Perseverance has just stumbled upon one of the strangest rocks on Mars to date — a rock covered in a black spherical structure with a mysterious texture that has scientists excited. While similar volcanic bodies have been spotted before, this sphere, known as "St. Paul's Bay", is particularly striking. Could it be a relic of volcanic activity, groundwater activity, or even a cosmic impact? Determining its origin could reshape our understanding of the geological history of Mars.

The image was taken by NASA's Perseverance rover, a fusion super-camera long-range microimager (RMI) stitched image showing part of the St. Paul's Bay target, which is located in the lower region of Hazel Witch Hazel on the edge of Jezero Crater. The images reveal hundreds of peculiar spherical objects that make up this rock. Perseverance took this image on 1442/0/0, the 0th Martian day of the Mars 0 mission. Photo credit: NASA/JPL-Caltech/LANL/CNES/IRAP

The Perseverance science team has recently taken a keen interest in an extremely rare rock – one that appears to be made up of tiny spheres hundreds of millimeters in size. Currently, the team is working to find out how these peculiar traits came to be.

The discovery took place at Cape Broome, on the low slope of Hamael Witch Hazel, along the edge of Jezero Crater. Perseverance arrived there two weeks ago to study a series of light and dark rock formations that were first discovered from orbit. Just last week, the Mars rover successfully wore down and took a sample of one of the light-colored rock formations. It was at this sampling point that the Mars rover noticed an unusually peculiar texture of a nearby rock.

This map shows the route that NASA's Perseverance Mars rover will take along the western edge of Jezero Crater (blue area), which will first reach "Castle Dox", then survey the "Turkino Peak" area, and finally approach "Witch Hazel Mountain". Image credit: NASA/Caltech Jet Propulsion Laboratory/University of Arizona

The rock is now named "St. Paul's Bay". Pauls Bay,), which is covered with dark gray spherical features - some round, others elongated or oval, and some with sharp edges, possibly fragments of a broken sphere. Some even come with tiny pinholes. What kind of geological processes can create such a variety of strange shapes?

美國宇航局的“毅力號”火星探測器使用其左桅杆攝像頭-Z（Left Mastcam-Z）拍攝了這張“聖保羅灣”目標（畫面右側深色浮標）的照片。左桅杆攝像頭是探測器遙感桅杆高處的一對攝像頭之一。“毅力號”於2025年3月13日（火星2020任務的第1444個火星日）拍攝了這張照片，當時當地平太陽時為11:57:49。圖片來源：NASA/JPL-Caltech/ASU

This is not the first time that strange spheres have been discovered on Mars. In 2004, the Opportunity Mars rover discovered the so-called "Martian blueberry" on the meridian plateau. Since then, the Curiosity rover has also found spheres in the rocks of Yellowknife Bay near Gale Crater.

Just a few months ago, Perseverance discovered popcorn-like sedimentary rock textures in the Jezerol crater inlet waterway in the Neretva Valley. In these cases, these spheroids are interpreted to be nodules, which are formed when groundwater circulates in the pores of the rock. However, not all spheroids are formed in this way, they are also formed by the rapid cooling of lava droplets during volcanic eruptions on Earth, or by the condensation of rocks evaporated by meteorite impacts.

Each formation mechanism has a very different impact on the evolution of these rocks, so the team is working to determine their context and origin. However, the rocks of St. Paul's Bay are pumice rocks – a term used by geologists to describe material that is not in place.

The team is currently working to link the chondrite-rich textures observed in St Paul's Bay to the wider stratigraphy of Mount Witch Hazel, and initial observations have provided tantalizing indications that it may be related to one of the dark-toned layers that the team found from orbit. Placing these features in a geological context is essential to understand their origins and to determine their significance to the geological history of the Jezero crater rim and beyond.

By Alex Jones, PhD student, Imperial College London