Download Report

46th Lunar and Planetary Science Conference (2015)
2481.pdf
EVIDENCE FOR LOCALIZED VARIATIONS IN OLIVINE WEATHERING ON MARS.
R.D. Hanna1 and V.E. Hamilton2, 1Jackson School of Geosciences, University of Texas, Austin, TX, 78712
([email protected]), 2Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302
Introduction: Previous studies have proposed
that widespread olivine weathering aided by aqueous
alteration leads to olivine-depleted sediment and soil
on Mars [1,2]. If this is true, olivine content and
thermal inertia (a proxy for particle size) should be
positively correlated, and analysis of THEMIS imagery has suggested that this is indeed the case [2].
The goal of our work is to re-examine this proposed
trend using larger, regional studies in addition to localized analyses and to quantify the possible correlation or non-correlation between thermal inertia and
olivine content. Our work to date has suggested that
on a global scale there is no evidence for such a trend
[3], but that within regions where relatively olivinerich surfaces are found, there is support for a positive
correlation [4]. For the final stage of our project we
searched for and examined areas that might not exhibit this simple olivine-weathering trend: 1) low
thermal inertia areas with significant olivine content
and 2) high thermal inertia areas with relatively low
olivine contents.
Data and Methods: We used data from the Mars
Global Surveyor Thermal Emission Spectrometer
(TES) and 2001 Mars Odyssey Thermal Emission
Imaging System (THEMIS) to determine thermal
inertia and olivine content of the surface in all areas
examined. For TES data, daytime bolometer-derived
thermal inertia [5] was used along with the total normalized olivine content derived via spectral analysis
from [6] for the same TES observation. In several
areas we also selected a pair of overlapping daytime
and nighttime THEMIS images for analysis. We limited the daytime THEMIS images to those with warm
surfaces (average IR surface temperature >250 K)
and selected nighttime images to maximize the overlap with the daytime image in the area of interest.
THEMIS thermal inertia was derived from the
nighttime images on a per-pixel basis using the observed surface temperature as a single-temperature
input to the KRC thermal model [7]. Olivine abundance from THEMIS daytime images was determined using non-negative linear least squares
(NNLS) fitting [8] of atmospherically corrected daytime THEMIS images [9]. The atmospheric correction will not completely eliminate contributions from
water ice clouds [9], so an atmospheric water ice
shape [10] was optionally included in the deconvolution end member set to account for variable water ice
across the image. All other deconvolution end members (apart from the globally homogenous high albe-
do surface (dust) dust shape [11]) were imagederived.
To locate olivine-bearing areas with relatively
low thermal inertia we produced a 2-degree-binned
map from TES detections of normalized olivine contents of at least 25% and having a TES thermal inertia of 350 J·m-2·K-1·s-1/2 or less (SI units are assumed
hereafter). To locate high thermal inertia deposits
with low olivine content, we examined each of the 39
intercrater bedrock locations from [12] for the presence of olivine using TES-derived olivine maps [6]
and 8-7-5 decorrelation stretched (DCS) THEMIS
images in which olivine appears as a distinctive magenta color [13]. We also used olivine and pyroxene
spectral parameter maps derived from the CRISM
[14] and OMEGA [15] instruments to examine the
mineralogy of several areas.
Results: We examined eight olivine-bearing, low
thermal inertia areas for this study. Two cases appear
to be spurious detections of slightly elevated olivine
contents, indicated by an along-track detection of
olivine in one orbit with an absence of comparable
olivine detections in other co-located orbits. In three
cases limited olivine was present in the area but not
consistently coincident with the lowest thermal inertia according to the TES. Two of the areas we examined contain olivine-bearing units with lower thermal
inertias that are interpreted to result from partial dust
cover. However, we found one area in Terra Cimmeria that contains olivine-bearing (~16%) crater ejecta
with a lower thermal inertia (~272) than the underlying unit (~352) with lower olivine content (~7%).
Out of the 39 intercrater bedrock locations from
[12], only one location, in a Noctis Labyrinthus
trough, exhibits no evidence of an olivine spectral
signature (Figs 1-3). The small size and large depth
(~5 km) of the trough relative the surrounding plateau
precluded a reliable atmospheric correction of a
THEMIS image for olivine content derivation, so 5
TES pixels covering the high inertia floor were used
to derive the mineralogy of the unit. Our results confirm an earlier study with OMEGA [16] that the floor
is generally basaltic with significant pyroxene and
plagioclase with no strong evidence of olivine.
Conclusions: We have found that relatively olivine-enriched regions do generally display a positive
correlation between olivine content and thermal inertia, suggesting that olivine-rich rock is weathering to
relatively olivine-poor sediment. However, we have
also found localized examples in which this simple
46th Lunar and Planetary Science Conference (2015)
olivine-weathering trend does not hold: 1) some relatively lower-thermal-inertia (< ~300) units retain a
significant olivine composition (e.g., Terra Cimmeria, 16%) and 2) not all bedrock is olivine-enriched
(e.g., Noctis Labyrinthus). This latter process might
be particularly important in regards to the late Amazonian volcanism of the Tharsis region (which is
mostly dust-covered) with which the olivine-poor
Noctis Labyrinthus unit is associated [16]. Therefore,
while evidence for olivine weathering may be common in dust-free regions of Mars there also exist localized examples where significant olivine weathering is not occurring. Thus, both geological and climactic factors are likely responsible for local variations in the role of aqueous alteration in the global
production of olivine-poor sediment.
References: [1] Bandfield, J.L et al. (2008) Geology, 36(7), 579-582 [2] Bandfield, J.L. et al.
(2011), Icarus, 211, 157-171 [3] Hamilton, V.E. et al.
(2010), LPSC XLI, abs #2239 [4] Hanna, R.D. and
Hamilton, V.E. (2014) LPSC XLV, abs #2576 [5]
Mellon, M. T. et al. (2000) Icarus, 148, 437-455 [6]
Koeppen, W. C. and Hamilton, V. E. (2008) JGR,
113. [7] Kieffer, H.H. (2013) JGR, 118, 3, 451-470
[8] Rogers, A.D. and Aharonson, O. (2008) JGR, 113
doi:10.1029/ 2007JE002995 [9] Bandfield, J.L. et al.
(2004) JGR, 109, doi:10.1029/2004JE002289 . [10]
Bandfield, J.L. et al. (2000) JGR, 105, 9573-9588
[11] Bandfield, J.L. and Smith, M.D. (2003) Icarus,
161, 1, 47-65 [12] Edwards, C.S. et al. (2009) JGR,
114 E11001 [13] Hamilton, V.E., and Christensen,
P.R. (2005) Geology, 33, 433-436. [14] Murchie, S.
et al. (2007) JGR 112, E05S03. [15] Ody, A. et al.
(2012) JGR 117, E00J14. [16] Mangold, N. et al
(2010) EPSL, 294, 440-450.
2481.pdf
Figure 1. Portion of CTX image G01_018678_1728_XN_07S099W
over Noctis Labyrinthus trough.
Figure 2. Thermal inertia derived from band 9 of THEMIS
image I07450009 over Noctis Labyrinthus trough. Color bar
represents thermal inertia from 80 (black-purple) to 600
(red) J·m-2·K -1·s-1/2 .
Figure 3. Modeled mineral group abundances for 5 averaged TES
surface spectra (OCK 3226) over highest thermal inertia area of
Noctis Labyrinthus trough floor.