Technology reused to probe new regions of Mars’ atmosphere

Using repurposed equipment, a team including researchers from Imperial College London was able to measure areas of the Martian atmosphere that were previously impossible to probe. This includes areas that can block radio signals if not properly considered, which is crucial for future Mars habitation missions.

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

To achieve this, ExoMars’ Trace Gas Orbiter (TGO) has teamed up with another ESA spacecraft orbiting the Red Planet: Mars Express (MEX). The two spacecraft maintain a radio link, so that when one of them passes behind the planet, the radio waves pass through the deepest layers of the Martian atmosphere.

Changes in the atmosphere’s refractivity (the way it bends radio waves) cause tiny but detectable shifts in the radio frequencies received by the spacecraft. By analyzing these shifts, scientists can determine the density of the lower atmosphere and the electron density in the ionosphere (a charged upper layer of the atmosphere). This technique is called radio mutual occultation.

Jacob Parrott, lead author of the study and a PhD student in the Department of Physics at Imperial College, said: “The MEX and TGO systems were not originally designed for this. The radio antennas we used were intended for communication between orbiters and rovers on the planet’s surface. We had to reprogram them in flight to carry out this new science.”

“This innovative technique is likely to be a game-changer for future missions, proving that mutual radio occultation between two orbiting spacecraft is a cost-effective way to extract more scientific value from existing equipment.”

A dream teamwork

Previously, radio occultation was accomplished by radio linking a Mars orbiter to large ground stations. The orbiter’s radio signal was monitored as the spacecraft “landed” (was occulted) behind Mars, meaning the signal passed through the layers of the planet’s atmosphere.

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

However, this method had only been used on Mars three times before, in 2007 by NASA for a hardware demonstration. This is the first time that this technique has been applied routinely on another planet.

Now that its viability has been proven, the scientists and engineers behind this work are looking at how to extend the use of this technique to future Mars missions.

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 Mars science in the future.

“There are currently seven spacecraft orbiting Mars. As the number of spacecraft increases, as it will in the coming decades, the number of radio occultation opportunities will increase rapidly. This technique will therefore be an increasingly important tool for studying Mars.”

More measures, more information

Occultation between spacecraft allows more measurements to be taken and new regions of the atmosphere to be probed.

Conventional radio occultation measurements on Mars require a radio link to a ground station, and the measurement location is therefore fixed relative to the slow motion of the Earth. This makes it difficult to capture global changes on Mars, because researchers often observe the same points.

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

Additionally, traditional radio occultation suffers from “occultation seasons,” where measurements are only possible for a few months per 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 overcomes these problems, allowing researchers to explore the full depth of Mars’ ionosphere around noon and midnight for the first time.