Might the martian heart still carry the wounds of its own violent genesis?
NASA’s InSight mission has provided seismic data showing that the Red Planet’s mantle is far from homogenous. Rather than the sleek, concentric layers one usually finds in planetary illustrations, scientists have discovered it to be speckled with old, jagged fragments some 4 kilometers across that have endured for over 4.5 billion years. These “geological fossils” are the remains of Mars in its prime, when massive impacts and giant oceans of magma resurfaced its interior.
The finding came out of a close examination of eight very distinct marsquakes, two created by recent meteorite impacts that had gouged out craters about 150 meters in diameter. When seismic waves from these events propagated through the planet’s mantle, InSight’s seismometer measured an unexpected lag in high-frequency arrivals. “The seismic signals showed clear signs of interference as they traveled through Mars’s deep interior,” said Dr. Constantinos Charalambous of Imperial College London. “That’s consistent with a mantle full of structures of different compositional origins leftovers from Mars’s early days.”
The reason is in the planet’s violent childhood. Within the disordered early solar system, Mars suffered multiple impacts from bodies sizable enough to be comparable to small planets. These blows ejected sufficient energy to melt large sectors of the crust and mantle, forming planet-scale magma oceans. Laboratory and modeling experiments of such oceans indicate they acted like molten rock “honey,” intensely convecting and crystallizing with time. As the molten layers cooled, crystals floated or sank based on their density, forming chemically differentiated zones. On Mars, the process was cut short: rather than being homogenized by active plate tectonics, as on Earth, the heterogeneity of the mantle was set in place under a stagnant crustal lid.
The size and spacing of these preserved pieces conform to what scientists term a “fractal” pattern a few big fragments with a lot of smaller ones in between a hallmark of catastrophic shattering. “You see the same effect when a glass falls onto a tiled floor as when a meteorite collides with a planet,” said Professor Tom Pike, who is also at Imperial College London. This trend indicates the energy of the impacts dwarfed the structural strength of the planet, breaking up and blending Mars and impactor material deep into the mantle.
The InSight measurements also highlight the importance of seismic wave transmission in the disclosure of planetary interiors. On Mars, some of the higher frequency waves seem to follow a more straight “seismic highway” through the mantle, going around the crust’s ability to dampen shock-produced vibrations. This lower transmission pathway, which was partly identified in a Cerberus Fossae impact study, is compelling scientists to rethink models for the planet’s internal make-up and structure.
Comprehension of Mars’s preserved mantle characteristics is also connected with general issues of planetary evolution. Magma ocean models, used on Earth, the Moon, and other rocky planets, suggest that initial large-scale melting and crystallization can establish the groundwork for a planet’s tectonic and thermal evolution. For Earth, mantle overturn and energetic convection eliminated most early heterogeneities. For Mars, small size and absence of plate tectonics preserved them. As NASA’s Jet Propulsion Laboratory’s Dr. Mark Panning pointed out, “InSight’s data continue to reshape how we think about the formation of rocky planets, and Mars in particular.”
The stagnant-lid regime that closed off Mars’s mantle could have been operating within the first 100 million years of the planet’s existence. Its lithosphere, modeled rheologically, implies that in dry conditions the present-day brittle–ductile transition is much deeper than on Earth, more than 100 kilometers in places, with more than 3,000 megapascals of strength sufficient to disallow plate boundary formation. In wetter early times, the lithosphere might have been thinner and weaker, and there may have been more intense mantle activity before cooling set the chaotic mantle fabric rigid.
To planetary scientists, the preserved shards are more than geological oddities. They are a unique, immediate record of the mechanical and chemical processes that were at work as the terrestrial planets were still coming together. And because Mars’s interior has been so slowly active, it provides a superbly preserved record a time capsule from the period when magma oceans dominated and planetary surfaces were still molten seas of rock.
