Surveying Mars
Martian oceans, v
he new Mars Global Surveyor
altimetry shows that the
heavily cratered southern
hemisphere of Mars is 5 km
higher than the sparely cratered
plains of the northern
hemisphere. Previous
suggestions that oceans formerly
occupied the northern plains as
evidenced by shorelines are
partly supported by the new
data. A previously identified outer
boundary has a wide range of
elevations and is unlikely to be a
shoreline but an inner contact
with a narrow range of elevations
is a more likely candidate. No
shorelines are visible in the
newly acquired, 1.5 metre/pixel
imaging. Newly imaged valleys
provide strong support for
sustained or episodic flow of
water across the Martian
surface. A major surprise,
however, is the near absence of
valleys less than 100 m across.
Martian valleys seemingly do not
divide into ever smaller valleys
as terrestrial valleys commonly
do. This could be due to lack of
precipitation or lack of surface
runoff because of high infiltration
rates. High erosion rates and
formation of valley networks
supports warm climates and
presence of large bodies of water
during heavy bombardment. The
climate history and fate of the
water after heavy bombardment
remain controversial.
T
3.20
In the Harold Jeffreys Lecture given to the Royal Astronomical Society on
12 February 2000, Michael H Carr describes new information and insights
from Mars Global Surveyor.
T
he purpose of this paper is to re-assess
the role that water has played in the
evolution of Mars in the light of the
new data from Mars Global Surveyor (MGS).
The Mars Global Surveyor spacecraft was
injected into Mars orbit on 12 September
1997. It was designed to operate in a sunsynchronous, circular orbit. However, in order
to minimize the fuel needed for orbit injection,
the spacecraft was initially placed in a highly
elliptical orbit, which became circular as the
atmosphere slowly decelerated the spacecraft.
This process took longer than originally
planned and systematic mapping from the predesignated orbit did not start until March
1999. Since then, the spacecraft has returned
thousands of high-resolution (1.5 m/pixel)
images, mapped the planet’s gravity field and
altimetery, determined the composition of the
surface with a 3 km spatial resolution, and discovered numerous, heretofore unsuspected,
remanent magnetic anomalies. These new data
are causing us to re-assess almost every aspect
of the evolution of Mars.
We have suspected, ever since Mariner-9
returned images of seemingly water-worn
channels and valleys incised into the Martian
surface, that water has played a major role in
the evolution of the planet (Masursky 1973).
Yet most aspects of the water story remain puzzling. Mars today is a cold, dry planet. The
atmosphere contains only minute amounts of
water and the only place on the surface where
water ice has been unambiguously detected is
at the north pole (Kieffer et al. 1976, Farmer et
al. 1976). Yet large flood channels indicate that
abundant water was formerly present at the
surface and branching valley networks appear
to imply that Mars was warmer in the distant
past. How much water was present, when the
warm conditions prevailed and what caused
the global climate to change are, however,
uncertain. Similarly, although we have excellent evidence for gigantic floods, we do not
know what caused the floods, what their climatic implications are, and what sized bodies
of water were left afterwards. Some researchers
have, for example, postulated that the floods
created ocean-sized bodies of water and
changed global climates (Baker et al. 1991);
others have suggested that the floods resulted
in only modest-sized bodies of water (Scott et
al. 1995, Carr 1996) that had only trivial
effects on climate. The new MGS data has
implications for all these issues.
The topography of Mars is now well defined
from the MGS altimetry (Smith et al. 1999).
The dominant feature is the 5 km difference in
elevation between the low northern hemisphere and the high southern hemisphere (figure 1). Most of the southern hemisphere is
rough and heavily cratered, having survived
from the early era of heavy bombardment. Its
topography is dominated by the southern parts
of the Tharsis bulge and the 8 km deep Hellas
Basin, with its 6000 km diameter rim. Much of
the northern hemisphere outside the volcanic
provinces of Tharsis and Elysium is an almost
level, sparsely cratered plain, within which is a
newly discovered, shallow, roughly 1500 km
diameter basin, centered at 45°N 250°W,
referred to as the Utopia Basin. At each pole is
a pile of layered deposits up to 3 km thick. The
observed drainage features, including both fluvial channels and lava flows, are to first order
consistent with the present topography. Since
most of the valley networks are ancient features (Carr 1996), the correlation indicates
that most of the topography dates back to very
early in the planet’s history.
Valley networks
Much of the ancient cratered highlands of
Mars are dissected by small valleys that form
branching networks. Their branching patterns
superficially resemble those of terrestrial rivers.
The valley networks are found mostly in the
Noachian terrain of the southern highlands,
but a few do cut younger units particularly
those of Hesperian age. While much of the
southern highlands is dissected, the drainage
pattern is mostly very open, with drainage densities that are two orders of magnitude lower
than typical terrestrial values. The valleys are
widely accepted as water-worn, although other
processes have been suggested. Because significant precipitation and surface runoff are not
possible under the present conditions with
mean daily temperatures ranging from 220 to
150 K, the valley networks have been taken as
June 2000 Vol 41
Surveying Mars
alleys and climate
1: The topography of Mars as determined from the laser altimeter on Mars Global Surveyor.
evidence of warmer conditions in the past. The
climatic implications of the valleys are, however, controversial, and not all researchers accept
that warmer conditions were necessary for
their formation (Squyres and Kasting 1994).
The valleys visible in the previously acquired
Viking imagery are typically 1–2 km across
and a few tens to a few hundred kilometres
long. One striking finding from the 1.5 m resolution imaging from MGS is that the Martian
valleys do not, in general, divide upstream into
ever smaller valleys as commonly happens on
Earth. With the new imaging we see the valleys
in greater detail but do not see arrays of previously unobserved valleys.
The near absence of drainage by valleys a few
hundred metres across or smaller could result
simply from erosion and burial. This is unlikely, however. Erosion rates since the end of the
Noachian have been very low as discussed
below. While some valleys may have been
buried by layered deposits that are common
throughout the highlands, small valleys are not
present where such layered deposits are absent,
and the outlines of the valleys that appear
more pristine do not show interruptions where
tributaries formerly present but now eroded
away or buried might have been. The small
tributary valleys appear to be scarce because
few formed. Two possible explanations are: (1)
No precipitation has occurred during the time
period recorded by the present landscape, there
has been no surface runoff, and all the valleys
June 2000 Vol 41
Martian ages
Ages of different parts of the surface of
Mars are estimated from the number of
superimposed impact craters. Noachian
surfaces date back to the era of heavy
bombardment which, by analogy with the
Earth and the Moon, is estimated to have
ended 3.8 ´109 years ago. The next
youngest era is the Hesperian, then the
Amazonian. Estimates of the date of the
Hesperian/Amazonian boundary range
from 1.8 to 3.5 ´109 years (Tanaka et al.
1992). The precise date is poorly known
because of uncertainties in the cratering
history of the inner solar system.
formed by release of groundwater or some
other mechanism. (2) There was precipitation,
but the relation between precipitation intensity, infiltration rates, slopes, and resistance to
erosion were such that the threshold for erosion by surface runoff was rarely achieved
(Carr and Malin in press).
The new MGS images provide excellent evidence for sustained erosion by modest-sized
streams. Nanedi Vallis is a 3 km wide, 1 km
deep valley that extends 800 km northward
from a source in an almost featureless plain
near 0°N 49°W (figure 2). A 200 m wide river
channel can be seen in places on the valley
floor. The river is likely to have meandered ini-
tially across the lava plains of Lunae Planum,
then sustained or episodic flow resulted in deep
incision of the meanders. Many of the small
valleys within the highlands had previously
been attributed to groundwater sapping, in
which valleys extend themselves headward by
erosion at spring sites. Such an origin is very
unlikely for Nanedi Vallis because of the difficulty in forming tight meanders in this way.
More likely, the valley formed as a result of
release of groundwater from some source
upstream of the section seen in figure 2. The
climatic conditions under which Nanedi Vallis
formed remain unclear. Rivers sustained by
groundwater may be able to flow for hundreds
of kilometres under an ice cover even under the
present frigid conditions (Wallace and Sagan
1979, Carr 1983, Squyres and Kasting 1994),
so that the valley does not necessarily imply
warm conditions in the Hesperian, when it
appears to have formed. An intriguing possibility is that at the time of formation of Nanedi Vallis, the groundwater table under Lunae
Planum was sustained by the presence of lakes
in the canyons to the south (McCauley 1978,
Nedell et al. 1987, Lucchitta et al. 1992) and
that movement of water through Nanedi Vallis
contributed to drainage of these lakes.
The valleys in the uplands range widely in
their state of preservation from those that are
barely discernible to those, like Nanedi Vallis,
that are almost perfectly preserved. In the
Viking images most of the upland valleys were
3.21
Surveying Mars
seen to have a rectangular cross-section. We
now see that the flat floors are due to later fill
and are not flood plains, as had been formerly
assumed by analogy with the Earth. Some
areas that appear densely dissected in the
Viking images are shown in the MOC images
to have poorly integrated drainage, with interconnected linear and rounded depressions.
These areas resemble the drainage in some
permafrost areas of the Earth where erosion is
by a combination of water, sublimation of
ground ice, and mass wasting. Some valleys
merge upstream with lines of depressions as
though fed by sub-surface streams. A particularly well developed drainage network is seen
on the inner wall of the crater Bakhuysen (figure 3). The tributaries start at the crater wall
and the surrounding terrain is undissected. The
valleys were possibly fed either from groundwater seeping from the crater wall or from
local melting of ice or snow that had collected
within the crater.
In summary, evidence for surface runoff is
rare. The valleys appear to have formed mainly from groundwater. One possibility is that
precipitation did occur after the present topography stabilized, but most of the water infiltrated into the ground and fed, via the groundwater system, the streams that cut the valleys.
Alternatively, little or no precipitation
occurred after the landscape stabilized and the
groundwater system was sustained by means
other than precipitation, such as basal melting
of polar ice deposits (Clifford 1993). The
abundant presence of water-worn valleys at
high elevations in the southern highlands indicates active recharge at least in the Noachian
and possibly in the lower Hesperian.
Outflow channels
Outflow channels are tens to hundreds of kilometres across, much larger than the valley networks. Most start at full size, commonly from
a rubble-filled depression. They have few tributaries. A distinctive characteristic is the presence of teardrop shaped islands with a pointed
prow upstream and a long tail downstream.
The valley floors typically have a longitudinal
scour of diverging and converging furrows.
Because of their abrupt beginnings, large size,
lack of tributaries, and numerous features in
common with large terrestrial floods, outflow
channels are almost universally accepted as
having formed by very large floods, much larger than any known on Earth. Rough estimates
of peak discharges range as high as 109 m3 s–1,
as compared with 2 ´107 m3 s–1 for the largest
terrestrial floods (Baker 1982, Robinson and
Tanaka 1990). Most of the floods appear to
have formed well after the end of heavy bombardment, as a result of massive eruptions of
groundwater. To achieve the high discharges
the eruptions must have been driven by large
3.22
2: Incised meanders of Nanedi Vallis at 5°N 48°W. The valley maintains a constant width of roughly 3 km for
most of its several hundred kilometre length. The Viking frame on the left is shown for context.
hydrostatic pressures (Carr 1979). Cold climate conditions, similar to those that prevail
today, were probably required so that pressures could build within groundwater trapped
below a thick permafrost zone
Because of their large sizes, the characteristics
of the outflow channels were well documented
from previous missions and the new MGS data
does not change significantly our perception of
these features. Only three points need be mentioned. The first is that the new altimetry
enables the channels to be traced farther into
the northern basin than was formerly possible.
Those that start to the southeast of Chryse can
be traced as far as 45°N and those that extend
northwest of Elysium can be traced into the
bottom of the newly discovered Utopia Basin
at 40°N 245°W. This may have implications
concerning the former presence of bodies of
water in these low areas. Secondly, the altimetry appears to confirm the presence of a large
Noachian channel that extends from the
Argyre basin into the low-lying areas of Margaritifer Sinus. It may imply that, some time
during the Noachian, the Argyre basin was
filled with water that overflowed to the north.
The third point is that the new altimetry and
images support the supposition that in places
there has been massive subsurface solution or
erosion in several places. The most striking
example is provided by Hebrus Vallis at 18°N
232°W (figure 4). The channel emerges fullsize from an irregular depression, extends
200 km to the northwest with typical outflow
channel characteristics. It then breaks up into
lines of deep discontinuous depressions. The
geometry resembles that in terrestrial karst
regions where rivers flow partly on the ground
and partly underground as a result of solution
of limestone. The geometry of Hebrus Vallis
and other similar valleys is puzzling in that the
thermal emission spectrometer on MGS has
not detected calcium carbonate anywhere on
the planet (Christensen 1998). Other examples
of large-scale subsurface flow are discontinuous fretted channels in northern Arabia and a
depression that extends northward from
Ganges Chasma to the source of the outflow
channel Shalbatana Vallis. In both these cases
the imaging suggests that the surface has partly collapsed to form a linear depression that
can be well seen in the altimetry. The example
near Shabaltana Vallis suggests that the lake
thought to have been formerly present in
Ganges Chasma (McCauley 1978, Nedell et al.
1987, Lucchitta et al. 1992) may have partly
drained northward below the surface.
Erosion
The evidence for a rapid decline in erosion
rates near the end of heavy bombardment is
unambiguous. Nearly all the craters in the
Noachian terrain that are tens of kilometres
across or larger must date from the era of
heavy bombardment. (Cumulative frequencies
for 10 km diameter craters on lunar surfaces
4.0 and 3.5 Gyr old differ by more than a factor of 10 (Neukum and Ivanov 1994). Similar
June 2000 Vol 41
Surveying Mars
3: Gullies incised into the wall of the 180 km diameter crater Bakhuysen. The
gullies start near the top of the crater wall. They could have formed as a result
of seepage from the crater wall or as a result of melting of snow or ice that
accumulated within the crater. The surrounding terrain is mostly undissected.
differences are expected for Martian surfaces.)
Most of these large craters are highly degraded, many being simply shallow rimless depressions. Even the large impact basins Hellas,
Argyre and Isidis have highly modified rims. In
contrast, many lower Hesperian surfaces,
which formed shortly after the end of heavy
bombardment (figure 5), have almost perfectly
preserved populations of craters less than
10 km in diameter (Carr 1992), with fine morphological details retained (Craddock and
Maxwell 1993). Estimates of erosion rates
after heavy bombardment rate range from
0.001 to 0.02 µm/yr as compared with estimates of 0.1–10 µm/yr for the Noachian
(Arvidson et al. 1979, Carr 1992). The latter
number is at the low end of terrestrial rates.
The cause of the 2–3 order of magnitude
decline in erosion rates is not known, but it is
difficult to see how climate change could not
have been involved. A warmer climate and an
active hydrological cycle during the Noachian
is also supported by the non-equilibrium distribution of water at the end of the Noachian and
early Hesperian, as discussed above.
High erosion rates in the Noachian must
June 2000 Vol 41
4: Hebrus Vallis at 20°N 234°W. The Viking image on the right is 120 km across.
The channel starts at an irregularly shaped depression. In its upper reaches it
has typical fluvial features but downstream it breaks up into discontinuous
lines of depressions as though flow was mainly subsurface.
have produced large amounts of eroded debris.
Thick deposits of wind-deposited, wind-eroded
material along the plains–upland boundary
between 120°W and 240°W were recognized
from the Viking images and etched deposits
overlying the cratered terrain were noted in
several areas, particularly north of Isidis (Scott
and Tanaka 1986, Greeley and Guest 1987).
The MOC images show that wind-blown
materials and layered, partly eroded deposits
are common at the surface throughout the
highlands, particularly in large craters and
basins. In addition, much of the terrain in the
30–45° latitude belts in both hemispheres
appear covered with easily erodible material
that mutes the topography of the underlying
surface. Thus poorly consolidated material is
abundant at the surface and probably represents material produced during the early era of
high erosion rates. Since the end of heavy bombardment, it has probably been constantly
moved around the planet by the wind, but little added to by subsequent erosion.
Climate change
The change in erosion rates, the disequilibrium
distribution of water and the presence of valley
networks all support a major change in climate
near the end of heavy bombardment (late
Noachian – early Hesperian). Climate modelers
have, however, had trouble explaining how
such a change could have occurred (Haberle
1998). Part of the problem is caused by Mars’
distance from the Sun and the likely low luminosity of the early Sun. Haberle (1998) estimates that to warm early Mars to 273 K, the
atmosphere must have had a very effective
greenhouse, needing to absorb 85% of the radiation from the surface, as opposed to the 56%
absorbed by the Earth’s atmosphere. Early
modelling by Pollack et al. (1987) suggested
that a CO2–H2O atmosphere at least 5 bars
thick was needed. Kasting (1991) subsequently
showed, however, that the vertical temperature
profiles within the atmosphere in the Pollack et
al. models were in error in that they had not
been adjusted to take into account condensation of CO2. Making the necessary corrections,
Kasting claimed that surface temperatures
could not reach 273 K, no matter how thick the
CO2–H2O atmosphere. More recently, Forget
and Pierrehumbert (1997) refuted this claim
3.23
Surveying Mars
and maintained that that CO2 clouds could so
effectively scatter infrared radiation from the
surface, that 273 K surface temperatures could
be achieved with as little as 1 bar of CO2.
Yet another problem with maintaining warm
temperatures on early Mars is that when surface temperatures get above 273 K and liquid
water becomes abundant, chemical weathering
accelerates, carbonates tend to form rapidly
and the atmosphere collapses on a short
timescale, on the order of 107 years (Pollack et
al. 1987). To maintain the atmosphere, the
CO2 would have to be recycled rapidly back
into the atmosphere, such as by volcanic burial (Pollack et al. 1987) or by impacts (Carr
1989). Finally, the atmosphere must be maintained against losses by impact erosion.
Melosh and Vickery (1989) showed that
impact erosion of the atmosphere would have
been very effective on early Mars, and Zahnle
(1993) invoked impact erosion to explain the
almost one hundred-fold, non-fractionating
depletion of non-radiogenic noble gases in the
Martian atmosphere, as compared with the
Earth. According to the Melosh and Vickery
model, early heavy bombardment could have
eliminated an atmosphere a few bars thick in a
few hundred million years.
I attempted to model how atmospheric pressure on early Mars might have changed as a
result of loss by weathering and impact erosion
and recycling of the CO2 in carbonates back
into the atmosphere as a result of burial and
heating by impacts and volcanism (Carr 1999).
The model assumes an arbitrary inventory of
CO2 at 4.4 ´109 years ago and steps forward in
time tracking the CO2 as it is lost to space by
impact erosion and redistributed between the
atmosphere and the ground. High rates of volcanism are expected on early Mars as a result
of the high heat flow (Schubert et al. 1992) and
appear confirmed by MGS observations of
sequences of layered rocks, several kilometres
thick, in the walls of Valles Marineris
(McEwen et al. 1999). The high burial rates
and steep thermal gradient on early Mars
would have enabled CO2 fixed as carbonates
to be returned to the atmosphere quite efficiently. However, the heat flow declines rapidly throughout the era of heavy bombardment
and by early Hesperian recycling of CO2
becomes trivial.
In the case of a strong greenhouse (Forget
and Pierrehumbert 1997), surface temperatures
and hence weathering rates are high during
heavy bombardment. As a consequence, much
of the CO2 resides in the ground as carbonates
and so is protected from removal from the
planet by impact erosion. According to the
model, at the end of heavy bombardment
0.5–1 bar of an originally assumed 6 bars of
CO2 is left in the atmosphere and 4 bars are in
the ground (figure 6). Globally averaged sur3.24
5: Evidence for a change in erosion rates early in Mars’ history. Small, minimally eroded craters are
superimposed on an 8 km diameter crater that is almost completely eroded away. Such relations are
common throughout the terrains that date back to heavy bombardment. Large, old craters are heavily
eroded while younger, much smaller craters are uneroded.
face temperatures are close to freezing. Most of
the 0.5–1 bar of CO2 in the atmosphere would
be subsequently lost to space by sputtering
(Luhmann et al. 1992). This model is roughly
consistent with what is observed. A climate
change occurs near the end of heavy bombardment as global temperatures fall below freezing. The atmosphere is slowly eroded away by
sputtering after heavy bombardment so that
average global temperatures, although below
freezing are not far below for some time, thereby possibly permitting formation of valley networks well into the Hesperian. Large amounts
of CO2 fixed as carbonates in the ground are
consistent with evidence given above for
underground movement of water, although
inconsistent with the lack of detection of carbonates by the thermal emission spectrometer
on MGS (Christensen et al. 1998). These models are, however, very simple and do not take
into account several important effects such as
formation of polar caps (Haberle et al. 1994).
In weak greenhouse models (Pollack et al.
1987), little weathering occurs because of the
low surface temperatures and much of the initial inventory of CO2 is lost to space by impact
erosion. Surface temperatures are well below
freezing throughout heavy bombardment.
Clearly, such models are inconsistent with high
rates of erosion and valley formation early in
the planet’s history.
A global aquifer system
Many of the large outflow channels appear to
have formed by eruption of water from below
the surface, and the characteristics of the valley
networks appear more consistent with origin
by groundwater seepage rather than surface
runoff. Clifford (1993) suggested that Mars
has a large capacity, globally interconnected
aquifer system. The aquifer is comprised mostly of a deep, porous megaregolith that was
formed during heavy bombardment. By analogy with the Moon, Clifford suggested that the
porosity declines with depth with a scaled
depth of 2.8 km, and estimated the total capacity of the aquifer is 0.5–1.5 km of water spread
evenly over the whole planet. We, of course,
have no means of telling what fraction of this
capacity is actually filled with water. Under
present heat flow and climatic conditions the
system is frozen to depths of roughly 2 km at
the equator and to roughly 6 km at the poles.
The model of Clifford should perhaps be
viewed as one that places upper limits on the
capacity, porosities and permeabilities of the
aquifer system. Comparison with the Earth
may be more appropriate than comparison
June 2000 Vol 41
Surveying Mars
320
5
310
ground
temperature (K)
pressure (bars)
4
3
2
impact
300
290
280
270
1
260
atmosphere
0
4.5
4
time before present (Gyr)
3.5
250
4.5
4
time before present (Gyr)
3.5
6: Some results from modelling the fate of an early atmosphere on Mars. In this model the planet is
assumed to have had 6 bars of CO2 4.4 ´109 years ago. Some of the CO2 is lost from the planet by impacts,
but most enters the ground as carbonates. Roughly 0.5–1 bar is left in the atmosphere at the end of heavy
bombardment and is mostly lost later by sputtering. The graph on the right shows globally averaged
temperatures according to the Forget and Pierrehumbert (1997) greenhouse model. It shows that global
temperatures fell below 273 K close to the end of heavy bombardment.
with the Moon. Permeabilities (and by inference porosities) on Earth decline rapidly with
depth until they reach the brittle to ductile
transition at roughly 12 km, where the permeability has fallen to 10–18 m2 (Manning and
Ingebritsen 1999). The permeabilities are
derived from chemical reactions and are for
regions and times of active metamorphism. In
stable cratonic regions of the Earth, significantly lower permeabilities are likely. On Mars
at the end of heavy bombardment, the heat
flow was roughly 2.5 times the present terrestrial heat flow (Schubert et al. 1992) so that the
depth to the ductile to brittle transition was
correspondingly shallower (roughly 4–5 km).
This depth may more realistically represent the
limiting depth of the aquifer system, since generation of a deep megaregolith effectively
ceased with the end of heavy bombardment.
Oceans
If Mars is or was water-rich and global temperatures were above freezing, then large bodies of water must have existed at the surface.
We have just seen that these two conditions are
likely to have been met in the Noachian. Moreover the high heat flows expected during the
Noachian (Schubert et al. 1992) would have
driven most of the near-surface inventory onto
the surface. Some observational support for
large bodies of water in the Noachian is provided by a Noachian outflow channel that
emerges from the Argyre impact basin, thereby
possibly implying that the basin overflowed.
The rationale for ocean-sized bodies of water
after the end of heavy bombardment is, however, far less convincing. Parker et al. (1989,
1993) mapped two discontinuous contacts
around the northern plains that they claimed
could have been shorelines of a former northern ocean. In addition, they argued that many
of the downstream features of the large outflow channels, particularly those that converge
June 2000 Vol 41
on the Chryse Basin, were consistent with their
having debouched into a body of water. Baker
et al. (1991) speculated further that the large
outflow channels created short-lived oceans
that temporarily changed the global climate,
thereby explaining some of the younger valley
networks. How such oceans formed, dissipated
and changed climates was, however, left largely unexplained.
The new altimetry from MGS shows that the
outer contact of Parker et al. (1989, 1993) has
a wide range of elevations (several kilometres)
and cannot be a shoreline unless Mars has been
far more active tectonically than is generally
believed. The inner contact, however, is much
more plausibly a former shoreline in that it has
a narrow range of elevations, particularly if the
sections near the volcanic regions of Tharsis
and Elysium are excluded (Head et al. 1999).
Terraces around the newly discovered Utopia
Basin provide strong support that this basin at
least could have been filled with water, at one
time. Similar “shorelines” have been proposed
for the interior of the Hellas basin. Arguing
against the ocean hypothesis is the lack of evidence for shorelines in the new high-resolution
imaging (Malin and Edgett 1999). Indeed, the
abundance of surfical deposits, clearly visible
in the MOC images at the latitudes proposed
for the shorelines, would make recognition of
shorelines extremely difficult even if they were
present. A possible reconciliation is that the
terraces around the Utopia Basin and possibly
elsewhere are traces of Noachian shorelines
that are now largely covered with younger
deposits and so are not discernible in the imaging. Such an interpretation would be consistent
with high erosion rates and possibly warmer
climates in the Noachian and low erosion rates
and cold climates in the post-Noachian.
The fate of the water present at the surface
near the end of heavy bombardment is puzzling.
Clifford (1993) suggested that the water could
be lost to the groundwater–ground-ice system
through the poles. When the climate changed,
the surface, including any bodies of water,
would have frozen. Ice at the surface would
slowly sublime and accumulate at the poles (figure 7). Ultimately, ice at the poles would
become so thick that the ice would melt at its
base and water could enter the underlying
aquifer system. Because of the high elevation of
the south pole, water from it could have steadily recharged the aquifer system underlying the
high-standing southern highlands, thereby providing a groundwater source for the postNoachian valleys. As the heat flow declined further and the frozen zone became thicker, then
access of groundwater to the surface would
have become more difficult and the rate of formation of valley networks would have declined,
as is observed. The large floods occurred subsequent to the main period of valley formation.
Most appear to have formed by massive eruptions of groundwater under large artesian pressures (Carr 1979). The pressures could have
been generated in at least two ways. The first
possibility is that groundwater became trapped
between the base of the global aquifer system
and the advancing base of the cryosphere (figure 7). The second possibility is that, as the heat
flow declined, water continued to enter the
global groundwater system through the poles,
thereby raising the global water table and creating large artesian pressures in low areas. Breakouts occurred in low areas at low latitudes
where the cryosphere was thinnest. The floods
could have involved huge volumes of water and
repetitively recreated ocean-sized bodies of
water as suggested by Baker et al. (1991). Alternatively, the floods could have resulted in only
modest-sized bodies of water that froze in place
to form permanent ice deposits in the lowest
parts of the northern plains (Carr 1996).
Summary
If Mars was water-rich as appears likely from
the presence of large flood features, and if,
near the end of heavy bombardment, surface
conditions were warm, as appears likely from
the high erosion rates, the presence of valley
networks, and the disequilibrium distribution
of water, then large bodies of water must have
been present at the surface during heavy bombardment. Terraces, revealed by the altimetry,
around the northern plains, and within the
Hellas and Utopia basins may be evidence for
these bodies. Visible evidence for the shorelines
appears to have been largely destroyed by subsequent erosion and deposition. During the
Noachian, an active hydrologic cycle probably
resulted in constant recharge of the groundwater system.
The climate history since the end of heavy
bombardment is controversial. The almost
complete absence of evidence for surface
3.25
Surveying Mars
other hand, have argued from erosion rates
(Carr 1979, 1996, 1999) that a dramatic
change in climate took place at the end of
heavy bombardment such that the surface
froze and has remained so ever since. According to this hypothesis the large floods are cold
climate features that left behind only small
bodies of water that froze and formed permanent ice deposits in low-lying areas. The
Noachian oceans dissipated near the end of
heavy bombardment largely by incorporation
of the water into the deep megaregolith by the
process of basal melting (Clifford 1987). ●
(a) Noachian age
2
elevation (km)
0
evaporation
–2
ocean
–4
impermeable
basement
–6
–8
–90
–60
–30
0
latitude
30
condensation on polar cap
2
(b) Early Hesperian age
polar layered deposits
cryosphere
elevation (km)
0
–2
90
60
basal melting
and infiltration
sublimation
–4
impermeable
basement
–6
basal melting
and infiltration
–8
–90
–60
–30
0
latitude
30
thickening cryosphere
90
(c) Late Hesperian age
2
catastrophic floods
0
elevation (km)
60
–2
–4
ground water trapped
impermeable
under high pressure
basement
–6
–8
–90
–60
–30
0
latitude
30
60
90
7: Diagram showing one possible sequence of events early in Mars’ history. (a) During heavy bombardment
(Noachian), Mars had a warm climate, and active hydrologic cycle, oceans, and high erosion rates.
(b) The climate changed at the end of heavy bombardment, the oceans slowly sublimed, and ice
accumulated at the poles. A cryosphere developed and water entered the groundwater system through
basal melting of the polar ice. (c) As the heat flow declined, the cryosphere became thicker and water was
trapped under high pressure below the cryosphere. Periodic breakouts of groundwater under high pressure
caused catastrophic floods.
runoff in the MOC images can be explained in
at least two ways: (1) climate conditions were
too cold for significant precipitation and surface runoff after heavy bombardment or (2)
warm climate conditions and precipitation did
occur after heavy bombardment but the permeability of the ground is such that most of the
precipitation infiltrated into the ground leaving
little surface runoff.
3.26
The fate of the planet’s water after heavy
bombardment is similarly controversial. Some
workers (Baker et al. 1991, Parker et al.1989,
1993, Head et al. 1999) have argued for the
presence of oceans long after heavy bombardment, at the time that most of the large floods
occurred, with the oceans possibly repeatedly
forming and dissipating and causing dramatic
climate changes (Baker et al. 1991). I, on the
Michael H Carr, US Geological Survey, Menlo
Park, CA 94025, USA. Tel: 605-329-5174. Fax:
605-329-4999. E-mail: [email protected]
Branch of Astrogeology MS-975.
References
Arvidson R E 1979 Nature 278 533–535.
Baker V R 1982 The channels of Mars University of Texas Press.
Baker V R et al. 1991 Nature 352 589–594.
Carr M H 1979 J. Geophys. Res. 84 2995–3007.
Carr M H 1983 Icarus 56 476–495.
Carr M H 1989 Icarus 79 311–327.
Carr M H 1992 Lunar Planet. Sci. Conf. XXIII 205–206.
Carr M H 1996 Water on Mars Oxford University Press.
Carr M H 1999 J. Geophys. Res 104 21 897–21 909.
Carr M H and Malin M C 2000 Icarus (in press).
Christensen P R et al. 1998 Science 279 1692–1698.
Clifford S M 1987 J. Geophys. Res. 92 9135–9152.
Clifford S M 1993 J. Geophys. Res. 98 10 973–11 016.
Craddock R A and Maxwell T A 1993 J. Geophys. Res. 98
3453–3468.
Farmer C B et al. 1976 Science 194 1339–1341.
Forget F and Pierrehumbert R T 1997 Science 278 1273–1276.
Greeley R and Guest J E 1987 U.S. Geological Survey, Misc Inv.
Map I–1802–B.
Haberle R M et al. 1994 Icarus 109 102–120.
Haberle R M 1998 J. Geophys Res. 103 28 467–28 480.
Head J W et al. 1999 Science 286 2134–2137.
Kasting J F 1991 Icarus 94 1–13.
Kieffer H H et al. 1976 Science 194 1341–1344.
Lucchitta B K et al. 1992 in Mars edited by H H Kieffer et al.
453–492, University of Arizona Press.
Luhmann J G et al. 1992 Geophys. Res. Lett. 19 2151–2154.
Masursky H 1973 J. Geophys Res. 78 4037–4047.
Malin M C and Edgett K S 1999 Geophys. Res. Lett 26
3049–3052.
Manning C E and Igebritsen S E 1999 Rev. Geophys. 37 127–150.
McCauley J F 1978 U.S. Geol. Survey Misc. Inv. Ser. Map I–897.
McEwen A S et al. 1999 Nature 397 584–586.
Melosh H J and Vickery A M 1989 Nature 338 87–489.
Nedell S S et al. 1987 Icarus 70 409–441.
Neukum G and Ivanov B A 1994 in Hazards due to comets and
Meteorites edited by T Gehrels, 359–416, University of Arizona Press.
Parker T J et al. 1993 J. Geophys. Res. 98 11 061–11 078.
Parker T J et al. 1989 Icarus 82 111–145.
Pollack J B et al. 1987 Icarus 71 203–224.
Robinson M S and Tanaka K L 1990 Geology 18 902–905.
Schubert G S et al. 1992 in Mars edited by H H Kieffer et al.,
147–183, University of Arizona Press.
Scott D M et al. 1995 U.S. Geol. Survey, Misc. Inv. Map I–2461.
Scott D M and Tanaka K L 1986 U.S. Geological Survey Misc Inv.
Map I–1802–A.
Smith D E et al. 1999 Science 231 1495–1503.
Squyres S W and Kasting J F 1995 Science 265 744–748.
Tanaka K L 1986 J. Geophys. Res. 91 E139–E158.
Tanaka et al. 1992 in Mars edited by H. H. Kieffer, 345–382,
University of Arizona Press.
Wallace D and Sagan C 1979 Icarus 39 385–400.
Zahnle K J 1993 J. Geophys. Res. 98 10 899–10 913.
June 2000 Vol 41