NEW EVIDENCE FOR KILOMETER-THICK AMAZONIAN ICE DEPOSITS IN AN IMPACT
CRATER IN PHLEGRA MONTES, MARS. J. W. Head1, J. L. Dickson1 and D. R. Marchant2, 1Dept. Geological
Sciences,
Brown
Univ.,
Providence,
RI
02912
USA
(james_head@brown.edu,
james_dickson@brown.edu), 2Dept. of Earth Sci., Boston Univ., Boston MA 02215 USA (marchant@bu.edu).
Introduction: Lineated valley fill (LVF) in fretted valleys
at the dichotomy boundary on Mars has been interpreted by
some as glacial in origin [1-4]. Unknown are 1) the original
thickness of the glacier ice, and 2) the amount of ice-surface
lowering, through sublimation, retreat and ice loss, to its
presently observed level. We recently addressed these questions through an analysis of an integrated LVF glacial
landsystem in the Protonilus Mensae-Coloe Fossae area of
the dichotomy boundary [3]. There, the elevation difference
between the upper limit (determined from a lobe of LVF
entering a box canyon at high elevations and being stranded
there) and the current surface of the LVF at the study site, is
~920 meters. We interpreted this difference to reflect the
minimum amount of ice-surface lowering of the valley glacier system during retreat. Consistent with a general lowering
of the surface are multiple moraines and/or trimlines, and
changes in LVF flow patterns indicating local reversals in
flow patterns as the ice retreated and decreased in thickness.
These data suggest that the major Late Amazonian LVF glaciation involved significantly larger amounts of glacier ice
than previously thought. Here we report on additional evidence for thick mid-latitude ice deposits in the Phlegra Montes area, at the same latitude band in the northern hemisphere
of Mars.
Phlegra Montes: The Phlegra Montes extend northward
from the Elysium rise into the northern lowlands for many
hundreds of kilometers (Fig. 1). These mountains, together
with numerous areas along the dichotomy boundary and
other regions in the 30-50 N latitude band, display geomorphic features (lineated valley fill, LVF; and lobate debris
aprons, LDA) interpreted to be indicative of 1) vapordiffusion-generated, pore-ice-assisted creep [5,6] or coldbased glaciation [1-4].
Located on the western flanks of Phlegra Montes is a ~32
km diameter impact crater with a smaller, ~8 km diameter
crater on its northeast rim (Fig. 1, arrow; Fig. 2). Although
containing a sharp rim crest, the crater has undergone significant modification and its floor is filled with concentric
ridged terrain arrayed around the interior, a texture typical of
craters characterized by "concentric crater fill" [5]. MOLA
altimetric profiles (Figs. 2, 4) reveal that the crater floor is
very asymmetric, with the the floor tilted poleward. The interior can be divided into five distinct segments from S-N,
each of which corresponds to a different geomorphic texture
(compare Fig. 2 and 4, bottom). From the southern rim of the
crater, these include: 1) the steep upper part of the southern
interior wall, about 3 km wide, that descends over 800 m
from the sharp rim crest (~-3060 m) to the base of the wall;
2) a shallow, convex-downward depression, about 4 km
wide, that corresponds to a zone of radial ridges and depressions; 3) a convex-upward portion of the floor, about 7.5
wide, characterized by concentric ridges and troughs, and
pitted terrain in its downslope portion (the lowest portion of
the floor, at ~-4304 m, separates this segment from the next);
4) a slightly convex-upward portion of the floor about 8 km
wide and sloping to the south that is characterized by concentrically textured and ridged material; 5) the northern wall
of the crater, about 5 km wide, rising about 1320 m to the
crater rim crest at -2880 m. These characteristics are very
similar to the interiors of modified craters in the mid-high
latitudes on Mars (concentric crate fill [5]), showing slope
asymmetries [7] and interior modification related to iceassisted downslope movement [8]. Uncertain, however, has
been the relative roles of 1) ice-assisted creep of crater wall
talus (diffusion, condensation, and freezing of atmospheric
water vapor during periods of climate change) versus 2) the
direct precipitation and accumulation of snow and ice to
produce debris-covered glaciers. In scenario 1), the deposits
are relatively local, and were dominated by pore ice and
some accretionalry ice. In scenario 2), significant accumulation results in glacial ice and flow, followed by climate
change, retreat and recession to the presently observed feature, the final remnnat of a debris-covered glacier.
The 8 km crater perched on the main crater rim (Fig. 2, 3)
provides data to help distinguish beween these two hypotheses. The interior is characterized by textures typical of
LDA/LVF-type fill (Fig. 3), but instead of forming on the
crater interior walls and flowing down across the breached
low point in the rim, down the wall, and onto the large crater
floor, it appears to have entered the small crater from the
opposite direction, the direction of the crater interior. In
addition, two smaller, but similar lobes, extend from the
northern rim crest of the small crater (at ~11 and 1 o'clock)
for several km, pointing northward. Taken together, these
observations suggest that the large lobe was empalaced from
the crater interior, and that the two small lobes represent
overflow from a once-filled small crater.
Interpretation: On the basis of these relationships, we interpret the lobe in the small crater to have been formed from
debris-covered ice that accumulated in the main crater interior and then flowed downslope northward into the smaller
crater on the rim, filling it sufficiently to cause two small
lobes to breach its northern rim. If this interpretation is correct, then additional areas of ice overflow should occur, and
indeed, such structures are seen along the relatively low eastern rim. Topography data provide information on the initial
configuration required for this scenario (Fig. 4). MOLA
profile 13024 suggests that a total thickness of ice of ~1000
m is required to fill the large crater, flow into, and breach the
northern rim of the small crater. This thickness is comparable
to that interpreted to have been present (and subsequently
lost) in Protonilus Mensae-Coloe Fossae area (~920 m).
What was the fate of this glacial ice? Detailed textures in the
crater lobe suggest lowering and some backflow in toward
the main crater. In the southern region of the crater floor at
the edge of the main crater wall, a zone of parallel ridges and
troughs, reminiscent of crevasses formed from glacial ice
descending over underlying topography, is observed. At least
480 m of ice loss from sublimation and retreat are implied
within the small crater, and an additional 530 m from the
main crater interior. These data add to evidence [3] that the
formation of Amazonian mid-latitude LDA/LVF deposits
involved ice sheets with thicknesses measured in hundreds of
meters to several kilometers.
References: [1] J. Head et al., EPSL, 241, 663, 2005; [2] J.
Head et al., GRL, 33, doi: 10.1029/2005GL024360, 2006; [3]
J. Dickson et al., Geology, in review, 2007; [4] J. Head & D.
Marchant, LPSC 37, #1127, 2006; [5] S. Squyres et al., Mars,
523, U. AZ Press, 1992; [6] T. Pierce & D. Crown, Icarus,
Northern Mid-Latitude Amazonian Glacial Ice Thickness: J. W. Head, J. . Dickson and D. R. Marchant
163, 46, 2003; [7] M. Kreslavsky & J. Head, GRL, 30, 1815,
2003; [8] M. Kreslavsky & J. Head, MAPS, 41, 1633, 2006.
Figure 1. MOLA topography over MOLA hillshade. Hecates
Tholus is in the southwest of the frame, while Phlegra Montes is
to the east.
Figure 2. MOLA topography over MRO CTX images
(P01_001553_2232 and P01_001619_2232).
Figure 3. MRO CTX subframe of image
P01_001553_2232.
Figure 4. MOLA track profiles from North to South across the
larger host crater. Profile locations are shown in Figure 2.