“On Aug. 31, the 1,610th sol of its mission, Perseverance captured an image of this rock, which looks like the head of a turtle coming out of its shell.” The Jet Propulsion Laboratory definition of NASA is as direct as it is suggestive. The photograph, taken in Jezero Crater, is the latest example of pareidolia man’s ability to recognize with familiar shapes in miscellaneous arrangements but behind the playfulness lies a genuine geological inquiry into how Mars’ surface was formed into such shape.
The photo was captured using the SHERLOC instrument of the rover, which is mounted on the robotic arm turret. SHERLEC is paired with WATSON (Wide Angle Topographic Sensor for Operations and eNgineering), a high-resolution imager, to capture targets after ultraviolet light has excited them. This union allows researchers to investigate surface texture at a micro-level, identifying mineral contours, grain boundaries, and erosion features that would otherwise go unseen by the human eye.
The capability of perseverance to capture such high-fidelity images is due to a sophisticated suite of optical systems. While Mastcam-Z, the rover’s dual zoom-able camera system, images terrain in high-resolution stereo from atop the mast, SHERLOC and WATSON take close-up context-rich photos. By synergizing these instruments, scientists are able to link macro-scale geological features with micro-scale mineralogical hints, building layered understanding of Mars’ environmental past.
The turtle-shaped rock is more than a curiosity. Its form can record the play of wind erosion, sediment deposition, and chemical alteration over billions of years. Wind is a master sculptor on the Red Planet. Although the planet’s atmosphere is only 1% as thick as Earth’s, it is strong enough to transport dust and sand-sized material, equivalent to sandpaper, slowly weathering out exposed rock. Over time, these processes can shape sharp edges, smoothed surfaces, and isolated shapes that encourage anthropomorphic interpretation.
In Jezero Crater, where Perseverance has been working since February 2021, the geological record is complicated. The western rim of the crater, including the Witch Hazel Hill area, is a combination of shattered igneous rocks ripped from the subsurface by ancient impacts and layered sedimentary deposits formed by water. Some of the rocks, for example, spherule-rich “Horneflya” and “St. Pauls Bay,” indicate groundwater interaction that resulted in mineral concretions. Others exhibit volcanic or impact origin. The formation process of the turtle-shaped rock is still being researched, but patterns on its surface could be connected to the same erosional and deposition histories.
Wind-driven aerosol processes on Mars can also align rocks in remarkably regular arrays. Experiments incorporating wind tunnel simulations, field measurements, and modeling have shown that small pebbles can travel upwind as sand is eroded in front of and deposited behind them. As the cycle is repeated over time, such a process can produce evenly spaced distributions, a phenomenon observed by earlier rovers. Even though the turtle rock is a single stand-alone shape, its direction and position may still signify such aeolian processes.
Perseverance’s image of the turtle rock is one of many examples of a vast scientific effort to chart the surface of the planet with unparalleled accuracy. Each photo, from whimsical sketch to mundane outcrop, is a data point in reconstructing Mars’ geologic and climatic past. As project scientist Katie Stack Morgan of the mission noted in describing the rover’s activity at the edge of the crater, “There are new and intriguing rocks everywhere the rover turns. It has been all we had hoped for and more.”
The fact that people were intrigued by the appearance of the turtle on Mars betrays a deeper truth: these visual memories of terrestrial shapes are portals into entrance into planetary science. Behind the entertaining spin-offs is the laborious work of deciphering mineral composition, erosion rates, and depositional regimes the road to determining if conditions for life ever occurred on Mars, and how they have been altered over time.
