Three Decades of Martian Surface Changes

Paul E. Geissler

U.S. Geological Survey

2255 N. Gemini Drive

Flagstaff, AZ 86001 USA

\(928\) 556-7257 voice

\(928\) 556-7014 fax

pgeissler@usgs.gov

26 manuscript pages

16 figures

1 color plate

0 tables

To be submitted to JGR-Planets

Draft July 9, 2004

Abstract

The surface of Mars has changed dramatically during the three decades
spanned by spacecraft exploration. Comparisons of Mars Global Surveyor
images with Viking and Mariner 9 pictures suggest that more than one
third of Mars\' surface area has brightened or darkened by at least 10%.
Such albedo changes could produce significant effects on solar heating
and the global circulation of winds across the planet. All of the major
changes took place in areas of moderate to high thermal inertia and rock
abundance, consistent with burial of rocky surfaces by thin dust layers
deposited during dust storms and subsequent exposure of the rocky
surfaces by aeolian erosion. At least three distinct mechanisms
contribute to aeolian erosion on Mars. Persistent winds dominate erosion
at low latitudes, producing diffuse albedo boundaries and linear wind
streaks generally oriented in the direction of southern summer winds.
Dust devils darken the mid- to high- latitudes from 45 to 70 degrees
during the summer seasons, forming irregular albedo patterns consisting
of dark linear tracks. Dark sand dunes in southern high latitudes appear
to be associated with regional darkening that displays diffuse albedo
boundaries. No surface changes were observed to repeat regularly on an
annual basis, but many of the changes took place in areas that alternate
episodically between high and low albedo states as thin mantles of dust
are deposited and later stripped off. Hence, despite the enormous extent
of alteration that has taken place during the intervals between
spacecraft missions, the face of Mars remains recognizable after a
century of telescopic observation.

1\. Introduction

One of the things that fascinates us most about Mars is its continually
changing surface. Prior to the era of spacecraft exploration, seasonal
variations and episodic changes were described by Earth-based observers
starting with the late 19^th^ century work of Schiaparelli, and were
widely believed to be caused by the growth and decay of martian
vegetation. Decades of telescopic photographic observations (e.g.,
Slipher, 1962, 1964) documented regional-scale variations, providing an
important historical context into which to place the surface changes
seen at close range by modern spacecraft. Many surface changes were
observed between the Mariner and Viking missions and from one year of
the Viking mission to another (Sagan et al.,1972, 1973; Veverka et al.,
1974, 1975, 1976, 1977; Thomas and Veverka, 1979). Most of these changes
were determined to have been produced by aeolian activity, through
deposition of dust during local and global dust storms and subsequent
darkening of the surface through erosion of the dust and transportation
of sand. However, many questions about martian surface changes remained
unanswered. Because of the short time span of the earlier observations,
it was difficult to determine which of the changes were seasonal
variations and which were long-term alterations. In many cases, owing to
the lack of high-resolution data, the exact causes of particular albedo
changes were uncertain. The mechanisms and implications of martian
surface changes were not well understood.

New data from the Mars Global Surveyor (MGS), Mars Odyssey and Mars
Express missions are now available to help clarify the frequency, extent
and causes of martian surface changes. For this study we have used MGS
Mars Orbiter Camera (MOC) daily global color images from the Wide Angle
Camera (WAC) to monitor the appearance of the martian surface for the
period from April, 1999 to January, 2003. These data enable us to
estimate the present day normal albedo of Mars under the clearest
atmospheric conditions, distinguish albedo changes over the past two
martian years, and identify longer term surface changes from the Viking
and Mariner 9 eras to the present. In addition, high resolution images
from the MOC Narrow Angle Camera (NAC) reveal the surface morphology of
variable features in unprecedented detail, enabling us to examine the
specific causes of martian surface changes. Our objectives are (1) to
map the surface changes that took place on Mars prior to the arrival of
Mars Global Surveyor, (2) to document any seasonal surface changes that
may have taken place over the two martian years of MGS observations, and
(3) to investigate the causes of martian surface changes by seeking
correlations between variable features and surface characteristics such
as elevation, geologic properties and predicted winds. From these
observations we hope to discover what the surface changes can tell us
about the interactions between the martian surface and the atmosphere:
Why do the variable features occur at the locations and seasons that
they do? Are there regular seasonal changes such as an annual \"wave of
darkening\" (de Vaucouleurs, 1954) that spreads from the spring
hemisphere across the equator? What constraints do surface changes place
on the strength of winds and the mobility of surface materials? What are
the roles of dust devils and sand dunes in altering the martian surface?

This paper presents a global overview of martian surface changes over
the last three decades and explores the range of phenomena that can
alter the albedo of the surface. In the next section we describe our
procedures for constructing global mosaics from the color MOC WAC images
and for producing a cloud-free summertime mosaic that can be compared to
the best available Viking and Mariner data products. In Section 3 we
examine the MGS images for seasonal variations and surface changes that
have taken place during the past two martian years. Section 4 points out
the major changes that have taken place between the end of the Viking
Mission and the arrival of Mars Global Surveyor, and quantifies the
areas affected in each case and the general setting of the variable
feature in terms of elevation, slope, thermal inertia, rock abundance,
and surface roughness. This section also presents a more detailed
examination of four specific surface changes that apparently differ from
one another in the manner and means by which they have altered. For
these case studies, we consider the historical behavior of the regions,
the presence of geomorphological features such as dust devil tracks,
wind streaks, dunes, and bedrock, and the influence of regional winds
predicted from theoretical models. The physical causes of surface
changes and the new questions provoked by this research are discussed in
Section 5. In the final section, we draw conclusions from these
observations about the extent and significance of surface changes over
the last few decades and about the variety of mechanisms that alter the
appearance of Mars over time.

2\. Approach

2.1. MOC global color mosaics

In order to determine the present day albedo of Mars and monitor any
annual variations, we processed more than 2,000 MOC WAC global mapping
swath color image pairs (a small fraction of the data available). These
were assembled into 21 global mosaics showing the appearance of Mars at
various seasons over a period of two martian years. Each mosaic is the
average of approximately 100 color observations spanning several
consecutive days. The individual images were first calibrated using
procedures of the U.S. Geological Survey ISIS software, making use of
preliminary Blue filter calibration coefficients derived by Erik
Eliason. Next the images were navigated (appended with geometric
information), assuming nadir pointing when necessary because of missing
or unusable camera pointing information, and then reprojected to a
common map projection. The data were next edited to eliminate images
that could not be correctly calibrated because of missing information
about gain changes during their exposures. Next the images were
corrected for limb darkening using a wavelength-dependent Lunar-Lambert
function. The coefficients L of the Lunar-Lambert correction were
derived empirically for the MOC Red and Blue filter images by creating
test mosaics and minimizing the mismatch at the seams; the best fits
were L=0.6 for Blue and L=0.25 for Red. The zero-phase-angle \"hot
spots\" were deleted from the images along with data taken with
incidence or emission angles greater than 84 degrees. Finally the images
were averaged to produce mosaics, and the seams removed via the
filtering procedure of Soderblom et al. (1978). The green images were
created by interpolating between the Red and Blue pictures. These
procedures produced low-resolution mosaics of acceptable quality for the
present purpose, but the results can be improved through better
calibration and more sophisticated photometric corrections that would
reduce the need for data redundancy. Even with the extensive averaging
of images that we employed, there remain artifacts in these mosaics in
the form of faint streaks in the orbit-track directions.

2.2. Construction of MOC cloud-free mosaic

Construction of a cloud-free, summertime mosaic was achieved by
combining the darkest pixels of all the individual mosaics acquired at
different seasons into a single color picture. This was done separately
for the Red and Blue filter data, and a synthetic green image was again
derived by interpolating the resulting Red and Blue mosaics. Seasonal
differences in overall brightness from one mosaic to another due to the
changing distance of Mars from the Sun were first removed by normalizing
the brightness of all the Red and Blue mosaics at low latitudes
unaffected by the polar hoods. Because of genuine surface changes that
were apparent after the global dust storm of 2001 (Section 3), only data
from the first 14 mosaics (up to phase E05) were used in the production
of the cloud-free mosaic.

2.3. Viking color mosaic data and processing and comparison to MGS

In Section 4 we will compare the MOC cloud-free mosaic to the U.S.
Geological Survey Viking color Mars Digital Image Model (MDIM) produced
by McEwen et al. (1994). Here we briefly review the steps that went into
creating the Viking mosaic in order to establish that the comparison
between these two products can be safely made. McEwen et al. began by
making mosaics of images from 57 individual Viking orbits using data
with phase angles ranging from 13 to 85 degrees. These single-orbit
mosaics were corrected for limb-darkening using a Minnaert function and
the best quality images were chosen to represent clear atmospheric
conditions and held constant while the others were adjusted in
brightness to match. Regions that were unusually bright in the Violet
filter images were assumed to be shrouded by atmospheric haze, and a
conservative estimate of the haze brightness was subtracted from both
the Red and Violet filter images. The images were next mosaicked by
placing the best quality images (generally those taken at the lowest
phase angles) on top, and the seams removed by the same filtering
procedure as used for the MOC mosaics. Finally, the Violet mosaic was
sharpened and the green mosaic interpolated from the Red and Violet
filter data.

For quantitative purposes, only the Red filter data from both Viking and
MGS were used to measure albedo changes by subtracting the coregistered
Viking Red MDIM from the MOC Red mosaic. We first applied a linear
contrast stretch to the Viking data to match the brightness of the MOC
image, computed by a least squares fit between the data sets. The
principal source of error in this comparison is the variation in phase
angle between the two data sets. This is particularly apparent in the
high southern latitudes, where topographic relief is enhanced in the
Viking mosaic and albedo variations are subdued. In places where there
was ambiguity over whether surface changes were real, we consulted
original Viking images and single-orbit mosaics that were acquired at
the lowest available phase angles in order to verify each case. An
independent estimate of albedo is provided by the broadband solar albedo
images acquired by the Thermal Emission Spectrometer (TES) on MGS and
similar images produced by the Viking Infrared Thermal Mapper (IRTM).
Although these data are of lower resolution, cover latitudes only
between 60°S and 60°N, and have their own issues concerning seasonal
differences and the presence of clouds and hazes, they provide a useful
reality check on the inferences drawn from the imaging data.

The surface areas affected by each change were computed by counting
pixels in equal area map projections. For each surface change we
estimated the total area that had visibly altered and also the area that
had altered by at least 10% of the mean planetary albedo. Where these
values differ greatly from the estimates derived from the TES and IRTM
solar albedo images, we list the areas estimated from both data sets.

3\. MGS to MGS Variations

Figure 1 shows a series of snapshots of Mars as it appeared over the
first two years of MGS observations. A color animation of these
observations can be viewed at
<http://astrogeology.usgs.gov/Projects/MartianChanges>. Many seasonal
variations can be seen that repeat from one martian year to the next.
All of the observed seasonal variations can be ascribed to atmospheric
effects or surface frost. The growth and retreat of the polar caps, the
appearance and disappearance of polar hoods, and the occurrence of
equatorial clouds atop the volcanoes were much as predicted from decades
of Earth-based observations (Capen, 1976; Beish, 2004). The great basins
of Hellas and Argyre brightened during southern hemisphere winter due to
deposition of surface frosts. Surface contrast was reduced during
periods of high atmospheric dust opacity, particularly during the global
dust storms of June to October 2001 (see Figure 1 phase E09).

No periodic changes in albedo patterns were detected. The boundaries of
bright and dark regions did not appear to shift on an annual basis.
However, a number of episodic changes took place during the global dust
storm of 2001. Most of these changes were relatively subtle, including
brightening (presumably due to dust deposition) along the west margin of
Syrtis Major (near 18°N, 300°W), portions of Nilosyrtis (40°N, 280°W),
northeast of Hellas (27°S, 270°W), and to the northwest of Elysium
(32°N, 235°W). Other areas darkened during the dust storm, perhaps
through removal of dust by unusually strong winds: darkening occurred
near the boundary between Hesperia and Elysium (7°S, 235°W) and at Sinai
Planum (16°S, 90°W). The brightening of Syrtis and darkening at Sinai,
although minor in extent, are important because these changes began to
restore the two areas to their former Viking-era appearance, reversing
the surface changes that took place between the Viking and MGS
observations (Section 4). While they are not "seasonal" changes per se,
they are among many areas of Mars that appear to alternate between
dust-covered and dust-free states repeatedly over time scales of
decades.

The two martian years of MGS observations highlight some of the hazards
to be avoided when looking for long term surface changes. In general,
albedo patterns do not alter on an annual basis but the contrast changes
dramatically with the season. The apparent surface albedos in dark areas
such as Syrtis and Acidalia depend strongly on atmospheric opacity.
Small-scale albedo patterns are easily obscured by dust storms, clouds
and hazes that can be misinterpreted as seasonal surface changes. The
MOC imaging results are consistent with the conclusions of Pleskot and
Miner (1981), who found no seasonal variations in surface albedo over
the martian year 1976-1978, which included the global dust storms of
1977. These authors suggested that the reported "wave of darkening" is
the result of contrast sharpening with increasing atmospheric clarity.

4\. Viking to MGS Changes

Much greater surface changes were seen over the 10 martian year interval
between Viking and MGS. Plate 1 shows a comparison between the Viking
color MDIM of McEwen et al. (1994), constructed of images taken from
1976 to 1980, and the cloud-free MOC mosaic made up of images acquired
from 1999 to 2001. An animation that blinks between these two pictures
can be viewed at <http://astrogeology.usgs.govProjects/MartianChanges>.
Several significant changes are evident from this comparison. In
general, the high latitude regions darkened during this period whereas
the equatorial regions brightened, consistent with the findings of Bell
et al. (1999) from Hubble Space Telescope (HST) observations. A small
bright band formed around the north pole of Mars, and the large dark
band surrounding it shifted towards the eastern hemisphere and expanded
along its southern margin. A large dark ring formed around the south
pole. Many of the classical albedo features along the equator shifted or
changed shape, including Syrtis, Alcyonius, Hyblaeus, Propontis,
Cerberus and Acidalia (all of these changes are described in detail
below). Such changes along sharp albedo boundaries are much easier to
spot than the more subtle changes in the albedos of broad regions or
those with diffuse albedo boundaries.

The surface changes are quantified in Figure 2, which displays the
difference between the present day appearance of Mars and its appearance
during 1976-1980 after subtracting the Viking era albedo from the MGS
measurements. In general, the visible imaging results (Fig. 2A) agree
with estimates derived from the broadband solar albedo channels of the
thermal imaging instruments (Fig. 2B), but there are several differences
between the data sets that may be explained by differences in bandpass,
season or phase angle. The visible imaging albedos are derived from the
Red filter data as described in Section 2; the bandpass of the Viking
visual imaging subsystem (VIS) Red filter was from 550 to 700 nm,
whereas the MOC Red filter spans a narrower range from 600 to 630 nm.
Both of these are much narrower than the albedo channels of the thermal
instruments, which were sensitive to wavelengths from 300 to 2900 nm.
The MGS TES albedo is from Christensen et al. (2001, Plate 14). The
Northern hemisphere measurements were made during late northern Summer
at solar longitudes (Ls) 100-150, prior to mission phase M02. The
Southern hemisphere observations were made in early southern Summer at
Ls 270-330 during phases M10 to M13. The Viking IRTM albedo is from
Pleskot and Miner (1981) and Kieffer et al. (1982) with refinements by
Christensen (1988). The Northern hemisphere IRTM measurements were
collected at Ls from 337 to 186, and the Southern hemisphere
observations Ls ranged from 120 to 360.

For this paper we will focus attention on the large scale surface
changes that can be seen in both the imaging and thermal instrument
albedo measurements. The locations of these major changes are shown in
Figure 3, which also shows the general relationships of the surface
changes to global topography and thermal inertia from MGS measurements.
Following are descriptions of each of these surface changes in the order
in which they are numbered in Figure 3. In addition to the albedos and
areas affected by each change, we describe the topographic setting of
each site and give information about the physical properties of the
surface derived from thermal measurements and laser altimetry. These
ancillary data are from several sources and were acquired at various
different times. We quote TES values for broadband albedo (Christensen
et al., 2001) and thermal inertia (Mellon et al., 2002). Rock abundances
are as derived from Viking IRTM analyses (Christensen,1986). MGS Mars
Orbiter Laser Altimeter (MOLA) data were used to obtain elevations
(Smith et al., 2003) and surface roughness estimates based on laser
pulse width measurements (Neumann et al., 2003; Deal et al., 2003).

4.1 Syrtis Major

Figure 2 shows that the northwest margin of this classical albedo
feature darkened during the interval between Viking and MGS. The
darkening took place in the vicinity of 20N, 290W. The interior of
Syrtis Major appeared instead to have brightened but, as pointed out
earlier, this appearance could be caused by atmospheric effects
unrelated to surface changes. The area along the edges of Syrtis that
can be shown to have visibly altered totals 428,000 km^2^, of which
62,000 km^2^ darkened by more than 10%. The TES broadband albedo in the
darkened zone ranges from 0.13 to 0.18 (I/F), to be compared to TES
albedos elsewhere in the vicinity from 0.10 in the dark center of Syrtis
up to 0.25 in bright, dust-covered regions. The changes took place on
the sloping margin of a broad plateau (Syrtis Major Planum), at
elevations ranging from 0 to 1000 m above datum. The region is locally
high in both TES thermal inertia (200 -- 400 J m^-2^ K^-1^ s^-½^,
henceforth abbreviated as "mks units") and IRTM rock abundance (10% to
20%), even though the rock abundance was calculated from Viking data
when the region was in its bright (presumably dust-covered) state. The
western margin of Syrtis brightened again after the global dust storm of
2001, partly restoring the region to its former Viking era appearance.
Frequent changes in the width of Syrtis Major have long been noted by
Earth-based telescopic observers (e.g., Capen, 1976), and can perhaps be
explained by cyclic deposition and erosion in this zone.

4.2 Nilosyrtis and Alcyonius

Several major changes occurred to the north of Syrtis. Most obvious is
the enlargement of a broad, dark band that covers latitudes from 50°N to
60°N and longitudes from 180°W to 300°W, that is best seen in the north
polar projection of Plate 2B. This band includes the classical albedo
features Nilosyrtis, Utopia, and Cebrenia. The albedo boundary that
separates the dark band from brighter regions shifted southwards,
particularly at Nilosyrtis (43°N, 275°W) where TES and IRTM data
indicate that the surface albedo dropped by up to 50%. Figure 4 shows an
overview of the changes that took place in Casius Quadrangle in the
period between the Viking and MGS observations. The northern low albedo
region (Nilosyrtis) greatly enlarged during this interval, and the
albedo boundaries between bright and dark terrain are sharply defined
but irregular in plan. An area of 1.1 million km^2^ was visibly altered,
and 688,000 km^2^ darkened by more than 10%. The TES albedo of the
darkened zone ranges from 0.11 to 0.16 with an average of 0.12. The
changes took place on the slopes of a shallow depression (Utopia
Planitia) in the plains of Vastitas Borealis, at elevations from -4950 m
to -3500 m. The TES thermal inertia in the changed area is moderate to
high, ranging from 220 to 310 mks units with an average of 275-285. The
Viking IRTM rock abundance is also high, from 16% to 25%. The MOLA
roughness of the region is low to moderate.

South of this band, a much smaller area of 161,000 km^2^ darkened when
the classical albedo feature Alcyonius (sometimes called Thoth) at 30°N,
265°W, enlarged. Unlike Nilosyrtis, the albedo along the southern
boundary of Alcyonius transitions gradually from dark to bright without
the complexity seen farther north. The TES albedo in the darkened zone
is 0.13 to 0.14, a reduction of 0.07 to 0.09 from its Viking appearance.
An area of at least 117,000 km^2^ darkened by 0.02 or more. Like many
low-latitude surface changes, the darkening of Alcyonius took place on a
sloping surface with relatively high thermal inertia and rock abundance,
suggesting that a thin veneer of dust was stripped off a bedrock surface
during periods of high winds. Elevations in this region range from -4500
m to -3500 m. The TES thermal inertia varies from 230 to 290 mks units,
averaging 245 to 265, and the IRTM rock abundances average 10% to 15%.

Historical records indicate that the growth of Nilosyrtis and Alcyonius
began before the arrival of Viking and is gradually restoring the region
to its appearance of 50 years ago. Telescopic images show extensive
areas north and east of Syrtis that were dark in 1954

(Slipher, 1962, Plate IX) but had brightened by 1969 (Martin et al.,
1992). Alcyonius was completely buried at the time Mariner 9 observed
the region in 1972, but had already begun to emerge by the arrival of
Viking. Both Alcyonius and Nilosyrtis enlarged over the course of the
Viking Mission, undergoing changes that affected tens of thousands of
square km from 1978 to 1980 (Figure 5).

The bedrock in this region is the Hesperian Vastitas Borealis Formation
of undetermined origin (Greeley and Guest, 1987). Superposed on these
rocks are many features shaped by periglacial processes including
irregular scalloped depressions formed by sapping or sublimation and
vast areas of polygonally patterned ground that feature small (50-100 m
diameter), regular polygons believed to be caused by the presence of ice
(ice wedges) or liquid water (mud cracks) in the sediments (Seibert and
Kargel, 2001). These polygons are quite distinct in the dark areas but
are faint or absent in bright areas, as if obscured by thin mantles of
dust. The transition between polygonally patterned ground and adjacent
smooth, brighter surfaces is quite gradual (see e.g. MOC NAC images
E0401425, E2301288).

High resolution MOC NAC images of the recently darkened areas in
Nilosyrtis present strong evidence for erosion by dust devils. Figure 6
shows a close-up of a newly darkened zone at 54.28°N, 292.69°W. At this
resolution (6.6 m/pixel), the darkest patches can be seen to be
crisscrossed with filamentary linear streaks that are diagnostic of
dust-devil tracks (Edgett and Malin, 2000; Malin and Edgett, 2001; Balme
et al., 2003). Dust devil tracks are absent from the nearby bright
regions and are also largely absent from lower latitude dark patches
such as Alcyonius. The tracks appear to form seasonally, commonly
appearing during northern Spring and Summer. A survey of MOC NAC images
from mission phases AB1 to E06 identified dust devil tracks in 107 NAC
images. Definitive nondetections were made in another 180 NAC images.
The distribution of detections (black) and nondetections (white) for all
these cases is shown in Figure 7. The dust devil tracks are strongly
zoned during all seasons, confined to latitudes \~45°N to \~65°N. In
areas such as Alcyonius that darkened outside of this latitude band,
dust devil tracks are absent and the surface is pervaded instead by wind
streaks indicative of persistent winds.

Northern Spring and Summer winds are light (\< 5 m/s) throughout the
region that underwent surface changes, according to the predictions of
the Geophysical Fluid Dynamics Laboratory Mars Global Circulation Model
(GFDL MGCM) (Wilson and Hamilton, 1996; Fenton, 2003). Southern Spring
and Summer winds are predicted to be lighter in the northern portion of
the region (the area where dust devil tracks are common) than in the
south (where no tracks are seen).

4.3 Hyblaeus

This classical dark albedo feature (also known as Aetheria) brightened
and a new dark patch appeared to the east, bordering the volcano
Elyseum. These changes took place on the sloping surface between Utopia
and Elyseum, at elevations ranging from --4400 m to --2000 m. The
northwestern slopes of Elysium also brightened during this interval
between Viking and MGS. The changes were centered at 25°N, 240°W but
affected an area of about 1.2 million km^2^. Approximately 304,000 km^2^
darkened by more than 10%, while at least 160,000 km^2^ brightened by
this amount. The TES albedo in the newly brightened area is 0.22 to
0.24, compared to 0.17 to 0.19 in the dark patches. The thermal inertia
ranges from 180 to 400, but averages 225 -- 250 mks units. The Viking
rock abundance prior to the change was slightly greater (10%-15%) in
areas that were dark at the time of the observations than in bright
areas (8%-9%). Mariner 6 and 7 images show the Hyblaeus albedo feature
absent in 1969, similar to its appearance in the MGS era (Edmonds and
Robinson, 2003).

4.4 Cerberus

The brightening at Cerberus (10°N, 205°W) provides an opportunity to
examine both the nature of dust deposition and the earliest stages of
erosion. Figure 8 shows an overview of the changes in the Cerberus
region. Several previous studies have pointed out that this classical
low albedo feature disappeared sometime between the end of the Viking
Mission and the HST observations of 1995 (Lee et al. 1996; Bell et
al.1999), probably before late 1990 (Moersch et al., 1997). The low
albedo materials, which extended across a region nearly 1400 km long and
250 km wide, are presently completely buried except for a few dark
patches in the eastern and western reaches. The (probable) new dust
deposits mantled an area of nearly 390,000 km^2^, of which 245,000 km^2^
brightened more than 10%. The TES albedo increased from 0.15 -- 0.18 in
the dark patches up to 0.23 -- 0.25 in the newly brightened zone. The
changes took place on the southern slopes of the Elysium rise, at
elevations from -2500 m to -1800 m. TES measurements indicate that the
zone occupies a bench of locally high thermal inertia averaging 250 --
290 mks units with a range of 220 -- 350. Viking IRTM rock abundances
were also locally high, averaging 9% to 11%. The MOLA roughness in this
area is average except for some relatively rough regions in the East,
near Cerberus Rupes.

Cerberus appeared dark and apparently unchanged from 1907 to 1954
(Slipher, 1962, Plate XXXV) but darkened and enlarged between Mariner 9
in 1972 and the arrival of Viking in 1978 (Chaikin et al., 1981). The
darkening and changes in direction of wind streaks were interpreted by
Chaikin et al. to be due to removal of bright dust. The region remained
dark in Viking Orbiter 1 observations acquired in 1980 (orbit 323S). Pic
du Midi telescopic observations ([Erard,
2002](http://www.bdl.fr/s2p/mars.html)) show Cerberus already absent in
1988. Possibly the feature was buried during the global dust storm of
1982.

Cerberus lies on the margin of early Amazonian lava flows from Elyseum
but is also the site of some of the youngest lava flows, channels and
alluvial deposits on Mars (Plescia, 1990, 2003; Burr et al., 2002a,
2002b; Berman and Hartmann, 2002). The classical albedo feature is cut
by two northwest-trending linear troughs, called the Cerberus Fossae,
believed to be volcanic fissures that are the source of both the lavas
and the floodwaters.

High resolution images show several morphological indications that the
new dust deposit covering the region is quite thin. Dust devil tracks,
produced by removal of a bright layer exposing a darker substrate,
appear sporadically in northern hemisphere summer (e.g., MOC NAC images
M0001273, M0002500, E1400795, E1601013, E1601635). Many small craters
that are located in the bright dust-covered regions have dark floors,
perhaps cleaned of dust by turbulent winds (MOC NAC images M0002500,
M0201973, M0204698, M0400964, M0402835, M1300846, M1301354, E1600925;
see also Figure 9). These imaging observations are consistent with the
interpretation of ground-based thermal data (Moersch, 1998) that a layer
of dust only 2 to 4 mm thick currently covers the Cerberus region.

Erosion at Cerberus is dominated by persistent southwesterly winds, as
shown by the numerous bright and dark streaks that extend hundreds of km
towards the southwest from topographic irregularities such as knobs,
impact craters and the Cerberus Fossae troughs. Southwesterly winds are
predicted by the GFDL MGCM to reach average strengths of up to 20 m/s
during southern Spring and Summer. Figure 9 shows the early stages of
erosion in an area near 13.96°N, 199.51°W that is still mostly covered
by dust. Incipient dark streaks are initiated at topographic obstacles
such as craters and knobs, while the interiors of the larger craters
have been filled with dust that has been organized into dune-like
ripples with wavelengths on the order of a few tens of meters. Elsewhere
in Cerberus, where erosion has progressed more fully, these dust-filled
craters lie windward of bright streaks representing the last of the dust
to be removed from the region. While dominated by persistent winds,
erosion is also contributed to by dust devils (as noted above) and sand
dunes locally appear to play a role as well. Several dark streaks arise
from "splotch craters", impact craters that are occupied by dark patches
on their downwind sides. Only one of these dark patches has so far been
imaged by MOC NAC; giant dark dunes with wavelengths of several hundred
meters are seen in image E1701373, inside the crater at 9.5°N, 210°W.
This crater heads a broad, diffuse dark patch that trails west-southwest
for more than 100 km from the crater rim. This zone of erosion could be
produced by winds interacting with the crater's topography, but it is
possible instead that sand escaping from the dark dunes acts to darken
the downwind surface. The potential mechanisms by which sand dunes might
darken adjacent terrain are discussed in Section 5.

4.5 Propontis

An area of about 190,000 km^2^ darkened in Arcadia Planitia near 37°N,
185°W. This change took place within a low, flat plain at elevations
from --4000 m to --3800 m. The low relief of this region demonstrates
that erosion is not restricted to sloping surfaces. The TES albedo of
the region is highly variable, ranging from 0.13 to 0.19. Imaging data
suggest that only 58,000 km^2^ darkened more than 10%, but the broadband
albedo estimates from TES and IRTM give a larger figure of 184,000
km^2^. The region is intermediate in TES thermal inertia, averaging 190
-- 230 mks units. The IRTM rock abundance in this zone was moderate
during the Viking era, ranging from 6% to 10%.

4.6 Southern Tropical Dark Band (Mare Cimmerium - Mare Sirenum)

Mars may have been in an atypical state during the tenure of the Viking
Orbiters, in the sense that there was a gap in the dark band that rings
the low southern latitudes of the planet. A broad zone centered at 30°S,
180°W was bright as late as 1980 (Viking orbit 323S) but darkened before
the arrival of MGS. Nearly 1.8 million km^2^ grew visibly darker,
reducing the albedo of this region by 21% to 36% to TES values of 0.13
to 0.15. Over the same period, many smaller dark patches were obscured
by new dust deposits over the equatorial zone from 10°S -- 25°S and from
160°W as far as 230°W. The boundary between the newly deposited bright
dust and the freshly eroded dark band is relatively abrupt and extends
over thousands of km (Plate 1) but does not correspond to any marked
variations in topography or thermal inertia. The changes took place in
the southern highlands on a sloping bench that gradually rises from 900
m to 2600 m in elevation. The sharp new albedo boundary does not
correspond to the margin of the rugged southern highlands with the
smooth northern lowlands, which is several hundred km to the north.
Neither is there a distinct change in thermal inertia across the albedo
boundary; the TES thermal inertia reduces only slightly from south to
north and remains in the range of 210 -- 260 mks units. Interestingly,
the albedo boundary appears in the global MOLA laser roughness map
(Neumann et al., 2003) as the division between a very rough region to
the south (in the low albedo zone) and an area to the north were the
laser returns were saturated, due to either high amplitude reflections
or anomalously long pulse durations indicating high roughness. The
simplest explanation is that the saturation is due to the higher albedo
of the bright tropical dust; future MOLA measurements may tell whether
the region is unusual in surface roughness.

4.7 Southern High-Latitude Band (Mare Chronium - Mare Australis) at
Phaethontis

The largest region that altered in appearance is a broad band of the
southern highlands that extends southwards from about 45°S to 70°S, an
area of some 16 million km^2^. This appears as a new dark ring around
the South Pole in the polar projection of Plate 1C. Both the MOC vs.
Viking imaging and the TES vs. IRTM albedo data show that this region
darkened during the past two decades. Imaging data measurements indicate
that at least 5.5 million km^2^ darkened by 10% or more. TES and IRTM
measurements would yield a much larger area darkened (at least 11
million km^2^ ), however the IRTM measurements do not extend past 60°S.
TES albedo in the newly darkened region is 0.13 to 0.15, a change of
-0.06 to -0.09 from the IRTM albedo. HST observations (Bell et al.,
1999) indicate that the change took place prior to 1995. This change is
the most difficult to map, because the boundaries of the recently
darkened area are diffuse and discontinuous. It was also regarded with
some suspicion, since the southernmost images of the Viking color MDIM
were data acquired at relatively high phase angles that show albedo
variations poorly. Comparison with low-phase-angle Viking images such as
those from orbit 507A (phase angle 21 degrees) confirms that some albedo
information was lost at these high southern latitudes in the
construction of the color MDIM. However, two Viking image sequences that
show the area of the southern high-latitude band at low phase angles
with good albedo definition verify that the darkening of the band is
genuine, at least over certain longitudes. A color sequence taken during
orbit 459A shows the region south of Sirenum at 39 degrees phase during
southern summer, at Ls 318 degrees. This sequence shows that the
latitude band from \~45°S to \~65°S was distinctly brighter than
present, at least over longitudes from 190°W to 240°W. Images from orbit
500A at 21 degrees phase suggest that the high latitude areas from 45°S
to 75°S that are presently dark over longitudes from 80°W to 160°W were
also bright at this time. Other sequences acquired on orbits 463A and
605A confirm albedo changes over the same longitudes (80°W to 240°W) but
in general, low phase coverage of the southern polar region by Viking
was sparse.

Based on the available low phase angle Viking images, together with the
support of the TES and IRTM measurements, we conclude that the darkening
of the southern high latitudes between Viking and MGS observations is
genuine. All together, the area affected by surface changes in the
southern highlands over the past two decades amounts to more than 40
million km^2^, nearly a quarter of the surface area of the planet. This
figure excludes the known surface changes in the equatorial and northern
latitudes of Mars and any activity in dust-covered regions such as
Amazonis and Arabia that may have experienced dust deposition and
erosion without detectable changes in albedo.

Figure 10 shows an overview of surface changes in Phaethontis
Quadrangle, covering a small portion of the new dark ring that appeared
across the southern high latitudes and also a sample of changes in the
tropical dark band. The bedrock in this region is mostly made up of
Plateau Sequence rocks of Hesperian and Noachian age, along with some
Hesperian volcanics and ridged plains (Scott and Tanaka, 1986). In the
north of the quadrangle, sand dunes and dust-devil tracks are absent and
the wind streaks indicate that the winds are from the northwest,
consistent with GFDL MGCM predictions of light (\< 5 m/s) southeasterly
winds during southern Summer. In contrast, most of the wind direction
indicators in the middle and southern portions of the quadrangle suggest
northwesterly winds, which reach average speeds of \~10 m/s during
southern Spring according to MGCM predictions. Dust-devil tracks are
common in southern Summer, occasionally reaching very high densities in
some locations (e.g., M1301057, E1100101, E1103012). As in Nilosyrtis
and Acidalia, the tracks are common in specific latitude bands and rare
or absent in others. Figure 11 shows that the tracks found in a cursory
survey of MOC NAC images are confined to latitudes from 45°S to 65°S, a
range similar to those determined from the northern hemisphere surveys
in the Nilosyrtis (Section 4.2) and Acidalia (Section 4.10) regions.

Much of the area that darkened in the southern half of Phaethontis
Quadrangle shows no distinct wind streaks or dust-devil tracks at the
resolution of the MOC NAC images. Instead, diffuse dark bands can be
seen extending downwind from "splotch" craters (impact craters occupied
by deposits of low albedo materials). In every case where MOC imaged one
of these splotches at high resolution, the images revealed that the low
albedo deposits were dark dunes clustered against the downwind margins
of the crater floors (Figure 12). The dunes are located in the
northwestern sides of the craters, the northwest walls of the craters
appear dark, and diffuse dark bands taper from the craters towards the
northwest, consistent with the northwesterly winds predicted by the GFDL
MDIM to reach strengths of up to 10 m/s during southern Spring. Figure
13 shows the locations of dark dunes identified in the released MOC NAC
images of the region. The MOC NAC data set is incomplete, and many of
the splotch craters that currently lack coverage by high-resolution
images are likely to contain dunes as well. Nevertheless, Figure 13
shows that dunes are ubiquitous throughout the darkened zone. Moreover,
dark dunes mapped globally from Viking images (e.g., Ward et al., 1985;
Peterfreund, 1985) and Mars Odyssey THEMIS images (Timothy Titus and
Rose Hayward, unpublished data) are distributed throughout the southern
high-latitudes where the dark circumpolar ring formed sometime between
the Viking era and the present.

4.8 Aonis Sinus

The largest brightening detected by TES took place on Daedalia Planum
near the southern end of Tharsis (30°S, 130°W). TES and IRTM albedo
measurements suggest that an area of 2.2 million km^2^ brightened,
whereas the MOC and Viking images give a smaller figure of 1.2 million
km^2^. The dust deposit formed a new bright gap in the dark band that
encircles the planet at southern tropical latitudes. TES albedo in the
newly brightened zone ranges from 0.21 up to 0.27. The change took place
on the steep terrain of the southern slopes of Tharsis, at elevations
from 1400 m to 6200 m above datum.

Aonis is another area of moderate to high TES thermal inertia, averaging
165 -- 290 mks units. IRTM rock abundances in this zone were mixed,
ranging from 5% to 10% prior to the albedo change.

Lowell Observatory images taken during clear atmospheric conditions show
Aonis Sinus bright in 1971 but dark during 1973 (Martin, 1974; note that
Martin et al., 1992, refer to a darkening of "Claritas"), suggesting
that this is another region that alternates between dust-covered and
dust-free states over time scales of decades. This episode of
brightening apparently took place prior to the Pic du Midi observations
of 1988.

4.9 Solis Lacus

The classical dark spot Solis Lacus (28°S, 90°W) is among the albedo
features most frequently reported to change on Mars, perhaps because it
is so easily recognizable as the "eye" of Mars. Dollfus (1965, Fig. 11)
noted changes in this region after the dust storm of 1956. Lee (1986)
described Viking era changes that took place after the 1977 dust storm.
As mentioned in Section 3, the region was the site of one of Mars\' most
recent changes during the dust storm of 2001. MOC images taken prior to
the 2001 dust storm show that the entire plateau south of the Valles
Marineris (centered at 20°S, 90°W) had darkened relative to its Viking
era appearance, affecting an area of 3.1 million km^2^. The changes took
place within a high plain rimmed by mountain ranges at elevations from
2500 m to 4300 m. The darkening was greatest to the south in Solis
Planum (28°S, 90°W) and the east in Thaumasia Planum (20°S, 60°W). TES
and IRTM measurements agree with the darkening in the south and east,
but do not show darkening to the north in Syria Planum (15°S, 110°W) or
Sinai Planum (15°S, 90°W), instead suggesting that brightening took
place in these regions. TES albedo is 0.12 - 0.17 in the darkened zones
and 0.18 to 0.25 in bright areas. Thermal inertia over the region
averaged 240 - 340 mks units. Rock abundance here was relatively high in
Viking measurements, averaging 6% to 17%.

New dust deposits emplaced during the 2001 dust storm brightened the
area to the east of Solis Lacus (Thaumasia Planum), partly restoring the
region to its Viking era appearance.

4.10 Acidalia

Several changes took place between Viking and MGS in the neighborhood of
the classical albedo feature Acidalia. This dark feature enlarged along
its margins and a new bright fringe appeared along its eastern edge
(30-40°N, 0-15°W). Brightening also occurred across the top of Acidalia
along the new north polar bright band seen in Plate 1B. Darkening took
place along Acidalia\'s southern margin (20-30°N, 40-50°W), along with
smoothing of the albedo boundary between the dark surfaces to the north
and the brighter surfaces to the south. Further darkening developed
along both the eastern and western margins, increasing the width of
Acidalia across latitude 35°N. There was also extensive darkening of the
region to the west of Acidalia, centered at 30°N, 60°W.

MOC images suggest an area of 2.2 million km^2^ was affected, however
TES albedo measurements suggest a much larger area of up to 4.6 million
km^2^ may have altered. TES albedo in the darkened region dropped from
\~0.24 to \~0.12. Most of the changes took place on the sloping rim of a
basin (Chryse Planitia) where the southern highlands meet the northern
plains, at elevations from -4200 m to -1400 m. In common with the other
zones of surface change, this is a region of moderate to high TES
thermal inertia (average 235 - 265 mks units), IRTM rock abundance
(average 11% - 19%), and MOLA roughness.

Aeolian erosion by persistent winds is chiefly responsible for the
surface changes along the southern margin of Acidalia. Figure 14 shows
an overview of surface changes in this region, which extend at least
2000 km in the east-west direction. Most of this expanse is occupied by
Arcadia Formation volcanics, Amazonian-aged deposits that were
subsequently modified by tectonism (Scott and Tanaka, 1986). A cursory
survey of high resolution MOC NAC images detected no dust-devil tracks
in the recently darkened zone (Figure 15); in common with the Nilosyrtis
region, dust-devil tracks in Acidalia were found only in higher
latitudes ranging from \~50°N to \~65°N. At lower latitudes, where most
of the darkening took place, the albedo is controlled by bright and dark
linear streaks that taper towards the southwest. Figure 16 shows a
close-up view of the newly darkened terrain at 26.10°N, 42.91°W. Most of
the dust has been removed from the region already, and the little that
remains is concentrated in the sheltered floors of impact craters, where
the dust is organized into small dune-like ripples. Bright streaks
extend downwind from the dust-filled craters towards the southwest. Much
larger dark streaks with the same southwesterly trend can be seen in
regional scale images (Figure 14) from both Viking and MGS, and the
albedo boundary dividing bright and dark surfaces can be seen to have
advanced in this direction during the interval between Viking and MGS.
The GFDL global circulation model predicts calm conditions during
southern Fall and Winter but strong (\> 10 m/s) southwesterly winds in
this region during the southern Spring and Summer, consistent with the
orientation of the wind streaks.

5\. Discussion

Some inferences from the global observations can help us begin to
interpret the causes of surface changes in general. First, all of the
albedo changes discussed above took place in areas of moderate to high
thermal inertia (Figure 3). The MOC vs. Viking imaging comparison
suggests that a portion of western Amazonis may have also brightened,
but this is not borne out by the TES and IRTM albedo measurements
(Figure 2). The correlation of surface changes with thermal inertia is
consistent with the widely held notion that the changes are caused by
deposition and erosion of bright dust that buries and exposes darker
bedrock. The dust deposits must be thin in comparison to low thermal
inertia regions like Amazonis and Arabia, because the thermal inertia of
the changeable zones remains high even when they are dust covered (e.g.,
Cerberus). Second, many of the areas that changed appearance between
Viking and MGS have histories of similar changes in the past, including
Syrtis Major, Nilosyrtis, Hyblaeus, Aonis Sinus, and Solis Lacus. Such
repeated episodes account for the impression of seasonal surface changes
that was long held by many ground-based observers. They also explain how
the surface of Mars remains recognizable after a century of telescopic
observation, despite the enormous extent of alteration that has taken
place between Viking and MGS: according to MOC/Viking imaging estimates,
a total area of 56 million km^2^ changed albedo by more than 10%, mostly
due to darkening in the southern highlands. Alternation of bright, dust
covered periods with darker states during which bedrock is exposed is
consistent with the deposition of thin dust layers during global and
regional dust storms that are subsequently stripped off by the wind.

But how does this erosion come about? Examination of the surface at
higher resolution suggests at least three distinct mechanisms may
contribute to the erosion of Mars\' surface. Persistent winds are
chiefly responsible for removing dust in low latitude regions including
Acidalia and Cerberus. Dark sand dunes are scattered throughout the
southern high-latitude band and apparently played a role in the
darkening of this vast region. Dust-devils evidently strip dust from the
northern Nilosyrtis Region in sufficient quantities to darken the
surface. It is not clear how dust-devils accomplish this darkening;
individual dust-devil tracks do not seem to persist for more than a few
months, based on repeated MOC NAC imaging of a dozen spots where the
tracks can be monitored. Perhaps it is the cumulative diminution of dust
in a region where dust-devils are active that produces an overall
darkening over time, rather than the accumulation of distinct dark
tracks on the surface. Other open questions concern the cause and extent
of the apparent latitudinal zoning of dust-devil tracks. Dust-devil
tracks in Nilosyrtis, Acidalia and Phaethontis are common during summer
seasons at latitudes from \~45 to \~70 degrees, but much rarer at lower
latitudes. While dust-devils and their tracks have been seen at all
martian latitudes, their contribution to erosion is apparently only
significant at mid- to high-latitudes. Balme et al. (2003) carefully
quantified dust-devil track densities in the great basins of Hellas and
Argyre. They noted that dust-devil tracks could be found at all
latitudes within their study areas (latitudes 15°S to 60°S) but found
greater track densities at higher latitudes (from \~45°S to \~60°S in
Argyre, and from \~35°S to \~60°S in Hellas; see their Figure 4). The
dust-devil track distributions in Hellas and Argyre are similar to the
qualitative surveys presented here, and support the suggestion of
latitudinal control on the frequency of dust-devils or the visibility or
persistence of their tracks. Further work is needed to clarify the cause
of this distribution. Perhaps dust devil tracks accumulate in areas that
experience only light summer winds.

The mechanism by which giant sand dunes darken adjacent terrain is
another matter of debate. The best studied examples of such albedo
changes are in Oxia Palus in western Arabia Terra, where conspicuous
dark streaks with bright margins extend southwards from numerous
dune-filled impact craters. These streaks were noted to have changed in
length, albedo, and orientation between observations made by Mariner 6
(1969), Mariner 9 (1972) and Viking 2 (1976-1980) (Thomas and Veverka,
1979). At least three hypotheses have been postulated to explain the
albedo changes in Oxia Palus: (a) deposition of fine dark particles
eroded from the dunes and carried downwind in suspension; the bright
margins would then represent the finest (dust-sized) fraction of the
sediment (Edgett, 2002). (b) deposition of coarse sand grains that
hopped (via saltation) or rolled (via traction) over the crater rims;
the bright margins of the dark streaks could be either dust remobilized
by the sand grains (Thomas et al., 1981) or red soil that was disturbed
and brightened by the passage of the sand grains (Mustard, 1995; Cooper
and Mustard, 1998). (c) erosion of bright dust and red soil downwind of
the craters by traction and saltation of sand grains escaping from the
dunes, revealing a darker substrate; the bright margins of the dark
streaks might then represent deposits of dust shed from the eroded
zones.

Edgett (2002) examined the dark streaks of western Arabia Terra in high
resolution MOC NAC images and found no evidence of morphological
differences between the dark streaks and the surrounding untinted
surface, ruling out significant deflation in the darkened zones. The
images did reveal several small bright streaks arising from dust-filled
craters, similar to those seen in recently eroded regions in Cerberus
(e.g., MOC image E0301587) and Acidalia (Figure 15). Edgett (2002)
favored downwind deposition by suspension, based on the absence of dunes
on the splotch crater walls and in the dark streaks downwind. On the
other hand, erosion of thin coatings of soil or dust in the lee of
splotch craters is permitted by the observations, as long as the layers
removed are much thinner than the scale of topography visible in the MOC
NAC images. Such erosion could be produced by the saltation and traction
of mobile sand grains originating from the dark dunes, provided that the
sand supply is small and does not exceed the removal rate. In either
case, the darkening of adjacent surfaces likely takes place when
materials are shed from the dark dunes and redeposited downwind.

Martian surface changes reveal the distribution of depositional and
erosional zones under present winds and give important clues to the
nature of surface materials and the physical process that alter the
appearance of the planet. The details of the surface changes provide
constraints on the direction and strength of the winds presently blowing
across the martian surface. The albedo changes affect the distribution
of surface heating by insolation, which in turn alters the circulation
of the winds that produce the surface changes. Because the most recent
changes resulted in darkening of the higher latitudes and general
brightening at lower latitudes, these changes are expected to produce a
more uniform overall distribution of surface temperatures. Only a
minimum estimate of the volume of sediment transported by winds on Mars
can be obtained from a study of surface changes. Erosion and deposition
of dust are likely to be as important in the bright, dust-covered
"continents" of Amazonis, Arabia, and Elyseum as elsewhere on the
planet, yet may produce no noticeable change in surface albedo.

6\. Conclusions

We constructed a cloud-free, summertime mosaic of Mars at it appeared to
MGS during the period 1999-2001 and compared it to earlier observations
from Viking, Mariner, HST and ground-based telescopes in order to
determine how the visible albedo of the planet has altered over time. We
quantified the areas affected and examined the topographic setting,
thermal inertia, and high resolution morphology of a few of the surface
changes identified in this way. From these considerations we suggest the
following interpretations:

1\. Extensive changes in albedo took place during the past three decades
of spacecraft exploration of Mars. Up to 1/3 of the surface area of the
planet altered in albedo by more than 10% since the Viking era, with the
most significant changes in the southern highlands. In general, the
martian surface tended to darken at higher latitudes while most of the
brightening took place in the equatorial regions. These albedo changes
could potentially alter the distribution of surface temperatures due to
insolation and the resulting global circulation of winds.

2\. All of the observed surface changes took place in areas of moderate
to high thermal inertia and rock abundance -- places where bedrock is
exposed at the surface and can be buried by dust and exhumed by the
wind. This suggests that the changes were caused by deposition of thin
dust layers during global and regional dust storms and their subsequent
removal by aeolian erosion. It is likely that dust deposition and
erosion also take place on thickly mantled, dust-covered regions of low
thermal inertial such as Amazonis and Arabia, but this suspected
activity does not alter the appearance of the already bright surface.

3\. Preliminary studies of high resolution images in variable regions
suggest that several distinct mechanisms contribute to the erosion of
dust on Mars. It is possible that further investigation of other martian
surface changes will reveal additional mechanisms.

> 3.1. Persistent winds dominate erosion at low latitudes, specifically
> at Alcyonius, Cerberus, Sirenum, and the southern margin of Acidalia.
> These winds produce linear bright and dark wind streaks with diffuse
> margins that typically trend in the direction of strong southern
> Spring and Summer winds. Remnant dust in these regions forms dunelike
> ripples inside impact craters, trailing bright streaks downwind as it
> erodes.
>
> 3.2. Dust devils apparently dominate erosion in the high northern
> latitudes. Recently darkened terrain in Nilosyrtis is criss-crossed by
> the dark tracks of dust-devils, forming irregular albedo boundaries
> with sharp margins. Some evidence for latitudinal zoning of dust-devil
> tracks is seen in both the northern and southern hemispheres.
> Dust-devil tracks in Nilosyrtis, Acidalia and Phaethontis are common
> during summer seasons at latitudes from \~45 to \~70 degrees, but much
> rarer at lower latitudes. Further work is needed to clarify the cause
> of this distribution.

3.3 Dark sand dunes are concentrated in southern Phaethontis Quadrangle
and are widely distributed throughout high southern latitudes, a region
that darkened without forming conspicuous albedo boundaries. Erosion of
the dunes apparently helped darken the region by staining the surface
with dark silt or scouring the region of dust through the saltation and
traction of sand particles.

4\. Surface changes occur episodically on Mars. No evidence was found
for seasonal changes in albedo patterns during two martian years of MGS
observations, although surface albedo contrast and obscuration by
frosts, clouds and hazes vary periodically on an annual basis. However,
many martian albedo features historically alternate between dust covered
and dust free states. Surface changes that took place between Viking and
MGS in Syrtis Major, Nilosyrtis, Hyblaeus, Aonis Sinus and Solis Lacus
reversed earlier changes or were restored to their former appearance
during the global dust storm of 2001. On time scales of decades, the
surface returns to an average or equilibrium appearance that ensures
that the face of Mars remains recognizable in general but different in
detail with each passing year.

Acknowledgements

Discussions with Tammy Becker, Glenn Bennett, Timothy Titus and many of
the scientific and technical staff of the U.S.G.S. Astrogeology Team in
Flagstaff contributed to this research. Philip Christensen and the TES
team are thanked for the use of TES albedo data. This work was supported
by the Mars Data Analysis Program.

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Figure Captions

1\. Time-series of MGS global monitoring images. This montage of red
filter MOC WAC mosaics shows the appearance of Mars at various times
during each of two martian years from 4/1999 to 1/2003. The mosaics are
shown in equal-area cylindrical map projections and are labeled with the
MGS mission phase and the solar longitude (Ls). Seasonal phenomena that
can be seen in these snapshots include the annual appearance of polar
hoods and surface frosts, brightening within the great basins of Hellas
and Argyre, and the occurrence of equatorial clouds adjacent to the
volcanoes of Tharsis and Elysium. The global dust storm of 2001 obscured
surface albedo contrast (especially during phase E09) and produced many
minor surface changes.

Plate 1. Comparison of Viking and MGS global mosaics of Mars. Shown here
are side-by-side comparisons of the color digital image mosaic assembled
by McEwen et al. (1994) from Viking images acquired during the period
1976-1980, and the cloud-free mosaic of MGS images from 1999 to 2001.
All of the large-scale surface changes discussed in this paper are
obvious in these renditions. Note in particular the new dark ring
centered on latitude 60°S that is apparent in Plate 1C.

Plate 1A. Viking color DIM (top) vs. MGS cloud-free mosaic (bottom) in
equal area cylindrical map projections.

Plate 1B. Viking (left) and MGS (right) views of the northern hemisphere
of Mars in polar orthographic map projections.

Plate 1C. Viking (left) and MGS (right) views of the southern hemisphere
of Mars in polar orthographic map projections.

2\. Differences between Viking and MGS albedos of Mars. These images map
the albedo changes that took place between Viking and MGS as seen by two
types of sensors. The upper image shows albedo changes computed by
subtracting the Viking red filter image mosaic of McEwen et al. (1994)
from the cloud-free red filter mosaic from MGS MOC. The image is scaled
so that black tones represent a decrease in albedo of -0.06 (I/F) and
white represents an increase of +0.03. The lower image shows a similar
result, produced by subtracting the Viking IRTM broadband albedo from
the MGS TES broadband albedo, stretched to portray a range of albedo
changes from -0.09 to +0.09. The images are presented in equal area map
projections from 60 S to 60 N, the limits of reliable IRTM coverage.
Both of the data products indicate darkening at higher latitudes,
particularly in the southern hemisphere, and brightening at tropical
latitudes.

3\. Locations of large-scale surface changes. Boxes outlining the major
surface changes described in the text are overlain on an MGS MOC red
filter mosaic (top), a gray-scale rendition of MOLA topography (middle)
and a map of thermal inertia from MGS TES (bottom). Note that all of the
major surface changes took place in areas of moderate to high thermal
inertia. The individual albedo features discussed in Section 4 are as
follows: 1. Syrtis Major. 2. Nilosyrtis and Alcyonius. 3. Hyblaeus. 4.
Cerberus. 5. Propontis. 6. Southern Tropical Dark Band (Mare Cimmerium -
Mare Sirenum). 7. Southern High-Latitude Band (Mare Chronium - Mare
Australis) at Phaethontis Quadrangle. 8. Aonis Sinus. 9. Solis Lacus.
10. Acidalia.

4\. Surface changes at Nilosyrtis and Alcyonius. Nilosyrtis is the name
given to the dark terrain in the northern part of this region. Alcyonius
is the isolated dark patch to the southeast of Nilosyrtis. Both of these
low albedo zones enlarged between the time of the Viking images (top)
and the MGS observations (bottom). Note the complex and sharply defined
albedo boundaries dividing bright and dark terrain. These images span an
east-west distance of 2440 km at 40°N latitude. The MGS mosaic shown is
from Caplinger (2002).

5\. Surface changes at Nilosyrtis and Alcyonius during the Viking era.
Two Viking Orbiter views taken on orbits 605A and 347S show changes in
albedo patterns already underway. Earlier Mariner 9 observations showed
Alcyonius buried, indicating that the surface changes in this region
have been going on for at least three decades.

6\. Dust-devil tracks in Nilosyrtis. A portion of a MOC NAC image (left)
shows a close-up view of recently darkened terrain in western Nilosyrtis
at 54.28°N, 292.69°W. The surface in the darkest parts of the picture
(near the middle of the footprint plot on the MOC WAC context image, at
right) is crisscrossed with filamentary linear streaks that are
diagnostic of dust-devil tracks. The width of the NAC image is 3.4 km,
and the context image is 133 km across. These images were processed by
Malin Space Science Systems.

7\. Nilosyrtis dust-devil track locations. A survey of MOC NAC images
from mission phases AB1 to E06 identified dust devil tracks in 107 NAC
images (solid black rectangles). Ambiguous cases, including vague or
partially erased tracks, are shown as open rectangles. Definitive
nondetections were made in another 180 NAC images (white rectangles).
The dust devil tracks are strongly zoned during all seasons, confined to
latitudes \~45°N to \~65°N. In areas such as Alcyonius that darkened
outside of this latitude band, dust devil tracks are absent and the
surface is pervaded instead by linear wind streaks indicative of
persistent winds.

8\. Surface changes at Cerberus. This classical albedo feature largely
disappeared during the interval between the Viking (top) and MGS
observations (bottom). The linear bright and dark wind streaks with
diffuse albedo boundaries are indicative of erosion by persistent winds.
These images span an east-west distance of 1700 km at 10°N latitude. The
MGS mosaic shown is from Caplinger (2002).

9\. Cerberus close-up. This portion of a MOC NAC image centered at
13.96°N, 199.51°W (left) shows dark-floored craters and incipient
erosional streaks that suggest a thin dust cover. The dust within the
largest crater is organized into dune-like ripples; elsewhere in
Cerberus, such dust-filled craters serve as sources for bright streaks.
The context image (right) shows dark erosional streaks that originate at
topographic irregularities such as craters and knobs. The width of the
NAC image is 3.0 km, and it is excerpted from a point 1/3 of the way
from the top of the footprint plot shown in the context image. These
images were processed by Malin Space Science Systems.

10\. Surface changes in Phaethontis Quadrangle. A broad region with
indistinct boundaries south of 50°S darkened between the imaging
observations of Viking (top) and MGS (bottom). This area is part of a
large, newly formed dark ring that encircles the south pole. Farther
north, both darkening and brightening have taken place at subtropical
latitudes from 30°S to 40°S. The images span an east-west distance of
2230 km at 50°S latitude.

11\. Phaethontis dust-devil track locations. The distribution of
dust-devil tracks in Phaethontis Quadrangle sampled during southern
summer is shown superimposed on the Viking image mosaic. Black
rectangles indicate the locations of NAC images in which dust-devil
tracks were observed, whereas white rectangles show the locations of
definite nondetections. The size of each rectangle indicates the
uncertainty in its location rather than the density of dust-devil
tracks. This cursory survey suggests that dust-devil tracks are
concentrated in the mid- to high-latitudes from 45°S to 65°S.

12\. Phaethontis close-up. Many of the dark-floored craters in
Phaethontis Quadrangle are occupied by fields of dark dunes that dwarf
the bright ripples made by dust. This example is at 52.87°S, 144.42°W.
The dune field is huddled to the northwest side of a small crater within
a larger crater. Tendrils of sand make their way up gullies on the
northwest side of the small crater. The northwest side of the larger
exterior crater is blackened, and an outlying dune appears on the
exterior crater wall. The MOC NAC image is 3.0 km across. These images
were processed by Malin Space Science Systems.

13\. Phaethontis dune locations. The locations of dark dune fields in
Phaethontis Quadrangle identified in MOC NAC images up to phase R09 are
shown superimposed on the Viking image background. Dark dunes are
scattered throughout the area that darkened between Viking and MGS
imaging observations. Many of the splotch craters that currently lack
high-resolution coverage are likely to contain dunes as well.

14\. Surface changes in southern Acidalia. The albedo boundary dividing
bright and dark surfaces advanced towards the southwest during the
interval between Viking (top) and MGS (bottom) imaging observations.
Linear wind streaks and diffuse albedo boundaries indicate erosion by
persistent southwesterly winds. These images span an east-west distance
of 2700 km at 30°N latitude. The MGS mosaic shown is from Caplinger
(2002).

15\. Acidalia dust-devil track locations. A cursory survey of northern
summertime high resolution MOC NAC images detected no dust-devil tracks
along the recently darkened southern margin of Acidalia. In common with
the Nilosyrtis region, dust-devil tracks in Acidalia were found only in
higher latitudes ranging from \~50°N to \~65°N. At lower latitudes,
where most of the darkening took place, the albedo is controlled by
bright and dark linear streaks that taper towards the southwest.

16\. Acidalia close-up. A close-up view of the newly darkened terrain at
26.10°N, 42.91°W shows that most of the bright dust has been removed
from the region already. The little dust that remains is concentrated in
the sheltered floors of impact craters, where it forms small dune-like
ripples. Bright streaks extend towards the southwest, downwind from the
dust-filled craters. These images were processed by Malin Space Science
Systems.