GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 1384, 10.1029/2001GL013832, 2002
Ancient lowlands on Mars
Herbert V. Frey,1 James H. Roark,2 Kelly M. Shockey,2 Erin L. Frey,3
and Susan E. H. Sakimoto4
Received 25 July 2001; accepted 13 September 2001; published 21 May 2002.
[ 1] Mars Orbiter Laser Altimeter (MOLA) data provide
compelling evidence that the martian lowlands, below the
smooth and sparsely cratered northern plains, are extremely old,
far older than the plains which cover them. The evidence is in the
form of a very large population of ‘‘Quasi-Circular Depressions’’
(QCDs), many of which are very evident in the MOLA elevation
data but generally not visible in available imagery. We interpret
these ‘‘invisible’’ QCDs to be buried impact basins. Cumulative
number versus diameter curves for lowland QCDs suggests the
buried lowland surface is older than the visible highland surface
and that the lowland plains are a relatively thin (1 – 2 km) veneer
overlying this much older surface. We conclude that the martian
lowlands have been low and stable for nearly all of martian
history.
INDEX TERMS: 6225 Planetology: Solar System
Objects: Mars; 5420 Planetology: Solid Surface Planets: Impact
phenomena (includes cratering); 5499 Planetology: Solid Surface
Planets: General or miscellaneous
1. Introduction
[2] The fundamental unsolved problem in martian crustal evolution is also the oldest: origin of the basic two-fold division of
ancient, high-standing, mostly southern cratered terrain and the
contrasting, smooth and sparsely cratered younger lowland terrain
in the north. The northern lowlands of Mars are covered with
plains units that are mostly Hesperian age and younger [Scott and
Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987].
Little is known about what underlies these plains, and explanations
for the origin of the lowlands allow both an older surface as well as
the possibility that the entire crust is similar in age to the younger
surface units. Here we show the northern plains are a thin veneer
overlying a surface that is much older, perhaps as old as the
highland surface to the south.
[3] Mars Orbiter Laser Altimeter (MOLA) data [Smith et al.,
1999] from the Mars Global Surveyor mission [Albee et al., 1998]
have confirmed the suggestion by McGill [1989] that a very large
and ancient impact basin lies in the Utopia region of eastern Mars.
This basin is largely responsible for the low elevation of this part of
Mars. Large impacts have been suggested as one way to form the
northern lowlands early in martian history [Frey and Schultz, 1988,
1990]. Internal causes, including plate tectonics, have also been
proposed [Wise et al., 1979; McGill and Dimitriou, 1990; Sleep,
1994, 2000; Zhong and Zuber, 2001]. Many internal mechanisms
have in common a relatively long duration (several hundred
1
Geodynamics Branch, Goddard Space Flight Center, Greenbelt,
Maryland, USA.
2
Science Systems and Applications, Inc. at the Geodynamics Branch,
Goddard Space Flight Center, Greenbelt, Maryland, USA.
3
South River High School, Edgewater, Maryland, USA.
4
University of Maryland Baltimore County at the Geodynamics Branch,
Goddard Space Flight Center, Greenbelt, Maryland, USA.
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL013832
million years) and in general suggest the lowland crust is relatively
young throughout its thickness like the plains themselves.
2. Quasi-Circular Depressions
[4] High precision gridded topographic data from MOLA have
revealed the presence a very large number of roughly circular
basins in both the martian highlands and lowlands [Frey et al.,
1999, 2000, 2001]. We call these Quasi-Circular Depressions
(QCDs). For many of these there is little or no evidence of a
corresponding structural feature seen in image data. Profile data
from the early portion of the Mars global Surveyor mission
revealed the first of these, a 450 km wide, 2 km deep basin in
the Arabia portion of Mars with no observable rim structure [Frey
et al., 1999]. We suggested this was a buried impact basin, and
suggested that similar features must be present elsewhere on Mars
that might be detectable as the mission continued to collect data.
We developed interactive computer graphics tools [Roark et al.,
2000; Roark and Frey, 2001] that shift and stretch the color
representation of the MOLA gridded data to systematically search
for such candidate buried impact basins and found that QCDs are
common over the highland portions of Mars. Furthermore, the
QCDs not visible on images generally outnumber visible impact
basins of the same size [Frey et al., 2000].
[5] We believe the MOLA-found QCDs without visible structural expression to be buried impact basins. This is supported by:
the widespread distribution of the features; their close association
with known impact basins, mostly in the heavily cratered highlands
of Mars; lack of association with obvious tectonic and volcanic
centers; generally circular, bowl-like shape, softened profiles and
subdued relief; and similarity of shape of their cumulative number
versus diameter curves (see Figure 3) to that for known impact
basins. The large number of buried basins found suggests that early
impact bombardment in the >200 km size range was much more
intense and early resurfacing which buried early impact basins was
more active than had previously been believed. This has important
implications for the stratigraphy and chronologies that are used to
describe martian evolution [Tanaka, 1986; Tanaka et al., 1992].
3. Lowland QCDs and Age of the Lowlands
[6] Figure 1 shows one example of a large number of QCDs in
the northern lowlands which are easily detectable in MOLA
elevation data but which are not visible in Viking imagery. Many
of these are isolated features, but, like craters and basins in the
highlands, they sometimes overlap. Contours help resolve complicated structures. We searched the entire northern lowlands for
QCDs >50 km using 32 x 64 pixels/degree gridded data. We
avoided both the polar caps and the area immediately along the
crustal dichotomy boundary zone where it is sometimes difficult to
decide where lowland plains begin and the transition zone ends.
[7] Our survey revealed 644 QCDs larger than 50 km diameter
in the northern lowlands (Figure 2). Only 90 of these are visible on
Viking MDIMs, including many nearly buried but partially protruding craters in the knobby plains of Amazonis. The largest of the
visible QCDs is a 365 km wide basin marked by an incomplete
ring of mountainous material called Erebus Montes which Schultz
22 - 1
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FREY ET AL.: ANCIENT LOWLANDS ON MARS
Figure 1. Examples of Quasi-Circular Depressions (QCDs). Left panels: Viking MDIMs. Right panels: stretched color and contoured
MOLA data. (Top) NW Utopia region. Contour interval 25 m. The large closed negative feature is 64 km across and 125 – 175 m deep.
East of this lies a smaller circular depression 34 km wide. Two QCDs half this diameter are seen to the south. None of these are visible in
the MDIM. (Bottom) Korolev area. Contour interval 50 m. Korolov is the 80 km wide, frost filled crater obvious in the MDIM. Two
QCDs of roughly similar size lie to the south. Both are #150 m deep and very obvious in MOLA topography, but have no expression in
the image data.
and Frey [1990] considered the inner ring of a much larger basin.
554 (85%) of the QCDs shown in Figure 2 are not readily visible in
Viking MDIMs. These likely buried basins range up to 1075 km in
diameter. Floor-to-rim relief in the buried features is typically
several hundred meters, compared with expected relief !1.6 km
for fresh impact craters !50 km in diameter [Garvin et al., 2000].
[8] QCDs are well distributed throughout the lowland plains.
This supports the hypothesis that the QCDs (both hidden as well as
visible) are impact basins. However, we found no QCDs of any
size north of the large central volcanic structure Alba Patera and
west and southwest of Olympus Mons. These are areas of relatively
young lava flows whose thickness may be great enough to
completely bury all relief associated with possible pre-existing
impact basins.
[9] Where basins smaller than 500 km wide are still detectable
by their relic relief, the overlying cover cannot exceed about
5 – 6 km or no relief would be preserved (based on [Garvin et
al., 2000]). Where 50 km wide QCDs are abundant, the plains are
likely less than 1.5 km thick. Based on the large number and
distribution of basins of this size and even smaller, the northern
lowland plains are generally 1 – 2 km thick, but in some places <1
and elsewhere >5 km thick, if the absence of large basins is due to
burial by the overlying plains.
[10] Figure 3 shows cumulative frequency curves for the northern lowland QCDs compared to QCDs in the Arabia highlands.
Over the diameter range 200 to 500 km, buried highland basins are
3 – 4 times more numerous (in a cumulative sense) than the visible
basins of the same size. Buried highland basins follow the "2
power law trend of visible basins over large diameters, but fall low
at smaller diameters (as expected for a buried population). Low-
land visible basins plot very low in Figure 3 compared to highland
visible basins, consistent with their overall Hesperian and younger
age. Buried basins in the lowlands plot much higher: at 100 km
diameter they are 15 times more abundant in a cumulative sense
than the visible lowland basins. More significantly, buried lowland
basins plot above visible highland basins for diameters >100 km.
At smaller diameters the lowland buried basins fall off from the "2
slope, as expected for a buried population.
[11] The nominal interpretation of the curves shown in Figure 3
is that the buried surface represented by the hidden lowland basins
(under the lowland plains) is older than the visible highland
surface. The highland surface on average is Middle Noachian
age [Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka
and Scott, 1987]. Therefore, the lowlands beneath the plains are
Early Noachian, dating from the earliest period of martian history,
and pre-dating most of what is seen on the martian surface. This is
a new and fundamental constraint on the age of the lowlands
basement beneath the plains deposits.
4. Implications
[ 12] These results imply not that only are the lowlands
extremely old, but apparently they have been stable (as a surface
to preserve craters) for nearly all of martian history. The implications of this are profound. The highland-lowland crustal dichotomy
is a primordial feature on Mars, dating from the Early Noachian,
not later as has often been thought (e.g., [McGill and Dimitriou,
1990]). This is consistent with the age of the Utopia impact basin
[McGill, 1989], which accounts for the dichotomy elevation difference in eastern Mars. QCDs older than Middle Noachian are
FREY ET AL.: ANCIENT LOWLANDS ON MARS
22 - 3
superimposed on this basin (Figure 2); therefore the Utopia basin
and the dichotomy predate the QCDs and must be Early Noachian
in age. The Isidis impact basin, which is Early Noachian and
superimposed on both the Utopia Basin and the dichotomy
boundary in eastern Mars (Figure 2), also implies the crustal
dichotomy dates from the Early Noachian.
[13] These results constrain not only when, but perhaps also
how, the crustal dichotomy was established. The existence of
Figure 3. Cumulative frequency plots for visible (open symbols)
and hidden (filled symbols) QCDs in the highlands and lowlands
of Mars. Black dotted lines show stratigraphic unit boundaries
based on Tanaka’s [1986] crater counts at 2, 5 and 16 km diameter,
extrapolated to larger diameters with a "2 power law. EH = Early
Hesperian, LN = Late Noachian, MN = Middle Noachian, EN =
Early Noachian. Blue symbols: Arabia highlands (see Figure 2).
Visible basin counts suggest a Middle Noachian age, in agreement
with geologic mapping, and closely follow a "2 power law (lower
dashed blue line). Buried highland basins (filled squares) follow
the same slope at large diameters, but bend away at lower
diameters, consistent with a buried population. Red symbols:
Lowlands. Visible impact basins lie very low, consistent with a
Hesperian and younger age for the lowland plains. Buried lowland
basins (filled red circles) plot above highland visible basins for
diameters >100 km. Based on these curves the buried lowlands are
Early Noachian in age, older than the visible Middle Noachian
highland surface.
Figure 2. Quasi-Circular Depressions (QCDs) in the northern
lowlands of Mars. Background is colored MOLA topography;
deeper lowlands are progressively deeper blues. The hemispherical
views are centered on 30W longitude and 50N latitude (top) and
210W longitude and 40N latitude (bottom). The north polar cap (P)
is shown near the top and the Elysium volcanic area (E) is near the
center in the view on the bottom. Of the 644 QCDs larger than 50
km diameter shown, only about 90 are visible impact craters.
Buried QCDs are widely distributed across the plains, but there are
several areas noticeably lacking these topographic lows. Especially
obvious is the region north of Alba (A) and the area west and
southwest of Olympus Mons (O). Note QCDs superimposed on the
Utopia (U) basin, and the Isidis (I) impact basin superimposed on
the dichotomy boundary. The Arabia highlands (Ar) provide a
comparison for cumulative crater counts (Figure 3).
buried basins does not directly indicate the mechanism by which
the lowlands were formed, but does favor processes that operate
quickly and early. Large size impact cratering [Frey and Schultz,
1988, 1990] associated with the end of accretion is consistent with
this requirement. Internal mechanisms, if they occurred, had only a
short time to operate before the formation of the preserved buried
impact basins in Utopia.
[14] It appears there were northern lowlands throughout essentially all of martian history. At whatever early time liquid water
may have been available on Mars, there was a basin in the north
into which it could drain. Mars may have had a primordial shallow
ocean in the Noachian, as Clifford and Parker [2001] suggest, in
which eroded ancient highland material may have been deposited
[Hynek and Phillips, 2001].
5. Conclusions
[15] MOLA data provide compelling evidence that the present
young lowland plains are a relatively thin veneer overlying a much
older surface. The lowlands appear to have been low and stable for
most of martian history. This means the fundamental character of
the martian crust, its highland-lowland crustal dichotomy, is a
primordial feature established soon after accretion and preserved
throughout all subsequent martian geologic evolution.
[16] Acknowledgments. We wish to express our admiration and
appreciation for the dedicated and exemplary efforts of the MOLA Instru-
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FREY ET AL.: ANCIENT LOWLANDS ON MARS
ment Design and Data Analysis Teams who made possible this superb data
set. We thank several members of the MOLA Science Team for discussions
and encouragement of this work, and also acknowledge the constructive
suggestions of two reviewers.
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"""""""""""
H. V. Frey, Geodynamics Branch Code 921, Goddard Space Flight
Center, Greenbelt, Maryland 20771, USA. ([email protected]. gov)
J. H. Roark and K. M. Shockey, SSAI/Geodynamics Branch Code 921,
Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
E. L. Frey, South River High School, Edgewater, MD 21037, USA.
S. E. H. Sakimoto, GEST/UMBC/Geodynamics Branch Code 921,
Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.