Sixth Mars Polar Science Conference (2016)
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GLACIAL AND PERIGLACIAL CHEMICAL WEATHERING ON MARS: NEW RESULTS AND NEW
QUESTIONS FROM FIELD ANALOG STUDIES AND MARS REMOTE SENSING. B. Horgan, N. Scudder,
A. Rutledge, and S. Ackiss, Purdue University – West Lafayette, IN ([email protected]).
Introduction: Ice has been a powerful physical
weather agent on Mars through geologic time, extending from the potentially extensive Noachian highlands
ice sheet to the widespread glacial and periglacial features observed at midlatitudes today. However, it is
less well understood how much chemical weathering
ice has caused on Mars, and how the mode of alteration has changed with the climate over time. Here we
summarize recent advances in our understanding and
identify remaining unknowns regarding glacial, periglacial, and polar weathering on Earth and Mars.
Glacial weathering on Earth: The flow of wetbased glaciers over their substrate causes significant
mechanical weathering underneath the glacier, and
generates large volumes of fine-grained materials (glacial flour, etc.). Aqueous geochemistry of glacial outwash has shown that the high surface area under the
glacier combined with large weathering surface area in
these materials drives significant chemical weathering
[1]. However, the mineralogy of the alteration products
produced in the subglacial, proglacial plain, and proglacial lake environments is poorly understood [2], so
we are currently unable to evaluate whether or not
glaciers played a major role in the chemical evolution
of the martian surface.
Glacial weathering of coarse grained sediments and
rocks may resemble subaerial pedogenic weathering by
snow and ice melt. In alpine and polar environments
on Earth, very high leaching rates during seasonal
snow melt tend to favor the rapid precipitation of poorly-crystalline phases, like the aluminosilicate allophane
[3]. Thus, alpine soils are commonly dominated by
poorly crystalline phases, which then mature into kaolin minerals [4]. Glacial weathering of sediments finegrained enough to be suspended in glacial outwash
may be more severe. Glacial flour in distal lakes on
Earth contains clay minerals that could be glacially
sourced [5], suggesting that glaciers could have been a
source of alteration minerals in lacustrine deposits on
Mars.
New glacial Mars analog studies: To better understand the alteration minerals that are characteristic
of glacial environments, we have established a new
Mars analog sites at the Three Sisters volcanic complex in central Oregon. The Three Sisters is the most
mafic persistently glaciated peak in the continental US,
making it an ideal site to study the effects of long-term
glacial alteration on rocks of Mars-like composition.
We will be conducting field work at the site prior to
the meeting, in August 2016 and will report first results from our field campaign in this presentation as
well as several others [6,7].
Preliminary results from reconnaissance campaigns
have suggested that the mineralogy of glacial weathering is unique and detectable by instrumentation on
Mars orbiters and rovers. In particular, we have found
that samples from the proglacial plain contain high
abundances of poorly crystalline silicates, most likely
iron-bearing. Remote sensing investigations of the
field site support this interpretation, showing enhanced
hydrated silica absorptions in the visible/near-infrared
(VNIR) [6] and high-silica phases in the thermalinfrared [7]. In our field campaign and following laboratory investigations, we will determine the composition of the poorly crystalline phases, as well as their
distribution in the glacial environment. We will acquire subglacial and supraglacial rocks and sediments,
proglacial plain and lake sediments, and suspended
sediments, in addition to fluid chemistry and field
VNIR spectra.
A major outstanding question related to glacial alteration is whether or not cold-based glaciers (frozen to
their based with limited or no melting) cause significant chemical weathering over long timescales. Based
on the lack of mechanical alteration in these systems, it
has generally been assumed that cold-based glaciers do
not cause chemical alteration, but this has never been
verified in the field.
Glacial weathering on early Mars: Abundant
morphologic evidence suggests that ice sheets and
glaciers were widespread during the Noachian and
Hesperian on Mars [e.g., 8], and that these ice sheets
were at least locally wet-based [9,10]. Thus, we might
expect that these ice sheets cause significant chemical
weathering on ancient Mars. However, the fate of these
weathering product is poorly understood. Some glacial
alteration products may have been transported into
lakes and deltas at lower latitudes. Intriguingly, poorly
crystalline alteration phases have been detected as a
major component of Noachian deposits sampled by
landed missions. Models of MER Mini-TES mid-IR
spectra of altered Clovis and Watchtower class rocks
in Gusev Crater include up to 50% of an unknown
poorly crystalline component [11]. Furthermore, nearly
every unit sampled for CheMin XRD analysis by Mars
Science Laboratory at Gale Crater, including lacustrine
and deltaic deposits, contains a significant poorly crystalline component of variable composition [12-14]. We
Sixth Mars Polar Science Conference (2016)
hypothesize that these poorly crystalline phases could
be the result of weathering by ice/ snow melt, perhaps
providing some limited geochemical support for sustained cold climates on early Mars.
An additional form of subglacial alteration that may
have been common under the extensive ice sheets of
ancient Mars is subglacial volcanism. Subglacial volcanism produces volcanoes with unique table-top morphologies known as tuyas, and possible tuyas have
been identified at both northern midlatitudes and
southern high latitudes on Mars [15,16]. New analyses
of CRISM VNIR spectra over the Sisyphi Montes region of Mars has shown that these possible tuyas exhibit a unique assemblage of minerals including zeolites, clays, oxides, and sulfates that together, strongly
suggest hydrothermal alteration due to subglacial volcanism [17]. Confirmation of these edifices as tuyas
may help us to reconstruct the past extent and thickness of the Dorsa Argentea ice sheet during the Hesperian on Mars.
Amazonian glacial/periglacial weathering: Results from orbiters and rovers have demonstrated that
weathering on more recent Mars terrains is generally
much more acidic than on ancient terrains, most likely
due to more restricted water:rock ratios and volcanic
emissions [e.g., 18,19]. This increased acidity appears
to extend to polar and periglacial weathering as well.
Some Amazonian mid-latitude mantles in the northern
plains exhibit extensive morphologic evidence for periglacial modification, and are appear to be a source of
the widespread dark and glass-rich sediments that cover much of the northern plains [20]. These sediments
exhibit spectral evidence for silica enrichement due to
prolonged acidic leaching, perhaps due to ice melt at
low water:rock ratios [21]. This suggests that this periglacial mantle experienced acidic weathering at some
point in its history. Acidic alteration also appears to
have occurred in the Amazonian deposits of the north
polar cap, Planum Boreum. Gypsum, a hydrated sulfate, has been detected throughout the north polar sand
sea as well as within the north polar layered deposits,
and is also most likely due to acidic alteration of mafic
sediments [22,23].
Global implications: One of the signatures of cold
climate weathering under either neutral or acidic conditions is enrichment in high-silica poorly crystalline
phases. Such phases have been observed across Mars
by both landers and rovers, including the high silica
component of the globally distributed TES Surface
Type 2 composition, as well as the high-silica detections by Mars Pathfinder in the northern lowlands.
Both of these detections are as yet unexplained, but we
hypothesize that they may be due to widespread glacial
and periglacial alteration throughout martian geologic
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history. This suggests that glacial alteration may have
been one of the major drivers in determining the observed surface composition of Mars.
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