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selected sites in great detail while also conducting planet-wide surveys to provide con-

text for interpreting the selected sites, to extrapolate from the intensively investigated

sites to regional and global patterns, and to identify which specific sites make the best

candidates for targeted examination.

One way this balance works is in the combination punch of orbital and surface mis-

sions. Mineral mapping by Mars Global Surveyor identified the hematite deposit that

made Meridiani Planum one of the top-priority targets selected as landing sites for the

Mars Exploration Rovers. The hematite suggested a possible water history. The rover

Opportunity's intensive examination of the composition and fine structure of rocks

where it landed confirmed that the site had been covered with water and added details

about the acidity of the water and the alternation of wet and dry periods at the site.

This "ground truthing" by the rover improves interpretation of current orbiters' observa-

tions of the surrounding region; the orbiters' observations add context for understand-

ing how the environment that the landing-site rocks reveal about a particular place and

time fits into a broader history. 

The mission of the Mars Reconnaissance Orbiter is another manifestation of combining

targeted inspection with wider surveys. This spacecraft will examine selected sites in

greater detail than any previous Mars orbiter -- with high enough resolution to see indi-

vidual rocks as small as some that Opportunity has drilled into, to identify minerals in

deposits no larger than the crater (dubbed "Eagle") where Opportunity landed and to

distinguish between buried layers as thin as about 7.5 meters (25 feet) thick. It will also

observe the entire planet every day and make regional surveys as context for the high-

resolution observations. The mission's goals include both providing information from

orbit about the history of water on Mars and also identifying the best sites for future

landings.

Myths and Reality

Mars caught public fancy in the late 1870s, when Italian astronomer Giovanni

Schiaparelli reported using a telescope to observe "canali," or channels, on Mars. A

possible mistranslation of this word as "canals" may have fired the imagination of

Percival Lowell, an American businessman with an interest in astronomy. Lowell found-

ed an observatory in Arizona, where his observations of the Red Planet convinced him

that the canals were dug by intelligent beings -- a view that he energetically promoted

for many years. 

By the turn of the last century, popular songs envisioned sending messages between

worlds by way of huge signal mirrors. On the dark side, H.G. Wells' 1898 novel "The

War of the Worlds" portrayed an invasion of Earth by technologically superior Martians

desperate for water. In the early 1900s novelist Edgar Rice Burroughs, known for the

"Tarzan" series, also entertained young readers with tales of adventures among the

exotic inhabitants of Mars, which he called Barsoom.

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Fact began to turn against such imaginings when the first robotic spacecraft were sent

to Mars in the 1960s. Pictures from the 1965 flyby of Mariner 4 and the 1969 flybys of

Mariner 6 and 7 showed a desolate world, pocked with impact craters similar to those

seen on Earth's Moon. Mariner 9 arrived in 1971 to orbit Mars for the first time, but

showed up just as an enormous dust storm was engulfing the entire planet. When the

storm died down, Mariner 9 revealed a world that, while partly crater-pocked like

Earth's Moon, was much more geologically complex, complete with gigantic canyons,

volcanoes, dune fields and polar ice caps. This first wave of Mars exploration culminat-

ed in the Viking mission, which sent two orbiters and two landers to the planet in 1975.

The landers included a suite of experiments that conducted chemical tests to detect

life. Most scientists interpreted the results of these tests as negative, deflating hopes of

identifying another world on where life might be or have been widespread. However,

Viking left a huge legacy of information about Mars that fed a hungry science commu-

nity for two decades. 

The science community had many other reasons for being interested in Mars, apart

from the direct search for life; the next mission on the drawing boards concentrated on

a study of the planet's geology and climate using advanced orbital reconnaissance.

Over the next 20 years, however, new findings in laboratories and in extreme environ-

ments on Earth came to change the way that scientists thought about life and Mars.

One was the 1996 announcement by a team from Stanford University and NASA's

Johnson Space Center that a meteorite believed to have originated on Mars contained

what might be the fossils of ancient bacteria. This rock and other Mars meteorites dis-

covered on several continents on Earth appear to have been blasted off the Red

Planet by asteroid or comet impacts. The evidence that they are from Mars comes

from gases trapped in them that unmistakably match the composition of Mars' atmos-

phere as measured by the Viking landers. Many scientists questioned the conclusions

of the team announcing the discovery of possible life in one Martian meteorite, but if

nothing else the mere presence of organic compounds in the meteorites increases the

odds of life forming at an earlier time on a far wetter Mars.

Another development shaping ideas about extraterrestrial life was a string of spectacu-

lar findings on how and where life thrives on Earth. The fundamental requirements for

life as we know it today are liquid water, organic compounds and an energy source for

synthesizing complex organic molecules. In recent years, it has become increasingly

clear that life can thrive in settings much harsher than what we can experience.

In the 1980s and 1990s, biologists found that microbial life has an amazing flexibility

for surviving in extreme environments -- niches that by turn are extraordinarily hot, or

cold, or dry, or under immense pressures -- that would be completely inhospitable to

humans or complex animals. Some scientists even concluded that life may have begun

on Earth in hot vents far under the ocean's surface.

This in turn had its effect on how scientists thought about Mars. Martian life might not

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be so widespread that it would be readily found at the foot of a lander spacecraft, but it

may have thrived billions of years ago in an underground thermal spring or other hos-

pitable environment. Or it might still exist in some form in niches below the currently

frigid, dry, windswept surface, perhaps entombed in ice or in liquid water aquifers.

Each successful Mars mission reads more pages of the planet's story. After years of

studying pictures from the Viking orbiters, scientists gradually came to conclude that

many features they saw suggested that Mars may have been warm and wet in an ear-

lier era. Two decades after Viking, Mars Pathfinder observed rounded pebbles and

sockets in larger rocks, suggesting conglomerates that formed in running water. Mars

Global Surveyor's camera has detected possible evidence for recent liquid water in

many settings, including hundreds of hillside gullies. Mars Odyssey's spectrometers

have found large amounts of ice mixed in with Mars surface materials. Observations by

Global Surveyor and Odyssey have also been interpreted as evidence that Mars is still

adjusting from a recent ice age as part of a repeating cycle of global climate change.

NASA's Mars Exploration Rover Opportunity established that rocks in at least one part

of Mars were formed underneath flowing surface water. Halfway around the planet, its

twin rover, Spirit, also found rocks extensively altered by water. The European Space

Agency's Mars Express has identified exposures of water-related minerals. That

spacecraft and telescopic studies from Earth have found traces of atmospheric

methane at Mars that might come from volcanic or biological sources. 

Three Ages of Mars

Based on what they have learned from spacecraft missions, scientists view Mars as

the "in-between" planet of the inner solar system. Small rocky planetary bodies such

as Mercury and Earth's Moon apparently did not have enough internal heat to power

volcanoes or to drive the motion of tectonic plates, so their crusts grew cold and static

relatively soon after they formed when the solar system condensed into planets about

4.6 billion years ago. Devoid of atmospheres, they are riddled with craters that are

relics of impacts during a period of bombardment when the inner planets were sweep-

ing up remnants of small rocky bodies that failed to "make it as planets" in the solar

system's early times.

Earth and Venus, by contrast, are larger planets with substantial internal heat sources

and significant atmospheres. Earth's surface is continually reshaped by tectonic plates

sliding under and against each other and by materials spouting forth from active volca-

noes where plates are ripped apart. Both Earth and Venus have been paved over so

recently that both lack any discernible record of cratering from the era of bombardment

in the early solar system.

Mars appears to stand between those sets of worlds, on the basis of current yet evolv-

ing knowledge. Like Earth and Venus, it possesses a myriad of volcanoes, although

they probably did not remain active as long as counterparts on Earth and Venus. On

Earth, a single "hot spot" or plume might form a chain of middling-sized islands such

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as the Hawaiian Islands as a tectonic plate slowly slides over it. On Mars there are

apparently no such tectonic plates, at least as far as we know today, so when volca-

noes formed in place they had the time to become much more enormous than the

rapidly moving volcanoes on Earth. Overall Mars appears to be neither as dead as

Mercury and our Moon, nor as active as Earth and Venus. As one scientist quips,

"Mars is a warm corpse if not a fire-breathing dragon." Thanks to the ongoing observa-

tions by current missions, however, this view of Mars is still evolving.

Mars almost resembles two different worlds that have been glued together. From lati-

tudes around the equator to the south are ancient highlands pockmarked with craters

from the solar system's early era, yet riddled with channels that attest to the flow of

water. The northern third of the planet, however, overall is sunken and much smoother

at kilometer (mile) scales. There is as yet no general agreement on how the northern

plains got to be that way. At one end of the spectrum is the theory that it is the floor of

an ancient sea; at the other, the notion that it is merely the end product of innumerable

lava flows. New theories are emerging thanks to the discoveries of Mars Odyssey, and

some scientists believe a giant ice sheet may be buried under much of the relatively

smooth northern plains. Many scientists suspect that some unusual internal process

not yet fully understood may have caused the northern plains to sink to relatively low

elevations in relation to the southern uplands.

Scientists today view Mars as having had three broad ages, each named for a geo-

graphic area that typifies it:

The Noachian Era is the name given to the time spanning perhaps the first 

billion years of Mars' existence after the planet was formed 4.6 billion years ago. 

In this era, scientists suspect that Mars was quite active with periods of warm 

and wet environment, erupting volcanoes and some degree of tectonic activity. 

The planet may have had a thicker atmosphere to support running water, and it 

may have rained and snowed.

In the Hesperian Era, which lasted for about the next 500 million to 1.5 billion 

years, geologic activity was slowing down and near-surface water perhaps was 

freezing to form surface and buried ice masses. Plunging temperatures 

probably caused water pooled underground to erupt when heated by impacts in 

catastrophic floods that surged across vast stretches of the surface -- floods so 

powerful that they unleashed the force of thousands of Mississippi Rivers. 

Eventually, water became locked up as permafrost or subsurface ice, or was 

partially lost into outer space.

The Amazonian Era is the current age that began around 2 billion to 3 billion 

years ago. The planet is now a dry, desiccating environment with only a modest 

atmosphere in relation to Earth. In fact, the atmosphere is so thin that water 

can exist only as a solid or a gas, but only temporarily as a liquid. Scientist are

now learning that over millions of years, the planet can vary its tilt, severely 

altering climate and perhaps stability of water on the surface.

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Apart from that broad outline, there is lively debate and disagreement on the details of

Mars' history. How wet was the planet, and how long ago? What eventually happened

to all of the water? That is all a story that is still being written.

Even if we ultimately learn that Mars never harbored life as we know it here on Earth,

scientific exploration of the Red Planet can assist in understanding the history and evo-

lution of life on our own home world. Much if not all of the evidence for the origin of life

here on Earth has been obliterated by the incredible pace of weathering and global

tectonics that have operated over billions of years. Mars, by comparison, is a compos-

ite world with some regions that may have histories similar to Earth's crust, while oth-

ers serve as a frozen gallery of the solar system's early days. 

Thus, even if life never developed on Mars -- something that we cannot answer 

today -- scientific exploration of the planet may yield critical information unobtainable

by any other means about the pre-biotic chemistry that led to life on Earth. Mars as a

fossil graveyard of the chemical conditions that fostered life on Earth is an intriguing

possibility.

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Historical Mars Missions

Mission, Country, Launch Date, Purpose, Results

[Unnamed], USSR, 10/10/60, Mars flyby, did not reach Earth orbit

[Unnamed], USSR, 10/14/60, Mars flyby, did not reach Earth orbit

[Unnamed], USSR, 10/24/62, Mars flyby, achieved Earth orbit only

Mars 1, USSR, 11/1/62, Mars flyby, radio failed at 106 million km (65.9 million miles)

[Unnamed], USSR, 11/4/62, Mars flyby, achieved Earth orbit only

Mariner 3, U.S., 11/5/64, Mars flyby, shroud failed to jettison

Mariner 4, U.S. 11/28/64, first successful Mars flyby 7/14/65, returned 21 photos

Zond 2, USSR, 11/30/64, Mars flyby, passed Mars but radio failed, returned no planetary data

Mariner 6, U.S., 2/24/69, Mars flyby 7/31/69, returned 75 photos

Mariner 7, U.S., 3/27/69, Mars flyby 8/5/69, returned 126 photos

Mariner 8, U.S., 5/8/71, Mars orbiter, failed during launch

Kosmos 419, USSR, 5/10/71, Mars lander, achieved Earth orbit only

Mars 2, USSR, 5/19/71, Mars orbiter/lander arrived 11/27/71, no useful data, lander burned up due to 

steep entry

Mars 3, USSR, 5/28/71, Mars orbiter/lander, arrived 12/3/71, lander operated on surface for 20 seconds 

before failing

Mariner 9, U.S., 5/30/71, Mars orbiter, in orbit 11/13/71 to 10/27/72, returned 7,329 photos

Mars 4, USSR, 7/21/73, failed Mars orbiter, flew past Mars 2/10/74

Mars 5, USSR, 7/25/73, Mars orbiter, arrived 2/12/74, lasted a few days

Mars 6, USSR, 8/5/73, Mars flyby module and lander, arrived 3/12/74, lander failed due to fast impact

Mars 7, USSR, 8/9/73, Mars flyby module and lander, arrived 3/9/74, lander missed the planet

Viking 1, U.S., 8/20/75, Mars orbiter/lander, orbit 6/19/76-1980, lander 7/20/76-1982

Viking 2, U.S., 9/9/75, Mars orbiter/lander, orbit 8/7/76-1987, lander 9/3/76-1980; combined, the Viking 

orbiters and landers returned 50,000+ photos

Phobos 1, USSR, 7/7/88, Mars/Phobos orbiter/lander, lost 8/88 en route to Mars

Phobos 2, USSR, 7/12/88, Mars/Phobos orbiter/lander, lost 3/89 near Phobos

Mars Observer, U.S., 9/25/92, lost just before Mars arrival 8/21/93

Mars Global Surveyor, U.S., 11/7/96, Mars orbiter, arrived 9/12/97, high-detail mapping through 1/00, 

now conducting third extended mission 

Mars 96, Russia, 11/16/96, orbiter and landers, launch vehicle failed

Mars Pathfinder, U.S., 12/4/96, Mars lander and rover, landed 7/4/97, last transmission 9/27/97

Nozomi, Japan, 7/4/98, Mars orbiter, failed to enter orbit 12/03 

Mars Climate Orbiter, U.S., 12/11/98, lost upon arrival 9/23/99

Mars Polar Lander/Deep Space 2, U.S., 1/3/99, lander and two probes, lost on arrival 12/3/99

Mars Odyssey, U.S., 3/7/01, Mars orbiter, arrived 10/24/01, currently conducting 

extended science mission and providing relay for Mars Exploration Rovers

Mars Express/Beagle 2, European Space Agency, 6/2/03, Mars orbiter/lander, orbiter conducting prime 

mission through November 2005, lander lost on arrival 12/25/03

Mars Exploration Rover Spirit, U.S., 6/10/03, Mars rover, landed 1/4/04 for three-month prime mission 

inside Gusev Crater, currently conducting extended mission.

Mars Exploration Rover Opportunity, U.S., 7/7/03, Mars rover, landed 1/25/04 for three-month prime 

mission in Meridiani Planum region, currently conducting extended mission

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Program/Project Management

The Mars Reconnaissance Orbiter project is managed by the Jet Propulsion

Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate, Washington. 

At NASA Headquarters, Alphonso Diaz is science associate administrator, Douglas

McCuistion is Mars program director, Dr. Michael Meyer is the lead scientist for the

Mars Exploration Program, Dr. Ramon DePaula is program executive for Mars

Reconnaissance Orbiter and Dr. R. Stephen Saunders is program scientist for the

Mars Reconnaissance Orbiter.

At the Jet Propulsion Laboratory, Dr. Fuk Li is the Mars program manager, Dr. Daniel

McCleese is Mars chief scientist, Jim Graf is Mars Reconnaissance Orbiter project

manager and Dr. Richard Zurek is Mars Reconnaissance Orbiter project scientist.

Lockheed Martin Space Systems, Denver, Colo., built the spacecraft and shares opera-

tional roles with JPL. Kevin McNeill is program manager at Lockheed Martin for the

Mars Reconnaissance Orbiter. 

8-02-05

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