Repurposed technology used to probe new regions of the Martian atmosphere

Using repurposed equipment, a team including researchers from Imperial College London have measured parts of the Martian atmosphere that were previously impossible to probe. This includes areas that can block radio signals if not properly accounted for – crucial for future Mars habitation missions.

The results of the first 83 measurements, analyzed by Imperial researchers and European Space Agency (ESA) colleagues 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). Both spacecraft carry a radio link, so as one passes behind the planet, radio waves travel through the deeper layers of the Martian atmosphere.

Changes in the refractivity of the atmosphere – the way it bends radio waves – cause small but noticeable 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 study author Jacob Parrott, a Ph.D. 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 designed for communication between orbiters and rovers on the planet’s surface. We had to reprogram them. while flying to perform this new science.

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

Dream teamwork

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

Using two orbital instruments to obtain this measurement is already a common way to investigate 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 on Mars three times before; by NASA in 2007 as a hardware demonstration. The new use by two ESA spacecraft marks the first time this technique has been routinely applied to another planet.

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

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

“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 increases rapidly. Therefore, this technique will be an increasingly most important to the study of Mars.”

More measurements, more insights

The shuttle from spacecraft to spacecraft allows more measurements to be taken 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 relative to the slow motion of the Earth. This makes it difficult to capture global changes on Mars, as researchers often look at the same spots.

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.

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

Mutual radio blocking overcomes these problems, allowing researchers for the first time to explore the entire depth of the Martian ionosphere around noon and midnight.