Dark Horizon Marks Intriguing Layer on Mars

HiRISE images reveal an intriguing marker horizon among the rocks near Mount Sharp is Gale Crater, which is more erosion resistant than layers above and below.

Beth Johnson
2 min readMay 19, 2022


IMAGE: The marker horizon (yellow arrow) is darker, smoother, and harder than the sulfate-bearing rocks surrounding it. HiRISE image merged to a Digital Terrain Model (DTM) perspective view showing approximately 1 kilometer of topography across Mount Sharp. Image features three times vertical exaggeration. CREDIT: NASA/University of Arizona

A press release was issued from the Planetary Science Institute that looks at an intriguing darker layer of rocks on Mars at Mount Sharp, near where Curiosity is roving. This layer is “darker, stronger, flatter” than the surrounding sulfate-bearing rocks and has been observed using orbital data from the HiRISE camera onboard the Mars Reconnaissance Orbiter. The layer is also weathering differently than the layers above and below, suggesting is made of a more erosion-resistant rock.

Because of the stand-out nature of this rock layer, geologists refer to it as a “marker horizon”. We use marker horizons on Earth to constrain time-stratigraphic features where we know that the layer formed during one single event or a specific period of time. That layer then marks the rocks below as deposited before the event and therefore [older], and the rocks above as deposited after the event and [younger]. Think of the KT boundary and the iridium that marks “below here are dinosaurs and above here, not so much.”

On Mars, that means something interesting happened to cause a break in the sedimentary deposits. Lead author Catherine Weitz explains: Some event occurred within Gale crater during the deposition of sulfate-bearing sediments that resulted in a different kind of rock unit. The marker horizon is distinct in appearance from the sulfate-bearing rocks above and below it, indicating an environmental change occurred for a brief time, such as a drier period, or perhaps a regional event like an explosive eruption from a nearby volcano that deposited ash across a large area which included Gale crater.

It could even be the same rocks as above and below but the marker horizon became harder and more resistant to erosion for some reason. As Weitz notes: All of these potential origins require the presence of at least some water to cause cementation that hardened the horizon. Our orbital observations currently favor [a hardened] sulfate or volcanic ash origin, but we will have to wait until the Curiosity rover reaches the horizon in the coming months before determining which origin is most plausible.

And that gives us something to look forward to. We’ll bring future results of Curiosity’s exploration right here on Daily Space.

This work was published in the Journal of Geophysical Research: Planets.

For more on this story, check out my interview with Cathy Weitz.

More Information

PSI press release

Orbital Observations of a Marker Horizon at Gale Crater,” Catherine M. Weitz et al., 2022 March 31, JGR Planets

This story was written for the Daily Space podcast/YouTube series. Want more news from myself, Dr. Pamela Gay, and Erik Madaus? Check out DailySpace.org.



Beth Johnson

Planetary scientist, podcast host. Communication specialist for SETI Institute and Planetary Science Institute. Buy me a coffee: https://ko-fi.com/planetarypan