486 PART 4^ |^ THE SOLAR SYSTEM
around Mars, Spirit found the mineral goethite, which only
forms in the presence of water.
One amazing bit of evidence of water on Mars is the analysis
of rock samples from the planet. Of course, no astronaut has ever
visited Mars and brought back a rock, but over the history of the
solar system occasional impacts by asteroids have blasted giant
craters on Mars and ejected bits of rock into space. A few of those
bits of rock have fallen to Earth as meteorites, and over 30 have
been found and identifi ed (■ Figure 22-21). Th ese meteorites
include basalts, as you might expect from a planet so heavily
covered by lava fl ows. Th ey also contain small traces of water and
minerals that are deposited by water. Chemical analysis shows
that the magma from which the rocks solidifi ed must have con-
tained up to 1.8 percent water. If all of the lava fl ows on Mars
contained that much water and it was all outgassed, it could cre-
ate a planetwide ocean 20 m deep. Th at isn’t quite enough to
explain all the fl ood features on Mars, but it does show that the
planet once had abundant water on its surface.
Mars has water, but it is hidden. When humans reach Mars,
they will not need to dig far to fi nd water in the form of ice. Th ey
can use solar power to break the water into hydrogen and oxy-
gen. Hydrogen is fuel, and oxygen is the breath of life, so the
water on Mars may prove to be buried treasure. Even more excit-
ing is the realization that Mars once had bodies of liquid water■ Figure 22-20
(a) Rover Opportunity photographed
these hematite concretions (“blueber-
ries”) in a rock near its landing site. The
spheres appear to have grown as miner-
als collected around small crystals in the
presence of water. Similar concretions
are found on Earth. (b) The layers in this
rock were deposited as sand and silt in
rapidly fl owing water. From the way the
layers curve and cross each other, geolo-
gists can estimate that the water was at
least ten centimeters deep. A few “blue-
berries” and one small pebble are also
visible in this image. (NASA.JPL/Cornell/
USGS)bb Visual-wavelength imageVisual-wavelength imagea Visual-wavelength image