MARTIAN GEOLOGY
Brine-driven destruction of clay minerals in
Gale crater, Mars
T. F. Bristow^1 *, J. P. Grotzinger^2 , E. B. Rampe^3 , J. Cuadros^4 , S. J. Chipera^5 , G. W. Downs^6 , C. M. Fedo^7 ,
J. Frydenvang^8 , A. C. McAdam^9 , R. V. Morris^3 , C. N. Achilles^9 ,D.F.Blake^1 , N. Castle^5 , P. Craig^5 ,
D. J. Des Marais^1 , R. T. Downs^6 , R. M. Hazen^10 , D. W. Ming^3 , S. M. Morrison^10 , M. T. Thorpe^11 ,
A. H. Treiman^12 , V. Tu^11 , D. T. Vaniman^5 , A. S. Yen^13 , R. Gellert^14 , P. R. Mahaffy^9 , R. C. Wiens^15 ,
A. B. Bryk^16 , K. A. Bennett^17 , V. K. Fox^2 †, R. E. Millken^18 , A. A. Fraeman^13 , A. R. Vasavada^13
Mars’sedimentary rock record preserves information on geological (and potential astrobiological) processes
that occurred on the planet billions of years ago. TheCuriosityrover is exploring the lower reaches of
Mount Sharp, in Gale crater on Mars. A traverse from Vera Rubin ridge to Glen Torridon has allowedCuriosityto
examine a lateral transect of rock strata laid down in a martian lake ~3.5 billion years ago. We report spatial
differences in the mineralogy of time-equivalent sedimentary rocks <400 meters apart. These differences
indicate localized infiltration of silica-poor brines, generated during deposition of overlying magnesium
sulfate–bearing strata. We propose that destabilization of silicate minerals driven by silica-poor brines (rarely
observed on Earth) was widespread on ancient Mars, because sulfate deposits are globally distributed.
S
mectitesareagroupofclayminerals
characterized by high surface areas and
negative electrostatic charge ( 1 ). These
properties promote intimate associations
with organic material during sedimenta-
tion, enhancing organic preservation in Earth’s
geological record ( 2 ). The increased temperature
and pressure experienced by sediments dur-
ing burial cause physical, chemical, and min-
eralogical changes—collectively termed burial
diagenesis—that break down mineral-organic
associations. A common burial diagenetic change
observed on Earth involves the transformation
of smectite into other minerals (typically illite
and chlorite), a process that coincides with
molecular reconfiguration and degradation
of associated organic matter ( 1 ). On Mars,
Noachian- to Hesperian-age [~4.1 to 3.2 bil-
lion years ago (Ga)] sedimentary rocks that
contain smectites but lack evidence for these
transformations could therefore preserve
information on geological or astrobiological
processes ( 2 , 3 ).
Since landing in Gale crater in 2012, the
Curiosityrover has traversed ~25 km across
the crater floor and the base of Aeolis Mons
(informally called Mount Sharp), a 5.5-km-high
mountain in the center of Gale. Rover mea-
surements have revealed smectite-bearing
~3.5-Ga sedimentary rocks with vertical thick-
nesses of several hundred meters (Fig. 1). The
smectites in these rocks are largely unaffected
by burial diagenetic transformations observed
in sedimentary basins on Earth and preserve
organic compounds ( 4 – 6 ). Bulk geochemical
indices and mineralogical trends in these rocks
predominantly reflect depositional processes
and conditions, as they coincide with sedi-
mentary indicators of changing environments
within the lake system that once occupied the
craterfloor~3.5Ga( 5 , 7 – 9 ).
Despite the limited evidence for burial
diagenesis of clay minerals, outcrops surveyed
byCuriositydisplay the influence of reactions
involving liquid water (aqueous alteration)
that postdate sediment deposition (and are
therefore referred to as diagenetic reactions).
Networks of millimeter- to centimeter-scale,
calcium sulfate–filled fractures and veins are
frequently found penetrating host rock ( 4 , 10 ).
In some cases, the passage of fluids has altered
the mineralogy and geochemistry of host
sediments, producing decimeter-scale“altera-
tion halos”adjacent to fractures ( 11 ). Some
rocks alongCuriosity’s traverse experienced
enhanced mobilization of trace elements and
mineralogical reactions that influence the phys-
ical and spectral properties of rocks ( 12 – 16 ).
X-ray amorphous materials have been detected
in all samples analyzed with the rover’s CheMinx-ray diffraction (XRD) instrument, with abun-
dances of ~15 to 70 wt %. This material could be
the product of multiple episodes of aqueous
interaction with sediment ( 11 , 13 , 17 ). Radio-
metric dating of jarosite—a potassium-bearing,
iron-sulfate mineral—provides evidence for in-
termittent aqueous alteration extending over a
period of at least half a billion years following
deposition ( 18 ).
The protracted history of diagenetic reac-
tions in Gale crater has made it challenging to
identify the sources and drivers of the post-
depositional passage of fluids ( 13 – 15 ). Natural
variations in the depositional mineralogy of
sediments, a function of changing sediment
sources, depositional setting, and lake condi-
tions ( 9 ), make the task more difficult. We
attempt to eliminate this extra complexity by
examining the influence of diagenetic reac-
tions on the mineralogy of a lateral transect of
ancient lake mudstones, deposited in similar
environments at the same time.Study area
Glen Torridon (GT) is a shallow west-to-east
elongated trough ~0.3 to 1 km wide, lying be-
tween the slopes of Aeolis Mons to the south
and the more resistant rocks of Vera Rubin
ridge (VRR) to the north (Fig. 2). Rocks ex-
posed within GT exhibit deep near-infrared
absorption features in orbital reflectance spectra,
which are attributed to smectites ( 19 , 20 ). The
clay minerals in GT are part of a sequence of
remotely detected hydrous and hydroxylated
minerals exposed on Aeolis Mons. These clays
are overlain by rocks enriched in sulfates and
iron oxides ( 19 , 20 ), a sequence that is widely
observed on Mars and is thought to have
formed during exhaustion of the planet’s near-
surface liquid water ( 3 , 21 ).
Two main geomorphological subunits in
GT were identified from orbital imagery
beforeCuriosityreached GT in January 2019:
a smooth ridged unit to the north, skirting
VRR, and a fractured unit to the south (Fig. 2).
DuringCuriosity’s traverse, rover imagery
showed that the smooth-ridged unit is covered
by a mixture of pebbles and sand. Occasional
outcrops expose thinly to thickly laminated
mudstones, which are sometimes fractured
and rubbly (the likely source of the pebbles)
and sometimes exhibit alternating recessive
and resistant laminations. The overlying frac-
tured unit primarily consists of sandstones.
The rock compositions and their physical fea-
tures (which, when grouped together, define a
distinguishable rock unit or facies) resemble
underlying and laterally equivalent deposits
( 8 , 22 , 23 ), indicating the continued pres-
ence of a lake with intermittent episodes of
deposition by rivers and of wind-blown sedi-
ment. Our analysis of rover imagery shows no
evidence of faulting or a depositional break
marking the boundary between VRR and GT.198 9JULY2021•VOL 373 ISSUE 6551 sciencemag.org SCIENCE
(^1) Eobiology Branch, NASA Ames Research Center, Moffett Field, CA
94035, USA.^2 Division of Geological and Planetary Sciences,
California Institute of Technology, Pasadena, CA 91125, USA.
(^3) Astromaterials Research and Exploration Science Division, NASA
Johnson Space Center, Houston, TX 77058, USA.^4 Department of
Earth Sciences, Natural History Museum, London SW7 5BD,
UK.^5 Planetary Science Institute, Tucson, AZ 85719, USA.
(^6) Department of Geosciences, University of Arizona, Tucson, AZ
85721, USA.^7 Department of Earth and Planetary Sciences,
University of Tennessee, Knoxville, TN 37996, USA.^8 Globe
Institute, University of Copenhagen, Copenhagen, Denmark.^9 Solar
System Exploration Division, NASA Goddard Space Flight Center,
Greenbelt, MD 20771, USA.^10 Earth and Planets Laboratory,
Carnegie Institution for Science, Washington, DC 20015, USA.
(^11) Jacobs Technology–Jacobs JETS Contract, Astromaterials
Research and Exploration Science Division, at NASA Johnson
Space Center, Houston, TX 77058, USA.^12 Lunar and Planetary
Institute, Universities Space Research Association, Houston, TX
77058, USA.^13 Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA 91109, USA.^14 Department of Physics,
University of Guelph, Guelph, Ontario N1G 2W1, Canada.^15 Los
Alamos National Laboratory, Los Alamos, NM 87545, USA.
(^16) Department of Earth and Planetary Science, University of
California Berkeley, Berkeley, CA 94720, USA.^17 U.S. Geological
Survey, Astrogeology Science Center, Flagstaff, AZ 86001, USA.
(^18) Department of Earth, Environmental Sciences and Planetary
Sciences, Brown University, Providence, RI 02912, USA.
*Corresponding author. Email: [email protected]
†Present address: Department of Earth and Environmental
Sciences, University of Minnesota, Minneapolis, MN 55455, USA.
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