A glacial geologic interpretation was recently presented for Argyre, which is herein extended to Hellas. This glacial event is believed to constitute an important link in a global cryohydric epoch of Middle Amazonian age. At glacial maximum, ice apparently extended far beyond the regions of Argyre and Hellas, and formed what is termed as the Austral Ice Sheet, an agglomeration of several ice domes and lobes including the Hellas Lobe. It is concluded that Hellas was apparently heavily glaciated. Also glaciation was young by Martian standards (Middle Amazonian), and ancient by terrestrial standards. Glaciation appears to have occurred during the same period that other areas on Mars were experiencing glaciation and periglacial activity. Glaciation seems to have occurred as a geological brief epoch of intense geomorphic activity in an era characterized by long periods of relative inactivity

Johnson, Natasha

Kargel, Jeffrey S.

Strom, Robert G.

English

NASA Technical Reports Server

 19°-I0774
GLACIAL GEOLOGY OF THE HELLAS REGION ON MARS; Jeffrey S. Kargel, Robert G.
Strom, and Natasha Johnson; Lunar and Planetary Laboratory, Univ. of Arizona, Tucson 85721
Introduction. We recently presented a glacial geologic interpretation for Argyre [1], which we now extend to
Hellas. This glacial event is believed to constitute an important llnk in a global cryohydric epoch of Middle Amazonian
age ............. '-- u,,, ........ ,[2., 31. At glacmJ maxtmum, Ice appa,¢nuy extended far beyond ,L. re,on.'; of A.r_,,r.e ,,,,t 14_lla._.and formed what
we term the Austral Ice Sheet, an agglomeration of several ice domes and lobes including the Hellas Lobe.
Regional landform associations. The Hellas Lobe originated on the summit and slopes of the giant shield
volcano Amphitrites Patera, and dominated northward inflow into the Hellas impact basin (Figs. 1 and 2). The Hellas-
facing side of Amphitrites Patera is dominated by a ridge-and-trough structure, indicating pervasive scouring perhaps
similar to that of glaciated lineated terrains in Canada [4]; thus, we term this region the lineated terrain. The erosive
agent appears to have been an aerially extensive, high-standing medium such as ice. It efficiently erased most craters
smaller than 2 kin, apparently at essentially a discrete point in geologic time [5]. The erosive agent flowed around
several of the largest craters, preserving their rims while severely degrading or completely removing their ejects
blankets, and producing deep upslope and lateral fluting. Orientations of lineatious are strongly correlated with large-
scale topography, thus confirming an essentially down-slope movement of the erosional medium. Subsidiary fluvial
erosion, probably by glacial meltwater, also occurred.
The lineated terrain passes northward into a complex region on the floor of Hellas including an area of
streamlined ridges and equant hills interpreted as a drumlinoid terrain; the orientations of drumlinoid features are
consistent with northward flow of ice from the lineatcd terrain. Long, sinuous ridges, believed to be eskers [6], are
also present but uncommon. Drumlinoid terrain grades northward into a region interpreted as hummocky ground
moraine formed by glacial till deposition and massive ice disintegration. Hummocky deposits form thick blankets of
material terminating northward along transverse cuspate ridges and escarpments, consistent with our interpretation
that these are moraines of southward derivation. The cuspate ridges spaced acrou the floor of Hellas mark terminal
and recessional margins of the former Hellas Lobe. At glacial maximum ice apparently flowed to and terminated near
the deepest part of the basin. Other ridges in the northern part of Hellas resemble lunar-type wrinkle ridges and have
not been mapped.
Smooth channel-fed plains, interpreted as pr0glacial lake deposits, are widespread on the floor of Hellas,
especially in the east-northeastern sector. Large channels entering the area from highlands to the east become subdued
and strongly anastomosinf, apparently indicating transition to a depositional regime as the channels entered the lake
area. Escarpments in the glaciolacustrine plains area may have been wave-cut, and thus possibly indicate former lake
levels. Moraine-like ridges in the vicinity of the gladolacustrine plains area are often flat-topped, also consistent with
erosional planation by wave activity.
Volume of the Hellas Lobe. The Hellas Lobe extended nearly 1500 km from Amphitrites Patera to its northern
terminus and covered an area of approximately 1.5 x 10' kma. We constructed a simple model of the ice profde based
on Bingham rheology and a combined parabolic and inclined plane solution incorporating isostatic adjustments. The
model thickness of ice on the slopes of Amphitrites Parers ranges from 450 to 900 m, consistent with the heights of
ridges and the towage of ice around large craters. On the basin floor the parabolic solution indicates ice thicknesses
ranging to nearly 4000 meters. The integrated volume of the Hellas Lobe model is 4.7 x 10skm s. If the Hellas Lobe
was typical of the entire Austral Ice Sheet, and if this ice sheet covered around 12% of the planet's surface (2/3 of
the area lying between the South Pole and 400 S. Lat.), then the total ice volume wound be 5.5 x 10_ inn3, equivalent
to a global layer of water 340 meters thick. This estimate is of the same order as the water budget of Mars calculated
on independent grounds [e.g., 7, and other references in 2].
Depth of erosion. The vertical scale of troughs and ridges in the lineated terrain suggests erosion to a depth
of order 500 m in the most strongly affected localities. We base an independent estimate on crater statistics. In a
separate abstract [5] we show that the lineated terrain exhibits a great deficiency of craters in the l-2-1un size class,
and a smaller deficiency between 2-4 Inn, a feature attributed to glacial erosion. The average ratio d:D _epth :
Diameter) for fresh, simple craters on Mars is 1:6, indicating d ,, 170 and 670 m for fresh craters having D ,, 1 and
4 kin, respectively. A crater would probably be obscured if erosion occurred to a depth of 0.6d. Thus, the cratering
record indicates that the lineated terrain has suffered widespread erosion to a depth of 100 m and locally to 400 m
(based on removal of most 1-kin and some 4-km preglacial craters). We take 200 m as a rough average erosional depth.
Chronology of glaciation. Statistics for fresh craters indicate a Middle Amazonian age of glaciation in Hellas.
The Middle Amazonian also saw extensive eryohydric activity elsewhere on Mars. The Argyre region apparently also
was glaciated during the Middle Amazonian, and lobate debris aprons (rock glaciers?) were widely deposited in the
Northern Hemisphere [3], collectively indicating a global humid epoch of glaciall-periglacial conditions within this
period. Earlier indications that fluvial erosion on Alba Patera may have been synchronous with glaciation now seem
more doubtful since this fluvial event may have occurred in the later part of the Early Amazonian or earliest Middle
Amazonian, before the glacial event [8]. The absolute chronology is more_epr_ob!ematic. However, in an accompanying
abstract we estimate that Midge Amazonian glaciation in Hellas probably occurred not earlier than 2300 million years
ago and not later than 250 million years ago. We cannot exclude the possibility that older episodes of glaciation may
also have occurred in this area.
We estimate the duration of glacial erosion in the lineated terrain by considering the average rate of glacial
erosion on Earth. Laurentide glaciation eroded an average of about 150 meters of rock from affected areas over the
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MARS GLACIATION Kargel, J.S., et al.
past 3 million years [9]. Approximately half of this period represents glacial epochs and half is intergladal. Thus, the
average rate of Laurentide glacial erosion calculated for the glacial epochs is about 0.010 cm y-'. At this rate 200 m
of erosion in the liaeated terrain would require 2 x 10s years of glaciation. Of course martian glacial erosion rates may
not have been similar to terrestrial rates. Amphitrites Patera, being a shield volcano, would likely be composed of
highly fractured volcanic rock, thus enhancing the efficiency of plucking and favoring high erosional rates and short
durations; warm-based glaciers, as we believe the Hellas Lobe was, would also favor high erosional rates. On the other
hand, the lower surface gravity on Mars would decrease shear stresses and erosional rates relative to the Earth (this
factor, however, is partially offset by a thickening of the martian ice sheet relative to terrestrial ice sheets also due to
the g factor). Thus, two million years is an order-of-magnitude estimate of the duration of Middle Amazonian glaciation
in the Hellas region.
Alternative mechanisms. We acknowledge that morphologies discussed here can be explained nonglacially. For
example, winds may have scoured the lineated terrain. Dust storm activity apparently cycles a meter or so of dust
between hemispheres on the relatively short precessional timescale, resulting in dramatic atmospheric effects but very
little actual erosion of rock, as testified by the preservation of many blocks at the Viking landing sites on terrains
which must be on the order of a billion years old [10]. It seems unlikely that hundreds of meters of erosion of apparently
basaltic lava in the lineated terrain could have occurred. Anyway, aeolian processes would not likely produce down-
slope orientations of lineations in the lineated terrain; the magnitude of the negative thermal anomaly associated with
Hellas is less than half that of the positive Tharsis anomaly where no orographically-associated aeolian erosion is
observed; at Hellas the maximum vertical component of mechanically- and thermally-forced orographic winds is on the
order of only a few cm see-' [11]. Furthermore, other processes would be required to explain young fluvial systems on
the lineated terrain and other giadal-appearing morphologies in the Hellas region. The regional associations and
ordered sequences of landforms, their relationships to large-scale topography, and the age of this event relative to other
proposed giacial/perigiacial events on Mars, are all well explained by giadation. Alternative hypotheses will have to
compete with the economy of the glacial hypothesis in explaining these important aspects as well as the morphologies
of individual hadforms.
Conclusions. 1) Hellas was apparently heavily glaciated. 2) Glaciation was young by martian standards (Middle
Amazonian), and ancient by terrestrial standards. 3) Glaciation occurred during the same period that other areas on
Mars were experiencing glaciation and periglacial activity. 4) Glaciation occurred as a geologically brief epoch of
intense geomorphic activity in an era characterized by long periods of relative inactivity.
References. 1. Kargel, J.S. and Strom, 1990, l, un. Planet. Sci. XXI, 597-598. 2. Baker et al., 1990, Nature. 3.
Strom et al., 1991, Lun. Planet. Sci. XXII (ABSTRACT), this volume. 4. Smith, H.T.U., 1948, Amer. J. Sci. 246, 503-
514. $. Johnson et al., 1991, Lun. Planet. Sci. XXII (ABSTRACT), this volume. 6. Karg¢l et al., 1991,
XXII (ABSTRACT), this volume. 7. Cart, M.H., 1987, Nature 326. 30-34. g. Gulicig V.C, and V.R. Baker, 1990,
(ABSTRACT) Lun. Planet. Sci. XXI. 443-4, and private comm. 9. Bell, M. and E.P. Laine, 1985, O.gth..R.f.L2_ 154-
174. 10. Arvidsoa, R., E. Guinness, and S. Lee, 1979, Nature 278. 533- 535. 11. Webster, P.J., 1977, Icarus 30, 626-649.
Figure 1. Distribution of selected
glacial features in Hellas.
Figure 2. Schematic ice flow lines
and former distribution of glacial
ice and a proglacial lake in Hellas.
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Glacial geology of the Hellas region on Mars

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