8. Mars: the red planet

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Physical properties of Marsa

Mass6.4185 x 1023 kilograms = 0.1074 ME
Mean radius3.3899 x 106 meters = 0.532 RE
Mean mass density3,933.5 kilograms per cubic meter
Surface area1.441 x 1014 square meters = 0.2825 AE
Rotation period or length of sidereal day24 hours 37 minutes 22.663 seconds = 8.86427 x 104 seconds
Orbital period686.98 Earth days = 668.60 Mars solar days
Mean distance from Sun2.2794 x 1011 meters = 1.52366 AU
Orbital eccentricity0.0933
Tilt of rotational axis, the obliquity25.19 degrees
Distance from Earth5.6 x 1010 meters to 3.99 x 1011 meters
Angular diameter at closest approach14 to 24 seconds of arc
Age4.6 x 109 years
Atmosphere 95.32 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon
Average global surface pressure0.056 bars = 560 pascals
Surface pressure at Viking 1 and 2 sites0.067 to 0.088 bars and 0.074 to 0.10 bars
Surface temperature range140 to 300 degrees kelvin
Average surface temperature210 degrees kelvin
Magnetic field strength (remnant)± 1.5 x 10-6 tesla = ± 0.05 BE
Magnetic dipole momentless than 10-4 that of Earth
a Adapted from H. H. Kieffer, B. M. Jakosky, C. W. Snyder and M. S. Matthews (eds.): Mars, University of Arizona Press, Tucson 1992. The symbols ME, RE, AE, and BE respectively denote the mass, radius, surface area, and magnetic field strength of the Earth.

Telescopic observations , early speculations and missions to Mars

An Earth-like planet

Mars spins on its axis with a rate and tilt that are almost identical to the Earth’s. The day on Mars is only 37 minutes longer than our own, and the polar axis on Mars is tilted at 25 degrees, about the same as Earth with its 23-degree tilt. Both planets therefore have four seasons – autumn, winter, spring and summer – although the Martian seasons last about twice as long since the Martian year is nearly two Earth years.

Fig. .. 

Mars is a relatively small planet, about half the size and one-tenth the mass of Earth. The total surface area of Mars is about equal to the land area of Earth, owing to the extensive oceans that occupy most of the terrestrial surface. Still, Mars is substantially larger than the Moon or Mercury, so we might expect it to be intermediate in many properties between Earth and the Moon.

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It is indeed the Earth-like appearance of Mars in a telescope that has intrigued humanity during the past few centuries. Its orbit is closer to the Earth than any other planet except Venus, enabling us to discern polar caps that change in size with the Martian seasons and dark markings that seasonally distort its ruddy face. A white polar cap grows in the local winter at each pole and recedes with the coming of local spring. Large grayish-green regions flourish in the summer and become dormant in winter, as many plants do on Earth. Their seasonal growth on Mars has been called the “wave of darkening” since a dark band moves from a polar cap toward the equator as the cap shrinks.

White clouds repeatedly form at certain locations on Mars, and clouds are not possible without an atmosphere. Both the Earth and Mars have shallow, relatively clear atmospheres, heated seasonally by varying sunlight. Thus, Mars is in many ways the planet most closely resembling the Earth. Both planets have an atmosphere, clouds, polar caps and seasons.

Early speculations about intelligent life on Mars

Over the past century, our fascination with Mars has been stimulated largely by the prospect that life may exist there, either in the past or the present. Large, seasonally-varying, dark regions seemed to suggest life, since water melting from the polar caps might cause hypothetical vegetation to grow and progress from the poles to the equator.

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During a particularly favorable opposition in 1877, when Mars was even closer to the Earth than during most other oppositions, the Italian astronomer Giovanni Schiaparelli (1835-1910), director of the Milan Observatory, reported that a maze of dark, narrow straight lines traverses the planet’s surface. He called them canali, the Italian word for “channels” or “canals”, assuming that they were natural features. Schiaparelli mapped them and gave the broadest ones the names of large terrestrial rivers, such as the Ganges and Indus.

At about the same time, a wealthy Bostonian, named Percival Lowell (1855-1916), convinced much of the American public that there is intelligent life on Mars. Rich enough to do as he pleased, Lowell built an observatory in the clear air of Flagstaff, Arizona with the specific intention of observing and explaining the Martian canals. When Lowell turned his telescope toward Mars in 1894, he found what he expected to see – a vast network of canals bordered by vegetation.

Most astronomers, however, could not see the canals, which had been glimpsed at the limit of telescopic detection, concluding that they were some sort of optical illusion if they existed at all. And no one ever succeeded in photographing the canals using telescopes on Earth.

The space-age odyssey to Mars

Due to distortion caused by the Earth’s atmosphere, the details of Mars remained hidden from view until spacecraft flew past it, and were then sent to orbit the red planet and land on its surface. The search for life on Mars has been one of the main driving forces behind all of these missions, which have successively gathered evidence for and against living things on the planet. We still do not know for sure whether life now exits on Mars, or if it once did, but the possibility of life on Mars remains deeply embedded in human consciousness.

The Martian atmosphere

A carbon-dioxide atmosphere

Astronomers have long known that Mars has an atmosphere. It is required for the formation and support of the clouds that have been observed telescopically since the 19th century. The seasonal waxing and waning of the polar caps also suggests the presence of an atmosphere on Mars. Gases are released into the Martian atmosphere when a polar cap warms up during the local summer and the cap becomes smaller; gases are extracted from the atmosphere during the winter growth of the cap.

The Viking 1 and 2 landers made detailed measurements of the composition of the Martian atmosphere. Carbon dioxide is indeed the principal constituent, amounting to 95.32 percent of the atmosphere at ground level, followed by nitrogen (2.7 percent) and argon (1.6 percent). When compared with Earth, there is much more argon in the Martian air.

Small amounts of oxygen, ozone and water vapor

Oxygen molecules are present in the Martian atmosphere, but in a miniscule amount of just 0.13 percent. In contrast, the Earth’s atmosphere is filled with breathable oxygen, amounting to 21 percent of our air. Detection of significant amounts of oxygen would have argued for the possibility of plant life on Mars, since oxygen is unstable in a planetary atmosphere and plants are needed to continuously supply it. The small amount of free oxygen that is now present on Mars is the by-product of the destruction of carbon dioxide by energetic sunlight. This process also results in the production of exceedingly small amounts of ozone.

Since there is so little ozone in the Martian atmosphere, it has no ozone layer, and the planet’s surface is exposed to the full intensity of the Sun’s ultraviolet radiation. The lethal rays would destroy any exposed microorganisms, so there might not be any live ones left on Mars. By way of comparison, the Earth has a thick ozone layer high in its atmosphere, which absorbs most of the dangerous ultraviolet sunlight and keeps it from reaching the ground.

Fig. .. 

There is now very little water vapor in the Martian atmosphere, about 0.03 percent near the surface. And that is about as much of the vapor that the atmosphere can hold. It is practically saturated with water vapor. When the temperature drops, water can condense and freeze out of the saturated air, forming low-lying mists or ground fogs in canyons and frosts on the surface.

The thin, cold Martian air

The surface pressure on Mars was first accurately determined when Mariner 4 passed behind the planet, and its radio signal penetrated the Martian atmosphere in order to reach Earth. From the manner in which the signal was altered, a surface pressure of about 0.005 bars was determined, compared to 1.000 bars at sea level on Earth. Warmed only by direct sunlight, without any pronounced greenhouse effect, the surface temperature on Mars averages 210 degrees kelvin, well below the freezing point of water at 273 degrees kelvin.

Under present conditions on Mars, liquid water is unstable and cannot stay on the surface of Mars. Because the temperature and pressure are so low, water on Mars is now stable only as ice or vapor. Over most of the surface, the temperature is usually below the freezing point of water, and when it warms above freezing the water turns almost directly into vapor. If liquid water was released onto the surface from the warmer interior, that water would survive for just a brief time before freezing into ice or evaporating explosively, turning into water vapor.

The winds of Mars

Why the winds blow

The Martian atmosphere responds to both the daily and seasonal temperature differences by generating winds that blow from hot to cold regions, transporting heat and trying to equalize the temperatures. And since a rise in temperature is equivalent to an increase in pressure, it is high-pressure air that rushes toward low pressure. This rush is the wind. The sharper the temperature or pressure difference, the stronger the wind.

Windblown dust and sand

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The icy Martian winds have swept up vast dunes of sand and fine-grained dust. Rippled dunes have piled up in basins, craters and chasms, but the dunes cover only a small percentage of the land on Mars, probably less than 1 percent. Both dark and light sand dunes are found on Mars. Starkly beautiful patterns are created when the polar caps warm up in local spring and summer, exposing dark sand dunes. Extensive dunes form a dark collar around the north polar cap in the local summer, while global winds tend to blow fine dust from the south to the north.

Dust devils and global dust storms

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As the powerful winds roar on an otherwise silent world, they occasionally stir up small, local dust storms, in much the same way that winds sometimes whip the terrestrial soils into towering columns called dust devils. The local dust storms form when the ground heats up during the day, warming the air immediately above the surface. The warm air rises in a spinning column that moves across the landscape like a miniature tornado, sweeping up dust that makes the vortex visible and leaving a dark streak behind. They have scratched tangled paths across some parts of Mars, often crossing hills and running across large sand dunes and through fields of house-sized boulders.

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Large dark areas, such as the elevated plateau Syrtis Major, apparently develop when the surface rocks are scoured by powerful, seasonal winds. On close inspection, these dusky areas dissolve into swarms of elongated light and dark streaks, often tens of thousand of meters long, pointing in the direction of strong prevailing winds. The light streaks consist of fine dust deposited on a darker terrain by the prevailing winds, on the downwind, or leeward, side of craters and hills. The dark streaks result from the removal of dust by strong winds to expose the underlying rock. When all the steaks in a given area are integrated and superimposed by the human eye or at the detector of a telescope, they form the larger, global features visible from Earth, in much the same way as dots in newsprint combine to make a picture.

Fig. .. 

Strong winds carry dust from the surface high into the atmosphere, forming yellow dust clouds that have been reported by telescopic observers for centuries. Numerous fleeting and localized dust storms occur each Martian year. They can occur at any season, but are more frequent in southern spring and summer. Small dust storms can form simultaneously at several points on the planet, and then coalesce with each other, producing dust storms larger by far than any seen on Earth. They sometimes grow and spread across the planet, engulfing the entire globe and shrouding it in an opaque yellow cloud.

Fig. .. 

Only a handful of these global dust storms have been observed, beginning with telescopic observations during the oppositions of 1922 and 1956. Mariner 9 arrived at Mars during the slow decay of one of them; until the dust settled, only the summits of volcanoes were visible to the spacecraft cameras. Two planet-encircling dust storms were observed during the Viking missions, when the sky above the landers turned dark red and the Sun was greatly dimmed. A global dust storm next hid the surface of Mars from view during the summer of 2001.

The polar regions

Seasonal polar caps

The seasonal caps were long thought to be composed of water ice, by analogy with the Earth’s polar caps. But the seasonally varying Martian caps are composed of frozen carbon dioxide, or dry ice. This is the same dry ice that is used on Earth to keep ice cream, lobsters and other things cold for days at a time. The winter on Mars is so cold that the carbon dioxide gas above each pole freezes and falls down to the ground.

The atmospheric carbon dioxide condenses in the winter when the temperature at the pole drops below 150 degrees kelvin, forming a large seasonal polar cap. It sublimates, or evaporates from solid ice to gas, when the polar temperature rises above 150 degrees in the spring and summer, returning to the atmosphere. This condensation of carbon dioxide during winter and its subsequent sublimation in the spring is what gives rise to the familiar waxing and waning of the Martian polar caps. The process is entirely analogous to the snowfall that blankets the Earth’s polar regions in the winter, and evaporates in the summer, except the “snowfall” on Mars consists of dry ice. It also accounts for the enormous seasonal change in the surface pressure on Mars; about 30 percent of the atmospheric carbon dioxide cycles into and out of the polar regions each year.

The residual, remnant, perennial or permanent caps

At both poles, the caps never completely disappear in the heat of the summer, when the temporary, seasonal deposits of dry ice sublimate back into the atmosphere. Residual, or remnant, polar caps are left behind. Since they remain throughout the Martian year, these residual caps have also been called perennial or permanent caps.

Fig. ..  Fig. .. 

The residual caps at the two poles have a split personality, with different sizes and compositions. At the south pole of Mars, the frozen carbon dioxide never entirely disappears, and a residual deposit of dry ice persists throughout the summer’s warmth. All the seasonal dry ice disappears during the northern summer, and the part that survives is about three times larger than the southern residual cap. Instead of frozen carbon dioxide, the residual cap at the north pole is composed of water ice.

Fig. .. 

Evaporation of carbon-dioxide ice in the summer reveals laminated terrain that extends horizontally for several thousand meters along the edges of both residual caps. Up to 20 layers have been exposed, each a few tens of meters thick, alternating between dark dust and bright ice. The extensive, regular polar layers were probably deposited during periodic climate change. It is estimated that the layered material was laid down at the rate of about 0.001 meters per year. So a layer that is several tens of meters thick took ten thousand to one hundred thousand years to accumulate, which is roughly comparable to the periodicity of great ice ages on Earth. The laminated terrain on Mars might be attributed to astronomical rhythms that have similarly created long-term, periodic changes in the climate of Mars, at least for the past few million years and perhaps longer.

Highland craters, lowland plains, and remnant magnetism

A world divided

The two hemispheres of Mars have distinctly different terrain. The planet’s southern half is extremely ancient, generally elevated, and highly cratered. Like the lunar highlands, most of the craters in the southern landscape on Mars date back to an intense bombardment by meteorites early in its history, estimated to about 3.9 billion years ago. The northern hemisphere, by contrast, consists mainly of younger, lower-lying, flat plains that are predominantly of volcanic origin and have been greatly transformed over the eons. It is as if the two hemispheres somehow fused together to form a divided world.

Heavily cratered highlands

Fig. .. 

The cratered scars of impacting meteorites can be found all over Mars, but the craters are more densely concentrated in the southern hemisphere where the terrain resembles the lunar highlands. The largest meteorites have gouged huge impact basins out of the Martian surface, throwing up mountains along their rims. The smooth floors of the biggest impact basins are light-colored, circular features that have been observed with ground-based telescopes. They retain their classical designations made more than a century ago - such as Argyre for the “silver” island at the mouth of the Ganges river and Hellas, the Greek word for Greece. The giant Hellas basin, some 2.3 million meters across, is covered with white frost in southern winter, forming a brilliant white disk seen from Earth.

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The inquisitive, close-up eyes of spacecraft detect numerous craters that are smaller than the impact basins. Some of these craters become frosted in the southern autumn and winter. Large craters on Mars are named after astronomers and scientists who have studied Mars, including Schiaparelli; smaller craters are named after villages on Earth.

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The ejected material surrounding some craters on Mars can be explained by supposing that the ground contained water or ice when they were formed. The flowing splashed pattern looks like that formed when you drop a pebble in the mud. On Mars, the heat of impact may have melted or vaporized water ice frozen in the Martian ground just below the surface, like the layers of permafrost underlying the Arctic landscapes of Earth. Or the impact might have released liquid water from the ground beneath the permafrost. The steam and liquid water then acted as a lubricant for the flowing debris and the muddy material sloshed outward like a wave until it dried and stiffened, or became cool and refroze.

Lowland volcanic plains

The extensive lowland plains that dominate the northern hemisphere of Mars are relatively flat and sparsely cratered. Most of them appear to be covered with lava flows, and are thus of volcanic origin. The plains of Mars are designated by the names of lands, followed by the Latin planita, meaning “a level surface or plain”. But they are not completely smooth. Volcanoes rise up in some of them, mesas and buttes in others; boulders or dunes give them a small-scale texture.

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The Latin term planum, meaning “plateau or high plain” follows the name of flat elevated regions, in contrast to the a low-lying planita. Most of the planitae are located in the northern hemisphere, while the plana are found just south of the equator. Another Latin name, terra, is used to designate an extensive land mass in the older, heavily cratered highlands.

Leftover magnetic fields

Earth has a global, dipolar magnetic field, like a bar magnet, with a north pole, a south pole and magnetic fields that loop between them. It is generated by the dynamo action of currents in its hot, molten metallic core. In contrast, there is not a global magnetic field on Mars, so its internal dynamo is now either extinct or much weaker than Earth’s. The Martian magnetic field exists only in local regions on the surface. Most likely, the local fields are fossil remnants of an early time when the Martian dynamo was sufficiently vigorous to magnetize the planet’s crust and maintain a dipolar field that is now imprinted in the ancient rocks.

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The dichotomy between the northern and southern hemispheres of Mars is retained in its leftover magnetism. The magnetic fields that have survived from the planet’s active youth are located within the heavily cratered southern hemisphere, and the northern lowlands are now largely free of magnetism. Magnetic fields were therefore present when the highlands were formed. But when the impacts stopped and the lowland plains originated, there was no magnetic field left – only the remnants currently preserved in the ancient highland crust.

The remnant magnetic fields are also missing from the very large, southern impact basins, such as Hellas and Argyre. When these impacts occurred, the internal dynamo must have been shut down, and the global fields were no longer present. Otherwise, the impacted material in the planet’s crust would have been magnetized.

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The magnetic fields that have survived on Mars are banded into a striped, bar code pattern of alternating magnetic polarity, each stripe extending up to 2 million meters from east to west. The magnetic field in one band points out of the planet, and the adjacent one points in; next to that it points out again and so forth. These data remind us of reversals in the Earth’s magnetic polarity that have been recorded in volcanic lava which has hardened into rock. The volcanic rock preserves the direction of the magnetic field at the time it solidified, and rocks of different ages indicate that the terrestrial magnetic field has regularly flipped or reversed direction, a few times every million years.

Towering volcanoes, immense canyons

Eons of volcanic activity

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When Mariner 9 and Viking 1 and 2 surveyed Mars in the 1970s, they revealed a mind-boggling world with kaleidoscopic beauty and variety. They replaced the bleak, drab, Moon-like view of a frozen, lifeless Mars, obtained during the partial, fleeting glimpses of previous spacecraft, with a new picture of a dynamic, living planet. Powerful forces have molded the face of Mars at an unsuspected scale, including giant volcanoes and immense canyon-lands that dwarf their terrestrial counterparts.

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As Mariner 9 settled into orbit around Mars, in November 1971, the planet was buried beneath a global dust storm. After circling the planet for a couple of months, the winds abated, the dust settled, and the spacecraft watched four high mountains emerge from the pall, each with craters at their summit.

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The volcanoes are perched on top of a vast uplift, known as the Tharsis bulge, which straddles the ancient uplands and lowland plains near the equator and overlies them both. The Tharsis bulge extends more than 2.5 million meters across, and it was formed roughly 2 billion years ago, after the division between the uplands and lowlands. Olympus Mons, the tallest mountain in the solar system, lies on the western edge of the Tharsis bulge. Three other tall volcanoes, named Arsia Mons, Pavonis Mons and Ascraeus Mons, crown Tharsis. They run diagonally across the equator along a ridge known as Tharsis Montes.

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These other three volcanoes are smaller than Olympus, but still much larger than any terrestrial volcano. With their gently sloping flanks and roughly circular summit calderas, the Martian volcanoes resemble the shield volcanoes of Hawaii, such as Mauna Loa, which has a similar slope but one third the height and one twentieth the volume of Olympus Mons. Such volcanoes are formed by the repeated eruption of lava that cascades down the flanks in thousand of individual flows.

Yawning chasms

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The turmoil associated with the formation of the Tharsis uplift and associated volcanoes opened up a network of vast canyons that are collectively known as Valles Marineris (Valleys of the Mariner). The colossal system of interconnected canyons, or chasmata, extends down the eastern flanks of the Tharsis bulge and along the Martian equator for 4 million meters, one-fourth the way around the planet. In places the chasms are as wide and deep, as Mount Everest is high. Their formation may be similar in origin to the rift valley in Africa, but on a much vaster scale. For Mars, it was as if a cosmic sculptor was trying to split the planet asunder.

Valles Marineris originates close to the summit of the Tharsis uplift, at Syria Planum, where the surface expansion and consequent stretching has produced the intricately fractured Noctis Labyrinthus (Labyrinth of the Night). As the name suggests, it is a maze of short, deep gashes intersecting at all angles.

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Further east the depressions become deeper, wider and more continuous. In the middle section of Valles Marineris they branch into three parallel canyons, the Ophir, Candor and Melas Chasmata, which are separated by intervening ridges. These canyons connect with the single long Coprates Chasma, which runs eastward and joins Eos Chasma. Yet further east the canyons become shallower, with evidence of past water flow, and finally the canyons terminate in the jumbled, blocky region called the Chaotic Terrain.

Liquid water on Mars

Cold, parched and wrapped in a thin, carbon-dioxide atmosphere, Mars today is a frozen, desiccated and inhospitable world. It cannot now rain on Mars, and liquid water cannot now remain on its surface. Yet, dry riverbeds, deep winding channels, and streamlined, washed-out landforms all provide compelling evidence for abundant liquid water on Mars in the distant past. In the planet’s early history, rivers ran across its surface and powerful floods coursed down its valleys, emptying into the russet plains and perhaps forming ancient lakes or seas. The ancient, water-cut features take two main forms, dubbed the valley networks and outflow channels. The valley networks seem to have been derived from the gradual flow of liquid water. The outflow channels were gouged out of the surface by the powerful rush of short-duration floods.

Dry river beds

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The valley networks have branching tributaries that connect into larger flows, so they look like dry river beds. The meandering flows increase in size downstream, and follow the local topography. They have lengths of up to hundreds of thousand of meters and widths of one to a few thousand meters, and they resemble river valleys on Earth. Of course, there is now no water in sight on Mars and these dry riverbeds were created long ago. They lie almost entirely in the ancient, heavily cratered highlands, and are rarely found in the younger, lowland plains. So, the valley networks are about as old as the highlands, dating back to 3.9 billion years ago.

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Scientists have debated whether the running water fell as rain, during a sustained period of warm, wet climate, or flowed just below the Martian surface, warmed by internal heat. Recent evidence supports the view that the valley networks formed by collapse into cavities formed by water running under the frozen, ice-rich surface. The branching tributaries of some valley networks end abruptly in box canyons, which indicate flooding rather than rain. Valleys that seem to have been deeply cut by the continual flow of water also have features that suggest formation by collapse rather than rainfall. Yet, other evidence points to a time when Mars was wrapped in a thicker and warmer atmosphere than now, with possible rainfall, rivers, lakes and maybe even an ocean.

Ancient, water-charged torrents

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Long, wide grooves have been gouged out of the equatorial regions of Mars, running downhill from the equatorial uplands to the lowland plains, measuring up to a million meters long. They tend to be narrow and deeply incised near their origins in the highlands and broad and shallow in the volcanic plains. Unlike valley networks, these enormous channels lack tributaries and are characterized by sculptured landforms such as scoured surface features, streamlined hills, and teardrop-shaped islands where the flowing water encountered an obstacle.

Possible ancient lakes and seas

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As we all know, water collects within holes in the ground, ranging in size from potholes in winter roads to stream-fed lakes and ocean basins. And if water once flowed across the surface of Mars, it would similarly pool in low-lying depressions, such as impact craters and basins, deep canyons and the northern lowland plains. At one time, they could all have been filled with water, forming ancient lakes and seas with perhaps a thin layer of ice on top, but all that now remains is their dried-out floors and sediment.

Where did all the water go?

Everyone agrees that water once flowed on the surface of Mars in large quantities, but the exact fate of all that water remains unknown. If spread uniformly over the surface, the amount of water involved in the flooding of the outflow channels would, by itself, cover the entire planet to a depth of 500 meters. Yet, there is no liquid water residing on Mars today, and the amount of water vapor in its atmosphere is negligible.

Some of the water might have evaporated in more clement times, to be later lost to space, but most of the water that once flowed on Mars probably persists today, frozen just beneath the surface or remaining liquid at greater, warmer depths. Some of the water is most likely buried as ice, frozen into the soil as permafrost. That would account for the muddy debris ejected from some impact craters. Vast stretches of ice could be hidden under dust and sand in the northern regions, outside the polar caps, frozen into the places that the outflow channels emptied their flows. The rest might be liquid water buried beneath the ice. Spring-like seeps and small gullies suggest that liquid water has been released from under the frozen surface of Mars in recent times, perhaps even today.

Running water in modern times

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Up until fairly recently scientists believed that water has not flowed on Mars for billions of years, but features identified in close-up images suggest that liquid water may still flow on Mars. The small, unassuming gullies have been carved into the steep, inside walls of some craters or valleys, with shapes that resemble gully washes on Earth. The Martian flow features emerge high up on the wall, run downhill in deep, winding channels, and fan out with an abrupt ending in an apron of dirt and rock.

Fig. .. 

The amount of water involved in each event can be estimated by measuring the volume of deposits with a conservative assumption of a 2-meter thickness, and by assuming that water accounted for just 10 percent of that volume. The rest of the debris flow is attributed to the dirt and rocks detected at the end of the gullies. It is thereby estimated that about 2,500 cubic meters of water have shaped each small gully, about the volume of an Olympic-sized swimming pool. That is equivalent to 2.5 million liters, and 660 thousand gallons, of water for each gully, enough to sustain future visitors to the planet.

Liquid water and life

Search for life on Mars

Viking 1 and 2 search for life

One of humanity’s most daring and imaginative experiments involved landing spacecraft on the surface of Mars, and searching for evidence of life there. The 3 billion-dollar gamble, in today’s dollars, began on 20 July 1976, when the Viking 1 lander came to rest on the western slopes of Chryse Planitia, the Plain of Gold, region of Mars. It appeared to have once been inundated by a great flood and was thus a promising place for life to have arisen. Six weeks later, the Viking 2 lander settled down in the Utopia Planitia region on the opposite side of the planet, near the maximum extent of the north polar cap, again a favorable site for water and possible life.

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How did the Viking landers test for life on Mars? The first, most obvious, moving creature test consisted of looking to see if any creatures were frolicking on the Martian surface. The cameras could detect anything down to a few millimeters in size if it came within 1.5 meters of the landers. Pictures were taken of all the visible landscape, from the stubby lander-legs to the horizon, for two complete Martian years, but the view was always one of a desolate, rock-strewn, wind swept terrain. A careful inspection of all these pictures failed to reveal any motions or shapes that could suggest life, not a single wiggle or a twitch, or an insect or worm. Unless they look like rocks, there are probably no forms of life on Mars larger than a few millimeters in size.

Of course, no one really expected that the Viking eyes would see living things, and each of the landers carried a $50 million biology laboratory designed to detect tiny, invisible microbes. Computerized devices inside the landers measured samples of the Martian soil for organic molecules and for signs of growth that might signal the presence of living microorganisms. The presence of microbes could be inferred if the Martian soil contained organic molecules with carbon in them. Such a test for organic molecules might be called a dead-body test, for soil would be expected to contain a higher proportion of organic molecules derived from dead bodies than from living ones. But not a single carbon compound was detected, even though the instruments could have spotted organic molecules at a concentration of one in a billion.

The other experiments on board the Viking landers searched for the vital signs of living microbes. They did this by exposing the soil to various nutrients, and sniffing the atmosphere to see if microbes ate the food and released gas. Something did emit carbon dioxide and oxygen gas, but it wasn’t alive.

After further experiments, scientists concluded that the biological tests failed to detect any unambiguous evidence for life on Mars. Instead of being produced by organisms of any kind, all of the results were attributed to non-biological, chemical interactions. Highly oxidized minerals in the Martian soil were reacting with the nutrients, breaking them up and liberating some oxygen gas and even more carbon dioxide.

Mars Pathfinder and Sojourner Rover

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Although the Pathfinder mission was not designed to look for direct signs of life, it did search for very indirect evidence of a formerly warm, wet Mars that might have supported life. The landing site in Chryse Planitia, the Plain of Gold, was chosen because it lies at the mouth of a large outflow channel, called Ares Vallis, apparently carved by catastrophic floods in the distant past. It is thought that running water flowed down the Ares Vallis and flooded the plain at the landing site between 3.6 and 4.5 billion years ago.

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One of the major unexpected results of Mars Pathfinder was the inability to detect chemical diversity in the rocks. The data are consistent with all measured rocks being chemically the same and covered by different amounts of the same dust. This was a surprise, because the landing site was expected to contain a wide variety of ancient rocks washed down by the flooding waters.

The distant, streamlined hills, known as Twin Peaks, appear to have been smoothed by water, and evidence of layered sedimentation shows up in both the nearby rocks and the Twin Peaks. Even the presence of sand, as opposed to smaller dust particles, suggests the widespread action of flowing water. The sand detected from Pathfinder was light in color, just like beach sand on Earth; and sand on Earth is formed by running water. In addition, magnets on Pathfinder found that the airborne dust is very magnetic, which can be explained if liquid water helped embed magnetic minerals in the dust.

Thus, there is abundant evidence that liquid water once flowed across Mars, and that the climate must have once been warmer and wetter than at present. Perhaps the planet had a thicker atmosphere in its early history, and conditions were then conducive to the survival of life.

The immediate vicinity of the Pathfinder landing site, however, appears to have been dry and unchanged for eons. The region seems to have remained almost unaltered since catastrophic floods sent rocks tumbling across the plain more than two billion years ago. It has apparently been untouched by water ever since the ancient deluge. Only the winds remained to erode and shape the surface.

Fig. .. 

The small rover, called Sojourner, added an important scientific component to the Pathfinder mission, along with elements of drama and excitement that captivated the public. The tiny vehicle weighed just 10.6 kilograms, about the same as a house pet, and had the overall size and rectangular shape of a small microwave oven. Equipped with six-wheel drive, with each wheel driven separately, Sojourner explored about 250 square meters of the Martian surface, measuring the chemical makeup of the rocks and soil. In contrast, each of the two Viking landers were shackled to one location, unable to roam across the surrounding terrain, somewhat like getting sick in bed on vacation.

Fig. .. 

The rover examined an array of Martian rocks of different shapes, sizes and texture, provided with colorful names like Barnacle Bill, Yogi, Scooby Doo, Casper, Wedge, Shark and Half Dome. It also analyzed the Martian soil in the vicinity, showing that it is very similar to the soil at the Viking 1 and 2 landing sites.

Possible life in a rock from Mars

Interest in the possibility of Martian life was heightened when scientists found possible signs of ancient, primitive bacteria-like structures inside just one of these rocks from Mars. This meteorite crash landed on the blue ice near the South Pole toward the end of the last ice age, resting there in frigid isolation for millennia, most likely compressed in the snow and later exposed. Then in 1984 a geologist spotted it in the Allan Hills region of Antarctica, bagged it, and sent it to the United States for analysis. Since it was the first meteorite to be processed from the 1984 expedition, the unusual rock has been designated ALH (for Allan Hills) 84001.

Fig. ..  Fig. .. 

The controversy and excitement centers on suggestions that microbial life took refuge in cracks within ALH 84001 a very long time ago. The microscopic and chemical evidence includes globules of calcium carbonate; the first organic molecules thought to be of Martian origin; several mineral features characteristic of biological activity; and what looks like fossils of extremely small bacteria-like organisms that lived on Mars billions of years ago. Some scientists argue that these observations collectively provide strong circumstantial evidence for past, primitive life on Mars; others reason that the evidence is not conclusive and that there are non-biological explanations for all of it.

The continuing hunt for extraterrestrial life

The mysterious moons of Mars

Discovery and prediction

Mars has two little moons that are so dark and small that they were undetected for centuries, even after the invention of the telescope. They were discovered in 1877 by Asaph Hall (1829-1907) using a 0.66-meter (26-inch) telescope at the United States Naval Observatory. He named the inner moon Phobos, the Greek word for “fear” and the outer one Deimos, Greek for “flight, panic or terror”, after the attendants of the Greek god of war, Ares, in Homer’s Illiad.

Fig. .. 

These are not the same kind of object as the Earth’s large and spherical Moon. Both moons of Mars are very small, with insufficient gravity to mold them into a spherical shape. They have a battered appearance, with a profusion of craters large and small. In fact, the surface of Phobos has been pounded into a layer of insulating dust at least a meter thick, enough to bury anything that tried to land on the small moon. Eons of meteorite impacts have apparently sandblasted it.

A maverick moon

Phobos is about as close to Mars as it can get. If it came much nearer to Mars, the planet’s gravitational forces would tear Phobos apart. In fact, the orbit of Phobos is steadily shrinking. If it continues to move toward Mars at the present rate, Phobos will either smash into the Martian surface or be torn apart by the planet’s gravity to make a ring around Mars in 50 million years. Because Phobos is about 4.6 billion years old, we are, astronomically speaking, catching a fleeting glimpse of the last few moments of its life.

Origin of the Martian moons

Fig. .. 

The irregular shapes, small sizes, and low mass densities of Phobos and Deimos closely resemble those of the numerous asteroids that orbit the Sun between the orbits of Mars and Jupiter. The two Martian moons and the asteroids have a battered appearance, with a profusion of craters large and small. Moreover, the surfaces of the Martian moons are as dark as some asteroids, known as the carbonaceous C-type, and nowhere near as lightly colored as the surface of Mars. Scientists therefore speculate that Phobos and Deimos were adopted from the asteroid belt.

Fig. .. 

Summary Diagram