Researchers have recreated the conditions of Mars' mantle in a laboratory, offering a new perspective on a mysterious layer detected beneath the planet's core. A study published on September 18 by scientists from the CNRS, the National Museum of Natural History, and Sorbonne University suggests that iron-rich magma forming around 1,550 kilometers beneath the surface could explain the seismic anomaly observed by the InSight mission. The researchers found that the rock begins to melt at this depth at a temperature about 100 degrees Celsius lower than previously thought. The resulting liquid, rich in iron, is dense enough to sink rather than rise, forming what could be an "upside-down volcano" at the base of the mantle. The InSight mission, launched in 2018 by NASA with CNES, aimed to study the interior of Mars. Its seismometer detected unusual seismic wave behavior at the boundary between the planet’s mantle and core, suggesting the presence of a partially melted layer. This layer, if confirmed, could be about 150 kilometers thick and would have significant implications for how Mars has lost heat over time and how its core has cooled. It might also preserve ancient materials from the planet’s early history. To investigate how and when the Martian mantle begins to melt, the team created a synthetic rock similar to the Martian mantle, rich in iron oxide. Using advanced laboratory techniques, they compressed and heated the sample to conditions found at 1,550 kilometers depth—around 180,000 times Earth's atmospheric pressure. Using X-rays and ultrasonic waves, they observed the formation of the first liquid, finding that melting begins at a lower temperature than previously thought. The liquid contains a higher concentration of iron oxide, making it denser than the surrounding solid rock, which could explain why it sinks rather than rises. This dense, sinking magma aligns with one of two models derived from InSight data but not the other, which would require an even higher concentration of iron. The researchers explain that the liquid can still move through solid rock by forming thin films between tiny grains. These films connect and form a network, allowing the liquid to migrate downward. However, the study acknowledges several uncertainties, including the volume of liquid required for such a layer, the amount of melting needed to achieve the observed density, and the exact composition of the Martian mantle. The study provides a plausible explanation but not definitive proof. The researchers emphasize that their findings offer a "plausible mineral physics framework" for the layer, consistent with how minerals behave under high pressure, but further data are needed. The InSight mission’s results could also have alternative explanations, such as irregularities in the mantle itself. Regardless, this study marks a significant step in understanding Mars’ interior, and similar layers may be common in other planets with differentiated structures, influencing their long-term evolution. Only new measurements will confirm whether this mysterious layer is indeed present at the bottom of Mars’ mantle.