The mission PerseverancePerseverance Continue to deliver spectacular results from the Jezero crater. Among the recent discoveries is the identification of singular structures in sedimentary rockssedimentary rocks Unity clay Bright Angelnicknamed “tasks leopardleopard By researchers.
Their detailed study brings precious clues to the chemistry of ancient Martian water, the sedimentary evolution of Lake Jezero and leaves open the porteporte potential biological activity in the past in March.
Intriguing textures in a sedimentary rock
Table of Contents
It’s the sample Sapphire Canyontaken from Cheyava Falls By Perseverance, which particularly intrigued the researchers. Thanks to its micro-imagery and spectroscopy instruments (Pixl XRF, Sherloc), the rover has indeed highlighted the presence of small spots from 200 µm to 1 mm in diameter, in the form of circular or crenellated fronts.
These structures have a contrastcontrast Marked with the clay matrix: a darker edge and a lighter heart, itself less red than the rest of the rock. Random distribution and morphologymorphology irregular from these spots exclude the hypothesis that these are sedimentary grains exogenousexogenous deposited in the clay deposit still not consolidated. Researchers see it as reaction fronts formed on-sitein other words post-depot chemical modifications.
Pixl analyzes reveal an iron enrichment, phosphorus and zinc in dark contours, and a heart enriched in sulfursulfurfer, nickelnickel and zinc. This chemical signature suggests Authoric precipitation processes involving oxid-reductive reactions and the presence of mattermatter organic.
Redox reactions at the heart of the scenario
Hurowitz’s article and al. (2025), published in the journal Naturethus offers a model in which theoxidationoxidation organic matter, present in sedimentssediments Shortly after their deposit, served as a redox engine. This oxidation would have provided electronselectrons necessary for the reduction of ferric iron (fe³⁺) contained in mineralsminerals ferrous ferrous rock (Fe²⁺). This fe²⁺ would then have reacted with the phosphatephosphate Available to form iron phosphates such as vivianitis and its oxidation products, responsible for the dark edges of tasks.
At the same time, the reduction of sulfate in sulfursulfurcoupled with the oxidation of organic matter, would have resulted in the precipitation of iron sulphides (for example, greigite), which is found in the hearts of spots. The local discoloration of the matrix, less red, corroborates the reduction of the fe³⁺.
These reactions would have taken place at low temperature, in conditions compatible with sediments saturated with water or weakly buried, which reinforces the interest of these structures as witnesses of the chemistry of Martian water on a microscopic scale.
An organic matter with multiple origins
This reaction scenario raises several questions, notably on the origin of organic matter in claye. One might think that “organic matter” necessarily rhymes with “biological activity”. But that is not true.
If life is of course a large producer of organic matter, many reactions abioticabiotic can also be at the origin. Three main scenarios are thus mentioned to explain their presence in Martian rocks:
- an exogenous contribution by meteoritesmeteorites or interplanetary dust;
- Abiotic synthesis on-site by reactions prebioticsprebiotics ;
- an ancient biological origin, produced by microorganismsmicroorganisms in an environment aqueousaqueous.
Whatever its origin, this organic matter was necessarily available very early to participate in redox reactions.
History of Mars: Water flows in the old Neretva valley (animation). © JPLraw
Land parallels and astrobiological implications
If for the moment no scenario can be privileged, it should be noted that this type of oxid-reductive reaction is well known on earth … and often associated with microbial activity.
Some bacteriabacteria (as Geobacter spp. or Shewanella spp.) Can use ferric iron (fe³⁺) as an electron acceptor in an environment poor in oxygenoxygenthus allowing the oxidation of organic matter. A reaction leading to the formation of vivianite, iron sulfides and the discoloration of the sedimentary matrix.
It is therefore not uncommon to find vivianite nodules in lakes or sediments. These analogues reinforce the idea that the “leopard spots” of Martian rocks could be the result of interactions between organic and mineral matter, with possibly the intervention of a biological activity.
Potential biosignatures
However, the authors remain cautious: these observations do not constitute direct proof that an past life existed on Mars. However, they are considered to be potential biosignatures – characteristics compatible with biological processes.
Confirmation of such a hypothesis will require a further analysis of samples, in particular their return to earth, where high -precision isotopic and spectroscopic techniques may (finally) decide.
