Seismic waves generated by a meteorite impact on the other side of Mars from where NASA's InSight lander sits have provided new information onthe Red Planet's deep interior.
The data indicates the presence of a previously unknown layer of molten rock surrounding a liquid metallic core - the planet's innermost component - that is smaller and denser than was previously predicted, said researchers.
Waves generated by quakes - including those caused by meteorite impacts - vary in speed and shape when journeying through different material inside a planet.
The meteorite impact that occurred in a Martian highland region called Tempe Terra on 18 September, 2021 triggered a magnitude 4.2 quake and left a crater about 130 metres wide.
It occurred on the opposite side of Mars from InSight's location in a plains region called Elysium Planitia.
"The importance of the far side impact was to produce seismic waves that traversed the deep interior of the planet, including the core.
"Previously, we had not observed any seismic waves that had transited the core. We had only seen reflections from the top of the core," said planetary scientist Amir Khan of ETH Zürich in Switzerland.
The behavior of the waves showed that previous assessments of Mars were missing something - the presence of a molten silicate layer about 150 km thick surrounding the core.
This molten region sits at the bottom of the interior portion of the planet called the mantle.
The researchers also recalculated the size of the core, finding that it has a diameter of about 3,350 km, and a volume about 30% smaller than previously thought.
The researchers said the mantle - a rocky layer sandwiched between the planet's outermost crust and core - extends about 1,700 km below the surface.
Unlike Mars, Earth has no molten layer around its core.
"The molten and partially molten layer is essentially composed of silicates (rock-forming minerals) that are enriched in iron and in radioactive heat-producing elements compared to the overlying solid mantle," said Henri Samuel, a planetary scientist.
The Martian core is made up mostly of iron and nickel, but also has some lighter elements such as sulfur, oxygen, carbon and hydrogen.
The fourth planet from the sun, has a diameter of about 6,791 km, compared to Earth's diameter of about 12,755 km.
Earth is almost seven times larger in total volume.
NASA retired InSight in 2022 after four years of operations.
"We have learned a lot about Mars by studying the unique seismic record provided by the InSight mission," Samuel said.
"Planets are rich and complex systems because they are a place where many different types of processes coexist and act on various spatial and temporal scales, and Mars is no exception."
