Martian butterfly crater reveals low angle impact and buried lava history

by Erica Marchand
Paris, France (SPX) Dec 08, 2025

ESA's Mars Express orbiter has observed an impact crater in Mars's northern lowlands whose shape resembles a butterfly, created when an incoming space rock struck the surface at a shallow angle. The main crater, which appears somewhat like a walnut from orbit, measures about 20 km east to west and 15 km north to south and sits within the Idaeus Fossae region. The impact threw material preferentially to the north and south, forming two lobes of ejecta that extend as raised, wing-like terrain.

Under typical impact conditions, debris spreads out in all directions, but in this case the oblique trajectory produced an oval crater and asymmetric ejecta. Some of the material forming the wings, especially to the north of the crater, appears smoother and more rounded than its surroundings, similar in form to a mudflow. This morphology indicates that the ejecta mixed with subsurface water or ice, which likely melted during the impact and allowed the debris to flow before solidifying again. Planetary geologists describe such deposits as fluidised ejecta, a style of impact deposit commonly observed on Mars.

Another butterfly-shaped crater, located in Mars's southern highlands at Hesperia Planum, has been documented previously, providing a second example of this type of low-angle impact structure on the planet. Together, these sites help researchers understand how impact angle and target conditions combine to shape craters and their ejecta blankets.

Crumpling lava

The terrain west of the crater contains a field of steep, flat-topped mesas that stand above a largely level plain. These rocky outcrops are remnants of once more extensive high-standing layers that have been progressively eroded, leaving only the most resistant material as isolated hills. Dark, laterally continuous bands exposed along the mesa flanks are interpreted as layers rich in iron- and magnesium-bearing minerals of volcanic origin, such as basaltic lava or ash.

Geological evidence suggests that Idaeus Fossae experienced repeated episodes of volcanic activity, with lava and ash accumulating in layers that were later buried and then exhumed by erosion. Additional signs of volcanic and tectonic modification appear as wrinkle ridges, folded structures that likely formed when broad sheets of lava cooled, contracted and were compressed, causing the surface to buckle. These ridges, together with the mesas and the impact structures, record a complex sequence of volcanic, erosional and impact processes in this sector of Mars's northern lowlands.

The broader setting of the butterfly crater lies near the Idaeus Fossae valley system, a network of troughs located a few kilometres west of the crater. One such valley cuts across the northern portion of the area, while more subdued valleys and ridges occur throughout the region. Most of the Idaeus Fossae system occupies ground adjacent to a steep, roughly 2 km high scarp that marks the boundary of the Tempe Terra plateau, a major tectonic and volcanic province in the Martian crust.

Mars Express has been orbiting Mars since 2003 and has built up a global data set of the planet's surface in colour, stereo and at high spatial resolution. Over more than two decades, the mission's observations have contributed to scientific understanding of Martian geology, climate history and water distribution. The High Resolution Stereo Camera (HRSC) instrument, developed and operated by the German Aerospace Center (DLR), acquires stereo and colour images that teams at DLR's Institute of Space Research in Berlin-Adlershof and the Planetary Science and Remote Sensing group at Freie Universitat Berlin process into visual products used for scientific analysis and public release.