The findings, published in the journal Science, may change what we know about the formation of rocky planets like Mars, Venus and the Earth.

The first four planets from the Sun are often depicted in textbooks as having smooth, layered interiors, with crust, mantle and core stacked like a millionaire's shortbread.

However, seismic anomalies detected on Mars by the InSight mission reveal that the mantle is far from smooth and contains rough lumps of ancient fragments up to 4km wide, preserving the planet's violent early history like geological fossils.

The solar system's rocky planets formed about 4.5 billion years ago when dust and rock orbiting the young Sun clumped together under gravity.

As Mars took shape, it was struck by giant objects the size of entire planets in cataclysmic collisions similar to the kind thought to have formed our Moon.

"These colossal impacts unleashed enough energy to melt large parts of the young planet into vast magma oceans," Constantinos Charalambous, an author of the study from Imperial College London, said. "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."

These cataclysmic collisions mixed fragments of the planet's crust and mantle from its "embryo" with debris from the impacting objects.

Then, as Mars cooled, these diverse chunks were trapped in a sluggishly churning mantle, "like ingredients folded into a Rocky Road brownie mix", the study said. However, the mixing of these "ingredients" was too weak to fully smooth things out.

Unlike the Earth, where plate tectonics constantly recycle the crust and mantle, the interior of Mars is sealed up beneath a stagnant outer crust, preserving a geological time capsule.

"The fact that we can still detect its traces after four and a half billion years shows just how sluggishly Mars's interior has been churning ever since," Dr Charalambous said.

Astronomers uncovered these lumps by analysing data from eight marsquakes detected by the InSight lander, including two triggered by recent meteorite impacts that left almost 150m-wide craters on the planet.

The lander is equipped with instruments to detect seismic waves on Mars travelling through the mantle.

The researchers found that seismic waves of higher frequencies took longer to reach the lander sensors from the impact site. This revealed that the interior was chunky rather than smooth.

"These signals showed clear signs of interference as they travelled through Mars's deep interior," Dr Charalambous said.

"That's consistent with a mantle full of structures of different compositional origins - leftovers from Mars's early days."

The latest findings, the study notes, have implications for our understanding of the histories of other rocky planets as well.

Strange blobs found inside Mars could be remnants of something ancient