The Moon's Giant Volcanoes Were Hiding in Plain Sight
Two rises on the Moon have been identified as enormous shield volcanoes. Their slopes are so slight that impact scars and younger lava helped them disappear into the landscape.
The Discovery
Planetary scientists have identified two broad lunar structures as giant shield volcanoes, each roughly 80 kilometres across. The Prinz-Harbinger and Theophilus shields are far larger than the confirmed lunar shields previously known, which were generally under 30 kilometres wide.
The peer-reviewed Geophysical Research Letters study combined spacecraft topography, imagery and gravity data to map their shapes and probe what lies below. A Nature research highlight published on 20 August drew attention to the pair.
What Makes Them Strange
These are not steep cones. Both rise only about half a kilometre across landscapes tens of kilometres wide, producing slopes of less than one degree. That low profile allowed craters, irregular terrain and later lava deposits to break up their outlines.
- Prinz-Harbinger measures about 74 by 72 kilometres and rises roughly 590 metres.
- Theophilus spans about 101 by 75 kilometres and rises roughly 541 metres.
- Both have summit depressions consistent with volcanic vents or collapse features.
On the Moon, a mountain can hide by barely looking like a hill.
The Implications
The researchers interpret the landforms as shields built by repeated flows of fluid lava. Gravity evidence beneath Prinz-Harbinger is also consistent with buried magmatic material, leading the team to propose that shallow magma storage may have fed at least one of the volcanoes.
That reservoir remains an interpretation of remote-sensing data, not a chamber directly sampled or imaged. But the scale of the shields expands the known range of lunar volcanism and suggests some eruption systems were more sustained and structurally complex than the small domes and shields had implied.
What Happens Next
The discovery came from re-reading existing spacecraft observations rather than landing at a new site. Applying the same combination of subtle topography, summit morphology and gravity measurements elsewhere could expose more volcanic giants flattened into the lunar background.
The Moon has been mapped in extraordinary detail. The surprise is that detail can still contain objects too large — and too gentle — to notice at first glance.