Researchers have discovered that Mars' liquid core is slightly denser and smaller than previously thought, and contains a mixture of iron and other metals. The discoveries, made possible by the first-ever detections of seismic waves on the Martian core, add to our understanding of the planet's genesis and history.

A new study has revealed fascinating insights into the liquid core at the heart of Mars, contributing to our understanding of the planet's genesis and development.

The study, led by the University of Bristol and published in the journal Proceedings of the National Academy of Sciences in the United States, reveals the first detections of sound waves travelling into Mars' core. Measurements of this acoustic energy, known as seismic waves, show that its liquid core is slightly denser and smaller than previously assumed, and is made up of iron and a variety of other elements.

The discoveries are all the more astounding given that the research mission was only supposed to last a little more than one Mars year (two Earth years). Despite Martian storms hastening dust accumulation and decreasing power to NASA's InSight Mars lander, NASA prolonged its stay, allowing geophysical data, including marsquake signals, to be collected until the end of last year.

"The extra mission time certainly paid off," stated lead author Dr. Jessica Irving, Senior Lecturer in Earth Sciences at the University of Bristol. We have made the first observations of seismic waves travelling through Mars' core. Two seismic signals, one from a very distant marsquake and one from a meteorite collision on the planet's far side, have allowed us to use seismic waves to study the Martian core. We've been listening for energy travelling through the heart of another planet, and we've finally heard it.

"These preliminary measurements of the elastic properties of Mars' core have aided us in our investigation of its composition." It is more than simply a ball of iron; it also contains a significant amount of sulphur, as well as other elements, including a trace of hydrogen."

The researchers utilised data from NASA's InSight lander, a robotic mission built to explore Mars' interior, to compare seismic waves travelling through the planet's core with those transiting Mars' shallower areas, and to predict the planet's internal features.

In 2018, the InSight lander placed a broadband seismometer on the Martian surface, allowing seismic phenomena such as marsquakes and meteorite impacts to be detected.  Seismologists, geodynamicists, and mineral physicists used observations of two seismic events in the opposite hemisphere from the seismometer to calculate the travel times of seismic waves that passed through the core versus seismic waves that remained in the mantle.

"So-called 'farside' events, meaning those on the opposite side of the planet to InSight, are intrinsically harder to detect because a significant amount of energy is lost or diverted away as waves travel through the planet," Dr Irving explained. To find and then use these events, we needed both luck and skill. In the first year of Martian operations, we identified no farside occurrences. This research would not have been possible if the mission had stopped at that point.

"The marsquake on Sol 976 was the most distant event discovered during the mission." The second farside event, S1000a - the first event discovered on day 1,000 of operations - was especially significant since it turned out to be a meteorite impact that we heard all the way through the globe, allowing us to pinpoint the source of the seismic waves. After the Marsquake Service (MQS) refined their skills on hundreds of days of Martian data, it took a lot of seismological knowledge from throughout the Insight Team to pull the signals out of the complicated seismograms acquired by the lander." 

"Detecting and understanding waves that travel through the very core of another planet is incredibly difficult, reflecting decades of efforts by hundreds of scientists and engineers from multiple countries," said co-author Ved Lekic, Associate Professor of Geology at the University of Maryland College Park in the United States. We had to use not only sophisticated seismic analysis techniques, but also understanding of how high pressures and temperatures influence the characteristics of metal alloys, leveraging the InSight Team's expertise."