If you’re fascinated by Mars, here’s the short version: magnesium–iron carbonates are chemical time capsules. They form when CO₂-rich water reacts with rock, and they lock away clues about ancient atmosphere, water chemistry, and — maybe — preserved organics. Knowing where rovers have actually drilled into rocks that contain Mg–Fe carbonates (rather than just seeing hints from orbit) is a big step forward.
In friendly guide I’ll walk you through the actual drill sites where rovers have encountered Mg–Fe (and closely related Fe-bearing) carbonates, the geological settings of those locations, what the drills and instruments did, what the mineral chemistry implies, and why these findings matter for Mars’ climate and habitability story. I’ll keep the science precise but readable — think geology explained over coffee.
Quick roadmap: the rovers that matter
Two rover programs are central to this story. NASA’s Curiosity (Mars Science Laboratory) has been exploring Gale Crater since 2012 and drills small powder samples into onboard labs (CheMin and SAM). Curiosity’s recent in-situ detection of an iron carbonate (siderite) in several drilled samples is a headline result. NASA’s Perseverance (Mars 2020), operating in Jezero Crater since 2021, drills and caches 6-cm long rock cores for possible return to Earth; it has encountered and collected rock cores from igneous and sedimentary units that contain magnesium- and iron-carbonates. I’ll treat each rover and its drill results in turn and tie the chemistry back to the places where the rocks were collected.
How rover drilling works — tiny cores, big science
Rover drills are not like oil rigs. Curiosity’s percussion drill produces a powder from a 1.6-cm diameter hole up to roughly 5 cm deep; that powder is delivered to internal instruments. Perseverance’s coring system extracts intact cylindrical cores about 13 mm in diameter and typically ~60 mm long and places them in sealed sample tubes for caching and (hopefully) future return to Earth. Curiosity’s powdered samples are analyzed on Mars by instruments that measure mineralogy and evolved gases; Perseverance’s cores are preserved for later, higher-precision study on Earth. The two approaches are complementary: Curiosity gives immediate, in-place mineralogical context; Perseverance preserves the actual rock material for detailed future lab work.
What do we mean by “magnesium–iron carbonates”?
Magnesium–iron carbonates are carbonate minerals containing Mg²⁺, Fe²⁺ (or both), and CO₃²⁻. Examples include solid solutions between magnesite (MgCO₃) and siderite (FeCO₃). They may form as discrete carbonate minerals, as mixed cation phases (Fe–Mg carbonates), or as alteration products in olivine-rich rocks. The presence of Mg and/or Fe in carbonates provides clues about the source rocks (e.g., olivine/ultramafic rocks release Mg and Fe) and about the redox and pH conditions during mineral formation.
Curiosity’s big contribution: siderite in Gale Crater
Curiosity made headlines when analyses of drilled powder from multiple sites in Gale showed evidence for an iron carbonate — siderite — in amounts substantial enough to be meaningful. The drill samples that produced the siderite signal came from specific drill locations on the slopes of Mount Sharp (the central mound within Gale Crater): among the named targets often discussed are Tapo Caparo, Ubajara, and Sequoia (and related drilled targets in that stratigraphic interval).
CheMin (an X-ray diffractometer in Curiosity’s belly) and the SAM evolved gas analyses provided the mineralogical and thermal decomposition fingerprints that point to siderite. The discovery is important because siderite forms when iron and carbonate come together in relatively reducing conditions — a record of past aqueous chemistry — and because the siderite appears in layers that also contain sulfates, suggesting later alteration and a dynamic history.
Exactly where in Gale did Curiosity drill for siderite?
Curiosity’s drilling campaign that yielded siderite focused on a stratigraphic package within Mount Sharp where sulfate-rich layers overlie other sediments. The specific drill targets reported in the peer-reviewed work include sites labeled in the mission’s shorthand such as Tapo Caparo, Ubajara, and Sequoia (these labels correspond to rock targets Curiosity examined and cored during its ascent). CheMin detected siderite in the powders from these drill holes, with abundances reported up to roughly ten percent by weight in some samples — not trace levels but a meaningful mineral component. This is in contrast to earlier carbonate detections that were thin coatings or trace phases; here, siderite appears as a genuine constituent of the rock fabric at these drilled depths.
Why was finding siderite in drill holes surprising?
For decades, orbital remote sensing had only found limited carbonate occurrences on Mars, and many models expected more carbonate sequestration of atmospheric CO₂ than was observed. Additionally, the sulfate layers that overlay the siderite-bearing rocks are bright and spectrally dominant at orbital wavelengths, so any carbonate deeper in the stratigraphy can be masked from orbit. That means the siderite was essentially hidden under terrain that looked carbonate-poor from space. Drilling revealed what the surface spectra could not: a subsurface record containing iron carbonates. This underscores how much of Mars’ mineral story can be invisible from orbit and how valuable direct sampling is.
How Curiosity analyzed the drill powders
Curiosity’s drill powders are delivered to CheMin for X-ray diffraction (to get mineralogy) and to SAM for evolved gas analysis and volatile chemistry. CheMin can quantify crystalline minerals present down to low percentages; SAM heats powders and measures gases released at characteristic temperatures (for example, a CO₂ release peak near ~400°C is consistent with carbonate decomposition). Combined, these instruments let scientists detect and quantify siderite even when orbital data missed it. Researchers reported siderite detections consistent with both CheMin mineralogy and SAM thermal evidence, boosting confidence in the identification.
Geologic context in Gale — what the rocks are telling us
The siderite-bearing drill targets sit within layered sediments that record changing aqueous environments. The presence of both sulfates and siderite suggests fluctuations in water chemistry, redox conditions, or fluid sources over time. One plausible narrative is that groundwater or shallow lake waters precipitated siderite under reducing or neutral conditions and later conditions became more oxidizing and sulfate-rich, obscuring the carbonate signature from orbit but leaving it preserved inside rock layers. This dynamic sequence offers a picture of an environment that changed over time, rather than a single, static climate regime.
Perseverance and Jezero: cores from olivine-rich, carbonated units
Perseverance has a different but complementary story. The rover has cored and cached dozens of samples in Jezero Crater’s ancient delta, fan margin, and crater floor, and analyses of traverse geology and cached samples indicate the presence of magnesium- and iron-rich carbonates in certain igneous and altered igneous rocks. High-level results from recent papers describe “carbonated ultramafic igneous rocks” encountered along Perseverance’s traverse — rocks composed of olivine plus Mg- and Fe-carbonates and phyllosilicates. That means Perseverance has both drilled carbonate-bearing rocks and collected intact cores that include Mg–Fe carbonate phases, providing the precious raw material for future laboratory work on Earth.
Which Perseverance drill cores are relevant to Mg–Fe carbonates?
Perseverance’s sampling campaign produced many named cores; among these, a handful have been emphasized in the science literature and mission releases for containing carbonated ultramafic material or hydrated carbonate signatures. One high-profile sample discussed in public and scientific communications is the July 2024 core nicknamed “Sapphire Canyon” (collected from a rock informally called “Cheyava Falls”), which has shown unusual mineral textures and has been highlighted as a promising sample with potential biosignature-preserving properties.
Beyond individual sample names, the broader floor and rim traverse of Jezero encountered olivine-rich units that appear to have been significantly carbonated; Perseverance’s cored samples from these units are therefore direct physical records of Mg–Fe carbonate occurrence. The peer-reviewed Science report on carbonated ultramafic rocks documents these lithologies and sample contexts.
SHERLOC’s role — detecting hydrated carbonate signatures in situ
Perseverance carries SHERLOC, a deep-UV Raman and fluorescence spectrometer optimized for detecting organic molecules and mineral vibrational signatures at micrometer scales. SHERLOC has reported detections consistent with hydrated carbonate phases in some margin sediments — the first in-situ identification of hydrated carbonate by a landed mission on Mars. Hydrated carbonates are especially interesting because they imply formation in the presence of liquid water under relatively mild conditions and because they are potentially better at trapping and preserving organics than anhydrous carbonates. Those SHERLOC observations add mineralogical nuance to the cored material and guide which cached samples should be prioritized for return.
What does “carbonated ultramafic” mean in Jezero?
Ultramafic rocks are rich in olivine and pyroxene — minerals high in magnesium and iron. If those rocks experienced alteration by CO₂-bearing fluids, their olivine can weather and release Mg²⁺ and Fe²⁺ that then recombine with carbonate ions to precipitate magnesite, siderite, or mixed Mg-Fe carbonates. The phrase “carbonated ultramafic” therefore describes igneous rocks that have undergone widespread carbonation: original silicate minerals converted (partially) into carbonate minerals. Finding these rocks in Jezero implies that local bedrock chemistry (olivine abundance) and fluid availability combined to produce Mg–Fe carbonates at some point in the crater’s history.
Details of the Perseverance drilling campaign (how cores were collected)
Perseverance’s drill acquires cylindrical cores approximately 13 mm in diameter and ~60 mm long. The rover abraded candidate targets to determine the best coring spots, then used a percussive-rotary coring operation to free the core and capture it in a sterile sample tube. The tubes are sealed and stored in an organized cache; some were deposited in a depot for potential future retrieval. Because the cores are intact, Earth labs can perform high-precision isotope geochemistry, mineral mapping, and organic detection in ways that are impossible with rover instruments alone. This coring approach contrasts with CheMin/SAM style analyses on powdered material but is the gold standard for long-term, contamination-controlled study.
Where in Jezero were Mg–Fe carbonates drilled? Geologic neighborhoods
Perseverance sampled across multiple geomorphic units: the western fan and margin units, the Bright Angel and Masonic Temple exposures near Neretva Vallis, and igneous exposures toward the crater rim. Carbonated ultramafic rocks have been reported from floor exposures and higher-elevation rim rocks, with more pronounced aqueous alteration lower in elevation. The samples that contained Mg–Fe carbonate phases were obtained from these olivine-rich, aqueously altered igneous rocks and from sedimentary margin rocks that show evidence of hydrated carbonate. The spatial diversity of these finds suggests carbonation was not strictly local to the delta but affected a broader part of the Jezero basin and its immediate surroundings.
How abundant are the carbonate phases in drilled samples?
Abundance varies by site. In Curiosity’s Gale samples, siderite was present up to roughly ~10% by weight in some drilled powders — significant for a mineral that had been previously elusive. In Jezero, published reports describe Mg–Fe carbonates as abundant enough to characterize in cored rocks, though the exact percentage varies with lithology and degree of alteration. The key point is that these carbonates are not merely trace coatings in many drilled sites; they can be primary alteration products within the rock matrix and therefore meaningful for geochemical budgets and preservation potential.
What the mineral chemistry implies (redox, temperature, and pH)
Iron-rich carbonates like siderite preferentially form under relatively reducing conditions where Fe²⁺ remains stable. Mg-rich carbonates form where Mg²⁺ is abundant and conditions favor magnesite or mixed Fe–Mg carbonate precipitation. The coexistence of clays, carbonates, and later sulfates implies variable pH and redox conditions through time — maybe neutral/alkaline conditions conducive to carbonate precipitation at one stage, followed by more acidic or oxidizing episodes that produced sulfates and iron oxides. Temperature indicators in the mineral assemblages and models suggest many of the carbonate formations occurred at low to moderate temperatures consistent with surface or near-surface aqueous alteration rather than high-temperature hydrothermal processes in most places.
Why these drilled carbonates matter for Mars’ carbon story
Carbonates lock CO₂ into rock. If sufficient carbonates formed on ancient Mars, they could explain how a once thicker CO₂ atmosphere was sequestered, contributing to planetary cooling. The discovery of measurable siderite in Gale drill holes suggests that portions of Mars did store carbon as carbonate more widely than orbital data had indicated. Likewise, Mg–Fe carbonates in Jezero indicate local sequestration in olivine-rich units. Together, rover drill results imply that carbonate sequestration was both locally important and sometimes underrepresented in orbital remote sensing, which helps refine models of Mars’ atmospheric evolution.
Preservation potential: can Mg–Fe carbonates hold organics?
On Earth, carbonates are known to preserve organic molecules and microtextures, sometimes exquisitely well. Hydrated carbonates and fine-grained carbonate muds are especially good at trapping organics. That’s why SHERLOC’s detection of hydrated carbonates in Jezero margin rocks is exciting: those rocks combine mineral hosts that can preserve organics with the low-energy depositional settings (deltas, shorelines) best for burying and protecting fragile molecules. Curiosity’s discoveries in Gale also show that carbonates can survive and be preserved beneath later layers. The drilled cores and powders therefore represent prime material for searching for preserved organic chemistry — but rigorous lab work on Earth is essential for unambiguous identification.
Limitations and caveats — what drilling can’t tell us yet
Rover instruments are powerful but have limits. Curiosity’s powders cannot deliver the ultra-fine isotopic measurements and microscopic imaging that Earth labs provide. Perseverance’s cached cores can be returned, but that requires Mars Sample Return (a complex, multi-mission effort) and time. Orbital masking by dust, surface coatings, or sulfate layers means we’re often watching the “top of the book” rather than the pages inside it; drilling reveals more but still only samples local heterogeneity. Finally, mineral identification sometimes rests on combining multiple indirect lines (CheMin + SAM, or SHERLOC + PIXL), so interpretations must be integrated and conservative. Nonetheless, the results to date are strong and game-changing.
What’s next — sample return and more in-place work
The obvious next step is to get the best carbonate-bearing cores back to terrestrial labs for isotope geochemistry, organic biomarker searches, and nanoscale imaging. Perseverance’s caching strategy was explicitly designed with that goal in mind. Meanwhile, continued in-situ analyses (SHERLOC, PIXL, SuperCam) and orbital reprocessing will refine maps of where Mg–Fe carbonates sit. Together these lines of work will help answer whether carbonates formed widely and early, what parts of the crust locked up ancient CO₂, and whether any organic chemistry was preserved in those rocks.
Bottom line: where have Mg–Fe carbonates been drilled?
In short: Curiosity drilled and recovered powders containing siderite (an iron carbonate) from drill holes in Gale Crater (notably in targets such as Tapo Caparo, Ubajara, and Sequoia within Mount Sharp). Perseverance drilled and cored multiple samples in Jezero Crater, collecting cores from olivine-rich and margin sedimentary units that contain magnesium- and iron-carbonates (reported as carbonated ultramafic rocks and hydrated carbonates detected in situ). Both rover programs therefore provide direct, drilled evidence of Mg–Fe carbonate occurrence on Mars — Curiosity via powdered drill samples analyzed on Mars, and Perseverance via intact cored samples cached for future lab study. These drilled finds are reshaping our understanding of Martian aqueous geochemistry and the fate of the ancient atmosphere.
Conclusion
Finding Mg–Fe carbonates in drilled materials is a watershed for Martian geoscience. It proves that carbon sequestration by carbonate formation happened in more places and ways than orbital views alone showed. It opens new paths for studying ancient water chemistry, climate evolution, and organic preservation. But science is a patient craft: final, decisive answers will come when carefully selected cores from Jezero and other promising samples are analyzed in terrestrial laboratories with instruments far more sensitive than any on a rover. Until then, the drilled powders and cores we already have are priceless clues — the pages of Mars’ chemical diary that we’re only now learning to read.
FAQs
Which exact drill holes produced iron-rich carbonate detections on Mars?
Curiosity’s powder samples from drill holes labeled in mission reports such as Tapo Caparo, Ubajara, and Sequoia on Mount Sharp in Gale Crater produced siderite detections via CheMin and SAM analyses. These were the key in-situ drilled detections that revealed iron carbonate in significant amounts.
Did Perseverance physically drill Mg–Fe carbonates or only detect them from orbit?
Perseverance physically cored and cached samples from olivine-rich and margin units in Jezero that contain magnesium- and iron-carbonates. In addition, SHERLOC and other instruments have detected hydrated carbonate signals in situ, and recent papers describe “carbonated ultramafic” rocks encountered and sampled during the traverse. Those cored samples are the real physicochemical material, now sealed for potential return to Earth.
How deep do the rover drills go when sampling carbonates?
Curiosity’s drill holes are roughly 1.6 cm wide and up to ~5 cm deep; powders from those holes feed the onboard labs. Perseverance’s cores are intact cylinders about 13 mm across and typically ~60 mm long (about 6 cm). Both approaches sample materials below the immediate weathered surface, where original minerals are more likely to be preserved.
Do these drilled carbonates prove there was life on Mars?
No single mineral proves life. Mg–Fe carbonates increase the chance that organics could be preserved, and some drilled samples contain organic signals or mineral contexts favorable for preservation. However, demonstrating biological origin requires multiple, independent lines of evidence (complex organics with biological patterns, isotopic fractionation consistent with metabolism, preserved microtextures in appropriate context). Those definitive tests are best done in Earth labs on returned samples.
What should we watch for next?
Watch peer-reviewed papers about specific Perseverance core analyses (especially those that describe “carbonated ultramafic” lithologies), mission updates on the prioritization of cached samples for return, and continuing Curiosity reports that further constrain the distribution and abundance of siderite in Mount Sharp. Once sample return happens, expect a flurry of high-precision isotope and organic chemistry results that will take years to fully digest, but that will be the decisive leap forward.
Thomas Fred is a journalist and writer who focuses on space minerals and laboratory automation. He has 17 years of experience covering space technology and related industries, reporting on new discoveries and emerging trends. He holds a BSc and an MSc in Physics, which helps him explain complex scientific ideas in clear, simple language.
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