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white polar caps as ice caps as Earth has. In the early part of tensive measurements of the Martian atmosphere, temperature
the twentieth century, the American astronomer who founded ranges, and chemistry. For about a year, Mariner 9 sent a flood
the Lowell Observatory in Arizona, Percival Lowell, published a of new and surprising information about Mars back to Earth.
series of popular books showing a network of hundreds of canals Mariner 9 found the surface of Mars not to be a crater- pitted
on Mars. Lowell and other respected astronomers interpreted surface as is found on the Moon. Mars has had a geologically ac-
what they believed to be canals as evidence of intelligent life on tive past and has four provinces, or regions, of related surface
Mars. Still other astronomers, however, interpreted the green- features. There are (1) volcanic regions with inactive volcanoes,
ish colors and the canals to be illusions, imagined features of one larger than any found on Earth, (2) regions with systems of
astronomers working with the limited telescopes of that time. canyons, some larger than any found on Earth, (3) regions of
Since canals never appeared in photographs, said the skeptics, terraced plateaus near the poles, and (4) flat regions pitted with
the canals were the result of the human tendency to see patterns impact craters. Surprisingly, dry channels suggesting former wa-
in random markings where no patterns actually exist. ter erosion were discovered near the cratered regions. These are
This speculation ended in the late 1960s and early 1970s sinuous, dry riverbed features with dry tributaries. At one time,
with extensive studies and probes by spacecraft (Table 15.3). Mars must have had an abundance of liquid water. Liquid wa-
Limited photographs by Mariner flybys in 1965 and 1969 had ter may have been present on Mars in the past, but none is to
provided some evidence that the surface of Mars was much like be found today. However, scientists using instruments on NASA’s
the Moon, with no canals, vegetation, or much of anything else. Mars Odyssey spacecraft found strong signals that ice—perhaps
Then in 1971, Mariner 9 became the first spacecraft to orbit enough to twice fill Lake Michigan—lies just beneath the surface.
Mars, photographing the entire surface as well as making ex- The atmosphere of Mars is very thin, exerting an average
pressure at the surface that is only 0.6 percent of the average
atmospheric pressure on Earth’s surface. Moreover, this thin
Martian atmosphere is about 95 percent carbon dioxide, and
TABLE 15.3 20 percent of this freezes as dry ice at the Martian South Pole
every winter.
Completed spacecraft missions to Mars
Does life exist on Mars? Two Viking spacecraft were sent to
Date Name Owner Remark Mars in 1975 to search for signs of life. The two Viking spacecraft
were identical, each consisting of an orbiter and a lander. After
Nov 5, 1964 Mariner 3 U.S. Flyby
11 months of travel time, Viking 1 entered an orbit around Mars in
Nov 28, 1964 Mariner 4 U.S. First photos
June 1976 and spent a month sending high- resolution images of
Feb 24, 1969 Mariner 6 U.S. Flyby
the surface back to Earth. From these images, a landing site was
Mar 27, 1969 Mariner 7 U.S. Flyby
selected for the Viking 1 lander. Using retrorockets, parachutes,
May 19, 1971 Mars 2 U.S.S.R. Lander
and descent rockets, the Viking 1 lander arrived on a dusty, rocky
May 28, 1971 Mars 3 U.S.S.R. Orbiter/lander slope in the southern hemisphere on July 20, 1976. The Viking 2
May 30, 1971 Mariner 9 U.S. Orbiter lander arrived 45 days later but farther to the north. The Viking
Jul 21, 1973 Mars 4 U.S.S.R. Probe lander contained a mechanical soil-retrieving arm and a minia-
Jul 25, 1973 Mars 5 U.S.S.R. Orbiter ture computerized lab to analyze the soil for evidence of metabo-
Aug 5, 1973 Mars 6 U.S.S.R. Lander lism, respiration, and other life processes. Neither lander detected
Aug 9, 1973 Mars 7 U.S.S.R. Flyby/lander any evidence of life processes or any organic compounds that
would indicate life now or in the past.
Aug 20, 1975 Viking 1 U.S. Lander/orbiter
The Viking spacecraft continued sending images and weather
Sep 9, 1975 Viking 2 U.S. Lander/orbiter
data back to Earth until 1982. During their six-year life, the or-
Jul 7, 1988 Phobos 1 U.S.S.R. Orbiter/Phobos
lander biters sent about 52,000 images and mapped about 97 percent of
the Martian surface. The landers sent an additional 4,500 images,
Jul 12, 1988 Phobos 2 U.S.S.R. Orbiter/Phobos
lander recorded a major “Marsquake,” and recorded data about regular
dust storms that occur on Mars with seasonal changes.
Nov 7, 1996 Global Surveyor U.S. Orbiter
Some answers about the geology and history of Mars were
Dec 4, 1996 Pathfinder U.S. Lander/Surface
rover provided by Mars Pathfinder. On July 4, 1997, a Pathfinder
lander and rover started sending images and data from the Ares
Oct 23, 2001 2001 Mars Odyssey U.S. Orbiter
Vallis area of Mars. The lander served as a base communications
Dec 25, 2003 Mars Express ESA Orbiter/Lander
station, but it also had cameras and measurement instruments of
Jan 4, 2004 Spirit U.S. Lander/Surface
rover its own. The first vehicle to roam the surface of another planet, a
skateboard-sized rover named Sojourner, was designed to move
Jan 25, 2004 Opportunity U.S. Lander/Surface
rover about and analyze the chemical makeup of the surface of Mars.
It was programmed to move from rock to rock by instructions
Mar 10, 2006 Mars Reconnaissance U.S. Orbiter
Orbiter relayed to it through the lander, then send analysis information
May 25, 2008 Phoenix Mars Lander U.S. Lander back to Earth—again through the lander. This provided data of
the geochemistry and petrology of soils and rocks, which in turn
15-7 CHAPTER 15 The Solar System 383