Following the discoveries made by the Opportunity rover at Meridiani Planum, members of the MER science team proposed that the Meridiani deposits are playa evaporites reworked by eolian processes. Alternate hypotheses have also been proposed to explain the deposits at Meridiani Planum, and these have highlighted serious problems with the provenance of the sedimentary material in the proposed playa hypothesis. These problems include: indications of cation-conservative weathering, the lack of a topographic basin, the intimate commingling of the most soluble and least soluble salts, and the overall scale of the deposit. These observations are important challenges to the playa scenario, and suggest that the sediment was derived from a different source

Michalski, J.

Niles, P.B.

NASA Technical Reports Server

THE ORIGIN OF THE MERIDIANI SEDIMENTS: THE KEY FOR UNDERSTANDING THE 
FORMATION OF SULFATES AND LAYERED DEPOSITS ON MARS.  P. B. Niles1, J. Michalski2, 
1Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058; 2Institut 
d’Astrophysique Spatiale, Université Paris Sud, Orsay, 91405, France. (paul.b.niles@nasa.gov) 
 
Introduction: Following the discoveries made by 
the Opportunity rover at Meridiani Planum, members 
of the MER science team proposed that the Meridiani 
deposits are playa evaporites reworked by eolian proc-
esses[3]. Alternate hypotheses have also been pro-
posed to explain the deposits at Meridiani Planum, and 
these have highlighted serious problems with the 
provenance of the sedimentary material in the pro-
posed playa hypothesis[3]. These problems include: 
indications of cation-conservative weathering, the lack 
of a topographic basin, the intimate commingling of 
the most soluble and least soluble salts, and the overall 
scale of the deposit[4, 5]. These observations are im-
portant challenges to the playa scenario, and suggest 
that the sediment was derived from a different source. 
Model Description: We propose a new model for 
the provenance of the sediments at Meridiani Planum, 
which is based in principal on geochemical ideas pio-
neered by Burns[8], and sedimentological ideas pro-
posed by Tanaka[9]. In our model, the sediments now 
located at Meridiani Planum were sourced from a 
nearby, massive dust/ice deposit that was located adja-
cent to Meridiani Planum. This massive dust/ice de-
posit formed through precipitation of ice around dust 
grains and aerosols during a period of high obliquity, 
and resembled the polar layer deposits that exist today 
in the martian north and south polar regions. Sus-
pended dust in a Noachian atmosphere provides nu-
cleation points for ice crystals to form which is an im-
portant driver for the precipitation of water ice in the 
present day polar caps[10] and would have been more 
prevalent on ancient Mars. 
Figure 1. An ice core from Antarctica with an aggregation of soil 
grains (Photo Credit: Hans Paerl, University of North Carolina at 
Chapel Hill). These grains have migrated through the ice to form this 
aggregate by means of radiant heating[2] 
Exposure of the ice deposits to sunlight during the 
summer seasons allowed for radiant heating of dark 
grains within the water ice matrix. A similar effect has 
been observed where radiant heating of soil grains, 
trapped in ice deposits in Antarctica, causes melting 
and migration resulting in the formation of aggre-
gates[2] (Fig. 1). The radiant heating led to the forma-
tion of thin water films sufficient to allow for reaction 
with the volcanic aerosols or sulphide minerals to cre-
ate acidic solutions. The ice matrix provided a physical 
barrier enclosing each grain or aggregate of grains 
within a closed system environment at low tempera-
tures with low water/rock ratios. The acidic solutions 
weathered silicate grains to form poorly crystalline 
aluminosilicates and sulfates[8]. The cold temperatures 
of the polar environment provided a mechanism for 
limiting water/rock ratios while simultaneously form-
ing more concentrated solutions through freezing of 
excess solution.  
A climactic shift (due to polar wander or obliquity 
changes) then led to conditions which favoured net 
sublimation of the massive ice deposit rather than 
deposition. The sublimation residue was made up of 
sand sized agglomerates of fine-grained, chemically 
weathered, and highly hydrated siliciclastic material 
mixed with sulfate salts. This material was reworked 
by eolian or impact activity. Burial of these highly 
hydrated phases caused the release of structural water 
that allowed limited diagenesis and blueberry forma-
tion to occur, but also prevented wholesale re-
equilibration of the deposit and complete conversion 
of jarosite to hematite[11]. Small amounts of addi-
tional water may also have been supplied by brief ice 
melting events allowing for the possibility for surface 
runoff[3]. 
The ice-weathering model resolves many prob-
lems of previous hypotheses including the large scale 
of the deposit, the source of the sediment, its composi-
tional similarity across large lateral distances, the 
cation-conservative nature of the weathering proc-
esses, the presence of acidic groundwaters on a basal-
tic planet, the accumulation of a thick sedimentary 
sequence outside of a topographic basin, and the low 
water/rock ratio needed to explain the presence of very 
soluble minerals and elements in the deposit.  
https://ntrs.nasa.gov/search.jsp?R=20090012000 2019-08-30T06:31:19+00:00Z
FORMATION OF SULFATES AT MERIDIANI PLANUM THROUGH ICE-WEATHERING: Niles and Michalski  
The cation-conservative nature of the weathering 
that modified the silicate materials has been noted by 
other authors[1, 5] and places a strong constraint on 
the provenance and weathering of the sedimentary 
material. The variations in the chemical composition of 
the deposit has been modelled as either 1) a mixture 
between a weathered basaltic component and a sulfate-
rich evaporite component[3], or 2) an addition of SO2 
to a pristine basaltic composition[5].  The fact that this 
assemblage has retained its basaltic cation abundances 
(Fig. 2) despite extensive modification to the original 
basaltic mineralogy suggests that the weathering took 
place under extremely low water/rock ratios[1, 4, 5]. A 
unique feature of weathering dusty material within a 
polar ice cap should be cation-conservative weathering 
on a centimeter scale due to the closed system envi-
ronment provided by the icy matrix. The cold tempera-
tures should act to limit water/rock ratios resulting in 
an intimate mixture of silicate and sulfate minerals[8]. 
This also provides an explanation for the close associa-
tion of minerals with different solubilities because 
each grain or small agglomerate of grains is weathered 
in its own encapsulated environment. Also, if the 
acidic solutions occurred as small pockets within a 
polar ice deposit, they would be effectively isolated 
from the alkaline basaltic crust of the planet. 
Implications: The formation process of the sedi-
mentary deposits in Terra Meridiani acted on such a 
large scale that it necessarily requires consideration of 
the broader regional and global geologic picture of 
Mars. Remote sensing studies have linked the Merid-
iani deposits to a number of other martian surface fea-
tures through mineralogic similarities, geomorphic 
similarities, and regional associations. Although they 
are separated by more than 1000 km; Aram Chaos and 
Valles Marineris each contain sulfate paired with 
hematite that has spectral features identical to the 
hematite found in Terra Meridiani[12, 13]. The ther-
mal infrared spectrum of each of these areas shows an 
alignment of the hematite c-axis, which is rare in ter-
restrial rocks and is probably caused by spherule 
growth[11]. The uniqueness of their thermal infrared 
signatures and the distance between these deposits 
suggest that all of these deposits share a common for-
mation process which must have acted over a large 
area of Mars.  
The weathering model proposed here could poten-
tially explain the origin of many layered sulfate depos-
its detected on Mars. Most of the sulfate deposits on 
Mars share common characteristics: they are layered, 
occur in mounds and ridges, and lack an obvious 
provenance[14]. In these deposits the sulfates are ex-
posed in spectrally detectable layered deposits, some 
of which may reflect the original morphology of lay-
ered ice deposits. Layered sulfate deposits may also 
have formed through eolian or impact reworking and 
transport of sublimation residue or through episodic 
fluvial processes driven by limited melting of ice. The 
modern polar sulfate deposit is mineralogically and 
geomorphologically different from the other sulphate 
deposits on Mars, but its proximity to a massive ice 
deposit and its distribution in an erg deposit make it a 
potential modern analog. This view of sulfate forma-
tion in Meridiani and elsewhere on Mars draws a di-
rect comparison between ancient and modern geologic 
processes and suggests sulfate minerals could have 
formed on Mars during a past cold and desiccated cli-
mate. 
Figure 2. Ternary plot of the molar Fe, S, and Al composition of 
Meridiani Planum sediments[1] and other martian materials[6, 7]. 
The arrow traces a constant FeO vs. Al2O3 ratio on the ternary dia-
gram. The Meridiani sediments plot along this constant ratio indicat-
ing that iron has not been fractionated from aluminium as one might 
expect from aqueous alteration in an open system. This shows that 
the Meridiani sediments can be seen as having a cation composition 
resembling basalt with SO2 added. The variation observed in the 
Meridiani sediments is solely due to changes in MgSO4 content. 
Uncertainties on data points are smaller than the size of the points. 
 
References: 
1. Clark, B.C., et al., Earth Planet. Sci. Lett. 240, 73-94 (2005). 
2. Paerl, H.W. and J.C. Priscu, Microbial Ecology 36, 221-230 
(1998). 
3. Squyres, S.W., et al., Science 313, 1403-1407 (2006). 
4. Knauth, L.P., D.M. Burt, and K.H. Wohletz, Nature 438, 1123-
1128 (2005). 
5. McCollom, T.M. and B.M. Hynek, Nature 438, 1129-1131 (2005). 
6. Morris, R.V., et al., J. Geophys. Res.-Planets 111 (2006). 
7. Morris, R.V., et al., J. Geophys. Res.-Planets 111 (2006). 
8. Burns, R.G., J. Geophys. Res. 92, 570 (1987). 
9. Tanaka, K.L., Icarus 144, 254-266 (2000). 
10. Clifford, S.M., et al., Icarus 144, 210-242 (2000). 
11. Golden, D.C., et al., Am. Mineral. 93, 1201-1214 (2008). 
12. Christensen, P.R., et al., J. Geophys. Res.-Planets 106, 23873-
23885 (2001). 
13. Bibring, J.P., et al., Science 317, 1206-1210 (2007). 
14. Bibring, J.P., et al., Science 312, 400-404 (2006). 
 


The Origin of the Meridiani Sediments: the Key for Understanding the Formation of Sulfates and Layered Deposits on Mars

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server