The ExoMars’ Trace Gas Orbiter has repurposed its antenna, allowing researchers to explore the Martian atmosphere in unprecedented ways. With the help of this modified equipment, a team, which includes researchers from Imperial College London, has been able to measure previously inaccessible parts of the Martian atmosphere. This is crucial for future Mars habitation missions, as it helps in accounting for areas that can block radio signals.

The initial 83 measurements, analyzed by Imperial researchers and colleagues from the European Space Agency (ESA), have been published in the journal Radio Science.

To achieve this breakthrough, ExoMars’ Trace Gas Orbiter collaborated with another ESA spacecraft, Mars Express (MEX). The two spacecraft are connected via a radio communication link, so when one goes behind the planet, radio waves can penetrate the deeper layers of the Martian atmosphere.

Small but noticeable changes in the way the atmosphere bends the radio waves lead to detectable shifts in the radio frequencies received by the spacecraft. This shift can be analyzed by scientists to determine the density of the lower atmosphere and the electron density in the ionosphere – a charged upper layer of the atmosphere. This technique is known as mutual radio occultation.

“The systems on MEX and TGO were not initially designed to do this – the radio antennas we used were made for communication between orbiters and rovers on the planet’s surface. We had to reprogramme them whilst inflight to carry out this new science,” said Jacob Parrott, a PhD student from the Department of Physics at Imperial and the lead author of the study. “This innovative technique is likely to be a game-changer for future missions, proving that mutual radio occultation between two orbiting spacecraft is an economical way to extract more scientific value from existing equipment.”

The technique of radio occultation previously involved using the radio link from a Mars orbiter to large ground stations on Earth. During occultation, the radio signal from the orbiter would be tracked as the spacecraft moved behind Mars, passing through the planet’s atmosphere layers. Employing two orbiting spacecraft to make this measurement is already a common practice for studying the Earth‘s atmosphere.

Thousands of such measurements occur between global navigation satellites, providing data for atmospheric monitoring and weather prediction. However, this technique had been used on Mars only three times before, by NASA in 2007, for a hardware demonstration. This marks the initial application of this technique to another planet by the two ESA spacecraft. Following its successful demonstration, the researchers and engineers involved in the project are exploring ways to extend the use of this technique in upcoming Mars missions.

Study co-author Dr. Colin Wilson, Project Scientist for the ExoMars Trace Gas Orbiter and Mars Express at ESA, said: “ESA has now demonstrated the viability of this technique, which could be transformational for Mars science in the future. There are currently seven spacecraft orbiting Mars; as the number of spacecraft increases, as it will in coming decades, the number of radio occultation opportunities will increase rapidly. Therefore, this technique will be an increasingly important tool for studying Mars.”

Spacecraft-to-spacecraft occultation enables additional measurements to be gathered and facilitates the investigation of previously unexplored areas of the atmosphere.

The use of conventional radio occultation measurements on Mars is constrained by the fixed measurement location relative to Earth’s slow movement, making it challenging to observe global changes on Mars and leading researchers to focus on the same areas. Additionally, this approach is limited to sampling near sunset and sunrise due to Earth’s proximity to the Sun, which restricts our view of Mars’ atmosphere.

Furthermore, traditional radio occultation is impacted by ‘occultation seasons,’ during which measurements are feasible for only a few months each year due to the spacecraft’s orbit. For instance, Mars Express was only able to conduct radio occultation for two months in 2022.

For the first time, researchers can explore the full depth of Mars’ ionosphere around noon and midnight for the first time thanks to mutual radio occultation, which helps overcome these challenges.

Journal reference:

1. Jacob Parrott, Håkan Svedhem, Olivier Witasse, Colin Wilson, Ingo Müller-Wodarg, Alejandro Cardesín-Moinelo, Peter Schmitz, James Godfrey, Olivier Reboud, Bernhard Geiger, Beatriz Sánchez-Cano, Bruno Nava, Yenca Migoya-Orué. First Results of Mars Express—ExoMars Trace Gas Orbiter Mutual Radio Occultation.