CHAPTER 22 | COMPARATIVE PLANETOLOGY OF VENUS AND MARS 485
from orbit reveal features that suggest subsurface ice. Gullies
leading downhill appear to have been eroded recently, judging
from their lack of craters; these may have been formed by water
seeping from below the surface. Some regions of collapsed terrain
appear to be places where subsurface water has drained away.
Photos taken over a period of years reveal the appearance of
recent landslides that may have been caused by water gushing
from crater walls and carrying debris downhill before vaporizing
completely. Instruments aboard the Mars Odyssey spacecraft
detected water frozen in the soil over large areas of the planet. At
latitudes farther than 60 degrees from the equator, water ice may
make up more than 50 percent of the surface soil.
If you added a polar bear, changed the colors, and hid the
craters in ■ Figure 22-19a, it would look like the broken pack ice
on Earth’s Arctic Ocean. Mars Express photographed these dust-
covered formations near the Martian equator, and the shallow
depth of the craters suggests that the ice is still there, just below
the surface.
Much of the ice on Mars may be hidden below the polar
caps. Radar aboard the Mars Express orbiter was able to penetrate
3.7 km (2.3 mi) below the surface and map ice deposits hidden
below the south polar region (Figure 22-19b). Th ere is enough
water there, at least 90 percent pure, to cover the entire planet to
a depth of 11 meters. Th e Phoenix probe, which landed in 2008
near Mars’s north pole, found water ice not only mixed with the
soil as permafrost, but also as small chunks of pure ice, indicating
that there once was standing water that had frozen in place.
Rovers Spirit and Opportunity were targeted to land in areas
suspected of having had water on their surfaces. Images made
from orbit showed fl ow features at the Spirit landing site, and
hematite, a mineral that forms in water, was detected from orbit
at the Opportunity landing site. Both rovers reported exciting
discoveries, including evidence of past water. Using its analytic
instruments, Opportunity found small spherical concretions of
hematite (dubbed blueberries) that must have formed in abun-
dant water. Later, Spirit found similar concretions in its area. In
other rocks, Opportunity found layers of sediments with ripple
marks and crossed layers showing they were deposited in moving
water (■ Figure 22-20). Chemical analysis of the rocks at the
Opportunity site showed the presence of sulfates much like
Epsom salts plus bromides and chlorides. On Earth, these com-
pounds are left behind when bodies of water dry up. Halfwaya Visual wavelength image bRadar imageOutline of
south polar capc■ Figure 22-19
(a) Like broken pack ice, these formations near the equator of Mars suggest fl oating ice that broke up and drifted apart. Scientists propose that the ice was
covered by a protective layer of dust and volcanic ash and may still be present. (ESA/DLR/F. U. Berlin/G. Neukum) (b) Radar aboard the Mars Express satellite
probed beneath the surface to image water ice below the south polar cap. The black circle is the area that could not be studied from the satellite’s orbit.
(NASA/JPL/ESA/Univ. of Rome/MOLA Science Team/USGS) (c) This view from the Phoenix lander shows the landscape of Mars’s north polar plain, including polygonal
cracks understood to result from seasonal expansion and contraction of ice under the surface. (NASA/JPL-Caltech/University of Arizona.)