40th Lunar and Planetary Science Conference (2009)
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PHOENIX LANDING SITE GEOMORPHOLOGY: SURFACE STABILITY AND IMPLCATIONS FOR THE MARTIAN LATITUDE-DEPENDENT MANTLE J. S. Levy1, J. W. Head1, D. R. Marchant2, 1Brown Univ. Dept. of Geological
Sciences, Providence, RI, 02906. 2Boston Univ. Dept. of Earth Sciences, Boston, MA, 02215. joseph_levy@brown.edu.
Introduction. The NASA Phoenix lander has provided
a unique first-look at permafrost processes operating on the
surface of the martian northern plains [1]. Critical results
from the mission include the detection of an icy surface present beneath thin, overlying, dry sediments [1-3]; complex
soil chemistry comparable to that observed in terrestrial cold
and arid terrains [4, 5]; and weather/climate conditions consistent with the generation of a range of cold-desert landforms [1-3]. Questions remain, however, as to the abundance
and origin of the observed ice, the history of liquid water at
the landing site, the age and stability of the landing site surface, and the relationship between the landing site and the
martian latitude-dependent mantle (LDM).
The martian northern plains are typical of martian LDM
surfaces [6-9]. Data from Phoenix can be used to test hypotheses accounting for the origin of the martian LDM in the
vicinity of the landing site by addressing questions raised by
each hypothesis (indicated parenthetically): 1) Young, latitude-dependent mantle: the LDM is meters-thick and formed
from precipitation of snow and dust, with dust forming a
sublimation lag, and with massive ice concentrated in underlying layers [6-13] (What processes have brought pebbles,
cobbles, and boulders at the site to the surface?). 2) Young,
latitude-dependent mantle—wet active layer: the LDM is
meters-thick and formed from precipitation of snow and dust,
with dust forming a sublimation lag—subsequently, formation of a wet active layer has mixed the sediment and brought
rocks to the top surface [14, 15] (Why are the ages of the
Phoenix landing site [16] and northern plains LDM surfaces
[7, 12, 17] considerably younger than the last predicted period of active layer activity in the northern plains [18]). 3) No
latitude-dependent layer—cold climate: Circumpolar deposits
are secondary ice [19]—polygons form from thermal cycling
of ice-cemented sediment and boulders accumulate at the
surface by long-term reworking of an old regolith (What
processes account for the <1 Ma age of the LDM surface?).
4) No latitude dependent layer—wet active layer: Martian
polar surfaces are actually ancient and have been enriched in
secondary ice by vapor diffusion—melting has occurred in
the recent geological past, causing cryoturbation of the deposits that bring boulders to the surface [1, 19, 20] (Occurring only during peak obliquity excursions [18], are active
layer processes sufficient to degrade craters?).
Here we present a range of observations of the Phoenix
landing site from the surface stereo imager (SSI) [2] that
provide insight into surface activity (cryoturbation of secondary ice versus vertical ablation of primary atmospheric ice)
and thermal contraction crack polygon dynamics.
Geomorphological Observations From SSI Suggesting Stable/Vertically Ablating Surfaces. Stable (nonchurning) and vertically ablating surfaces are typical of extremely ice-rich permafrost surfaces in some portions of the
Antarctic Dry Valleys—particularly those in which sublimation is a dominant water phase transition—and provide a
guide to geomorphic features that may suggest stable, vertically ablating surfaces on Mars [21-24]. Pitted rocks are present at the Phoenix site, and have been interpreted as vesicular boulders (Fig. 1) [25]. Alternatively, these boulders may
have been pitted by a range of surficially-acting physical
processes, such as salt weathering, which typically forms
mm-to-cm-scale pits on static rocks in Antarctic environments on timescales from ~1-4 Ma [22, 26-28], consistent
with predictions of syngenetic LDM and boulder emplacement and arrangement [34]. Next, although preliminary cobble and boulder counts at the Phoenix site have been interpreted to indicate thorough reworking of the surface by cryoturbation [3, 20], the presence of cobbles and boulders overlying fine sediments in polygon interiors (rather than embedded in them) (Fig. 2) strongly suggests the formation of “desert pavement” surfaces, typical of vertically ablating arid
surfaces [24, 29-31]. Pavement surfaces are ablational, consistent with the paucity of depositional ripples at the site reported by [25]—although light-toned, smooth “dust pools”
are abundant around the lander, analogous to flat-lying sand
accumulations typical of Antarctic polygonally patterned
terrains [32]. Additionally linear groups of boulders and cobbles (Fig. 3) present at the Phoenix site appear analogous to
the surface expression of relict polygon trough locations
found in vertically-ablating Antarctic tills [30]. Lastly, tightly
clustered groups of cobbles and boulders at the landing site
appear analogous to disintegrated boulders (or “puzzle
rocks”) common in Antarctic, arid terrains—clasts that are
weathering in-situ with minimal surface disturbance [27].
Ongoing Sublimation-Style Polygon Activity. Troughs
emerging from polygon interiors and orthogonally intersecting polygon margin troughs (Fig. 4) indicate that the Phoenix
landing site surface has undergone more than one generation
of polygon trough formation, consistent with models predicting ongoing thermal contraction cracking [33]. Ongoing fracturing, preferential sublimation of buried ice at fracture locations, and periodic slumping of clasts into polygon troughs
(typical of terrestrial sublimation polygons [30]) may account
for polygons observed at the site with cobble/boulder-covered
interiors (Fig. 5) overlying fines transitioning into troughs
with finer grained rock cover.
Conclusions. Geomorphological analysis of the Phoenix
landing site suggests a history for permafrost at the site characterized by the recent dominance of ice removal by sublimation, ongoing thermal contraction cracking, and limited cryoturbation by either wet or dry processes. These observations
are consistent with global observations of the martian latitude-dependent mantle suggesting the vertical ablation of
excess ice in the martian subsurface originally of primary
atmospheric origin.
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al. (2008) AGU Fall Mtg., Abstr. U14A-04. [6] Head, J.W.,
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40th Lunar and Planetary Science Conference (2009)
33, doi:10.1029/2006GL025946. [13] Kuzmin, R.O., et al.
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Fig. 3. Linear arrangements of surface clasts cross-cutting
extant polygon interiors and troughs (highlighted), interpreted to be the surface expression of relict polygon troughs.
Analogous features form in vertically-ablating Antarctic terrains. Portion of PHX image 17106.
Fig. 4. High-centered, round-shouldered polygon bounded
by depressed troughs, and cross-cut by a narrower trough
connecting the polygon center to the bounding troughs. Multiple generations of fractures suggests ongoing thermal contraction cracking. Portion of PHX image 17106.
Fig. 1. Pitted boulder in near-foreground of PHX image
17106. Pits may be vesicles, or salt-weathering pits typical
of boulders on stable surfaces in Antarctica.
Fig. 2. Cobbles and pebbles overlying fine material at the
Phoenix landing site. The surface texture is analogous to
“desert pavement” surfaces typical of arid and cold terrains
on Earth, and are largely formed by aeolian processes. Portion of PHX image 17106.
Fig. 5. Polygon with dense interior clast coverage and
troughs with finer-scale clasts, suggesting ongoing shedding of interior cobbles/boulders by gentle processes such as
slumping, and transport of fines by aeolian activity, typical
of terrestrial sublimation polygons.