Water ice is exposed in the martian north polar cap, and is occasionally exposed beyond the cap boundary. Orbital gamma ray spectrometry data strongly imply the presence of water ice within meters of the surface at latitudes north of approximately 60 deg. We have examined midlatitude areas of the northern plains displaying evidence of residual ice-rich layers, and report possible present-day exposures of ice. These exposures, if confirmed, could constrain the latitudinal and temporal stability of surface ice on Mars

Allen, Carlton C.

Kanner, Lisa C.

NASA Technical Reports Server

EXPOSURES  OF  WATER  ICE  IN  THE  NORTHERN  MID-LATITUDES  OF  MARS. 
C. C. Allen1 and L. C. Kanner2          1NASA Johnson Space Center, Houston, TX  77058   carlton.c.allen@nasa.gov 
2Brown University, Providence, RI  02912   lisa_kanner@brown.edu 
 
 
Introduction:  Water ice is exposed in the martian 
north polar cap [1], and is occasionally exposed 
beyond the cap boundary.  Orbital gamma ray 
spectrometry data strongly imply the presence of water 
ice within meters of the surface at latitudes north of 
approximately 60o [2].  We have examined mid-
latitude areas of the northern plains displaying 
evidence of residual ice-rich layers, and report possible 
present-day exposures of ice.  These exposures, if 
confirmed, could constrain the latitudinal and temporal 
stability of surface ice on Mars. 
Ice-Rich Layer:  Polygonal features with 
dimensions of approximately 100 m, bounded by 
cracks, are commonly observed on the martian 
northern plains.  These features are generally attributed 
to thermal cracking of ice-rich sediments, in direct 
analogy to polygons in terrestrial polar regions [3,4].  
We mapped polygons in the northern mid-latitudes 
(30o to 65o N) using all Mars Orbiter Camera (MOC) 
narrow-angle images (resolution ~ 5 m / pixel) from 
9/97 through 9/03 [5].  While MOC images show that 
polygons are scattered across the northern plains, 74% 
of such images are centered in western Utopia Planitia 
(40o to 50o N; 258o to 288o W; Fig. 1). 
  
 
 
Figure 1.  MOC images showing polygons – latitude 
range 30o to 65 o N 
This region largely overlaps the Late Amazonian 
Astapus Colles unit, characterized by polygonal terrain 
and nested pits “consistent with periglacial and 
thermokarst origins” [6].  Other authors suggest that 
this morphology reflects the presence of a generally 
continuous ice-rich mantle [3,4].  Ice stability models 
[7], orbital spectrometry [2], and the occurrence of 
thermokarst morphologies [5] indicate that the ice is 
concentrated below 1 m and is currently subliming. 
In a follow-on study McBride et al. [8] measured 
all of the impact craters, with diameters between 100 
m and 4 km, occurring on polygonal terrain between 
30o and 65o N.  The size-frequency distribution of the 
craters larger than 1 km is concordant with the 
distribution for larger craters (> 8 km) in western 
Utopia (N. Barlow, personal communication), 
indicating preservation of a late Hesperian crater 
population.  Of the craters on polygonal terrain, 97% 
predate the polygonal cracks, indicating Amazonian-
age deposition or activation of an ice-rich layer.  The 
crater size-frequency distribution on polygonal terrain 
shows a marked deficiency of craters smaller than 1 
km, suggestive of mantling.  A subset of such craters 
with diameters between 460 m and 1.12 km are buried 
to their rims by polygonal terrain, indicating that the 
ice-rich layer is locally 30 to 40 m thick [8].  These 
findings are in accord with models of obliquity-driven 
deposition and sublimation of ice-rich mantles in the 
northern mid-latitudes of Mars [9].  One recent model 
of the climate during high obliquity epochs specifically 
predicts ice accumulation in western Utopia [10]. 
Bright and Dark Polygonal Cracks:  Many 
polygons, particularly in the northern portion of the 
study area, are bounded by cracks significantly 
brighter than the polygon centers.  Other polygons, 
particularly farther south, are bounded by dark cracks 
(Figs. 2,3). 
A survey of the longitude range 240o to 300o W 
showed that polygons at latitudes between 60o and 65o 
N are generally demarcated by bright cracks.  Dark 
cracks bound polygons at latitudes of 35o to 60o N [5]. 
A more extensive examination of all MOC images 
(1997 through 2003) covering 30o to 65o N supported 
these initial results.  At latitudes of 55o to 65o N, 45 
MOC images show polygons – with 73% showing 
bright cracks.  Between 40o and 55o N, 141 images 
show polygons – with 66% showing dark cracks [11].  
The farthest north polygon bounded by dark cracks 
https://ntrs.nasa.gov/search.jsp?R=20070017882 2019-08-30T00:46:15+00:00Z
lies at 54o N; the farthest south polygon bounded by 
bright cracks lies at 50o N. 
 
 
Figure 2.  Bright polygonal cracks  (MOC E0300299) 
 
 
400 m
44˚N  273˚W
 
Figure 3.  Dark polygonal cracks  (MOC  M0401631) 
Bright and Dark Spots:  Many of the highest 
resolution MOC frames showing polygons in the 
northern plains also show small numbers of bright and 
dark spots scattered across the terrain (Figs. 4,5).  The 
spots are particularly noticeable in western Utopia 
Planitia, the area with the highest concentration of 
polygons. 
  
42o N
276o W
Dark  Spot
Bright  Spot
400 m
 
Figure 4.  Dark and bright spots on polygons  (MOC  
S03001) 
 
 
 
Figure 5.  Bright spots on polygons  (MOC E0401564) 
 
 
The spots range from tens of meters to 
approximately 100 meters in diameter.  Many of the 
bright spots are centered on cracks bounding the 
polygons.  Some, but not all, of the dark spots are also 
centered on cracks.  The spots are uniformly bright or 
dark at MOC resolution.  Neither type shows evidence 
of raised rims nor ejecta blankets.  The spot 
morphologies are distinctly simpler than those of the 
freshest small impact craters imaged by MOC [12; Fig. 
6]. 
 
400 m 
   400 m 
 
 
48o N   278o W 
65o N  293o W 
 
 
Figure 6.  Recent (2002 or 2003) 23-m diameter 
impact crater in Arabia Terra  (MOC  S1502724) 
 
Discussion – Bright and Dark Polygonal 
Cracks:  Sublimation polygons have been formed and 
preserved in the upper reaches of the Antarctic Dry 
Valleys [13].  These polygons form under strongly 
sub-freezing conditions, without involvement of liquid 
water.  Exposed ice and snow may be preserved in 
these features, particularly in the bounding cracks (Fig. 
7). 
 
 
 
Figure 7.  Polygons in old glacial till deposits from 
McKelvey Valley, Antarctica; snow preferentially 
preserved in the cracks; polygons ~ 10 m across   
(photograph by David Sudgen) 
 
Commonly in Antarctica exposed ice is removed 
by sublimation, owing to the low humidity and 
katabatic winds.  The polygon morphology is 
characteristically preserved even when no ice is visible 
at the surface.  Ice remains at the cores of these 
polygons, where the ice is shielded from sublimation 
by overlying sediments [13]. 
The martian polygons formed at latitudes of 55o to 
65o N characteristically have bright bounding cracks, 
which we interpret as exposed water ice.  Thermal 
models and orbital spectrometry both indicate that the 
stability line for near-surface water ice is currently at 
latitudes north of 60o. 
The majority of polygons in our survey located 
between 40o and 55o N show dark bounding cracks.  
These are interpreted as polygons from which the 
exposed ice has been removed by sublimation.  This 
interpretation indicates that the long-term stability 
limit for exposed ice, even in deep cracks, lies near 55o  
N. 
Alternatively, the polygonal cracks may be bright 
due to concentrations of bright minerals, including 
salts.  These salts could be initially frozen into the ice, 
and preserved in the polygonal bounding cracks when 
the ice sublimes. 
Discussion – Bright and Dark Spots:  The bright 
spots are also interpreted as exposed ice, due to their 
prevalence on terrain mapped as ice rich.  The dark 
spots are interpreted as former bright spots, which 
have darkened as the exposed ice is lost by 
sublimation.      If the bright spots are indeed 
composed of water ice this is a reasonable 
interpretation, since these spots are prevalent at 
latitudes around 40o – 50 o N, well south of the ice 
stability line.  This interpretation implies that the 
bright spots are unstable and may be short-lived. 
The bright spots may be formed by any of several 
mechanisms.  One possible origin is by small meteorite 
impacts.  Analogous bright craters dot the surfaces of 
the icy satellites of Jupiter, where impacts excavate 
ice-rich material from beneath dark surfaces [14].  If 
the spots are formed in this way, they represent a 
population of craters considerably smaller than the 23-
m diameter crater shown in Fig. 6. 
Another possibility is that ice rose to the surface 
along bounding cracks.  Such “ice diapirs” have been 
proposed for features on Mars, Europa, and Enceladus 
[15,16,17]. 
Several authors [18,19,20] have recently suggested 
that the bright spots may be the martian equivalents of 
pingos, ice-cored mounds found in periglacial regions 
on Earth.  Pingos in Canada’s Mackenzie river delta 
are characteristically 10’s to 100’s of meters in 
diameter, roughly the scale of the martian features.  
Terrestrial pingos from which the ice core has melted 
often collapse to form depressions similar to the 
martian dark spots. 
Finally, the martian bright spots may be composed 
of bright salts, remaining from the sublimation of ices 
or brines.  Salt deposits might be stable over 
considerably longer periods than exposed ice.  
Future Observations:  The ground-penetrating 
radars currently in orbit on Mars Express and the Mars 
Reconnaissance Orbiter (MRO) should be able to 
confirm the presence and measure the depth of the 
interpreted ice-rich layer that forms the Astapus Colles 
unit.  If this layer is confirmed it will strengthen the 
interpretation of bright polygon bounding cracks and 
bright spots as exposed ice.   
HiRISE images of the northern plains are showing 
unprecedented details of the polygonal cracks.  These 
images also have the potential to reveal ejecta 
morphologies around the small bright spots.  Future 
HiRISE images that include bright spots, compared to 
MOC images taken years earlier, will illustrate the 
temporal stability of the spots.   
The MRO CRISM spectrometer, with multiple 
spectral bands and a spatial resolution around 20 m, 
should allow mineralogical identification of the 
material exposed in the polygonal bounding cracks and 
in the bright spots.  These data may conclusively 
demonstrate whether these features are composed of 
ice, salt, or a mixture of both. 
The Phoenix lander – scheduled for launch in 
August, 2007 – will likely be targeted to a region of 
small (~ 10 m diameter) polygons, bordered by bright 
cracks, in the northern plains.  This lander could 
collect samples of material in these cracks and provide 
detailed chemical analyses.  
Implications:  If the bright material in cracks and 
spots is indeed exposed ice, its presence in the martian 
mid-latitudes will serve as an additional test of ice 
stability models and the sensitivity of orbital 
spectrometry.  The bright spots can provide an 
indication of near-surface ice independent of 
polygonal and thermokarst terrain.  Recent ice diapirs 
or pingos, if confirmed, will provide further 
demonstrations that Mars is an active planet with a 
complex water cycle.  Alternatively, if the bright 
features prove to be salts, their analyses will provide 
key geochemical data concerning the recent martian 
cyrosphere. 
 
References:  [1] Kieffer H.H. et al. (1976) Science, 
194, 1341-1344.  [2] Boynton W.V. et al. (2002) 
Science, 297, 81-85.  [3] Seibert N.M. and Kargel J.S. 
(2001) GRL, 28, 899-902.  [4] Mangold N. et al. 
(2004) JGR, 109, E08001.  [5] Kanner L.C. et al. 
(2004) LPS XXXV, Abstract #1982.  [6] Tanaka K.L. et 
al. (2005) Map 2888, USGS.  [7] Mellon M.T. and 
Jakosky B.M. (1993) JGR, 98, 3345-3364.  [8] 
McBride S.A. et al. (2005) LPS XXXVI, Abstract 
#1090.  [9] Head J.W. et al. (2003) Nature, 426, 797-
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Abstract #1778.  [11] Kanner L. (2004) Geology 
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GSA Bull., 114, 718-730.  [14] Greeley R. et al. (2000) 
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Exposure of Water Ice in the Northern Mid-lattitudes of Mars

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