316 Encyclopedia of the Solar System
TABLE 1 Mars MissionsMission Nation Launch Date FateMariner 4 US 11/18/1964 Flew by 7/15/1965; first close-up images
Mariner 6 US 2/24/1969 Flew by 7/31/1969; imaging and other data
Mariner 7 US 3/27/1969 Flew by 8/5/1969; imaging and other data
Mars 2 USSR 5/19/1971 Crash landed; no surface data
Mars 3 USSR 5/28/1971 Crash landed; no surface data
Mariner 8 US 5/8/1971 Fell into Atlantic Ocean
Mariner 9 US 5/30/1971 Into orbit 11/3/1971; mapped planet
Mars 4 USSR 7/21/1973 Failed to achieve Mars orbit
Mars 5 USSR 7/25/1973 Into orbit 2/12/1974; imaged surface
Mars 6 USSR 8/5/1973 Crash landed
Mars 7 USSR 8/9/1973 Passed by Mars
Viking 1 US 8/20/1975 Landed on surface 7/20/1976; orbiter mapping
Viking 2 US 9/9/1975 Landed on surface 9/3/1976; orbiter mapping
Phobos 1 USSR 7/7/1988 Lost 9/2/1988
Phobos 2 USSR 7/12/1988 Mars and Phobos remote sensing
Mars Observer US 9/22/1992 Lost during Mars orbit insertion
Pathfinder US 12/4/1996 Landed 7/4/1997; lander and rover data
Global Surveyor US 11/7/1996 Into orbit 9/11/1997; imaging and other data
Mars Odyssey US 4/7/2001 In orbit 10/24/2001; imaging, remote sensing
Spirit Rover US 6/10/2003 Landed in Gusev 1/3/2004
Opportunity Rover US 7/7/2003 Landed in Meridiani 1/24/2004
Mars Express Europe 6/2/2003 In orbit 12/25/2003; imaging, remote sensing
Mars Reconnaissance
OrbiterUS 8/12/2005 In orbit 3/10/2006; imaging, remote sensingmore Earth-like than Moon-like.Mariner 4was followed
by two moreMarinerspacecraft in 1969 (Table 1), which
seemed to confirm Mars’ lunar-like characteristics. How-
ever, our perception of Mars changed dramatically in 1972
when systematic mapping by theMariner 9orbiter space-
craft revealed the planet that we know today. As mapping
progressed, huge volcanoes, deep canyons, enormous dry
riverbeds, and extensive dune fields came into view, and a
complex, variegated geologic history became apparent. Ex-
ploration of Mars continued in the 1970s as both the USSR
and the United States sent landers to the surface and other
vehicles to the planet. Exploration in the 1970s culminated
with theVikingmission, which successfully placed two lan-
ders on the surface and two other spacecraft into orbit. By
the end of theVikingmission, almost all the surface had
been photographed from orbit at a resolution of about 250
m/pixel and small fractions with resolutions as high as 10
m/pixels. In addition, theVikinglanders had carried out a
variety of experiments directed mostly toward detecting life
and understanding the chemistry of the soil.
In the early 1980s, our understanding of Mars was fur-
ther enhanced when it became clear that we had samples of
Mars in our meteorite collections here on Earth. A group
of meteorites called SNCs (which stands for Shergotty–
Nakhla–Chassigny and is pronounced snicks) were initially
suspected of being of martian origin because they were
basaltic and were 1.3 billion years old. These meteorites
could not have come from the Earth because their oxy-
gen isotope ratios are distinctively different from terrestrial
ratios. The only plausible body that could have been vol-
canically active at that time and supplied the meteorites
was Mars. A martian origin was later confirmed by finding,
trapped within the meteorites, gasses that are identical in
composition to those in the martian atmosphere as mea-
sured by theVikinglanders. The meteorites are believed
to have been ejected from Mars by large impacts and sub-
sequently captured by Earth after spending several million
years in space. We have since added to the collection, and
there are now about 30 known martian meteorites.
All these meteorites are basaltic, and all but one have
ages significantly less than the age of the planet. The ex-
ception, ALH84001, is 4.5 billion years old. In 1996, it
was tentatively suggested that carbonate globules within the
meteorite, together with some disequilibrium mineral as-
semblages, polycyclic aromatic hydrocarbons (PAHs), and a
number of different types of very small segmented rods that
resemble some terrestrial nanofossils, might be the result
of biologic activity. This suggestion has, however, received