It is not enough to study the surface of a planet to understand how it forms, we need to study the interior too. NASA’s InSight is helping scientists do just that by recording Marsquakes, the small tremors coming from inside of the Red Planet. The quakes can be used like a sonogram to see what Mars is like on the inside and compare this with likely scenarios.

"The exploration of the deep interiors of Earth, Mars and other planets is one of the great frontiers of science," said Nishida. "It's fascinating partly because of the daunting scales involved, but also because of how we investigate them safely from the surface of the Earth."

The most common idea is that the core of Mars is made of an iron-sulfur alloy, just hot enough to be molten at around 1,500°C (2,732°F), and is under immense pressure, roughly 130,000 times the pressure at sea level (13 gigapascals). As reported in Nature Communications, the question the researchers wanted to answer is how fast a certain seismic wave would move through the planetary core. Now, they have an answer thanks to a Kawai-type multianvil press, a machine capable of exerting enormous pressure onto tiny samples.

By then hitting those samples with X-rays, researchers can estimate properties. In this case, the team worked out that a P-Wave inside of Mars moves at 4,680 meters (15,354 feet) per second, or about 13 times faster than the speed of sound.

"Due to technical hurdles, it took more than three years before we could collect the ultrasonic data we needed, so I am very pleased we now have it," lead author Keisuke Nishida, an assistant professor from the University of Tokyo's Department of Earth, said in a statement. "The sample is extremely small, which might surprise some people given the huge scale of the planet we are effectively simulating. But microscale high-pressure experiments help exploration of macroscale structures and long time-scale evolutionary histories of planets."

While several tremors on Mars have been detected, it is too early to tell the exact properties of the planet. Earth is a lot more active than Mars and, even then, there are still many uncertainties about what’s below our feet. This work provides some important parameters for further investigation.

"Taking our results, researchers reading Martian seismic data will now be able to tell whether the core is primarily iron-sulfur alloy or not," said Nishida. "If it isn't, that will tell us something of Mars' origins. For example, if Mars' core includes silicon and oxygen, it suggests that, like the Earth, Mars suffered a huge impact event as it formed. So, what is Mars made of and how was it formed? I think we are about to find out."