OrbitalHub

Valles Marineris runs nearly 3,900 kilometres across Mars, reaches 600 kilometres wide in places, and drops more than nine kilometres from rim to floor. NASA calls it the largest canyon system in the solar system, and by the numbers it is not a close call: nearly nine times longer than the Grand Canyon, more than twenty times wider, roughly five times deeper (NASA, NASA Science). For most of the last two decades it has been treated as scenery, a feature that formed in the first billion years of Martian history and has been sitting quietly ever since. A paper published in Nature Communications on 15 October makes that assumption harder to hold. Jung-Hun Song of Imperial College London and colleagues report direct seismic evidence that the ground deep beneath the canyon is still moving, and that the structure controlling where it moves was inherited from the canyon’s own formation billions of years ago (Nature Communications).

The evidence comes from a single earthquake. On sol 976 of the InSight mission, 25 August 2021, the lander recorded an event its catalogue calls S0976a. It was unusual in two ways that made it useful. It arrived from the far side of the planet, roughly 146 degrees away, more than any other well-located marsquake InSight had found, and it was the only event in the mission’s catalogue that could be pinned to Mars’s western hemisphere. It was also puzzling. A seismic signal on Mars does not automatically mean a fault broke. Meteoroid impacts, landslides, and mass wasting all send vibrations through the crust, and the earlier analysis of S0976a had placed it near western Valles Marineris without settling on a cause. Nobody had shown that it was a marsquake at all, rather than something falling over.

Song’s team went back to the raw seismograms and asked what kind of motion would produce that signature. The answer turned on a problem of geometry. Working out how a fault ruptured normally requires many seismometers spread around the source, sampling the radiation pattern from many directions. InSight was one station, sitting roughly 8,000 kilometres away on the opposite side of Mars, so it sampled the pattern once, badly. The trick the team used is that seismic waves bouncing off the planet’s surface arrive as echoes, and each echo corresponds to a different angle between the wave and the surface it left. By jointly inverting the first-arriving P and S echoes, called PP and SS, the team recovered the fault geometry from a single station, validated first against a terrestrial event recorded the same way.

The solution is a predominantly double-couple strike-slip mechanism: blocks of crust sliding horizontally past each other on a steep fault. Across the acceptable solutions, 82.2 percent fall in the strike-slip family, with nodal plane strikes clustering around 48 and 143 degrees and dips steep enough to be nearly vertical. The best-fitting mechanism gives fault strikes near 60 and 330 degrees. The pressure axis sits roughly parallel to the canyon trough, the tension axis roughly perpendicular to it, which reads as horizontal extension across the canyon combined with shear along its length. Depth is the other half of the answer. The source lies in the middle to lower crust, above a Moho estimated at 55 to 65 kilometres beneath western Valles Marineris, with the allowed depth range running from about 20 to 40 kilometres depending on which interior velocity model is used. That is intact crust, not the loose material at the surface.

Rejecting the alternatives took as much work as finding the mechanism. The team ran impact-like explosions, single-force landslide models, and combinations of double-couple and non-double-couple sources, comparing each against the observed waveforms. None of the impact or landslide models reproduced the PP and SS arrivals. A landslide of the size needed to generate an event this big on Mars would have required a failure volume of at least 10 million cubic metres, which should have left a fresh deposit hundreds of metres to a kilometre across. The team checked every available post-event Context Camera image from the source region and found no fresh crater and no new collapse. The seismic data and the orbital imagery pointed the same way, which is what makes the result convincing rather than merely interesting: two kinds of evidence converging on one answer.

The geology says something the seismology alone cannot. Western Valles Marineris is crisscrossed by long-lived fault systems, and the stress orientation from the focal mechanism matches what those structures would experience today. Song’s team mapped faults, grabens, wrinkle ridges, and boulder-fall sites from high-resolution imagery and found a bimodal population of grabens with dominant strikes near 15 and 115 degrees, defining an orthogonal fault set. The P and T axes from the earthquake line up with those dominant trends. The best interpretation is slip on inherited fabric, not the creation of something new: the fault that broke in 2021 was already there, and the present-day stress field is now favourable for it to slip again.

What supplies that stress is a more open question, and the honest answer is a combination. Three sources are invoked. Long-term planetary cooling supplies a background horizontal compression, which no amount of waiting makes go away. Tharsis, the enormous volcanic rise west of the canyon, loads the lithosphere and flexes it, modifying both the magnitude and the orientation of the horizontal principal stresses. And the canyon itself, having been deeply incised, has unloaded the crust vertically, which reduces vertical stress and produces horizontal compression. At the source depth the basaltic crust carries lithostatic pressures of several hundred megapascals, a confining pressure that suppresses tensile opening and favours brittle shear failure instead. The paper is explicit that the relative contributions of flexural loading and dynamic support cannot be separated uniquely from this event alone.

Two results change what the data means beyond this canyon. The first is the aftershock question, and the answer is that InSight would not have seen one. The authors modelled aftershock production and wave propagation and found that ground displacements from a marsquake in western Valles Marineris would rarely exceed the ambient noise level at the lander, a consequence of geometric limits on long-distance propagation across Mars, including core shadowing. The probability of detecting even a single sizeable aftershock within 50 sols is 0.1 percent, which is a strange thing to have calculated, and I am glad they ran the numbers. The absence of follow-up events in the catalogue therefore says nothing about whether the fault is quiet, which is a limit of the instrument rather than of the planet. The second is the comparison across three regions. InSight’s seismograms now resolve three distinct styles of present-day deformation on Mars: strike-slip faulting beneath Valles Marineris from S0976a, compressional faulting in southern Elysium Planitia from S1222a, and normal faulting beneath Cerberus Fossae. Three different mechanisms in three different places means deformation is partitioned across a mechanically heterogeneous shell rather than driven by one planet-wide stress pattern.

The broader implication is about stagnant-lid planets, which is most rocky worlds. Mars has no plate tectonics; its outer shell behaves more like a single lid that cools slowly rather than recycling. Whether faults formed early in a planet’s history can stay weak enough to matter billions of years later has been argued about theoretically and never directly confirmed, because the geological record alone cannot distinguish a fault that reactivated today from one that has been dead since the Hesperian. A dated, located seismic event with a fault mechanism is exactly the missing measurement. Fault zones in a stagnant lid heal slowly, and stress fields evolve slowly, so the two together can keep old structures marginally close to failure indefinitely. The motion at the bottom of Valles Marineris is happening on the same fault that was already there when the canyon opened, under a stress field that has drifted slowly since.

Song put it plainly: “faults deep beneath the canyon can still slip and generate marsquakes” (Lamont-Doherty Earth Observatory, Columbia Climate School). The InSight mission ended in 2022 and its seismometer is off, but the recordings it returned are still producing results five years on. This one is the clearest evidence yet that a planet’s oldest fractures are still part of its present.