http://www.psrd.hawaii.edu/Mar03/Meridiani.html
Gray Iron Oxide on Mars
Planetary Science Research Discoveries
March 13, 2003
--- A deposit of gray hematite in
Terra Meridiani may suggest that
water once circulated through the
rock layers in this region of Mars.
Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology
In our continuing coverage of the exploration of Mars and NASA's strategy of
"follow the water," PSRD highlights recent research of the hematite deposit
in Terra Meridiani. The iron oxide mineral called hematite (Fe2O3) forms on
Earth in several ways, most involving water. For this reason, the
announcement in 2000 of the discovery of crystalline gray hematite near the
Martian equator was, in a word, a watershed for planetary geologists and
astrobiologists interested in unraveling the history of water and climate on
Mars. Detection of the hematite in Terra Meridiani is one of the key
discoveries of the Thermal Emission Spectrometer (TES) instrument on the
Mars Global Surveyor (MGS) mission. Using TES spectral data in combination
with image and topographic data, Brian Hynek, Raymond Arvidson, and Roger
Phillips (Earth and Planetary Sciences and McDonnell Center for the Space
Sciences at Washington University in St. Louis) recently made detailed
regional analyses of Terra Meridiani to better understand the origin and
significance of the hematite in the Martian environment. Using Earth as a
guide to hematite formation, researchers think the hematite could have
formed on Mars by thermal oxidation of iron-rich volcanic eruptive products
during eruption or it could have formed by chemical precipitation when
iron-rich water circulated through the preexisting layers of volcanic
material (ash). Hynek and his coauthors as well as other researchers
studying data from Terra Meridiani prefer the chemical precipitation
hypothesis because it is most consistent with their observations of the
geology of the region.
Reference:
Hynek, B. M., Arvidson, R. E., and Phillips, R. J. (2002) Geologic
setting and origin of Terra Meridiani hematite deposit on Mars. Journal
of Geophysical Research, v. 107, no. E10, 5088, doi: 1029/2002001891.
--------------------------------------------------
How hematite was found on Mars
Crystalline gray hematite (the coarse-grained form of the iron oxide we call
rust when it's powdery and red) was found by scientists analyzing remote
sensing data gathered by the Thermal Emission Spectrometer (TES) instrument
during the early (1997-1998 aerobraking) phase of the MGS mission. The TES
instrument measures the infrared energy emitted by surface materials and by
CO2, water ice, dust, and water vapor in the atmosphere. Phil Christensen
(Arizona State University) and a team of scientists studied over four
million TES spectra. After removing the effects of the atmosphere, they were
left with the distinctive spectral curves of the surface materials. These
were compared with spectra measured in the laboratory for a wide variety of
minerals. Shown below, the average spectrum for Terra Meridiani matched the
laboratory spectrum for hematite. The depth and shape of the hematite
fundamental bands reveal that this hematite is the crystalline and
coarse-grained (greater than about 10 micrometers in diameter) gray variety
of the iron oxide.
[spectra of hematite and Terra Meridiani compared]
Christensen and the TES science team identified hematite in
the TES data by the presence and shape of dips in the curve
(called oxide fundamental vibrational absorption features)
centered near 300, 450, and >525 cm-1 and by the absence of
dips (silicate absorptions) in the 1000 to 1400 cm-1 region.
The gap in the TES spectrum is the location of an
atmospheric absorption band, which gives no information
about the surface. The two curves have been offset for
easier comparison.
--------------------------------------------------
Hematite in Terra Meridiani
The major hematite deposit was found in a relatively low, smooth area
spanning latitudes approximately 1 oN to 3 oS and longitudes 8 oW to 1 oE in
the region called Terra Meridiani (variously named Meridiani Planum or Sinus
Meridiani.)
[topography of Terra Meridiani] [TES data showing distribution of hematite]
Image on the left is a topographic map (from the Mars Orbiter Laser
Altimeter, MOLA) of the relatively smooth Terra Meridiani region and its
neighboring cratered terrain to the south. Image on the right is a Viking
photomosaic superimposed with TES-derived hematite abundance. Detectable
hematite abundances range from approximately 5% to 15%. Corresponding
latitude and longitude lines are shown in both images for easier
comparison.
The Meridiani region has layered deposits covering ancient cratered terrain.
The hematite occurs in one layer within a 600-meter-thick stack of layers
composed of materials probably the size of coarse sand based on thermal
conductivity studies and TES data. The layers are visible for hundreds of
kilometers, which is remarkable on this planet where so much of the surface
is obscured by dust. This layered terrain was probably even more extensive
before it was eroded or buried. The thinness of the layers suggests they
were deposited at regular intervals, but for how long nobody knows. What the
layers are made of is an ongoing issue. Debates on the plausible origins of
the layered materials generally center on whether they are sedimentary
deposits or volcanic ash. Evidence is inconsistent for either origin because
there is a lack of a topographic basin and large volcanoes in this region.
Nevertheless, Hynek and co-authors favor a volcanic origin contending that
if the ash came from volcanic air fall, then the source vents for the ash
could simply be buried or distant, even several thousand kilometers away. A
discussion of the hypotheses for the origin of the hematite deposit will
follow the next section, which describes the geologic units mapped by Hynek
and his colleagues.
--------------------------------------------------
Mapping the hematite
The key geologic units mapped by Hynek and colleagues in Terra Meridiani are
plains, etched, and interior layered crater deposits (see map below). All
these units are geologically distinct from and bury preexisting cratered
terrain.
[Terra Meridiani deposits mapped by Hynek et al.]
Typical surface features are shown in the MOC images below; their locations
are indicated on the map of Terra Meridiani as black squares numbered I, II,
III, and IV. Each image links to a web page at Malin Space Science Systems
with options for downloading full-sized versions.
[hematite unit] [etched plains] [P1-P3 plains] [subdued cratered unit]
Plains units (P1, P2, P3) The units are all smooth, widespread, and somewhat
eroded by wind. The upper most unit (P3) is a bright cliff-forming unit
about 200 meters thick. The middle plains unit (P2) is dark basaltic plains,
contains hematite, and is less than a few hundred meters thick. According to
the TES data this unit has about 10 to 15% hematite. The lowermost unit is
P1. It is about 200 meters thick. (See images I and III above.)
Etched unit (E) is characterized by many different deposits that appear to
lie between and below the plains units. Extensive erosion has made it
impossible to connect individual layers between outcrops of the etched unit.
The material forms ridges, mesas, pits, and troughs. (See image II above.)
Interior layered crater deposits (I) are mounds of thinly layered material
found inside craters. They look like they may be isolated outcrops of the
plains and etched units.
Cratered deposits are mapped as various units: dissected (Cd), subdued (Cs),
undivided (Cu). All are rough but degraded meaning that crater rims are no
longer sharp, but are rounded due to erosion. (See image IV above.)
Based on superposition of units and crater counts (more craters means the
surface has been exposed longer to impacts), Hynek and colleagues report
that the layered plains and etched unit have a probable age of Late Noachian
to Early Hesperian, that is about 3.7 to about 3.5 billion years old.
--------------------------------------------------
Was it chemical precipitation from iron-rich fluids?
What happened in Terra Meridiani to cause the formation of the
coarse-grained gray hematite? What sequence of events led to stacks of thin,
parallel-bedded deposits with only one hematite-rich layer? The events must
not have been so common or else we'd expect to see more occurrences of gray
hematite on Mars. As it stands now, at a detection limit of several percent,
TES data show very limited exposures of gray hematite. Near-global mapping
of TES data by Christensen and colleagues reveals deposits of gray hematite
in only three places on Mars: Terra Meridiani, Aram Chaos (2oN, 21oW) and
small scattered exposures in Valles Marineris.
In Terra Meridiani some layers appear loosely packed and easily eroded while
other layers are more resistant to erosion and form cliffs and flat-topped
hills. People continue to look at the data for evidence to explain the
layering as volcanic, wind-blown, or water-laid deposits. The hematite might
have formed at the same time as the layers (primary formation) or it may
have formed long after the deposition of the layers (secondary formation).
Hynek and coauthors considered the plausible origins of hematite that have
been proposed and compared them to their own observations of Terra
Meridiani. The table below shows a summary of their findings.
Consistency of proposed hypotheses with observations and regional mapping
of Terra Meridiani hematite by Hynek, Arvidson, and Phillips
Observations
Origin Proposed hypothesis consistent with Observations inconsistent
hypothesis with hypothesis
no obvious closed basins or
obvious sources for lake
deposits; the age of the
deposits may be too old to
correspond with proposed
"warm and wet" conditions
precipitation from smooth, on Mars that would be
iron-rich, layered, easily necessary to deposit 600
Primary low-temperature eroded deposits meters of lakebeds; only
waters, as in a of constant one layer of the 600
lake or sea thickness meter-thick stack is
hematite-rich; lack of
other minerals in TES data
that would be expected in
lake deposits (evaporites)
or in banded iron
formations (quartz, chert,
carbonates)
different
large areal extent (>105
precipitation from erosional
iron-rich, patterns within km2); lack of associated
units; possible hydrothermal-alteration
Primary high-temperature cemented products in TES data; lack
waters, in
hydrothermal joints; of evidence of tectonic
systems association processes or other obvious
with outflow heat sources
channels
materials have
volcanic
compositions
(basaltic,
thermal oxidation andesitic);
of volcanic layers are
deposits: iron-rich widespread; lack of nearby volcanoes;
lava flows, there are Martian lavas are generally
ignimbrites, air possible flow far less susceptible to
Primary fall features in erosion than these
Note: this some of the deposits; layers have
hypothesis does not layers; layers nearly constant thickness
require liquid are thin, flat, and conform to preexisting
water to cause smooth, and topography
oxidation drape the
preexisting
topography;
some layers are
susceptible to
erosion
different
erosional
patterns within red not gray hematite is
Secondary ground water - units; possible more probable; sharp
leaching boundaries that correlate
cemented
joints; layers with local topography
are widespread
existence of
one hematite
layer in a 600
meter stack; lack of associated
ground water - different hydrothermal alteration
Secondary hydrothermal erosional products in TES data; lack
alteration along patterns within of evidence of tectonic
permeable layers units; possible processes or other obvious
joint systems; heat sources
association
with outflow
channels
coatings of
iron-rich rock by hematite red not gray hematite is
Secondary weathering from coatings are more probable; lack of
surface and/or common on Earth substantial atmospheric
atmospheric water water on Mars
Of the primary formation options, Hynek and coauthors think it's plausible
that the hematite could have formed as iron-rich deposits that were oxidized
during eruptions from distant volcanoes. This kind of thermal oxidation of
volcanic ash during eruption does not require water. In this scenario,
however, a wider distribution of hematite-rich ash would be expected but is
not seen in the TES data. Perhaps some hematite-rich ash layers are still
buried or perhaps they have been eroded away.
Alternatively, the hematite may have formed by a later secondary mechanism
in preexisting ash beds. In this case, Hynek and his colleagues favor
precipitation of the hematite when iron-rich fluids circulated within the
layered volcanic ash. This kind of secondary formation of the gray hematite,
they say, is most consistent with their regional geologic, topographic, and
spectral observations.
Researchers are continuing to study a variety of remote sensing data of
Terra Meridiani. Daytime infrared images from the Thermal Emission Imaging
System (THEMIS) on the Mars Odyssey spacecraft allow new views of the
layered terrain in better detail. Christensen and the THEMIS team see
different temperatures for the different layers. They think this could
indicate the layers have different physical properties (such as particle
size, mineral composition, or density) perhaps due to changes in when or how
fluids circulated through. Other researchers are still looking for evidence
of surface water in the region (paleolake basins, for example) or trying to
evaluate the potential of the hematite deposits to preserve microfossils.
New studies and results will be reported at the annual Lunar and Planetary
Science Conference in Houston, March 17-21, 2003.
--------------------------------------------------
Likely landing location
Orbital MOLA topographic data and TES compositional
data have given us testable hypotheses of how the
hematite formed. We may soon be able to test these ideas right on the spot.
Terra Meridiani is a leading candidate for further exploration by one of the
upcoming Mars Exploration Rovers (MER) because of its distinctive mineralogy
and its relative safety as a landing site (in terms of low wind shear, low
abundance of boulders, and low slope angles). After years of detailed
evaluations of potential landing sites, debates and recommendations on
scientific merit and safety issues, and a final peer review taking place
this month we will finally hear the announcement of the two site selections
in early April, 2003. Launch window for the first MER spacecraft opens May
30, 2003 for an early January 2004 landing. The second MER craft is
scheduled for launch on June 24, 2003 followed by a late January landing.
The data gathered by these robotic field geologists will help scientists
here on Earth read the stories of the rocks, stories told in the language of
texture, chemistry, and mineralogy about the role of water and whether or
not the environment may have ever been suitable for life.
--------------------------------------------------
ADDITIONAL RESOURCES
ASU Spectral Library from Arizona State University Thermal Emission
Spectroscopy Laboratory. Lists thermal infrared (2000 - 380 cm-1)
emission spectra of over 150 pure minerals, with an emphasis on common
rock-forming minerals.
Christensen, P. R., Bandfield, J. L., Clark, R. N., Edgett, K. S.,
Hamilton, V. E., Hoefen, T., Kieffer, H. H., Kuzmin, R. O., Lane, M.
D., Malin, M. C., Morris, R. V., Pearl, J. C., Pearson, R., Roush, T.
L., Ruff, S. W., and Smith, M. D. (2000) Detection of crystalline
hematite mineralization on Mars by the Thermal Emission Spectrometer:
Evidence for near-surface water. Journal of Geophysical Research, v.
105 (E4), p. 9623-9642.
Christensen, P. R., Morris, R. V., Lane, M. D., Bandfield, J. L., and
Malin, M. C. (2001) Global mapping of Martian hematite mineral
deposits: Remnants of water-driven processes on early Mars. Journal of
Geophysical Research, v. 106 (E10), p. 23,873-23,885.
Hynek, B. M., Arvidson, R. E., and Phillips, R. J. (2002) Geologic
setting and origin of Terra Meridiani hematite deposit on Mars. Journal
of Geophysical Research, v. 107, no. E10, 5088, doi: 1029/2002001891.
(pdf)
Lunar and Planetary Science Conference March, 2003 program with
abstracts, including sessions on Mars (available as pdf files.)
Mars Exploration Rovers (MER) - Athena instrument payload from Cornell
University.
THEMIS image of Terra Meridiani from the 2001 Mars Odyssey - Thermal
Emission Imaging System from NASA/JPL/Arizona State University.
Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS).
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