the red planet: Mars Express (MEX). Both craft maintain radio contact so that when one passes behind the planet, radio waves penetrate the deeper layers of the Martian atmosphere.

Changes in the atmosphere’s refraction—how it bends radio waves—cause small but detectable shifts in the radio frequencies received by the spacecraft. By analyzing this shift, scientists can determine the density of the lower atmosphere and the electron density in the ionosphere—the charged upper atmosphere. The technique is called mutual radio occultation.

Lead study author Jacob Parrott, Ph.D. student from the Department of Physics at Imperial, said: “The systems on MEX and TGO weren’t originally designed for this – the radio antennas we used were made for communication between orbiters and rovers on the planet’s surface. We had to reprogram them while in flight to carry out this new sciences.

“This innovative technique is likely to be a game-changer for future missions, demonstrating that mutual radio cloaking between two orbiting spacecraft is an economical way to get more science value from existing equipment.”

The teamwork of dreams

Previously, radio occultation was performed using a radio link from a Mars orbiter to large ground stations on Earth. The radio signal from the orbiter would have been monitored when the spacecraft “went in” (became cloaked) behind Mars, meaning the signal passed through the layers of the planet’s atmosphere.

Using two orbiting craft to make this measurement is already a common way of investigating the Earth’s atmosphere: thousands of such measurements take place between global navigation satellites, where the data they provide is used for atmospheric monitoring and weather forecasting.

However, this method had only been used three times before on Mars; NASA in 2007 as a hardware demonstration. The new use by two ESA space probes is the first time the technique has been routinely applied to another planet.

Now its viability has been proven, and the scientists and engineers behind the work are investigating how to expand the use of the technique in future missions to Mars.

Study co-author Dr. Colin Wilson, ExoMars Trace Gas Orbiter and Mars Express Project Scientist at ESA, said: “ESA has now demonstrated the viability of this technique, which could be transformative for future Mars science.

“There are currently seven spacecraft orbiting Mars; as the number of spacecraft increases, and also in the coming decades, the number of radio occultation opportunities will increase rapidly. Therefore, this technique will become an increasingly important tool for the study of Mars.”

More measurements, more insights

Spacecraft-to-spacecraft cloaking allows more measurements to be made and allows new regions of the atmosphere to be probed.

Because conventional radio occultation measurements on Mars involve a radio link to a ground station on Earth, the measurement location is fixed due to the slow motion of the Earth. This makes it difficult to capture global changes on Mars, as researchers often look at the same places.

Additionally, this method can only sample near sunset and sunrise due to Earth’s proximity to the Sun, limiting our view of the Martian atmosphere.

In addition, traditional radio occultation suffers from an “occlusion period” when measurements are only possible for a few months of the year due to the spacecraft’s orbit. For example, Mars Express could only perform a radio occultation for two months in 2022.

Mutual radio occultation overcomes these problems, allowing researchers to probe the full depth of the Martian ionosphere around noon and midnight for the first time.