Radio signals measure unfathomable parts of the Martian atmosphere

Reprogrammed technology on board ESA’s Trace Gas Orbiter and Mars Express spacecraft has made it possible to measure parts of the Martian atmosphere that were previously impossible to probe.

This includes areas that can block radio signals if not properly taken into account, which is crucial for future habitation missions on Mars. The results of the first 83 measurements are published in the journal Radio Science.

Trace Gas Orbiter (TGO) and Mars Express (MEX) maintain a radio link, so that when one passes behind the planet, radio waves pass through the deeper layers of the Martian atmosphere.

Changes in the atmosphere’s refractivity (how it bends radio waves) cause tiny but detectable changes in the radio frequencies received by the spacecraft. By analyzing this change, scientists can determine the density of the lower atmosphere and the density of electrons 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 student in the Department of Physics at Imperial College London, said in a statement: “The MEX and TGO systems 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 reprogram them in flight to carry out this new science.

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

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

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

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

Now that its feasibility has been demonstrated, the scientists and engineers behind the work are looking at how to expand the use of this technique 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 demonstrated the feasibility 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 increases rapidly. Therefore, this technique will be an increasingly important tool for studying Mars.”

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

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 points.

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

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

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

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