We have collected laboratory thermal IR spectra of the mineraloid allophane and aluminosilicate gels. Using those spectra to model regional TES spectra, we suggest that several areas of Mars contain significant amounts of allophane-like weathering products. The presence of allophane on Mars indicates that 1) significant Al sources, such as feldspar or glass, were weathered; 2) weathering on Mars produced poorly-crystalline aluminosilicates, rather than easily identifiable crystalline minerals; and 3) some Martian weathering proceeded under moderate pH environments, suggesting acid weathering is not the only major alteration mechanism on Mars

Golden, D. C.

Kraft, M. D.

Ming, Douglas W.

Rampe, E. B.

Sharp, T. G.

NASA Technical Reports Server

DETECTION OF ALLOPHANE ON MARS THROUGH ORBITAL AND IN-SITU THERMAL-
INFRARED SPECTROSCOPY.  E. B. Rampe1, M. D. Kraft1, T. G. Sharp1, D. C. Golden2, D. W. Ming3, P. R. 
Christensen1, and S. W. Ruff1, 1School of Earth and Space Exploration, Arizona State University, Box 871404, 
Tempe, AZ 85287, Liz.Rampe@asu.edu; 2ESCG-JE23, Houston, TX; 3NASA-Johnson Space Center, Houston, TX. 
 
Introduction:  We have collected laboratory 
thermal IR spectra of the mineraloid allophane and 
aluminosilicate gels.  Using those spectra to model 
regional TES spectra, we suggest that several areas of 
Mars contain significant amounts of allophane-like 
weathering products.  The presence of allophane on 
Mars indicates that 1) significant Al sources, such as 
feldspar or glass, were weathered; 2) weathering on 
Mars produced poorly-crystalline aluminosilicates, 
rather than easily identifiable crystalline minerals; and 
3) some Martian weathering proceeded under moderate 
pH environments, suggesting acid weathering is not 
the only major alteration mechanism on Mars. 
Background: Allophane is a poorly-crystalline, 
hydrous aluminosilicate that forms by low-T aqueous 
alteration of feldspar and volcanic glass at near-neutral 
pH [1].  It is a clay mineraloid with variable 
composition (Si/Al ratios range from ~0.5-1) [1].  
Aluminosilicates with greater Si/Al ratios can also 
precipitate, and, here, we refer to those products as 
“aluminosilicate gels.” 
Allophane has been inferred for Mars from 
chemical models and thermal-infrared (TIR) 
spectroscopy [2-6].  It may be a significant constituent 
of light-toned soils in Meridiani and chemically 
weathered rock surfaces in Gusev crater, based on 
phase models of APSX data [2,3].  Allophane or other 
poorly-crystalline aluminosilicates may comprise the 
high-silica phases modeled in global Thermal 
Emission Spectrometer (TES) data; however, the lack 
of allophane in spectral libraries has impeded its 
detection in TES models.  Consequently, high-silica 
materials have been modeled as phases with similar 
compositions and structures, including volcanic glass, 
phyllosilicates, zeolites, and amorphous silica [7-11], 
which have different geological implications than 
allophane.  TIR spectra of allophanes were not 
previously available because natural allophanes are 
difficult to isolate.  Thus, we synthesized allophanes 
and other aluminosilicate gels to acquire their TIR 
emission spectra and test for the presence of allophane 
on Mars by modeling TES and Mini-TES spectra. 
Methods: Allophanes and gels were synthesized 
according to [12,13].  Compositions and structures 
were analyzed by atomic absorption spectroscopy, X-
ray diffraction, transmission electron microscopy, and 
transmission FTIR spectroscopy.  TIR emission spectra 
of compressed pellets were measured at the Mars 
Space Flight Facility at Arizona State University.  Four  
distinct allophane and gel spectra were added to a 
spectral library commonly used to model TES spectra 
[14], and this library was used to model nine regional 
TES spectra identified by [15] using the same rigorous 
routine outlined by Rogers and Christensen (2007) 
[14].  The four allophane and gel spectra were added to 
spectral libraries used to model Mini-TES spectra of 
fresh and altered rocks from Gusev (Adirondack, 
Clovis, Wishstone, and Watchtower) [16]. 
Results: Laboratory analyses of our synthetic 
products are consistent with natural and synthetic 
allophanes and aluminosilicate gels [1].  Significant 
abundances of allophanes and gel were modeled in 
TES spectra from three regions of Mars.  Mini-TES 
spectral models suggest some altered rocks in Gusev 
crater may contain allophane or another 
aluminosilicate weathering product. 
TES Models. Significant abundances (>10 vol.%) 
of allophane and aluminosilicate gel were modeled in 
regional spectra of Northern Acidalia, Solis Planum, 
and Meridiani Planum, and the composition of the 
modeled allophanes and gel differ between each 
region.  Models with allophane/gel have better RMS 
errors than models without.  Allophane/gel 
components replace specific secondary silicates in 
models of each region.  High-silica gel replaces 
phyllosilicates in models of Northern Acidalia; high-
silica gel and Al-rich allophane replace phyllosilicates 
and volcanic glass in Solis; and high-silica allophane 
replaces aluminous opal in Meridiani.  This confirms 
that some high-silica phases previously identified from 
TES spectral models, particularly phyllosilicates and 
amorphous silica, could be allophane [4-6]. 
The identification of allophane in TES spectral 
models affects the modeled pyroxene mineralogy in 
Solis and Meridiani but does not affect the overall 
interpretation of major mineral groups.  Models 
without allophane identify primarily orthopyroxene 
and high-calcium pyroxene in Solis and Meridiani, 
respectively [14].  The addition of allophane to the 
library results in the identification of pigeonite in 
models of both regions (Figure 1).  Pigeonite has been 
identified in Meridiani from OMEGA visible/near-IR 
data and Mini-TES spectral models of the basaltic sand 
component [17,18].  We suggest that the addition of 
more accurate secondary phases to spectral libraries 
can improve modeled igneous mineralogy, therefore 
improving interpretations of aqueous alteration and 
petrologic histories. 
https://ntrs.nasa.gov/search.jsp?R=20110005445 2019-08-30T14:24:14+00:00Z
Mini-TES Models. High-silica gel is identified in 
models of Clovis and high-silica gel and Al-rich 
allophane are identified in models of Wishstone; 
however, the addition of allophane does not improve 
RMS errors and the identification of allophane and gel 
are affected by small variations in the modeled 
wavelength range.  Additionally, allophane and gel 
primarily replace plagioclase feldspar and minor 
amounts of high-silica phases in the models, including 
volcanic glass and zeolite.  The iron mineralogy of 
Watchtower suggests that this sample is the most 
altered, and phase models of APXS data indicate it 
should have the most allophane [3], but our models of 
Watchtower do not identify allophane or gel.  The 
replacement of igneous minerals by allophane, the 
dependence on modeled wavelength range, the lack of 
improvement of model fit, and the absence of 
allophane in models of Watchtower indicate 1) 
allophane may not be present in these rocks and the 
libraries may still lack the poorly-crystalline 
aluminosilicate phase(s) present in these samples, such 
as imogolite or hisingerite; or 2) allophane is present 
but weathering creates mineralogical and textural 
complexities that cause non-linear spectral mixing [5] 
so that allophane cannot be detected. 
Implications for Aqueous Alteration on Mars: 
In-situ chemical data from APXS shows global 
variation in Fe and Mg abundances, which have been 
used as evidence for recent global acidic alteration at 
low water-to-rock ratios and the preferential 
dissolution of olivine [19].  Conversely, the 
identification of allophane and gel in TES spectral 
models of Northern Acidalia, Solis, and Meridiani 
suggest recent near-neutral aqueous alteration at low-
T and low water-to-rock ratios.  Furthermore, the 
presence of a secondary aluminosilicate requires the 
dissolution of an aluminum-rich phase, such as 
volcanic glass and/or plagioclase.  Global acidic 
chemical weathering and the dissolution of olivine 
cannot account for the precipitation of allophane.  We 
must reevaluate our ideas of global weathering 
mechanisms on Mars to explain the presence of 
allophanes and gels.  We propose that distinct regional 
weathering mechanisms occur on Mars, resulting in the 
precipitation of different allophanes and gels in 
different Martian regions.  Recent discoveries of 
carbonates in Gusev and at the Phoenix landing site 
further support regional weathering at near-neutral 
conditions [20,21]. 
References: [1] Wada K. (1989) Soil Sci Soc. Am., 
no. 1, 1051–1087. [2] Clark B. C. et al. (2005) EPSL, 
240, 73-94. [3] Ming D. W. et al. (2006) JGR, 111, 
E02S12. [4] Hamilton V. E. et al. (2008) JGR, 113, 
E12S43. [5] Kraft M. D. (2009) Ph.D. Thesis, Ariz. 
State Univ. [6] McDowell M. L. and Hamilton V. E. 
(2009) JGR, 114, E06007. [7] Bandfield J. L. et al. 
(2000), Science, 287, 1626-1630. [8] Wyatt M. B. and 
McSween H, Y. (2002) Nature, 417, 263-266. [9] 
Kraft M. D. et al. (2003) GRL, 30, DOI10.1029/ 
2003GL018848. [10] Ruff S. W. et al. (2004) Icarus, 
168, 131-143. [11] Michalski et al. (2005) Icarus, 174, 
161-177. [12] Ohashi F. et al. (2002), Clay Mins., 37, 
451-456. [13] Wada S.-I. et al. (1979), JSS, 30, 347-
355. [14] Rogers A. D. and Christensen P. R. (2007) 
JGR, 112, E01003. [15] Rogers A. D. et al. (2007) 
JGR, 112, E02004. [16] Ruff S. W. et al. (2006) JGR, 
111, E12S18. [17] Glotch T. D. and Bandfield J. L. 
(2006) JGR, 111, E12S06. [18] Poulet F. et al. (2009) 
Icarus, 195, 106-130. [19] Hurowitz J. A. and 
McLennan S. M.  (2007) EPSL, 260, 432-443. [20] 
Morris R. V. et al. (2010) Science, 329, 421-424. [21] 
Boynton, W. V. et al. (2009) Science, 325, 61-64. 
 
a 
 
b 
 
Figure 1. a) Grouped modeled mineral abundances in 
Meridiani from models without allophane by [13]. b) 
Grouped modeled mineral abundances in Meridiani from this 
study.  Note the similarities between the two models, except 
for the identification of pigeonite in this study (b). 


Detection of Allophane on Mars Through Orbital and In-Situ Thermal-Infrared Spectroscopy

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server