Lunar and Planetary Science XLVIII (2017)
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POTENTIAL MARS ANALOGUE MINERALS’ REFLECTANCE CHARACTERISTICS UNDER
MARTIAN CONDITIONS. J. T. Poitras1, E. A. Cloutis1, and P. Mann1. 1Dept. of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, Canada R3B 2E9; [email protected],
[email protected].
Introduction: Analogue spectra of potential Martian minerals that have been exposed to Mars-like surface conditions are needed to better analyze remote
sensed data received from orbiters, rovers, and landers.
Previous studies have shown that minerals may undergo spectral structural and compositional changes when
exposed to Mars-like surface conditions of lowpressure CO2 [1]. These changes could thus influence
their identification in remotely sensed data. In order to
find the expected spectral “fingerprint” of various minerals that may be present on Mars, we are continuing
our spectral-compositional-structural investigation of
minerals exposed to a Mars-like surface environment
so that their spectral characteristics can be determined.
One of the NASA Mars 2020 rover mission’s goals
is to find evidence of past or present life on the surface
of Mars. Targets of interest will be characterized using
a suite of instruments, including the spectral reflectance capabilities of the rover’s SuperCam [2], for
acquisition and possible later return to Earth. In order
to more robustly identify minerals potentially associated with biological processes and/or indicative of habitability, we are focusing on developing a spectral library of such materials.
As a continuation from previous studies [see 3],
new experimental runs are focusing more on astrobiologically important minerals. These include hydrated
species and minerals whose formation is dependent
upon aqueous environments. The current sample suite
comprises phosphates, oxalates, nitrates, nitrites, borates, hydroxides, carbonaceous chondrites, and micas.
All of these have been identified or speculated to exist
on Mars [4, 5, 6, 7, 8, 9, 10, and 11].
Here we present the results for a borate, nitrate, and
oxalate. Borates have been found to be essential for the
formation of life and pre-biotic chemistry on earth [12,
13, 14]. Boron has been found in Martian meteorites
and is speculated to be held in Martian clays [8]. Nitrate is often associated with terrestrial biological processes [15], and has also been observed in Martian
meteorites [16]. Lastly oxalates, which could be found
in carbonaceous meteorites, are speculated to be continually delivered to the Martian surface and could
play several important geochemical roles on Mars [5]
and were therefore included in this investigation.
In this experimental run, ammonium nitrate, oxalic
acid, and tincalconite (a borate) were subjected to simulated Martian conditions (i.e., 5 Torr CO2) for 133
days. The minerals were characterized before, during
and after the simulated environmental trial to determine their stability and spectral characteristics for remote sensing identification.
Methods: A suite of 28 minerals were selected
based on previous detections and expectations or relevance of their detection for astrobiology, three of
which are described in Table 1. The experimental run
was conducted at the University of Winnipeg’s
HOSERLab. Samples were ground by hand in an alumina mortar and pestle, then dry-sieved to a grain size
of <45 µm. Fine-grained samples were used to maximize surface area and ths enhance any reactions. The
powdered samples were loosely pressed into Al disks
and placed in one of two 12 cm diameter sample turntables. Each sample well was 8mm in diameter and 6
mm deep, with 16 samples held in each turntable.
Sample disks were placed in the Mars Environmental Chamber (ME), as described by Craig et al. [17]. A
150-watt QTH light was positioned for 30° incident
angle and an Analytical Spectral Devices (ASD) spectrophotometer (350-2500 nm) was used to collect reflectance at a 0° emission angle. Sample reflectance
was measured relative to a Spectralon® 100% diffuse
reflectance standard, and a total of 500 individual spectra were averaged together to improve signal to noise.
Table 1: Selected Minerals
Mineral
HOSERLab ID
Ammonium Nitrate1 NIT004
Oxalic Acid
1
Tincalconite
1.
2
Synthetic
2.
Formula
NH4NO3
ART002
HO2CCO2H
BOR002
Na2B4O5 (OH)4•3H2O
Received from Minerals Unlimited
Results: Below we describe the results for the
spectra shown in Figure 1 normalized at 1250 nm.
NIT004 appears to be stable over the 133 day period.
The second overtone of the NH4+ asymmetrical stretch
appears at ~1563 nm and an additional overtone combination appears at ~1634 nm [18]. Neither shows any
appreciable change in depth, position, or shape. Other
features are likely due to N-O combinations, located at
~1270, ~2036, and ~2135 nm which do not show any
appreciable change in characteristics.
Oxalic acid (ART002) has hydration bands as expected near 1400 and 1900 nm. These absorptions dis-
Lunar and Planetary Science XLVIII (2017)
appear almost immediately after exposure to Mars surface conditions, and new features are seen: at 1646 and
1698 nm, and a broad triplet feature at 2231 nm. Applin et al. [5] explored this compound and include reflectance spectra up to 4100 nm which appear to be
unchanged by exposure. This compound appears to be
unstable at Mars surface conditions.
The spectra of BOR002 show distinctive H2O- and
OH-associated absorption at 980, 1400, 1450, and
~1900 nm. These features show some loss of detail in
terms of the sharpness of the band minima, but are not
lost. The 1900 nm region band’s minimum shifts to
lower wavelengths by ~10 nm after exposure. The
2140 and 2190 nm features are due to the second overtones of the B-O symmetric and asymmetric stretches
[19, 20]. The first feature becomes shallower and the
second feature is nearly lost. This mineral appears to
be unstable in the simulated Martian surface conditions, likely changing to a different hydrated borate.
Both NIT004 and BOR002 exhibit spectral changes
in the lower wavelength region. The absorption at
~800 nm, which is likely associated with small
amounts of Fe that are present in the samples, is lost.
Absolute Reflectance
NIT004
ART002
BOR002
Pre-Exposure
Day 1
Day 133
350
650
950 1250 1550 1850 2150 2450
Wavelength (nm)
Figure 1: Reflectance spectra of NIT004, ART002,
BOR002. Verticle dashed bars indicate regions of interest.
Discussion: Results indicate that even after a short
duration exposure to simulated Martian conditions
there are measurable spectral changes. While the sample suite is quite diverse, the three samples presented
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here are representative of the array of changes seen for
the full sample suite.
Previous studies have found that absorption bands
associated with H2O are generally the least stable absorption bands in reflectance spectra of samples exposed to Mars surface conditions, and the latest experimental run is consistent with previous results. The
1900 nm region H2O absorption feature in the BOR002
spectra shows a rapid decrease in depth and change in
position. Since there are 3 molecules of H2O in the
ideal formula for tincalconite, this could indicate both
a loss of H2O and a restructuring of the mineral. X-ray
diffraction and Raman analyses will be applied to determine what these changes are due to.
The NIT004 sample appears to be stable with the
exception of the 800 nm band. Since ammonium and
nitrate can be associated with biological processes, its
spectral stability is a promising find for Martian exploration concerning extant or extinct life.
The oxalate displays the most dramatic spectral
changes of the entire sample suite. All of its absorption
bands show dramatic changes after the first day. X-ray
diffraction and Raman data will be used to help identify the cause of the spectral changes.
Results from the full sample suite will provide
more insight into the stability and spectral detectability
of minerals of astrobiological importance on Mars.
References: [1] Cloutis E. A. et al. (2008) Icarus,
195(1), 140-168. [2] Mustard J. F. et al. (2013)
http://tinyurl.com/glsvzsp .[3] Poitras J. T. et al. (2016)
LPS MMXVI, Abstract #2294. [4] King P. L. and
McSween H. Y. (2005) JGR, 110(E12). [5] Applin D.
M. et al. (2015) Earth Planet. Sci.420, 127-139. [6]
Sefton-Nash E. et al. (2012) Icarus, 221, 20-42. [7]
Palomba E. et al. (2009) Icarus, 203, 59-65. [8] Stephenson J. D. et al. (2013) PloS one, 8(6), e64624. [9]
Bell III J. F. et al (2000) Lunar Planet. Sci. 31. Abstract 1227. [10] Yen A. S. (2006) JGR, 111, E12S11.
[11] Ehlman B. L. (2009) JGR, 114, E00D08. [12]
Neveu M. (2013) Astrobiology 13, 391-403. [13] Furukawa Y. et al. (2015) Astrobiology, 15(4), 259- 267.
[14] Kim H. J. et al. (2011) J AM Chem Soc.
133(24):9857-68. [15] Straub K. L. et al.(1996) Appl.
Environ. Microbiol. 62(4), 1458-1460. [16] Grady M.
M. et al. (1995) JGR Planets 100(E3), 5449-5455. [17]
Craig M. et al. (2001) LPS MMI, Abstract #1368. [18]
Boer G. J. et al. (2007) J. Quant. Spectrosc. Radiat.
Trans. 108, 17-38. [19] Hunt G. R. et al. (1972) Mod.
Geol. 3, 121-132. [20] Cloutis E. A. et al. (2016) Icarus, 264, 20-36.
Acknowledgements: HOSERLab and this study
have been supported by the Canadian Space Agency,
NSERC, CFI, MRIF, and the University of Winnipeg.