Mars (planet)
| I | INTRODUCTION |
Mars
(planet), fourth planet in distance from the Sun in the solar system.
Mars is of special scientific interest because of its similarities to Earth. It
has an atmosphere with seasons and changing weather, and its surface shows
evidence of ancient water and volcanoes. The length of its day and the tilt of
its axis are similar to those of Earth. Mars takes about two years to circle the
Sun at an average distance of 228 million km (141.7 million mi). The possibility
of life on Mars, now or in the distant past, is one of the major questions in
astronomy. More space probes have been sent to Mars than to any other planet.
Mars is named for the Roman god of war. It is sometimes called the red planet
because it appears fiery red in Earth’s night sky, the result of rusty,
iron-oxide mineral dust that covers its surface.
Mars is a relatively small planet, with a
diameter of about 6,794 km (4,222 mi) or about half the diameter of Earth. Mars
has about one-tenth Earth’s mass. The force of gravity on the surface of Mars is
about three-eighths of that on Earth. Mars has twice the diameter and twice the
surface gravity of Earth’s Moon. The surface area of Mars is almost exactly the
same as the surface area of the dry land on Earth. Mars is believed to be about
the same age as Earth, having formed from the same spinning, condensing cloud of
gas and dust that formed the Sun and the other planets about 4.6 billion years
ago.
Mars has two moons, Phobos and Deimos, which
are named after the sons of the Roman god Mars. These tiny bodies are heavily
cratered, dark chunks of rock and may be asteroids captured by the gravitational
pull of Mars. Phobos orbits Mars once in less than one Martian day, so it
appears to rise in the west and set in the east, usually twice each day. Deimos
has the more ordinary habit of rising in the east and setting in the west.
| II | OBSERVATION FROM EARTH |
Mars appears as a fairly bright, red, starlike
object in Earth’s night sky. Because of the relative movements of Earth and Mars
around the Sun, Mars appears to move backward in the sky for a short time around
opposition, which is the time when the two planets are closest. As Mars and
Earth orbit the Sun, the distance between them varies from about 56 million km
(about 35 million mi) at their closest approaches to about 375 million km (about
233 million mi) when the planets are on opposite sides of the Sun. This change
in distance causes the apparent size of Mars to vary by more than a factor of 5
and its brightness to vary by a factor of 25. Because the orbit of Mars is
elliptical and not circular, Earth and Mars approach each other more closely
during some orbits than others. For example, in late August 2003 Earth and Mars
passed closer to each other than at any time since 1924. The two planets will
not get that close again until the year 2287.
When Mars is viewed through a telescope, it
looks like a reddish-orange disk. When Mars is close to Earth, an observer with
a telescope can usually see white ice caps at the north and south poles of Mars.
These polar caps grow and shrink throughout the Martian year, just as the polar
caps of Earth do. The darker areas of Mars’s surface may look greenish to the
telescope observer, but this is an optical illusion caused by the contrast in
color between the dark patches and the redder, brighter areas. Scientists
believe that the dark areas are regions of relatively unweathered dark rocks and
sand, while the bright areas are regions with deposits of dusty, fine-grained
oxidized iron minerals. Scientists now believe that the “canals” people observed
on Mars during the 19th century are actually another optical illusion, caused by
the mind’s tendency to draw connections between irregular patches in a fuzzy
image.
The Hubble Space Telescope (HST) provides the
clearest Earth-based views of Mars, and astronomers use it to study the
composition of the surface and to monitor the weather on the planet. HST has
provided detailed images of local and global dust storms, enormous spiral-shaped
water ice cloud systems, and changes in the bright and dark surface markings
that have occurred since the first detailed images were taken during the 1970s.
The telescope also has enabled spectroscopic measurements that provide
comprehensive information on atmospheric chemistry and on the nature and
variability of ices and minerals on the surface. Using HST images and other
data, astronomers have determined that the atmosphere of Mars is generally
cooler and clearer when the planet is farther from the Sun and warmer and
dustier when it is closer. There also appear to be longer-term trends in the
Martian climate, but as is the case for Earth’s climate, scientists are only now
beginning to untangle the complexities required to understand and perhaps one
day even predict climate changes on Mars. Orbiting spacecraft around Mars
furnish constant data about the planet. However, they orbit so close to the
planet and are in a fixed orientation relative to the Sun that they cannot see
features in the early morning or late afternoon parts of the Martian day. As a
result, astronomers still need telescopes like the HST to study Mars,
particularly its early morning and late afternoon cloud formations.
| III | ORBIT AND ROTATION |
Mars orbits the Sun at an average distance
of about 228 million km (141.7 million mi), or 1.524 astronomical units (AU). An
AU is equal to the average distance between the Earth and the Sun, or about 150
million km (93 million mi). However, Mars’s orbit is more elliptical than
Earth’s—its nearest point to the Sun (perihelion) is about 42 million km (26
million mi) closer than its farthest point (aphelion), compared with only a 5
million km (3 million mi) difference between perihelion and aphelion for
Earth. Mars’s year, or the time it takes to revolve once around the Sun,
is about two Earth years long (687 Earth days). Mars receives less than half the
amount of sunlight Earth does and is much colder.
Mars is tilted on its axis by about 25°
(Earth is tilted at 23.5°). This tilt gives Mars seasons similar to Earth’s
seasons. The elliptical orbit of Mars, however, causes the planet to have
seasons of unequal lengths. For example, the southern hemisphere’s summer on
Mars is about 25 days shorter than the northern summer. The intensity of
sunlight also changes substantially during the Martian year: solar heating
during the southern summer, when Mars is closer to the Sun, is 40 percent more
intense than in the northern summer. During the warmer spring and summer period
in the southern hemisphere, great dust storms have sometimes been observed
through telescopes as bright yellow clouds. Sometimes white clouds of water
vapor are visible, especially during the northern summer when Mars is near its
farthest point from the Sun and its thin atmosphere is the coldest.
Like Earth, Mars turns counterclockwise on
its axis (from west to east) when seen from its north pole and orbits the Sun in
a counterclockwise direction. It takes Mars 24 hours and 37 minutes to rotate
once on its axis (its sidereal day). Its solar day (the time between when the
Sun next crosses the noon point in the sky) is about 24 hours and 39 minutes—its
orbital motion around the Sun adds two minutes to its rotation period. (Earth’s
solar day (24 hours) is four minutes longer than its rotation period.) The
Martian solar day is sometimes called a sol.
| IV | THE INTERIOR OF MARS |
The density of Mars is about 30 percent less
than that of Earth (3.94 g/cm3 vs. 5.52 g/cm3). Based on
spacecraft measurements of the Martian gravitational field,
scientists believe that the planet’s interior consists of a crust, mantle, and
core like Earth’s interior. While the relative sizes of these components are not
known for certain, the planet’s lower density combined with spacecraft mapping
of the structure of its gravity field suggest that the planet’s iron-rich core
and mantle are a smaller fraction of its volume than in the case of Earth. Mars
therefore probably has a relatively thick crust compared to Earth. Beneath the
Tharsis bulge, an area of volcanic activity in the northern hemisphere, the
crust may be as thick as 130 km (80 mi). But the crustal thickness appears to
vary significantly. For example, beneath the landing site of the United States
spacecraft Viking 2, it may be as thin as 15 km (9 mi).
The Martian core is probably much like
Earth’s, consisting mostly of iron, with a small amount of nickel. If other
light elements, particularly sulfur, exist there as well, the core may be larger
than presently thought. From studying Earth’s magnetic field and core,
scientists theorize that the motions of the liquid rock in Earth’s core generate
its magnetic field. Mars does not have a significant magnetic field, so
scientists believe that Mars’s core is probably solid. However, spacecraft data
indicate that Mars probably did have a strong magnetic field early in its
history, suggesting that the core of Mars may have been at least partially
liquid at one time.
Tectonics on the Earth is dominated by the
relative motions and collisions of a few dozen large, moving lithospheric
plates. Earthlike plate tectonics does not appear to be active on Mars today.
However, there is considerable debate over whether Mars may have had plate
tectonics in the distant past, when the core may have been molten. Ancient
magnetic field patterns preserved in the crust show some similarities to
magnetic field patterns that arise from plate tectonic processes on Earth.
Because Mars is so much smaller than Earth, however, its more rapid cooling and
crustal thickening after formation may have favored the creation of a one-plate
planet rather than Earthlike plate tectonics.
Heat that melted at least some of the Martian
interior has sculpted parts of the planet’s surface. In some places molten rock
broke through the crust to form volcanoes. In other places, large-scale motions
of the partially molten mantle cracked the crust to form large rifts and canyon
systems. Scientists do not know if the interior of Mars is still geologically
active. No evidence for active volcanism or tectonic movement has been found on
the planet. However, images from orbiting spacecraft suggest that some of the
Tharsis volcanoes have been periodically active in the last 100 to 350 million
years, and perhaps as recently as 2 million years ago. Smaller volcanic cones
discovered around the north pole may have erupted as recently as 1 million years
ago.
Additional details about the Martian interior
may have to await a time when more sophisticated spacecraft or even astronauts
bring instruments such as seismometers to the planet, providing information
similar to that which scientists routinely obtain for Earth’s interior
today.
| V | THE SURFACE OF MARS |
The surface of Mars would be a harsh place for
humans, but it is more like the surface of Earth than that of any other planet.
The temperature on Mars never gets much warmer than the temperature at
Antarctica, and it is usually much colder. At the surface the average
temperature is about -55°C (about -67°F) and at the extremes it ranges from
about -140° to 15°C (about -225° to 60°F). The surface’s famous reddish color
comes from iron oxide minerals in the dust, similar to rust on Earth. The most
interesting surface features of Mars include two very distinct hemispheres, an
enormous bulge called Tharsis littered with volcanoes and cut by an enormous
rift valley, channels apparently carved by water, and polar ice caps similar to
Earth’s.
| A | Distinct Hemispheres |
The northern and southern hemispheres of
Mars have different characteristics. The southern hemisphere has many impact
craters and has a generally much higher elevation than the northern hemisphere.
The southern highlands are probably the oldest terrain on Mars, dating back to
the early history of the solar system when large impact events were much more
common than they are today. The southern highlands, with their pervasive
craters, resemble the surface of the Moon.
Hellas Planitia is a giant impact basin in
the southern hemisphere. The impact of a large asteroid formed the basin long
ago. At 6 km (3.8 mi) deep and with a diameter of about 2,000 km (about 1,250
mi), it is the largest and deepest basin on Mars. A few other large basins and
thousands of large craters can be found on the surface, mostly concentrated in
the lunar-like southern highlands.
The northern hemisphere of Mars contains a
much wider variety of geologic features, including large volcanoes, a great rift
valley, and a variety of channels. The northern hemisphere also contains large
expanses of relatively featureless plains. Radar and topographic studies of the
northern hemisphere by Mars orbiters have revealed ancient impact craters
beneath the plains, however, indicating that the underlying crust may be the
same age as the southern highlands. Astronomers do not know why the northern and
southern hemispheres of Mars are now so different; figuring out the reason is an
important goal of Mars exploration.
| B | The Tharsis Bulge |
Mars has an enormous bulge in its surface
called Tharsis. Tharsis is 10 km (6 mi) high and 4,000 km (2,486 mi) wide, and
contains giant volcanoes and valleys. The largest volcano in the solar system,
Olympus Mons, is located in the Tharsis region. It is over 21 km (13 mi) high
(more than twice as high as Earth’s Mount Everest) and covers an area comparable
to the state of Arizona. Near it, three other volcanoes almost as large—Arsia
Mons, Pavonis Mons, and Ascraeus Mons—form a line running from southwest to
northeast. These four volcanoes are the most noticeable features of Tharsis.
Another volcano, Alba Patera, is also part of the Tharsis bulge but is quite
different in appearance. It is probably less than 6 km (4 mi) high but has a
diameter of more than 1,600 km (1,000 mi). None of these volcanoes appears to be
presently active, but there is some evidence of small eruptions in the last 100
to 350 million years, and perhaps as recently as 2 million years ago.
The Tharsis bulge has had a profound effect
on the appearance of the surface of Mars. It includes many smaller volcanoes and
stress fractures in addition to the large volcanoes. Its presence affects the
weather on Mars and its formation may have changed the climate by changing the
rotational axis of the planet. Valles Marineris (named for the U.S. Mariner
spacecraft that discovered it) is the most notable stress feature associated
with the Tharsis bulge. It is a great rift valley and interconnected canyon
system extending from the Tharsis region to the east-southeast. Valles Marineris
is about the same length as the distance from New York to California (about
4,000 km or 2,500 mi). This canyon system reaches widths of 700 km (440 mi) and
depths of 7 km (4 mi) in some places. High-resolution spacecraft images have
revealed a spectacular variety of layered landforms in and around the canyon
system. These layers may represent different episodes of volcanic eruptions, or
they may be sedimentary deposits laid down when the canyons were possibly
water-filled. The origin of this enigmatic layering on Mars is presently
unknown, but most astronomers agree that understanding it will be critical to
understanding the history of the planet.
| C | Water Channels |
Two main types of channels, valley networks
and outflow channels, can be found on Mars. Both were probably formed by the
action of liquid water. These channels are unrelated to the “canals” thought to
be seen in early telescopic views of Mars.
Valley networks are similar in general
appearance to streambeds on Earth and occur in the southern highlands. These
channels may date from a time early in Mars’s history when the atmosphere was
thicker and liquid water could flow readily on or near the surface.
High-resolution images reveal important differences between these Martian valley
networks and terrestrial valley networks, however. Specifically, Martian valley
networks do not appear to have formed from rainfall or surface runoff, but
instead may have formed primarily from the action of underground liquid water. A
small number of valley networks, however, observed at the highest resolution by
Mars Global Surveyor, look like they may have been formed from rainfall or
surface runoff. Mars Global Surveyor images of Eberswalde Crater southeast of
the Valles Marineris canyon system also show a fan-shaped deposit that closely
resembles a river delta, further suggesting that water sometimes flowed for an
extended period of time.
Outflow channels, formed by giant
floods, occur primarily on the boundary between the southern highlands and the
northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft
landed in 1997, is one of these outflow channels. An important difference
between outflow channels and valley networks is that outflow channels appear to
have been formed quickly by the sudden and catastrophic release of enormous
volumes of liquid water, with no particular requirements on climatic
conditions.
Small-scale water events may still be
occurring on Mars. Outflows of liquid water may have formed gullies seen on the
walls of craters by the Mars Global Surveyor. Comparison of images taken between
1999 and 2005 showed fresh flows of bright material on the inner walls of
several small craters. These flows could result from subsurface liquid water
erupting onto the surface. Under current conditions of extreme cold and low air
pressure, liquid water cannot exist for long on the surface. Water mixed with
salts that lowered its freezing point or water erupting after being under
pressure from overlying rock layers might flow for a short distance,
however.
| D | Ice Caps |
Mars has small, permanent ice caps at its
north and south poles that increase in size with the addition of seasonal ice
caps during the winter of each hemisphere. The polar caps in the north and south
are quite different from one another. The northern permanent ice cap is composed
of water ice and is about 1,000 km (about 620 mi) across. A seasonal cap of
frozen carbon dioxide adds to the northern ice cap in the northern winter. The
southern permanent ice cap is one-third the diameter of the northern cap because
summer in the southern hemisphere is warmer than in the north. The southern
seasonal cap is larger than the northern cap—more carbon dioxide is frozen out
in the south than the north because Mars is farthest from the Sun, and therefore
coldest, in the southern winter. Carbon dioxide may also make up some of the
southern permanent cap. Radar on the Mars Express found evidence for deep layers
of frozen water under the south pole. If the amount of ice apparently indicated
was melted, it could cover the entire planet in almost 11 m (36 ft) of
water.
Both polar caps and their surrounding
deposits show spectacular, fine-scale striped layering of dust, rock, and ice to
the limits of the resolution of the best available pictures. Like similar
layering found in Earth’s polar regions, these Martian polar layers may provide
evidence of both short-term and long-term changes in the planet’s climate. The
true origin of the Mars polar layering is unknown at present, but it may have
been caused by climate cycles similar to ice ages on Earth. Understanding the
polar layering is yet another important motivator for continued exploration of
the planet.
| E | Subsurface Water Ice |
Instruments on the Mars Odyssey orbiter
suggest that large deposits of water ice may lie below the Martian surface in
the mid-latitudes. The data show that 50 percent or more of the uppermost meter
of the soil may be made up of ice in some places. The depth and thus the total
amount of this subsurface ice are not known, however. Ice may also exist in the
subsurface near the equator. A region of Elysium Planitia near the equator has
been interpreted from Mars Express images as a possible “frozen sea” (resembling
pack ice seen on Earth) covered in dusty soil or volcanic ash only a few
centimeters thick. However, the features in this region could also be platy or
fractured lava flows without any associated subsurface ice.
| VI | THE ATMOSPHERE OF MARS |
| A | Composition |
The atmosphere of Mars is 95 percent
carbon dioxide, nearly 3 percent nitrogen, and nearly 2 percent argon with tiny
amounts of oxygen, carbon monoxide, water vapor, ozone, and other trace gases.
Earth’s atmosphere is 78 percent nitrogen and 21 percent oxygen, with about 1
percent argon and only 0.03 percent carbon dioxide. The larger relative
proportion of argon in the air on Mars may indicate that its atmosphere was much
thicker in the past. Compared to other gases, argon is a relatively heavy gas
that is not as easily lost into space over time. As a noble gas, argon does not
combine chemically with other substances in the atmosphere or on the
surface.
In 2004 scientists reported detecting
small amounts of methane in the Martian atmosphere, at about 10 parts per
billion. The observations are controversial, however, because such small levels
of abundance are involved. If the methane abundance is real, then some recent or
ongoing process on Mars may be releasing fresh methane, because otherwise the
gas would quickly break down from the ultraviolet radiation from the Sun.
Possible sources for the methane on Mars could include volcanic or hydrothermal
activity, chemical reactions between water and minerals in the crust, ancient
deposits of methane ice, or even biological activity. However, there is
currently no strong evidence for active volcanic or hydrothermal processes on
Mars, which should also release sulfur dioxide (SO2), a gas not yet
detected there. The apparent concentration of methane in places where subsurface
water ice is thought to be present might argue more for a water-rock chemical
reaction origin. At present there is no real consensus on the presence or origin
of methane on Mars.
The pressure of the Martian atmosphere
varies with the seasons, ranging from 6 to 10 millibars, or about 1 percent of
the air pressure at Earth’s surface. The variation in pressure occurs because in
the fall and winter the temperature gets so low at the poles of Mars that carbon
dioxide snows out of the atmosphere and forms meters-thick deposits of dry ice
on the surface. In the springtime as the surface warms up, the dry ice
evaporates back into the atmosphere. The pressure also varies with altitude just
as it does on Earth and is about ten times lower on the top of Olympus Mons than
on the floor of Hellas Planitia.
| B | Clouds |
Even though the Martian atmosphere
contains less than 1 percent as much water vapor as Earth’s atmosphere, clouds
and frosts form on Mars and have been studied in detail by telescopes and
spacecraft. Wave clouds, spiral clouds, clouds formed near topographic obstacles
such as volcanoes, wispy cirrus-like clouds, and a wide variety of hazes and
fogs have all been observed. Along with the dust storms and related clouds,
these features all reveal the Martian atmosphere to be quite dynamic.
Data collected by the Mars Global Surveyor
spacecraft indicate that much denser water ice clouds can form on the night side
of Mars not visible from Earth. These clouds are about five times as thick as
the water ice clouds sometimes seen on the day side and form lower in the
atmosphere, creating a foglike layer above parts of the surface. A clue to the
presence of such clouds comes from the ground temperature on the night
side—during the Martian summer some areas near the equator in the northern
hemisphere can be up to 20°C (68°F) warmer than predicted. The clouds overhead
prevent some of the heat built up during the day from radiating away into space
at night. Most of these water ice clouds quickly disappear after dawn.
Clouds consisting of carbon dioxide ice
crystals also form in the Martian atmosphere, mainly over the polar regions in
winter when the temperature is lowest. The CO2 ice crystals strongly
scatter thermal radiation, reducing the loss of heat into space over the
poles.
| C | Sky Colors |
Rovers and landers on the surface of Mars
have photographed the planet’s striking sky colors, including sunrises and
sunsets. On Earth, the mid-day sky is blue because of the way air molecules
efficiently scatter blue light in our dense, relatively clear atmosphere. When
the Sun is low in Earth’s sky, sunlight travels through much more of the air and
so scattering of red light by dust and haze particles becomes more important
than scattering by air molecules and our skies turn reddish and pink. On Mars,
the sky color is almost totally determined by dust (only a feeble amount of
molecular scattering is possible in the thin Martian atmosphere). Based on their
sizes and shapes, dust particles scatter different colors of light more
efficiently in certain directions, however. The mid-day Mars sky is reddish to
pink because scattering of red light by fine dust particles is most efficient in
that geometry. When the Sun is low, scattering of blue light is more efficient.
Thus, the pinkish mid-day skies give way to bluish sunsets. Martian sky colors
are to some extent opposite to those of Earth.
| D | Winds and Dust Storms |
During most of the year wind speeds are
fairly low—about 7 km/h (about 4 mph)—but during dust storms they can exceed 70
to 80 km/h (40 to 50 mph). These winds often originate in large basins in the
southern hemisphere and carry great volumes of dust from the basins to other
regions. The dust is not sandy, as in a sandstorm on Earth, but has the
consistency of flour. The largest of these storms can cover the entire planet
and last for months. An unusually large dust storm covered the planet in 2001
and was the largest storm seen since 1971. Smaller local or regional dust storms
can occur any time during the Martian year. Some scientists think dust storms
may generate a strong static electric charge that affects the chemistry of the
soil, creating hydrogen peroxide that would break down organic substances on the
surface or methane in the air.
Dust devils are also an important feature
of Martian weather. These swirling columns of dust and sand occur during the
Martian summer when surface temperatures can warm to 20°C (68°F). Starting as
heated air at ground level, Martian dust devils can reach heights of 10 km (6
mi) and rotate at 30 meters per second (70 mph). Although the force of such
winds is weak because of the planet’s thin air, dust devils on Mars can grow
vastly larger than dust devils on Earth—ten times as large as Earth tornados.
Because the rotating winds transport dust into the atmosphere, dust devils may
have a major effect on the planet’s overall weather and climate. Dust devils
also may carry a strong static electric charge. Such electrified dust devils
could pose a hazard to future robotic or human explorers, coating equipment with
dust and sand, interfering with radio communications, and even discharging
lightning.
| VII | THE MARTIAN PAST |
Space probes have provided scientists with
enough information to decipher some of the history of Mars as a planet. Surface
features indicate that the environment on the surface of Mars has changed
dramatically over time, from geologically active early periods when a relatively
thick atmosphere and liquid water may have been present to today’s frozen world
with a thin atmosphere.
| A | Formation of Mars |
The chemical composition of Mars is
similar overall to that of the Earth, although there are important differences
in the abundances of iron and of volatile elements like water and sulfur. These
differences probably exist because Mars likely formed further away from the Sun
than the Earth, in a different region of the disk of rock, ice, and gas from
which all of the planets formed.
Like Earth, Mars went through a period of
massive bombardment from asteroids and comets from its formation about 4.6
billion years ago until about 4.2 billion years ago. Conditions would have been
hostile to the rise of life, but the bombardment also provided Mars with some of
the same chemical building blocks that made life possible on Earth. These
substances include organic carbon compounds.
| B | Climate History |
While there is still intense debate and
scientific study of Martian climate change, a number of models of the planet’s
evolution have been proposed to try to match the steady stream of new
observations.
| B1 | Earthlike Mars Model |
In one model, Mars appears to have had
more Earthlike surface conditions between about 4.2 and 3.5 billion years ago. A
thick CO2 atmosphere may have trapped more solar heat through the
greenhouse effect, allowing the surface to warm up. Along with higher
atmospheric pressure, the warm temperatures allowed water to remain liquid for
long periods of time and to possibly cover extensive areas of the surface. Water
reacted with the surface and subsurface rocks, creating clays and other hydrated
minerals and possibly also carbonate rocks.
Volcanic eruptions over time released
large amounts of sulfur dioxide (SO2) into the atmosphere, slowly
changing the chemical environment. The added sulfur turned the surface water
acidic, dissolving most of the clays and any carbonate rocks that may have
formed during earlier more Earthlike periods. When the sulfur-rich surface water
evaporated it left behind deposits of salty sulfate minerals.
| B2 | Dry Mars Models |
Other models of the Martian past paint
a “drier” picture of early Mars. In these scenarios, liquid water may have
existed on the surface or in the shallow subsurface for perhaps only
intermittent periods or only in small regions of the planet. These models lead
to less optimistic implications for the possibility of life on Mars but are
still consistent with the information that space probes have obtained.
Astronomers focus significant efforts on trying to distinguish between these
very different models for the Martian past, including designing future missions
to try to resolve the controversy.
| C | Thinning Atmosphere |
Regardless of the specific details of the
climate of early Mars, it seems clear to most astronomers that beginning around
2.5 billion years ago Mars began to evolve into the frozen, dry world it is
today. It became a planet with little volcanic activity and a carbon dioxide
atmosphere too thin to allow liquid water on the surface or to sustain a
significant greenhouse effect. Exactly what could have happened to most of the
thicker early atmosphere that might have existed is still a mystery. One theory
is that loss of the planet’s magnetic field after Mars’s liquid core began to
turn solid allowed charged particles from the solar wind to collide with the
atmosphere, knocking atoms and molecules off into space. Other theories propose
that part of the atmosphere may have been blown away in a catastrophic impact
event, or that the gases reacted with water and are chemically combined in rocks
and minerals on the surface and in the subsurface.
| D | Loss of Liquid Water |
Scientists also wonder where the liquid
water that appears to have formerly existed at the Martian surface went. Some
astronomers believe that it seeped into the ground and is still there as ice or
possibly liquid water in the subsurface today. Others think that much of it may
have evaporated and slowly trickled off into space as sunlight broke apart the
water vapor molecules over long periods of time. Determining the history of the
Martian atmosphere and finding out whether sizable quantities of water still
exist below the surface are among the most important goals of Mars exploration
today.
| E | Shifting Axis Tilt |
A major difference between Earth and Mars
is the stability of the tilt of the planet’s axis. Due to gravitational effects
of the Moon, Earth maintains a relatively stable 23.5 degree tilt on its axis,
with only a slight wobble (obliquity) between about 22.1° to 24.6° over a
timescale of millions of years. Without a large moon, and being closer to the
strong gravitational pull of Jupiter, Mars has a much more dramatic wobble to
its axis, resulting in a tilt that may shift from 0 degrees to 60 degrees and
causing major climate shifts over time, between massive ice ages and relative
warm periods. When the tilt of Mars is nearly vertical, the polar caps may
expand almost to the equator. At its greatest tilt, one pole is warmed almost
directly by the Sun, likely releasing the deposits of frozen water and gas into
the atmosphere.
| VIII | SPACECRAFT MISSIONS TO MARS |
Space probes have provided the most
detailed information about Mars. But getting a spacecraft to Mars and operating
it there successfully is a difficult and risky process. Beginning in the 1960s
and into the 21st century, dozens of spacecraft were launched to Mars. A
significant number of probes have failed, mainly from technical problems. The
most successful missions returned vast amounts of data about the chemical and
physical characteristics of Mars and a large number of digital photographs of
its surface. Several missions are currently returning data from Mars as part of
an international effort to intensively study the planet from orbit and from the
surface.
| A | United States Exploration of Mars |
The U.S. exploration of Mars by the
National Aeronautics and Space Administration (NASA) falls into two separate
phases. A series of Mariner and Viking missions were sent to Mars during the
1960s and 1970s. NASA then abandoned the exploration of Mars for a number of
years but began sending some spacecraft to the planet during the 1990s and in
the early part of the 21st century.
| A1 | Mariner and Viking |
NASA launched its first Mars
spacecraft, called Mariner 3 and 4, in 1964. Mariner 4 was successful and
performed the first flyby of Mars in July 1965, taking dozens of close-up
pictures and other measurements. These pictures had a powerful impact because
the only features seen in the images taken of the parts of the southern
hemisphere that Mariner 4 happened to pass over were impact craters like those
on the Moon. These first close-up images did not reveal any evidence of the
advanced civilizations that people in the 19th and early 20th centuries imagined
might exist on Mars, or even any interesting and potentially Earth-like geologic
or atmospheric features that modern astronomers were hoping to see. The 1969
flybys of Mariners 6 and 7 took much more detailed pictures of the Martian
surface as well as measurements of the planet’s gravitational field and
atmospheric composition. Even these more extensive views of the red planet,
however, were just glimpses that did not reveal the true character of Mars.
Mariner 9, launched in 1971, was the
first spacecraft to orbit Mars, and the resulting detailed and systematic study
from orbit revealed the enormous volcanoes, canyons, and enigmatic channels that
have come to characterize the modern view of Mars. Much of Mariner 9’s mission
was hampered by a global dust storm that shrouded most of the surface from view
during much of 1971. However, once the dust settled, Mariner 9’s ultimate legacy
was showing that the planet was much more like Earth than the Moon.
NASA launched an even more ambitious
series of probes to Mars—Viking 1 and 2—in 1975. These spacecraft provided
scientists with incredible high-resolution views of the planet’s surface and
atmosphere. The Viking probes included orbiters, which mapped Mars and made
global studies of its geology and meteorology, and the first successful landers,
which measured the composition of the surface, studied the planet’s daily and
seasonal weather patterns, and searched for signs of life. The Viking Landers
revealed a landscape much like some of the cold, dry deserts of Earth, except
that the soils were found to be completely sterile, and the environment overall
much too harsh for Earth-like organisms to survive.
| A2 | Later Missions |
After a 17-year interval, NASA
launched its next Mars mission, Mars Observer, in 1992. Observer was lost just
three days before it reached Mars. Its replacement, Mars Global Surveyor (MGS),
was launched in 1996 and successfully went into orbit around the planet in 1997.
The MGS operated until the end of 2006, when the probe lost contact with Earth.
It carried instruments to measure the composition and topography of the surface
and to monitor weather conditions in much more detail than scientists can from
Earth. MGS also carried cameras that can resolve details as small as 1.5 m
(about 5 ft) on the surface. Some of the MGS images reveal erosion patterns on
the planet’s surface, which appear to have been formed by relatively recent
near-surface liquid water. This discovery is both exciting and puzzling, because
ice, not liquid water, is expected to exist at such low pressures and
temperatures.
MGS provided images of an enormous
number of dunes and other windblown landforms that appear to be the only active
geology on the planet today. It also discovered the many enigmatic layered
deposits at the poles and discovered and mapped remnants of a once-strong
planetary magnetic field preserved in certain parts of the Martian surface.
Other important results from MGS include global mapping of the planet’s
topography to an accuracy better than is available for most of Earth’s
topography, global mapping of volcanic and other minerals on the surface, and
daily mapping of the planet’s clouds and polar caps. In addition, in 2001 the
spacecraft captured detailed multi-instrument measurements of the largest dust
storm observed on Mars since 1971.
In 1997 the Mars Pathfinder spacecraft
became the third successful mission to land on Mars. Pathfinder consisted of a
lander and a small rover named Sojourner. The lander took digital camera images
of the geology of the landing site and studied the weather conditions on Mars.
The rover, a separate autonomous spacecraft about the size and weight of a
microwave oven, was able to travel a few meters per day around and away from the
lander, taking close-up images and chemical measurements of surface materials
that were inaccessible to the lander itself. Pathfinder operated for 83 Martian
days and discovered evidence, preserved in the geology and chemistry of the
rocks and soils at the landing site, for the action of liquid water long
ago.
NASA launched two spacecraft to Mars
in 1998 and 1999. The first spacecraft, Mars Climate Orbiter, reached the planet
in September 1999 but crashed into Mars instead of orbiting the planet because
of a navigational error. The second spacecraft, Mars Polar Lander, reached Mars
in December 1999, but it too crashed into the planet’s surface. Engineers
believe the craft fired its landing rockets too early. Mars Polar Lander also
carried two independent surface penetrator probes called Deep Space 2, but these
also failed to perform successfully.
Another U.S. mission to Mars, an
orbiter called Mars Odyssey, went into orbit around the planet in late 2001.
Odyssey carries instruments to make geochemical measurements of the surface and
to map the planet’s rock and mineral deposits in greater detail than MGS. This
orbiter began its primary mapping mission in early 2002 and discovered evidence
of extensive subsurface ice deposits later that year.
NASA launched the Mars Reconnaissance
Orbiter in August 2005, and it successfully went into orbit around Mars in 2006.
Its mission is to perform even higher resolution imaging and atmospheric
studies, and partly to search for landing sites for future missions in the Mars
Exploration program. By early 2007, after only a few months of operation, it had
returned more scientific data than all other spacecraft yet sent to Mars,
including images of objects on the surface as small as 25 cm (1 ft) across.
| A3 | The Mars Exploration Rover Mission |
The Mars Exploration Rover mission
consisted of two spacecraft, Spirit and Opportunity, which were highly capable
rovers equipped with the scientific instruments needed to determine whether
liquid water once existed on some parts of the Martian surface. After being
launched from Cape Canaveral, Florida, in June 2003, Spirit landed safely on
January 4, 2004, in the Gusev Crater about 15 degrees of latitude south of the
Martian equator. The rover was deployed on the surface on January 15 to begin
exploration of the crater, which scientists believe may be the bed of an ancient
lake. The Opportunity spacecraft, launched from Cape Canaveral in July 2003,
landed on Mars three weeks after Spirit, on January 24, 2004, in a shallow
crater. The rover was deployed on January 31 in an equatorial region known as
Meridiani Planum, where mineral deposits suggested the previous existence of
liquid water. Cameras on the Opportunity rover sent back images of the first
exposed bedrock on the surface of Mars.
The two identical rovers differ
significantly from the earlier Sojourner rover on the Pathfinder mission. Each
Mars Exploration Rover is 1.6 m (5.2 ft) long and weighs 174 kg (384 lb),
whereas Sojourner was 65 cm (2 ft) long and weighed 10 kg (22 lb). Both rovers
are capable of traveling many times farther than Sojourner, which traveled about
the length of a football field. The Mars Exploration Rovers also carry their own
telecommunication devices to return data to Earth or to other Mars-orbiting
spacecraft, unlike Sojourner, which had to relay its data to the Pathfinder
lander. The six-wheeled Mars Exploration Rovers have a suspension system that
enables them to ride over rocks bigger than 26 cm (10 in) and to tilt about 30
degrees without turning over.
The landing sites were selected after
long and careful study by planetary geologists. Gusev Crater is about the size
of the state of Connecticut. The floor of the crater measures 160 km (100 mi) in
diameter. A geologic feature that resembles a river valley leads directly into
the crater through a breach in its southern rim. Some scientists theorize that
flowing water created the valley about 2 billion years ago and may have pooled
in the crater to form a lake. The Meridiani Planum landing site, also near the
equator, is about halfway around the planet from Gusev Crater. Planum is
Latin for “plains,” and Meridiani Planum is one of the flattest and smoothest
places on Mars. The Mars Global Surveyor orbiter discovered that this region is
rich in coarse-grained hematite, a type of iron oxide mineral. On Earth,
coarse-grained hematite usually forms in association with liquid water, although
it can also be formed by volcanic lava.
In addition to cameras that can
provide both panoramic and microscopic images of the surface, the Mars
Exploration Rovers carry a variety of scientific instruments for measuring the
composition of soil and rocks. An adjustable arm on each rover even features the
equivalent of a geologist’s rock hammer: a rock-abrasion tool that uses a
grinding wheel to remove dust from rocks. It can penetrate as deep as 12 mm (0.5
in).
The use of the grinder tool on bedrock
found in the Meridiani Planum crater where Opportunity landed enabled NASA
scientists to conclude in March 2004 that features found in the bedrock could
best be explained by the existence of liquid water sometime in the past. The
grinding tool revealed that within the bedrock outcropping, a feature the
scientists named El Capitan, were dense deposits of sulfate salts, which on
Earth are left behind by receding groundwater or the evaporation of ocean water.
One of Opportunity’s spectrographic instruments also detected the presence of
jarosite, a sulfate mineral containing iron and potassium that can only be
formed in the presence of water. Opportunity’s microscopic camera also imaged
so-called vugs (cavities) in the rock that may have been produced when salt
crystals formed in rocks that were in briny water. The salt crystals then
dissolved, leaving behind the cavities.
The scientists also found evidence of
geologic patterns called cross-beds, which can occur when water currents cause
rock layers to be deposited at an angle to other layers, and they found puzzling
pebble-sized spherical grains of rock similar to those that result when minerals
form out of porous, water-soaked sediments. The combination of all these
findings led the scientists to conclude that only the existence of liquid water
in this area of Mars in the distant past could explain all of these features.
Later studies indicated that upwelling groundwater could also have been the
source of the salt and mineral deposits left by evaporation, rather than
exclusively a shallow surface lake or sea.
The two rovers continued exploration
well beyond their original 90-Martian day (sol) missions. In late 2006 Spirit
and Opportunity both passed the 1,000 sol mark. Although some mechanical and
software problems have occurred, the rovers have collected a wealth of
additional data. Winds have periodically cleared dust from the solar power
panels, allowing the rovers to continue operating. However, Spirit is further
away from the equator and is more affected by reduced sunlight during the
Martian winters.
The area in Gusev Crater where Spirit
landed turned out to be a relative disappointment to some scientists. Although
Gusev Crater may have been filled with water early its history, much of its
modern surface is now covered in volcanic rocks much like those seen by the
earlier landers Viking and Sojourner. Rocks more relevant to the possible watery
past of this area were found when Spirit moved away from its landing site and
climbed during 2005 and 2006 through the Columbia Hills (named for the Columbia
space shuttle and crew, which perished in 2003). The hills may be a jumble of
bedrock thrown up by an ancient impact and thus may reveal earlier periods of
Mars’s history. Spirit was able to examine a variety of rocks and found
additional evidence of minerals formed in the presence of water. It climbed down
the hills later in 2006. One of Spirit’s six wheels no longer turns and must be
dragged, hampering the rover’s ability to explore and climb. Scientists are
hopeful Spirit will continue to make important discoveries even with limited
mobility. Indeed, the dragged wheel has occasionally dug up exposures of salty,
sulfate-rich soils in certain areas. Rover scientists are trying to figure out
if these salts came from evaporation of salty surface water or from the
precipitation of minerals from subsurface volcanic gases. These soils are
covered by a layer of dust and may not have been discovered if not for the wheel
problem.
Opportunity is still able to move with
all six wheels, despite becoming stuck for weeks in a ripple of sand in 2005. In
late 2006 Opportunity reached the edge of Victoria Crater, an 800 m (0.5 mi)
wide impact crater that exposes much deeper layers of Martian bedrock than seen
before by the rovers. Opportunity began to work its way around the rim of the
crater, taking a series of detailed photographs of the exposed layers of rock
inside the crater. These overlapping images will allow scientists to create 3-D
views of the layers and the crater slopes. At some point in the future, the
rover may descend into the crater to perform a closer examination of the rocks.
Intriguingly, some of the exposed layers resemble the extensive layering seen in
photos taken by the Mars Reconnaissance Orbiter (MRO) of Candor Chasma canyon in
the Valles Marineris. The high-resolution MRO photos showed apparent evidence
that liquids or gas once flowed between underground layers of rock.
| B | Exploration of Mars by Other Countries |
| B1 | The Soviet Missions |
From 1960 to 1971 the Union of Soviet
Socialist Republics (USSR) sent 12 probes to Mars before their first partial
successes with missions Mars 4, 5, and 6 in 1973. The Soviets did not explore
Mars again until the Phobos missions in 1988. The Phobos probes were primarily
designed to study the planet’s moon Phobos. Phobos 1 was lost on its way to
Mars, but Phobos 2 went into Martian orbit and sent back information on the
composition of both Phobos and Mars.
Russia continued scientific study of
Mars after the dissolution of the Soviet Union in 1991, although on a more
modest scale than the Soviet space program. To date, the most ambitious Russian
mission involved an orbiter called Mars 96, which suffered an unsuccessful
launch and crashed back to Earth. While no firm plans for future Russian Mars
missions have been announced, Russian astronomers continue to participate as
collaborators on Mars missions of other nations.
| B2 | ESA and Japanese Missions |
Many other nations have participated
in Mars exploration, either by contributing scientific knowledge and
instrumentation to missions led by the United States and Russia or by launching
their own spacecraft. The European Space Agency (ESA) successfully placed the
Mars Express spacecraft in orbit around Mars in December 2003 after launching it
from the Baikonur Cosmodrome in Kazakhstan in June 2003. However, ESA officials
were unable to make contact with the spacecraft’s lander, the British-built
Beagle 2, after it separated from the Mars Express orbiter and descended to the
surface of Mars. The lander, named after the ship that carried British
naturalist Charles Darwin to the Galápagos Islands, was to use its instruments
to search for past or present signs of life on Mars. Nevertheless, the seven
remote-sensing instruments on board the Mars Express orbiter are returning
valuable data about both the Martian surface and atmosphere. In January 2004 ESA
scientists said the Mars Express instruments had directly detected the existence
of water ice on the Martian surface. Mars Express instruments have also found
evidence for clays, sulfates, and other minerals on the surface, supporting and
extending the results from the Mars rovers.
Japan also launched a spacecraft to
Mars, but like so many other Martian missions, it failed. The spacecraft called
Nozomi was launched in July 1998 and initially went into orbit around the Sun.
In December 2003 Japanese space officials announced that malfunctions had caused
the spacecraft to go off its intended trajectory to Mars and that they were
abandoning efforts to correct its course.
| C | Future Exploration of Mars |
The United States and other countries
are in the midst of an ambitious, long-term program of Mars exploration. NASA’s
Phoenix Mars Lander mission is scheduled for launch in 2007. The Phoenix Mars
Lander mission is intended to land a spacecraft in an ice-rich area of the north
polar region of Mars and to scoop up samples of soil for detailed analysis. It
is part of the Mars Scout program, which is conceived as a series of low-cost
missions to Mars. The Mars Science Laboratory in 2009 is intended to put a rover
on Mars that can travel over a region at least 20 km (12.5 mi) wide. A possible
sample-return mission from the surface of Mars is being studied for a potential
2014 or 2016 launch, although no specific plans have yet been announced for this
mission.
ESA is considering an advanced Mars
rover called ExoMars as part of its Aurora planetary exploration program. If
approved for launch, ExoMars would search for signs of present or past life on
Mars. That mission would launch in 2011 and land in 2013. A European-led
sample-return mission from the Martian surface is also under study, possibly in
collaboration with NASA and other countries. Russia and China have announced
plans to study joint missions to return samples to Earth from the moon Phobos
and possibly from the surface of Mars.
What would be the most ambitious Mars
mission yet was announced in January 2004 by U.S. president George W. Bush, who
called for the establishment of an astronaut base on the Moon that would provide
a launching pad for a later human mission to Mars. NASA announced more details
about this new round of manned space exploration in 2006 in what is being called
the Constellation Program. The primary vehicles would be the manned Orion space
capsule, which carries a crew of six, and the Ares booster rocket. Flights to
Mars would require a much more powerful version of the booster called Ares V.
Plans for this ambitious return of humans to the Moon and then on to Mars are
still being intensely discussed and debated in the United States, and specific
launch and mission schedules have not yet been established.
| IX | SEARCH FOR LIFE ON MARS |
Mars is the most Earth-like place in the
solar system besides Earth itself, and so it is only natural to wonder if the
similarities extend to the existence of life. People have speculated about the
possibility of life on Mars for centuries, and one of the major justifications
for sending spacecraft to Mars is the search for direct evidence of past or
present life.
| A | Early Speculations |
Astronomers have often fueled the
speculation that life may exist on Mars. For example, the 19th century Italian
astronomer Giovanni Schiaparelli reported that he saw long, straight markings on
Mars that he called canali (Italian for “channels”). He and other
astronomers of that era also reported seeing evidence for seasonal color changes
on Mars that could be interpreted as evidence for vegetation. Some astronomers
of the early 20th century, as well as American entrepreneur and amateur
astronomer Percival Lowell, turned Schiaparelli’s canali into the
now-famous “canals,” forever changing the public’s perception of the red planet.
Lowell believed that the canals indicated the existence of an advanced
civilization on Mars. He wrote several books and magazine and newspaper articles
on the subject and lectured extensively about his theory around the country to
sold-out audiences. He proposed that the canals were a planetary-scale
irrigation project, carrying water from the wet polar regions to the dry
equatorial deserts. As telescopes improved, however, and as it became possible
to record photographs of Mars on film instead of relying on human vision alone,
astronomers found it more and more difficult to see repeatable evidence for
Lowell’s canals. Close-up images of Mars from the Mariner spacecraft finally
proved that the canals did not exist, although numerous channels carved
naturally by flowing water were discovered.
Scientists now know that windblown dust
causes the color changes and that the canals are no more than an optical
illusion caused by the limitations of human eyesight at the telescope. But
Lowell’s beliefs about civilization on Mars have had a powerful and lasting
effect on human perception of the planet. British author H. G. Wells’s The
War of the Worlds (1898) and American actor and director Orson Welles’s 1939
nationally-broadcast radio hoax based on that novel put a sinister face on our
interplanetary neighbors. American author Edgar Rice Burroughs’s series of Mars
books, starting with A Princess of Mars (1912), provided a more
benevolent expansion of the influence of Lowell’s ideas and inspired a
generation of would-be planetary explorers. For a while in the 1970s, some
people even thought there were human faces and pyramid-like structures carved
into landforms in places on Mars, until better images revealed these, too, to be
optical illusions. Even today, science fiction stories, movies, and television
shows about Mars and Martians continue to be popular around the world.
| B | Current Scientific Knowledge |
A major focus of the Viking missions was
to search for actual scientific evidence of life. Several instruments on the
Viking landers were designed specifically to detect organic molecules in the
soil, and to test soil samples for evidence of metabolism, growth, or
photosynthesis of possible Martian life forms. Even though all of these
experiments were sensitive enough to have been able to detect life even in the
most arid, cold, or otherwise hostile environments on Earth, none of them showed
any convincing evidence for the presence of life on Mars.
Most scientists today think that it is
highly unlikely that there is any life on the surface of Mars. Conditions at the
surface are extremely hostile to life as we know it. Temperatures are usually
well below the freezing point of water, and the atmosphere is extremely thin and
dry. Without a protective ozone layer like Earth’s, ultraviolet radiation bathes
the surface and would destroy any organic molecules exposed there. Static
electricity generated by dust storms and dust devils may also create hydrogen
peroxide, which can break down organic chemicals.
However, a growing number of scientists
believe that some form of life could possibly exist on Mars today in more
protected environments such as underground or inside pores and cracks in
rocks—places sheltered from the extreme conditions of the surface and where
liquid water could exist even at very low temperatures. This new appreciation
for the possibility of life on Mars has been driven by the discovery, only in
the last decade or so, of simple life forms on Earth tenaciously surviving and
in some cases even thriving in what used to be considered inhospitable
conditions. On Earth, life has been found at great depths on the ocean floor,
deep underground in volcanic rocks, in highly acidic cave waters, in
near-boiling hot springs, and in almost permanently frozen tundra sediments. If
life can maintain a foothold in even these extreme environments on Earth, then
it may also be able to exist on Mars. See also Hydrothermal Vent.
A more basic question is whether
conditions on Mars were ever Earthlike enough, and for a long enough time, for
some form of life to have evolved. Liquid water has been essential for life as
we know it to evolve and survive on Earth. The Spirit and Opportunity Rovers
both found evidence that liquid water once existed on and below the surface of
Mars billions of years ago, but the results seem to indicate that water would
have been extremely acidic, resembling dilute sulfuric acid. Although
preexisting Martian microorganisms could possibly have adapted to high acidity,
it is less clear that life itself could start in such a harsh environment.
Ancient terrains examined from orbit hint that conditions more favorable for the
evolution of life may have existed at an earlier period on Mars before volcanic
eruptions changed the surface chemistry with large amounts of sulfur. Detailed
images from the Mars Reconnaissance Orbiter also show evidence that water or
other fluids may have flowed underground between layers of rock, providing a
possible environment for microorganisms. If life once existed on Mars but died
out, fossils may exist in ancient sediments.
| C | Meteorites from Mars |
Rare types of meteorites found on Earth
have a chemistry very similar to rocks on Mars. Scientists think these
meteorites are chunks of Mars that were blasted into space by large impacts on
the planet’s surface. The rocks went into orbit around the Sun and eventually
fell to Earth as meteorites. Some of these meteorites have been studied for
possible signs of Martian life.
In 1996 a group of NASA scientists
announced that a meteorite thought to have come from Mars contained possible
fossil evidence of bacteria-like life forms. The meteorite traveled through
space to Earth millions of years ago after being blasted from the surface of
Mars, probably by the impact of a large meteor. The scientists’ evidence was
based on the presence of certain chemicals and minerals—as well as microscope
pictures of bacteria-like features—within the meteorite. Intense scrutiny of
this meteorite by other scientists has not provided support for this theory,
however, and most scientists believe that the meteorite may somehow have been
contaminated by Earth life. Even though it is still possible that the rock does
contain fossilized bacteria from Mars, most scientists now focus instead on what
can be agreed upon about this mysterious meteorite. Liquid water probably once
flowed through the meteorite, sources of heat such as meteorite impacts or
volcanism acted upon it, and it probably contains at least some simple organic
molecules that may have originated on Mars. These conditions—liquid water, heat,
and organic molecules—are the requirements for life to exist on a planet.
In 2006 a group of scientists suggested
that microscopic etch marks found in another meteorite from Mars could be
possible evidence that Martian bacteria had once fed on minerals in the rock.
Underground bacteria found in Earth leave similar etch marks when they extract
chemical energy from iron minerals in rock. The meteorite was a volcanic rock
that is thought to have been exposed to water about 600,000 years ago on Mars
and later blasted into space by a large impact. No evidence of DNA was found in
the sample.
Even if these kinds of Martian meteorites
do not preserve actual evidence of life on Mars, their most important message
may be that Mars is one of the few places in the solar system where we know that
the conditions were habitable and life was at least possible. The goal now is to
figure out how and where to look for more convincing evidence such as traces of
organic material. Robotic Mars missions such as the Mars Science Laboratory,
ExoMars, and sample returns are the next logical steps.
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