VIKING STEREO IMAGES GIVE US FIRST 3-D VIEW OF THE SOUTH POLE OF MARS


Oblique perspective view
of the South Pole of Mars
SCIENCE CONTACTS:

P.M. Schenk
Lunar & Planetary Institute, Houston, TX 77058
281 486-2157
(schenk@lpi.usra.edu)

J.M. Moore
NASA Ames Research Center, Moffett Field, CA 94035
650 604-5529
(jmoore@mail.arc.nasa.gov)


Stereo images of Mars obtained 20 years ago have provided the first detail view of the topography of the south polar region of Mars (shown above in a perspective view from a new 3-D animation of the region) and the Mars Polar Lander landing zone.

These 3-D views and new topographic maps reveal that the thickness and volume of icy materials at the south pole is roughly similar to that of the icy deposits found at Mars' north pole. Together, the north and south polar deposits account for less than 1/5th of the water inferred to have flooded Mars in the ancient past. These new data indicate that most of these waters must have disappeared to somewhere other than the two polar caps.


Color Mosaic and Geologic Map of South Polar Region of Mars

Figure 1a

Color Mosaic of the South Polar region of Mars. This image is nearly 3000 kilometers wide. The bright region is permanently covered by carbon dioxide ice. Extensive layered deposits surround this ice cap. In early December 1999, Mars Polar Lander (MPL) will land within the curved box.

Figure 1b

Geologic Map of the South Polar region of Mars (simplified from Tanaka and Scott, 1987). White areas are residual, or permanent, ice deposits; blue areas are layered deposits; pink and purple areas are smooth plains material; the dark brown arc is the rim of the 850-kilometer-wide Prometheus impact basin.


The new elevation maps of the south pole were produced from Viking Orbiter stereo images taken in 1977 and 1978. Drs. Paul Schenk of the Lunar & Planetary Institute (Houston) and Jeffrey Moore of NASA Ames Research Center (Moffett Field, CA), will report their findings at the 30th Lunar and Planetary Science Conference, Houston, TX, on March 18, 1999.

Unused before now, these Viking stereo data were reexamined in order to determine the thickness and volume of these deposits. They were also reexamined in order to provide the earliest possible topographic data to assist in selecting a safe landing site for the Mars Polar Lander (MPL'98). MPL is due to land near the south pole in December 1999 to search for water and carbon dioxide ice, but MOLA (the altimeter onboard Mars Global Surveyor) will not begin mapping the region until this spring and summer. The Viking data also fill an expected gap in the MOLA data south of 86 S latitude, which includes part of the residual ice deposits.


3-D Views of the Mars Polar Dome and the Mars Polar Lander Landing Zone

Figure 2

3-D stereo view of the south polar cap of Mars. The bright material is ice, most likely frozen carbon dioxide (dry ice).

Vertical exaggeration factor=5
Vertical resolution=80 meters.

Figure 3

3-D stereo view of part of the MPL landing site. Images obtained in martian spring. Bright patches are carbon dioxide frost. These frost deposits, while confusing in monoscopic images, provide high-contrast patterns that facilitate topographic discrimination in stereo analyses.

Vertical exaggeration factor=2
Vertical resolution=160 meters.

Figure 4

3-D stereo view of the Mars Polar Lander landing site region. These images were obtained in the frost-free martian southern summer.

Vertical exaggeration factor=4
Vertical resolution=300 meters.


From these stereo views, Schenk and Moore were able to construct topographic relief maps of the south polar region using computer software designed at LPI to extract topography from stereo data. These maps have spatial resolutions of 1.5 to 3 kilometers and vertical resolutions of 80 to 160 meters.

The most prominent feature in the new topographic map is a broad convex dome approximately 500 kilometers in diameter with a maximum height of 3 kilometers above the surrounding plains. This dome, referred to here as the South Polar Dome (SPD) but also known as "The Wart", extends nearly 100 kilometers beyond the bright permanent ice cap. The maximum elevation of the south polar dome is located ~250 kilometers from the south pole itself.

The South Polar Dome is abruptly truncated along one margin by a prominent 1 kilometer high scarp. This scarp is flanked by a broad 500-meter-de ep and 100-kilometer-wide moat-like depression. The scarp and depression are interpreted as evidence that the layered deposits were originally much more extensive but have retreated due to erosion. Evidence for retreat of the North Polar Cap has also been cited by the MOLA team.


Topographic Maps of the South Polar Region of Mars

Figure 5a

Digital Elevation Model of the South Polar Cap of Mars. Brighter areas are higher in elevation. Red outlines correspond to geologic boundaries; curved box is MPL landing zone.

Figure 5b

Color-coded Digital Elevation Model of the South Polar Cap of Mars. Red is high, blue is low. Rougly 3 kilometers of relief is portrayed.

Figure 6

Sketch map showing major physiographic regions of the south polar region. The medium-weight outline is the edge of the layered deposits. The heavy lines are the exposed rims of impact craters and basins. The circled cross is the south pole of Mars.

Figure 8a

Topographic cross-section across the South Polar Dome and layered deposits. At left are smooth plains, at right are layered deposits.

Figure 8b

Topographic cross-section across the South Polar Dome and layered deposits. In both views, one side of the dome is truncated by a prominent 1 kilometer high scarp.


The dark "spiral" bands that cross the bright ice deposit are shown in these new data to be outward facing scarps several hundred to 1000 meters high. These scarps form a series of descending terraces. The bright residual surface ice deposits are located on these shallow terrace slopes. These scarps also indicate that the south polar dome and layered deposits were originally more extensive. In the north polar cap, similar dark bands have been shown by the MOLA team to be deep asymmetric troughs.


Figure 8c

Topographic cross-section across an outward-facing scarp within the South Polar Dome (SPD) of Mars. and layered deposits. This scarp is approximately 1 kilometer high. The crest of the SPD is to the right.


The South Polar Dome forms only part of the southern layered deposits. Beyond the Dome, the layered deposits form a vast 1000 by 1500 kilometer wide dissected plateau 1 to 2 kilometers high. This plateau would cover an area the size of Oregon. This mega-plateau is smooth to gently undulating, except for a series of outward facing scarps near the outer margins, and several shallow tongue-shaped troughs or reentrants. These scarps and troughs are several hundred meters high and can extend for up to 500 kilometers.


Figure 9

Topographic cross-section across the northen edge of the south polar layered deposits. In this region, the layered deposits form 300 kilometer wide plateau that stands 2 kilometers above the surrounding plains. This plateau has been proposed as a landing site for the Mars Polar Lander

Figure 10

Three views of a 300-kilometer wide plateau in the SP layered deposits. (Left) View obtained in mid-spring. Seasonal frost deposits are seen usually in topographic lows. (Middle) View obtained in mid-summer. Dark deposits are probably sand or dust. Sun is from the bottom. (Right) DTM topographic map of part of this region. Bright values are high. The plateau is shown to be roughly 2 kilometers high.


The troughs and scarps subdivide the layered deposit mega-plateau into a series of smaller plateaus typically 100 to 300 kilometers across. One of the largest of these plateaus (shown above) has been proposed as a landing site for MPL. Our maps suggest that the region is relatively safe (at kilometer scales) for landing.


Figure 11

Topographic cross-section across an closed depressions within the south polar layered deposits. These depressions are up to 1 kilometer deep and may have been formed by wind erosion.


From these topographic maps, Schenk and Moore estimate that the volume of material in the South Polar Dome is 300,000 cubic kilometers. The volume of all the southern layered deposits is approximately 1.5 million cubic kilometers, roughly comparable to that found within the northern layered deposits by the MOLA team.

Vast flood waters once raged over the surface of now-dry Mars. If the southern layered deposits are mostly icy, then the total estimated volume of ice deposits at both the north and south poles of Mars has been doubled to nearly 3 million cubic kilometers, roughly the volume of the Greenland ice cap. This amount of frozen water would still be less than 1/5th of the minimum estimated volume of the ancient northern ocean, proposed to have flooded large parts of Mars. Other hiding places for this missing water would need to be found, either underground or permanently lost to space.