Workshop on Martian Gullies: Theories and Tests (2008)
8007.pdf
LATE-STAGE GULLY MODIFICATION ON MARS: POLYGONALLY-PATTERNED GROUND,
PERMAFROST, AND GULLY WATER SOURCES. J. S. Levy1, J. W. Head1, D. R. Marchant 2, J. L. Dickson1,
and G. A. Mogan 1, 1Brown University Department of Geological Sciences, 324 Brook Street, Box 1846, Providence,
RI, 02912, joseph_levy@brown.edu, 2Boston University Department of Earth Science, 675 Commonwealth Avenue,
Boston, MA, 02215.
Introduction: Gullies on Mars are a class of
young features, initially interpreted to have formed by
surficial flow of released groundwater [1, 2], and
which may still be active [3]. Alternative hypotheses
for the origin of gully-carving fluids include melting of
dust-rich snow deposits [4], melting of atmospherically
emplaced ice [5], and melting of ice-rich permafrost
terrains at high obliquities [6]. Recent fieldwork in the
Antarctic Dry Valleys (ADV) has reported on the topdown melting of snow as the primary source for water
currently flowing through gully channels and hyporheic zones [7-10]. Analysis of composite-wedge polygons in the gullied terrain indicates that polygonally
patterned ground enhances local the accumulation of
windblown snow (and thus, increases the amount of
meltwater in the gully systems), assists in the transport
of gully-related meltwater (by altering the local icecement table topography to concentrate channelized
flow), and mediates in the storage of gully meltwater
(in the distal hyporheic zone) [11]. Further, polygons
were shown to have pre-dated the gullies studied, and
to have remained active during the entire process of
gully formation, implying the continuous presence of
an impermeable permafrost layer beneath the gullies
throughout their evolution [11]. These observations
provide a baseline for analyzing gully-polygon interactions on Mars, and suggest that analysis of surface
permafrost features can be used to infer the state of the
shallow subsurface in martian gullied terrains.
Gully-Polygon Interactions on Mars: A survey of
HiRISE images of the martian surface from the primary science phase, spanning 30°-80° north and south
latitude was conducted. Of the 537 images studied, 118
feature gullies, and 70 feature gullies in conjunction
with polygons. Analysis of gully-polygon suites reveals the presence of several analogous landforms
between ADV and martian gully-polygon systems.
Alcove Polygons. As in the ADV, polygons are
common in gully alcoves [11] (Fig. 1). Polygons in
gully alcoves are commonly outlined by bright deposits which are present preferentially in polygon troughs.
Annexed Polygon Troughs. In ADV gully systems,
active and abandoned gully channels show evidence of
the diversion of gully-channel water flow into previously incised polygon troughs—a process referred to
as polygon trough annexation [11]. In inactive annexed
polygon troughs, gully-channel fluvial deposits are
present above composite polygon wedge structures
[11]. Annexation widens and deepens polygon troughs,
and results in less angular trough intersection angles
[11]. Although active fluvial processes were not observed in the 537 HiRISE images studied (with the
possible exception of [3]), several features were observed which were 1) continuous and sub-linear 2)
present on polygonally patterned surfaces in widened
and curved polygon troughs, and 3) had a texture
similar to gully-fan surface textures (Fig. 2). These
features are interpreted as being analogous to terrestrial
annexed polygon troughs.
Fan Overprinting and Dissection. Gully-fan deposits in the ADV overprint and embay compositewedge polygons and are dissected by a network of
continuously expanding polygon cracks [11]. Several
gully fans observed in HiRISE images embay polygonally patterned ground on gullied slopes (Fig. 3). Further, polygon troughs contiguous with the surrounding
trough network are expressed through some gully fan
surfaces (Fig. 3). Generally, such fan surfaces are of
small spatial extent (>50% the surface area of the associated gully alcove), and have subdued topography (no
extensive relief is present between fan surfaces and
off-fan surfaces).
Gully-Polygon Stratigraphy and Implications:
Taken together, these observations of gully-polygon
suites on Mars suggest the following stratigraphic and
temporal relationships between gullies and polygons:
1) polygons post-date alcove excavation in some gullies, making polygons the youngest landscape element
present; 2) polygon troughs have been annexed by
some gully channels, indicating channel formation in a
polygonally patterned surface; and 3) fan embayment
and dissection implies that gully fan deposits formed
on a polygonally patterned surface which has undergone continuous thermal contraction cracking during
fan aggradation. Polygon-gully stratigraphic relationships suggest that these martian gullies have formed
and evolved on slopes underlain by continuous shallow
permafrost during the most recent period of gully activity. No evidence of catastrophic water release was
observed. These lines of evidence suggest an atmospherically emplaced, top-down source for gully volatiles involved in the most recent stage of martian gully
evolution. Polygon and permafrost enhancement of
gully processes analogous to those observed in the
Workshop on Martian Gullies: Theories and Tests (2008)
8007.pdf
ADV is consistent with cold, ice-depositional climate
conditions modeled to have prevailed at gully-polygon
sites during the last ~5-10 My [12].
References: [1] Malin, M.C. & K.S. Edgett (2000) Science, 288, 2330-2335. [2] Malin, M.C. and K.S. Edgett,
(2001) JGR, 106, 23,429-23,540. [3] Malin, M.C., et al.
(2006) Science, 314, 1573-1577. [4] Christensen, P.R.
(2003) Nature, 422, 45-48. [5] Hecht, M.H. (2002) Icarus, 156, 373-386. [6] Soare, R.J., et al. (2007) Icarus,
191, 95-112. [7] Dickson, J.L., et al. (2007) LPSC 38,
Abstr. #1678. [8] Head, J.W., et al. (2007) LPSC 38,
Abstr. #1617. [9] Levy, J.S., et al. (2007) LPSC 38, Abstr. #1728. [10] Morgan, G.A., et al. (2007) LPSC 38,
Abstract #1656. [11] Levy, J.S., et al. (2007) Antarctic Science, in review. [12] Forget, F., et al. (2007) 7th
Mars, Abstr. #3028.
Figure 2. Annexed polygon troughs in PSP_001846_
2390 (58.7 N, 82.4 E, Ls = 152.2: northern summer)
indicated by arrow. Bright material in sinuous polygon
troughs has a similar texture to nearby fan deposits. Illumination from right.
Figure 1. Alcove of gully in PSP_001882_1410 (38.7
S, 194.0 E, Ls = 153.7: southern winter). The origin
of bright material preferentially present in polygon
troughs remains an active area of inquiry. Illumination
from left.
Figure 3. Gully fan deposits in PSP_001846_2390 in
contact with polygonally patterned ground. Fan deposits
embay some polygons, and fill some polygon troughs.
The underlying polygon network is visible through the
fan, suggesting continued dissection of the fan surface.