The Mystery of Aganippe Fossa

Snake scar on Mars

This image from ESA’s Mars Express shows Aganippe Fossa, a fascinating groove at the base of Mars’s colossal Arsia Mons volcano. Credit: ESA/DLR/FU Berlin

A fascinating feature takes centre stage in this new image from ESA’s Mars Express: a dark, uneven scar cutting through the marble ground at the base of a gigantic volcano.

This scar, known as Aganippe Fossa, is an irregular, approximately 600 km long feature known as a graben: a trench-like groove with steep walls on either side.

Aganippe Fossa cuts through the lower flank of one of Mars‘s largest volcanoes, Arsia Mons. Mars Express regularly observes Arsia Mons and its nearby companions in the Tharsis region, home to several of Mars’s giant volcanoes. This includes Olympus Mons, the tallest volcano in the solar system (visible on the context map accompanying this new image, as is Arsia Mons).

Arsia Mons itself is 435 km in diameter and rises more than 9 km above the surrounding plains. For context, the highest dormant volcano on Earth, Ojos del Salado on the border between Argentina and Chile, is less than 7 km high.

Broader view of Aganippe Fossa

This image shows Aganippe Fossa, a winding groove at the base of the giant Arsia Mons volcano on Mars, in a broader context. The area outlined by the larger white box shows the region captured by the High Resolution Stereo Camera on board ESA’s Mars Express orbiter on 13 December 2023 during orbit 25189, while the smaller white box shows the part of the surface seen in these new images. Credit: NASA/MGS/MOLA Science Team

Seeping lava

We still don’t know how and when Aganippe Fossa formed, but it seems likely that it was formed when magma rose from beneath the colossal mass of the Tharsis volcanoes, stretching and tearing the Martian crust.

In this image, Mars Express’s High Resolution Stereo Camera (HRSC) captures two different types of terrain: so-called hilly terrain, which consists of many irregularly shaped hills and valleys all close together, and lobate terrain, which consists of gently rolling cliffs and rocky scree.

Topography of Aganippe Fossa

This colour-coded topographic image shows Aganippe Fossa, a winding groove at the base of the giant Arsia Mons volcano on Mars. The image was created using data collected by ESA’s Mars Express on 13 December 2023 and is based on a digital terrain model of the region, from which the topography of the landscape can be inferred. Lower parts of the surface are shown in blue and purple, while higher areas are shown in white and red, as indicated on the scale at the top right. Credit: ESA/DLR/FU Berlin

These terrains are characteristic of Arsia Mons’ ring-shaped ‘halo’, a 100,000 square kilometre disk around the base of the volcano, possibly associated with ancient glaciers. Interestingly, this halo is only built up on the northwestern flank of the volcano, probably because prevailing winds from the opposite direction determined where ice settled over time.

Windblown dust and sand have also shaped this patch of Mars, creating interesting zebra-like patterns on the right of the image as darker material is deposited on lighter ground (or vice versa!). The surface here also shows evidence of lava flows, dating back to the time when the volcano was active.

Aganippe Fossa in 3D

This stereoscopic image shows Aganippe Fossa, a winding groove at the base of the giant Arsia Mons volcano on Mars. The anaglyph provides a three-dimensional view when viewed through red-green or red-blue glasses. Credit: ESA/DLR/FU Berlin

Exploring Mars

Mars Express has been orbiting the Red Planet since 2003. It is imaging the Martian surface, mapping its minerals, identifying the composition and circulation of its thin atmosphere, investigating the subsurface of the Martian crust, and studying how various phenomena in the Martian environment interact.

The spacecraft’s HRSC, responsible for these images, has revealed much about the diverse surface of Mars over the past 20 years. The images reveal everything from wind-sculpted ridges and grooves to sinkholes on the flanks of colossal volcanoes to impact craters, tectonic fractures, riverbeds and ancient lava pools. The mission has been enormously productive throughout its lifetime, delivering a far more complete and precise understanding of our planetary neighbor than ever before.

The High Resolution Stereo Camera (HRSC) on the Mars Express spacecraft is an advanced imaging system designed to study Mars in great detail. Launched by the European Space Agency (ESA) in 2003, the HRSC produces high-resolution, three-dimensional images of the Martian surface, allowing scientists to study the planet’s topography and morphology in unprecedented detail. This camera system uses stereo imaging techniques to produce color images along with topographic maps, allowing researchers to analyze the geology, composition, and physical processes of Mars. The HRSC has been instrumental in providing insights into the planet’s past water activity, volcanic activity, and other dynamic processes.

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