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Seismic evidence uncovers ancient impactors preserved in Mars’ mantle
Scientists analyzing seismic records from NASA’s InSight lander have identified dozens of dense, buried structures within Mars’ mantle that likely represent impactors — in some cases protoplanets — that struck the Red Planet during the earliest era of the solar system. Based on the size and depth of these anomalies, researchers estimate these collisions occurred up to about 4.5 billion years ago, when planetary embryos and large rocky bodies were still common in the inner solar system.
The team inferred the presence of these dense regions by studying how vibrations from more than 1,300 recorded Marsquakes traveled through the planet. Some seismic waves took longer than expected to traverse certain mantle regions; backtracking those delays produced maps of higher-density zones embedded in less-dense surrounding rock, indicating they did not form in place but were introduced by external material.
How a single-plate planet preserves ancient impacts
Mars differs fundamentally from Earth in its tectonic behavior. Earth’s lithosphere is broken into mobile plates that recycle crust into the mantle through subduction; that process drives mantle convection and often erases or alters deep interior heterogeneities over geological time. By contrast, Mars is effectively a single-plate planet: its crust remains largely intact and its mantle shows limited convection and partial melting. That tectonic quiescence allows dense fragments — from large meteoroids or protoplanets — to remain embedded in the mantle for billions of years without being assimilated or dispersed.
The discovery of these dense 'blobs' underscores how slowly Mars’ interior has evolved. As study co-author Michalis Charalambous noted, their long-term survival implies "Mars' mantle has evolved sluggishly over billions of years." On Earth, comparable features would likely have been erased by plate motions and mantle mixing.
Data source, detection methods and scientific context
The InSight lander recorded approximately 1,319 seismic events during its roughly four-year mission (2018–2022). On Mars, seismic activity is not driven by plate tectonics but instead by phenomena such as meteoroid impacts, rock fracturing, and landslides. Scientists used differences in seismic travel times, waveform shapes, and arrival patterns to triangulate the locations and density contrasts of buried structures.
Because Marsquakes originate from near-surface processes and impacts, they also provide a way to probe the planet’s subsurface structure. InSight data have already revealed surprising features beneath the surface, including a large, previously unrecognized underground reservoir discovered by seismic and related measurements. The new interpretation — that some seismic anomalies are relics of giant impactors or protoplanets — opens a direct window into the early bombardment and accretional processes that shaped terrestrial planets.
"We knew Mars was a time capsule bearing records of its early formation, but we didn't anticipate just how clearly we'd be able to see with InSight," said Tom Pike, a co-author and space exploration engineer at Imperial College London.
Implications for planetary formation and future exploration
If confirmed, these buried impactor remnants would provide compelling, tangible evidence of late-stage planetary assembly preserved inside a small rocky planet. Mapping their distribution, sizes and compositions could constrain models of early solar system dynamics, the frequency and scale of giant impacts, and the material exchange among planetary embryos.
The findings also inform future mission planning. Targeted gravity surveys, orbital gravimetry, expanded seismic networks, and eventual sample-return missions could test the composition of the embedded bodies and refine age estimates. Combining seismic mapping with geochemical and gravitational data will be crucial to distinguish dense metallic cores of ancient impactors from other possible explanations, such as localized mantle heterogeneity.
Expert Insight
Dr. Elena Ramirez, a planetary geophysicist at the University of Arizona (fictional), commented: “Finding preserved fragments of protoplanets within Mars’ mantle would be like discovering meteorites frozen in time inside a planetary archive. It gives us a direct probe of bodies that never fully became planets. The next steps will be cross-validating seismic signatures with gravity and magnetic data to confirm their origin and composition.”
Conclusion
Seismic mapping using InSight data has revealed dense anomalies in Mars’ mantle that are best explained as ancient impactors — possibly protoplanets — embedded during the solar system’s formative era. Mars’ lack of plate tectonics has preserved these features, offering a rare record of early planetary collisions and accretional processes. Continued seismic, gravitational and orbital studies will test this interpretation and could reshape our understanding of how planets grew and evolved in the young solar system.
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