Planetary Sciences Research Spotlight

"Knobby Terrain" a Sign of Mars's Explosive Past

Newly identified knobby terrain related to ancient volcanoes on Mars hint that pyroclastic ash and rock flowed down slopes early in the red planet's history.


The red planet’s upper crust is brittle and weak. Planetary geologists often attribute this to effusive eruption—lava pouring out of volcanoes onto the ground—early in Mars’s history that later weathered down. However, some have suggested that the friable materials were created by widespread ash-laden explosive volcanoes that were eroded by geologic processes over the course of Martian history.

Scientists have had a tough time sorting out these different origin theories because ancient volcanoes were ultimately buried by lava from more recent eruptions. However, several volcanic regions identified in recent years are home to some of the red planet’s oldest volcanoes. These ancient regions, Circum-Hellas and Arabia Terra, also hint at a history of explosive eruptions.

Huang and Xiao have identified 75 additional ancient volcanoes across Mars’s surface. Using high-resolution satellite images and heat-sensing instruments, the researchers found a unique knobby terrain in 17 of those volcanoes. The pair also noticed similarities to eroded features on Earth called ignimbrites, which form as a result of huge explosions of pyroclastic ash and rock flowing down volcanic slopes. The comparison is supported by spacecraft data showing that the features are made of a fine-grain material. Spectral data also indicate that the knobby features were eroded by liquid at some point in the past.

The scientists interpret their observations as a sign that these knobby features formed in pyroclastic explosions early on Mars and were later eroded. If the finding is correct, it would lend support to the theory that explosive volcanism was dominant on Mars in the distant past. (Geophysical Research Letters, doi:10.1002/2014GL061779, 2014)

—Eric Betz, Freelance Writer

Citation: Betz, E. (2015), “Knobby Terrain” a sign of Mars’s explosive pPast, Eos, 96, doi:10.1029/2015EO024831. Published on 27 February 2015.

© 2015. The authors. CC BY-NC 3.0