Water ice is exposed in the martian north polar cap, but is rarely 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 60deg. We have examined mid-latitude areas of the northern plains displaying residual ice-rich layers, and report evidence of present-day surface exposures of water ice. These exposures, if confirmed, could con-strain the latitudinal and temporal stability of surface ice on Mars

Allen, Carlton C.

Kanner, Lisa C.

NASA Technical Reports Server

PRESENT-DAY  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], but is rarely 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 residual ice-rich layers, and 
report evidence of present-day surface exposures of 
water ice.  These exposures, if confirmed, could con-
strain the latitudinal and temporal stability of surface 
ice on Mars.   
Ice-Rich Layer:  Polygonal features with dimen-
sions of 100+/-30 m, bounded by cracks, are com-
monly 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 65 oN) using all 
MOC narrow-angle images (resolution ~ 5 m / pixel) 
from 9/97 through 9/03 [5].  While MOC images show 
polygons scattered across many areas of the northern 
plains, 74% of such images showing polygons are cen-
tered in western Utopia Planitia (40o to 50oN; 258o to 
288oW).  This region largely overlaps the Late Ama-
zonian Astapus Colles unit, characterized by polygonal 
terrain and nested pits “consistent with periglacial and 
thermokarst origins” [6].  Other authors suggest that 
this concentration of polygons indicates the presence 
of a generally continuous ice-rich mantle [3,4].  Ice 
stability models [7], orbital spectrometry [2], and the 
occurrence of thermokarst [5] indicate that the ice is 
concentrated below 1 m in depth and is currently sub-
liming. 
We also measured all of the impact craters, with 
diameters between 100 m and 4 km, occurring on po-
lygonal terrain between 30o and 65o N [8].  The size-
frequency distribution of these craters larger than 1 km 
is concordant with the distribution for larger craters (> 
8 km) in western Utopia (N. Barlow, personal commu-
nication), indicating preservation of a late Hesperian 
crater population.  Of the craters on polygonal terrain, 
97% predate the polygonal cracks, indicating Amazo-
nian-age deposition or activation of the ice-rich layer.  
The size-frequency distribution of craters 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 recent models of 
obliquity-driven deposition and sublimation of ice-rich 
mantles in the northern mid-latitudes of Mars [9]. 
Bright and Dark Polygon Cracks:  During our 
initial survey MOC images, a striking dichotomy of 
polygon albedo was noted.   Many of the polygons, 
particularly in the northern portion of the study area, 
are bounded by cracks that are significantly brighter 
than the polygon centers.  Other polygons, particularly 
farther south, are bounded by dark cracks (Fig. 1). 
 
400 m 400 m
65oN  293oW 44˚N  273˚W
 
Figure 1.  Bright and dark polygon cracks 
 
A survey of the longitude range 240o to 300oW 
showed that polygons at latitudes between 60o and 
65oN are demarcated by bright cracks.  Dark cracks 
bound polygons at latitudes of 35o to 60oN [5]. 
A complete examination of all MOC images (1997 
through 2003) showing polygons in the latitude band 
30o to 65oN supported these initial results.  At latitudes 
of 55o to 65oN 45 MOC images show polygons.  Of 
these images 73% show polygons with bright cracks.  
Between 40o and 55oN a total of 141 images show 
polygons.  Of these images 66% show dark cracks 
[10]. 
Bright and Dark Spots:  Many of the highest 
resolution MOC frames showing polygons in the 
northern plains also show small numbers of bright 
spots and larger numbers of dark spots scattered across 
the terrain (Fig. 2).  The spots are particularly notice-
able in western Utopia Planitia, the area with the high-
est concentration of polygons.  No systematic mapping 
of these spots has yet been conducted, so the latitu-
dinal variation of spot albedo is currently unknown. 
https://ntrs.nasa.gov/search.jsp?R=20070006554 2019-08-30T00:29:24+00:00Z
42o N
276o W
Dark  Spot
Bright  Spot
400 m
 
Figure 2.  Dark and bright spots on polygons 
 
The spots range from tens of meters to approxi-
mately 100 meters in diameter.  Many of the bright 
spots are centered on the cracks bounding the poly-
gons.  Some, but not all, of the dark spots are also cen-
tered on cracks.  The spots are uniformly bright or 
dark at MOC resolution.  Neither type shows evidence 
of raised rims nor ejecta blankets.  The spot morpholo-
gies are distinctly simpler than those of the freshest 
small impact craters imaged by MOC [11]. 
Discussion:  Sublimation polygons have been 
formed and preserved in the upper reaches of the Ant-
arctic Dry Valleys [12].  These polygons form under 
strongly sub-freezing conditions, without involvement 
of liquid water.  Exposed ice in these features may be 
preserved, particularly in the bounding cracks.  How-
ever, in some cases exposed ice is removed by subli-
mation, 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 sedi-
ments. 
The martian polygons formed at latitudes of 55o to 
65oN 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 be-
tween 40o and 55oN show dark bounding cracks.  
These are interpreted as polygons from which the ex-
posed ice has been removed by sublimation.  This in-
terpretation indicates that the long-term stability limit 
for exposed ice, even in deep cracks, lies near 55oN. 
The bright spots are also interpreted as exposed 
ice, due to their prevalence on terrain mapped as ice-
rich.  One possible origin is formation by small mete-
orite impacts.  Analogous bright craters dot the sur-
faces of the icy satellites of Jupiter, where impacts 
excavate ice-rich material from beneath dark surfaces 
[13].  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 
[14,15,16]. 
Bright spots are rare relative to dark spots.  If they 
have the same origin, this observation suggests that the 
bright material darkens with time.  If the bright spots 
are indeed composed of water ice this is a reasonable 
interpretation, since these spots are prevalent at lati-
tudes around 40o N, well south of the ice stability line.   
Future Observations and Implications:  The 
ground-penetrating radars currently in orbit on Mars 
Express and MRO should be able to confirm the pres-
ence and measure the depth of the ice-rich layer form-
ing 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.   
One of the first images released from the HiRISE 
camera on MRO shows details of polygons and a pos-
sible bright spot in Utopia Planitia, and additional im-
ages should augment and improve upon MOC cover-
age.  The CRISM spectrometer on MRO, with 59 spec-
tral bands and a spatial resolution around 20 m, should 
allow identification of the material exposed in the 
polygon bounding cracks and in the bright spots. 
If the bright material is indeed exposed ice, its 
presence in the martian mid-latitudes will serve as an 
additional test of ice stability models and the sensitiv-
ity of orbital spectrometry.  The bright spots can pro-
vide an indication of near-surface ice independent of 
polygonal and thermokarst terrain.  Recent ice diapirs, 
if confirmed, will provide another demonstration that 
Mars is an active planet with a complex water cycle. 
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, 797-801. [10] Kanner L. (2004) 
Geology Comps Paper, Carleton College (unpublished). [11] 
Malin M.C. et al. (2006) Science, 314, 1573-1577. [12] Mar-
chant D.R. et al. (2002) GSA Bull., 114, 718-730. [13] 
Greeley R. et al. (2000) Planet. Space Sci., 48,  829-853. 
[14] Hoffman N. (1999) 5th Intl. Conf. Mars, Abstract #6008. 
[15] Collins G.C. et al. (1999) LPS XXX, Abstract #1434. 
[16] Pappalardo R.T. and Nimmo F. (2006) LPS XXXVII¸ 
Abstract #2113. 


Present-day Exposures of Water Ice in the Northern Mid-latitudes of Mars

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