A giant collision in Mars’ early history created a global magma ocean and buried large fragments of debris deep within the young planet. Credit: Vadim Sadovski / Imperial College London
New research has revealed the core of Mars is not as smooth and uniform as previously thought. Instead, it’s a chunky mismatch of scraps from ancient collisions which have been preserved in the mantle.
The team of international researchers used seismometers on Mars to detect the billions-of-years-old fossilised fragments in the planet’s mantle – the massive layer of rock located between the core and outer crust. The fragments help piece together understandings of how Earth’s neighbouring planet evolved.
The solar system’s rocky planets were formed from dust and rocks orbiting the Sun around 4.5 billion years ago. After the red planet had taken shape, Mars was struck by giant, planet-sized objects in several impactful collisions.
“These colossal impacts unleashed enough energy to melt large parts of the young planet into vast magma oceans,” says Dr Constantinos Charalambous, the lead researcher of the study from Imperial College London in the UK.
“As those magma oceans cooled and crystallised, they left behind compositionally distinct chunks of material – and we believe it’s these we’re now detecting deep inside Mars.”
Mars’ interior is often depicted as having 3 smooth, distinct layers – like a cake. But the researchers found that there were possible fragments from the impacting objects scattered and mixed with pieces from the planet’s mantle and crust in the molten interior of Mars.
They suggest that as the planet cooled down after the early collisions, the fragments were trapped, creating a sluggish and slow churning mantle. The researchers likened this to a chef mixing the ingredients in rocky road.
Geological activity means that tectonic plates on Earth continuously descend into the mantle and re-emerge as lava in other places. However, Mars’ stagnant crust sealed up early. This preserved its interiors, creating a geological time capsule for scientists.
“Most of this chaos likely unfolded in Mars’s first 100 million years,” says Charalambous.
“The fact that we can still detect its traces after 4.5 billion years shows just how sluggishly Mars’s interior has been churning ever since.”
The researchers noticed that the mantle chunks followed a pattern, with a few large fragments surrounded by many smaller ones. The largest of the fragments was up to 4km wide.
“What we are seeing is a ‘fractal’ distribution, which happens when the energy from a cataclysmic collision overwhelms the strength of an object,” says Professor Tom Pike, study co-author also from Imperial College London.
“You see the same effect when a glass falls onto a tiled floor as when a meteorite collides with a planet: it breaks into a few big shards and a large number of smaller pieces. It’s remarkable that we can still detect this distribution today.”
The research team was alerted to these fragments in Mars’ mantle thanks to seismic vibrations picked up by NASA’s InSight mission.
The InSight lander detected the seismic waves as they travelled through the mantle. When analysing 8 seismic events, 2 of which had been triggered by meteorite impacts, the research team noticed that waves of higher frequency took longer to reach the sensors from the site of impact.
“These signals showed clear signs of interference as they travelled through Mars’s deep interior,” says Charalambous.
“That’s consistent with a mantle full of structures of different compositional origins – leftovers from Mars’s early days.”
The InSight lander launched in 2018 to study Mars’s interior structure and ended its data collecting mission in 2022.
“It’s exciting to see scientists making new discoveries with the quakes we detected,” says Dr Mark Panning from NASA’s Jet Propulsion Laboratory, the lab which led the InSight Mission.
“InSight’s data continues to reshape how we think about the formation of rocky planets, and Mars in particular.”
The study, published in Science, offers a unique perspective as to what might be hiding deep beneath the surface of these planets. The researchers suggest the findings could also have implications for how scientists understand the formation of other rocky planets like Venus and Mercury.