The New Mars

But Mars had become another world entirely.

John Barnes: In the Hall of the Martian King (2003)

The information gained on Mars in the past 10 years is more abundant than all the knowledge accumulated during the preceding millennia. However, this does not mean that we know Mars through and through. With just a hint of exaggeration it can be said to be typical for research made with space probes that the information sent by a craft launched to solve one question stirs up ten new questions. The Red Planet still harbors lots of secrets and riddles on both large and small scales. However, in the beginning of the twenty-first century we already have quite a comprehensive general view of the world next to our own.

Topographic map of Mars based on measurements made with the MOLA instrument onboard Mars Global Surveyor. White areas are the highest, purple the lowest.

Benumbed planet

The temperature on the surface of Mars is at places well below –100°C, but it is not that warm in the bowels of the planet, either. Right after the birth of the Solar System all the larger bodies were in a molten state. The heat for melting came from the shrinkage of the primordial gas and dust cloud, impacts of smaller bodies, and the decay of short-lived radioactive elements. Just as in the case of Earth the heavy elements sank to the center of Mars to form a iron–nickel core, and the light ones rose to the surface to form a thin crust.

The radius of Mars’s core is 1,300–2,300 km or 40–65% of the radius and 15–30% of the total mass of the planet. Mars does not have a global magnetic field anymore, so the core is supposedly completely solidified. Based on studies of Martian meteorites and measurements made by space probes it has been deduced that earlier it did have a magnetic field, which was created by the rotating solid inner core whisking the material in the molten outer core. There is still some remnant left on the surface as a distant memory of the magnetic field long gone. The existence of the magnetic field is also proved by aurorae observed by Mars Express. However, because of the weakness of the field these are much dimmer and only local, restricted to smaller areas than the magnificent northern and southern lights around the polar regions of Earth.

Mars has a thick crust and an iron–nickel core, the diameter of which is about half of the diameter of Mars.

Mars must have cooled down rather quickly for the solid crust to form. The early bombardment of meteorites abated rapidly some 3.8 billion years ago, and there are still clear signs of the ancient events on the surface of Mars. The landscape was shaped by meteorite impacts peppering the entire surface with craters of different sizes. In addition to asteroids and smaller rocks, Mars was hit by innumerable icy comets. These were supposedly the source of most of the planet’s water, which originally might have been even more abundant than on Earth.

The end of the meteorite bombardment also marked the end of the Noachian era, the oldest of the three Martian eras begun at the time of the birth of the Solar System. The eras – Noachian, Hesperian, and Amazonian – have been named after certain terrains formed during those periods: Noachis Terra, Hesperia Planum, and Amazonia Planitia.

Gradually the crust of Mars has thickened so that nowadays it is tens, according to some estimates maybe even hundreds, of kilometers thick. The crust did not have time to break into separate plates like the crust of Earth, and therefore Mars is said to be a “one-plate planet.” The interrupted tectonic evolution of Mars is manifested by all kinds of bulges, ruptures, and wrinkles on the surface of the planet.

There was still some heat left underneath the solid crust created by the decay of long-lived radioactive elements. For the next couple of billions of years Mars was a very volcanic world. Eruptions spewed not only lava but all kinds of gases that helped create the atmosphere of the planet. During those times the large scale terrains of Mars such as lava plains, highlands, and depressions were also born. This era, the Hesperian, was a very long period, ending only 1.8 billion years ago. Since then the Red Planet has gone through the Amazonian period during which formations like Olympus Mons, the Tharsis volcanoes, and the vast dune fields in the polar areas were born.

By about a billion years ago Mars had cooled considerably. Because of its small size the planet could not maintain internal heat, like Earth. The furnace inside Mars died down, and the planet began to fall into a long, icy sleep. The core solidified, which made the magnetic field disappear so that the solar wind was able to strip the atmosphere off into space. The large variations in the orbit and in the inclination of the axis of rotation made the situation all the worse. The atmosphere became thinner, the climate got colder, water disappeared, and Mars turned into a freezing cold world. Time was stopped – almost.

A bissected world

On a large scale, Mars is divided into two very different hemispheres. The southern and part of the northern hemisphere is ancient, rugged terrain covered with countless craters. The rest of the planet, the northern polar areas and their surroundings, are very smooth. There are only a few craters here and there, in places none at all, and the elevation could vary by only some tens of meters across distances of hundreds of kilometers. Furthermore the northern hemisphere is “lower” than the southern one. In approaching the equator from the south the terrain slopes continuously down on the large scale. The southern hemisphere is on average a couple of kilometers higher than the “zero level.” Because of the lack of any sea on Mars to use for determining heights, the zero level is an artificial “plane” defined by atmospheric pressure. North of the equator, inclined by about 30°, there is a border line with a distinct drop to the northern plains, which completely lie below the zero level.

The great depression of the northern hemisphere may be the result of a huge impact that created a crater almost half a planet wide. If the theory holds, this event occurred very early in the history of Mars, possibly around the same time as the cosmic collision that gave birth to the Moon, the satellite of Earth. Even though the origins of the depression could be explained by an external factor, the reason for the smoothness of the surface has to be found on Mars itself. The only equally smooth regions on Earth are the deserts and ocean bottoms with deposited layers of material leveling off the terrain with time. Some scientists consider the conclusion inevitable: there was a heaving ocean on the northern hemisphere of Mars. Then again the smoothness might have originated in ancient lava flows. The roughness left behind was later smoothed by the dust and sand blown around by winds.

With its MARSIS radar system ESA’s Mars Express has studied the area of Medusa Fossae lying near the equator, exactly on the borderline between the highlands and lowlands. There are few craters in the area, so the formation must be very young, perhaps among the youngest on the surface of the Red Planet. The Medusa Fossae formation also exhibits some of the most mysterious deposits on Mars. Previously it was thought that these deposits might be ash originating from catastrophic eruptions differing from those that created the huge volcanoes on Mars. Previously, it was also thought that the deposits composed of easily eroded materials might be rather thin, but the results of Mars Express reveal that there is a thick layer of more than 2.5 km covering the solid rock.

The Medusa Fossae Formation, which is found near the equator, might be among the youngest deposits on the surface of the Red Planet. It is practically devoid of impact craters, a telltale sign of young age.

In addition to measuring the thickness of the deposits MARSIS has also determined their electrical properties, which gives us clues to the composition and structure of the material. The data suggest that the layers are made of porous, low-density stuff. This would be difficult to explain if they consisted of volcanic ash or wind-blown dust, both of which would have been compressed under their own weight. However, if there is water ice mixed into the soil, it could explain the curious properties.

There should not be much ice in the equatorial regions of present-day Mars, but if it were buried deep below the surface, it could have been preserved there, since in the past the inclination of the axis of Mars was larger than it is now, making the equatorial regions much colder. If this really were the case, the future studies of the buried ice could yield information on the climate and atmospheric conditions over the past 3–4 billion years, as well as whether there was life in the distant past.

An image of Olympus Mons based on laser measurements made by Mars Global Surveyor. The vertical scale is tenfold.

A realm of giants

The diameter of Mars is only half that of Earth, but regarding many Martian formations it is a kind of an XXL version of our planet. The highest mountain on Mars – and in all of the Solar System – is Olympus Mons, which rises some 27 km above the surrounding plains and has a diameter of about 600 km. This makes the mountain about three times higher than the highest peak on Earth, Mount Everest. However, despite its height Olympus Mons is – apart from the steep precipice along the edge of the mountain, with a height of about 7 km in places – very gentle. The rise of the slope is so gradual that any potential climber would have difficulty noticing the land going up.

The giant volcano was formed of lava welling up from the innards of the planet at a “hot spot” residing on a weak point of the crust. Lava poured down the gentle slopes, and the mountain gained height layer by layer. This is the way Mauna Loa on Hawaii, the biggest volcano on Earth, with a height of 9 km measured from the ocean bottom and a diameter of 120 km, was born. Unlike Mars, the plate tectonics on Earth make the hot spots move (or actually it is the other way around: the hot spots stay put, the plates are moving), so shield volcanoes such as Mauna Loa do not have enough time to grow that big. Mars lacks plate tectonics, so once a mountain begins to grow it gains height perhaps for hundreds of millions of years, so long as there is a source of lava. The smaller gravity and thicker crust of Mars also make the existence of high mountains possible; on Earth a mountain over 20 km high would have started to sink back into the mantle a long time ago.

The total area of Olympus Mons is more than 80% that of Finland.

The caldera on top of Olympus Mons, created when the emptied lava chamber collapsed, is 90 km long and 60 km wide. In fact there are several calderas overlapping each other. They were born at different times over a period of a few hundred million years. In case a future Mars explorer were to stand on the edge of the caldera on some distant day, he or she would not even be able to see the opposite edge, since it would be below the horizon. The same goes for the mountain itself. It is so vast that it is impossible to perceive it from the surface of the planet at all. If you are situated at such a distance from the mountain that you could see it all, the mountain would be beyond the horizon.

The height of the Tharsis region and its gigantic volcanoes shows on a map based on measurements made with the MOLA instrument of the Mars Global Surveyor. In 1879 Giovanni Schiaparelli gave Olympus Mons the name Nix Olympica, “Snows of Olympus,” because telescopes showed it at times as a white spot. While Mariner 9 photographed Mars in 1972 the dust storm covering the planet began gradually to settle, so the mountains began to loom through the dust as four dark spots. At first they were called Groucho, Chico, Harpo, and Zeppo, after the Marx brothers.

East of Olympus Mons there are three somewhat smaller volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, in a straight line. They have heights of “only” 15 km. Nevertheless, the summits of the trio rise nearly as high as the summit of their gigantic neighbor, because they are in the Tharsis region, which is at a height of some 10 km. This region, covering nearly one sixth of the total area of Mars, is in reality a bulge on the planet with a diameter of 4,000 km formed as early as 3–4 billion years ago. It might have been born as a result of several consecutive eruptions distributing vast amounts of lava on the surface layer by layer, but more probably it was raised by molten rock accumulating under the crust. One clue in favor of this theory is the network of cracks surrounding the region. The whole planet was about to split up along those clefts. On the other side of Mars there is a similar bulge, Elysium, with apparently the same kind of origins, but it has a diameter of only 2,500 km and a height of about 3 km.

The Tharsis volcanoes are lined up for the same reason the Hawaiian islands are. Both were born on a hot spot, but unlike Olympus Mons, the spot beneath the Tharsis mountains has been slowly moving. The reason for this is not known, since there are no plate tectonics on Mars. The volcanoes are much younger than the region all around them. Arsia Mons, the most southerly of them, had its last eruption 700 million years ago, Pavonis Mons 300 million years ago, and Ascraeus Mons 100 million years ago. The latest lava flows on Olympus Mons are younger still; they are at most only 30 million years old, possibly just a few million years old. Thus Mars has been volcanically active through most of its history, so it might still be.

A planetary scar

The eastern edge of the Tharsis region marks the starting point of Valles Marineris, a huge valley system, stretching across nearly one fifth of the circumference of the planet. The same volcanic activity swelling out of one side of the planet has torn the other side up. Valles Marineris is 4,000 km long, more than 600 km wide, and 7 km deep. Seen from one edge the opposite side would be below the horizon. It has often been compared with the Grand Canyon, which has a length and width of about one tenth, and a depth of about one fourth of those of Valles Marineris, but the Grand Canyon was carved by water. Based on size and origins a better analogy would be to the East African Rift Valley, which is a huge crack in the crust, just like Valles Marineris.

Valles Marineris has been modified by water since its formation. The water has carved branching tributaries leading to the large gorges. Landslides and depressions have also eaten up the canyon walls, giving birth to new, smaller ravines. There are thick sediments at the bottom of Valles Marineris, supposedly formed in standing water. At some stage the valley system may have been a huge lake or rather a sea, which has contributed to the present appearance of Valles Marineris.

Valles Marineris on the equator of Mars. The western end is dominated by Noctic Labyrinthus, the Labyrinths of the Night, and the eastern end by “chaotic terrain,” which is a starting point for riverbeds ending up at Chryse Planitia.

Lost keys?

It is a proven geologic principle that the present is a key to the past. The processes observed today are similar to what they used to be in the earlier days. But what if the lock has been changed? During its millions and billions of years of history Mars has changed much more radically than Earth. Even though the physical processes might have generally been the same, the factors affecting them could have been very different from what they are nowadays. Based on phenomena observed at present it might not be possible to draw justifiable conclusions on what happened on the surface of the planet long ago.

For example, the surface of Mars is very rocky. The rocks are of different sizes, colors, structures, and chemical compositions. Nevertheless, they are all thought to be material created by volcanic activity churned up by meteorite bombardment billions of years ago. It is difficult to test this theory, though, because meteorites seldom fall these days. Plus they are small rocks, not huge boulders, and volcanic activity is intermittent, if not nonexistent.

Opportunity found an iron–nickel meteorite about the size of a basketball.

The surface of Mars has also been worked on by big floods. For example, Pathfinder/Sojourner landed in Ares Vallis, thought to be a vast floodplain. The images from the surface and the measurements made by the rover have validated the findings from the images taken from orbit: an ancient flood has brought a large number of rocks to the area with features suggesting that they came from very different regions. There is no liquid water on Mars anymore – at least not on the surface – so the effect of floods on the formations and other features of the terrain cannot be observed directly.

The meteorite craters on Mars are much more worn out than for example, those on the surface of the Moon. Despite their age of billions of years the lunar craters still look very fresh because there is no atmosphere, wind, or water to wear them out. Mars, on the contrary, used to have a much thicker atmosphere than it has now, and large amounts of water, which, together with volcanic activity, have affected the appearance of the craters. But not anymore; nowadays the surface is changed only by the dust carried by the winds.

The size of the Martian polar caps varies with the seasons. Images taken with the Hubble Space Telescope show the northern winter changing into spring and summer.

A “living” world

There might not be life on Mars, there might never have been, but it is not a completely dead planet. Even though the atmosphere of Mars is very thin, it is thick enough to ensure that there is always something happening on the surface of the planet. While the footprints left by the astronauts on the surface of the Moon will look nearly the same even after millions of years, the dust carried by the winds on the surface of Mars is continuously covering up old formations and exposing new ones.

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter has recorded seasonal changes in the south polar cap. When sunlight warms the soil beneath the ice, carbon dioxide frost starts to vaporize at its base. The gas is trapped under the frost until it is released in an outburst that makes the expanding gas cool and partially refreeze to form bright, wind-swept fans. (L_s = 180° is the southern spring equinox; L_s = 270° the southern summer solstice.)

Mars’s atmosphere consists almost exclusively (95%) of carbon dioxide. The amount of nitrogen is less than 3% and of argon about 1.6%. There is a tiny amount of water in the form of water vapor, and the oxygen is next to negligible. The atmospheric pressure on the surface is about 6 mbar, or 1/200 of the atmospheric pressure at sea level on Earth. In spite of the modest density and low pressure the greenhouse effect caused by the atmospheric carbon dioxide has raised the surface temperature by about 6°C.

The atmospheric conditions and phenomena are continually affected by dust, which is always present in small amounts, but during dust storms in large quantities. The dust absorbs the radiative energy from the Sun and warms up the atmosphere. Going up from the surface the temperature drops so that at an altitude of 40 km it is some 60°C lower than on the surface. In theory the temperature should drop even faster, but it is slowed down by the dust floating in the atmosphere.

A large part, even as large as one third, of the carbon dioxide in the Martian atmosphere participates in the seasonal cycle between the polar areas. In the wintertime, carbon dioxide sublimates in the polar cap to form a layer a few meters thick, which in the spring sublimates back into the atmosphere, beginning a migration to the other pole. The southern polar cap is made entirely of carbon dioxide, so in the summer it disappears completely, but at the northern pole there is also water ice, which forms a permanent polar cap.

Just like carbon dioxide a part, although a small part, of the water ice sublimates in the spring and summer into water vapor, which is sometimes visible high in the atmosphere as thin clouds or after an especially cold night on the ground as a thin layer of frost vanishing with the first rays of the rising Sun.

The great dust storm of 2001. The images of Hubble Space Telescope show the same hemisphere of Mars. During the storm dust covered up the details of the surface almost completely.

Alive and kicking. Mars Reconnaissance Orbiter’s HiRISE camera has recorded Martian avalanches in action on the edge of layered deposits at the north pole. The avalanches consist of fine–grained ice and dust with perhaps large boulders falling down a steep cliff over 700 m high. They are most probably caused by the melting of the ices in the spring.

Nowadays the winds in the thin atmosphere are a very important if not the only factor in the slow change of the formations on the surface of Mars. In the low gravity the strong winds have an easy job of taking dust and sand up with them to grind away rocks and cliffs. The winds also cause sand to drift into dunes in places covering very large areas. For example, the northern polar cap is surrounded by an almost continuous dune field.

The winds also affect the visibility of the surface features of Mars from Earth. Astronomers peering at Mars with their telescopes from the seventeenth century onward developed imaginative theories on why the appearance of the dark and light areas, the albedo features, change from one opposition to the other. Today we know that the dust carried by the winds is to blame. For example, Syrtis Major, the first feature identified on Mars, has changed considerably in the course of the centuries. In reality the dark area is a lava plain sloping gently upward. For short periods it is partly covered by light dust before being blown again away by the winds.

A Theory of the End of the World

The dust storms on Mars and their effects on the conditions of the planet gave rise in 1983 to a theory on nuclear winter, a prolonged period of cold weather following a large scale nuclear war. According to the theory the vast fires lighted by the nuclear explosions would release so much smoke, soot, and dust into the atmosphere that the Sun would be blacked out for months if not years. The mean temperature could set as much as 20°C, which would cause a global winter lasting for several years. The theory is known as TTAPS after the initials of the last names of the scientists – Richard Turco, Brian Toon, Thomas Ackerman, James Pollack, and Carl Sagan – who developed it. Nuclear winter theory played an essential role in the arms limitations realized in the 1980s.

Dusty gusts

The big dust storms are the most effective means of transporting dust around the planet. Usually their occurrences concentrate at post-perihelic time, when Mars is closest to the Sun and there is summer in the southern hemisphere. In certain areas there are small dust clouds almost all the time, but every now and then the clouds gather to form a storm that engulfs the whole planet. It was one of those storms that covered the surface of Mars in November 1971, when Mariner 9 arrived at Mars, and the latest – though a lot smaller than the one in 1971 – was in mid-summer of 2007 in the southern hemisphere of Mars. By the latter half of June (by our calendars) dust clouds had appeared near the eastern end of Valles Marineris, and within a few weeks the whole planet was obscured by dust raised by both local and regional storms.

In broad outlines the cause of the dust storms is known rather well, but there is much to learn in the details. Usually they begin in the Hellas Basin which – with its depth of 9 km and diameter of 2,300 km – is so vast that it affects the global circulation of the atmosphere. For example, in this case tiny stormlets began to raise enough dust into the atmosphere that it started to absorb the heat of the Sun. This caused the upper layers of the atmosphere to warm up, according to measurements by 20–40°C. But closer to the surface the temperature stayed low, since only some 20% or less of the solar radiation was reaching the surface of the planet. The temperature difference created between the upper and lower atmosphere gave rise to strong winds with velocities of almost 500 km/hour blowing between cold and warm masses of air. This raised more and more dust in the air, and the dust absorbed more and more heat, and so on. A Martian dust storm is a treadmill giving itself a push: the warming of the upper atmosphere is at the same time the result and the cause of the dust storms.

A dust storm may envelop Mars for months, but finally even the strongest of storms will lose vigor. When the temperature difference finally evens out, the winds start to calm down, and there is no more new dust rising in the air. In the low gravity of Mars the dust floats in the air for a long time, even if the storm subsides and there are small dust clouds here and there just like before the big storm. Dust particles with a diameter of about 1 µm, or one thousandth of a millimeter, usually fall back to the surface in a few months, but smaller particles might remain up in the thin air for years. That is why there is always some dust in the atmosphere; the Martian sky is never completely clear.

An unsteady planet

One reason for the changes on Mars is the very large variations in the inclination of the axis of rotation, which is caused by the huge gravity of Jupiter, the effects of which are not balanced by a large satellite, as in the case of Earth. The position of the axis of Mars changes every 100,000 years by about 20° from the current value, but the variation is chaotic and unpredictable. During the past 10 million years the inclination of the axis of rotation might have changed between 0° and 60°.

These changes affect the seasonal variations and the severity of the climatic conditions. When the axis is more or less in an upright position the polar areas receive less radiative energy from the Sun and it is colder there; at those times a very large part of the atmospheric carbon dioxide and water vapor sublimate into the polar caps. When the axis is more tilted, the polar areas receive more radiation, and larger amounts of the gases in the polar caps are released into the atmosphere. At present the sublimation of water vapor from the northern polar cap during the summer makes it thinner by only some tenths of a millimeter, but when the inclination of the axis is large, the cap may lose dozens of centimeters of ice. The increase of atmospheric carbon dioxide and water vapor will make the greenhouse effect stronger, and the climate may warm up considerably. According to some theories there is an ice age on Mars at the moment, the end of which will bring more favorable weather conditions with it.

Earlier the notion of the variations in the inclination of the axis of rotation of Mars and their magnitude was mainly based on theoretical calculations. In 2005 scientists got more direct evidence for the first time that the position of Mars has really changed a lot in the past billions of years. The Canadian astronomer Jafar Arkani-Hamed studied the large impact basins on the surface of Mars and noticed that five of them – Argyre, Hellas, Isidis, Thaumasia, and Utopia – lie in a straight line, or to be more precise, in a great circle. Three of the basins are easily seen in images and in topographic maps, but the other two were found only after studying the deviations in the gravitational field of Mars. At the site of the impacts the gravity is slightly stronger than in the surrounding areas, since due to the impacts the material at those sites has a higher density.

According to Arkani-Hamed the basins were formed after an asteroid with a diameter of about 1,000 km came close to Mars and broke up, with the fragments hitting the surface of the planet lined up in a row – much the same way as the pieces of the comet Shoemaker-Levy 9 hit Jupiter in 1994. The inclination of the axis of Mars’s rotation at that time can be deduced from the fact that the fragments followed the same orbit and hit different spots only because the planet had time to rotate a little between the impacts. This makes the line of the basins parallel with the ancient equator of Mars.

What makes Arkani-Hamed’s results even more interesting is that the impacts occurred before the formation of the Tharsis region. According to the current view there was lots of water on Mars in those days, flowing in rivers and standing in lakes and seas. If the inclination of the axis of rotation of Mars really was at that time what the theory indicates, it differs from the current inclination by nearly 60°. The areas now near the equator would have been close to the poles, so because of the cooling of the climate a thick layer of permafrost must have formed in those areas with possible layers of groundwater underneath it. And they might still lurk at depths of several kilometers beneath the surface.