Recycled technology used to explore new parts of Mars’ atmosphere

Using the repurposed equipment, a team including researchers from Imperial College London measured parts of the Martian atmosphere that were previously impossible to study. These include areas that could block radio signals if not properly accounted for, which is crucial for future missions to Mars.

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

To achieve this, ExoMars’ Trace Gas Orbiter (TGO) teamed up with another ESA spacecraft orbiting the Red Planet, Mars Express (MEX). The two craft maintain a radio link, allowing radio waves to cut through the deeper layers of Mars’ atmosphere as one passes behind the planet.

Changes in the atmosphere’s refractivity (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 (a charged upper layer of the atmosphere). The technique is called mutual radio occultation.

Lead author of the study Jacob Parrott, a PhD student from Imperial’s Department of Physics, said: “The systems on MEX and TGO weren’t originally designed to do this – the radio antennas we used were made for communications between orbiters and rovers on the planet’s surface. We had to reprogram them in flight to do this new science.

“This innovative technique will likely be a game changer for future missions, proving that mutual radio occultation between two spacecraft in orbit is an economical way to extract more scientific value from existing equipment.”

Dream Teamwork

Previously, radio occultation was performed using a radio link from a Mars orbiter to large ground stations on Earth. The orbiter’s radio signal would be monitored while the spacecraft “set” (occulted) behind Mars, meaning that the signal passed through the layers of the planet’s atmosphere.

Performing these measurements using two spacecraft is already a common way to study the Earth’s atmosphere: thousands of such measurements are made by global navigation satellites. The data they collect is used to monitor the atmosphere and predict the weather.

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

Now that its feasibility has been proven, the scientists and engineers behind the work are investigating how to deploy the technology more broadly on future missions to Mars.

Study co-author Dr Colin Wilson, Project Scientist for the ExoMars Trace Gas Orbiter and Mars Express at ESA, said: “ESA has now shown that this technique is feasible, which could have a major impact on Mars science in the future.

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

More measurements, more insights

Occultation between spacecraft allows more measurements to be made and new parts of the atmosphere to be explored.

Because conventional radio occultation measurements on Mars require a radio link to a ground station on Earth, the measurement location is fixed relative to the slow motion of the Earth. This makes it difficult to record global changes on Mars, because researchers often look at the same spots.

Furthermore, this method only allows us to take samples around sunset and sunrise, because Earth is close to the sun, which limits our view of the Martian atmosphere.

Furthermore, traditional radio occultation suffers from “occultation seasons”, where measurements are only possible for a few months each year due to the spacecraft’s orbit. For example, Mars Express was only able to perform radio occultation for two months in 2022.

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