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Interior of Mars
As with the other rocky planets, heavier elements inside Mars have sunk towards the centre while lighter ones have risen to the surface, forming a layered structure of core, mantle and crust. Estimates based on the Martian gravitational field suggest that the planet’s core is lighter than those of the other inner planets, probably due to a significant amount of sulfur mixed with its iron and nickel. The lack of a strong magnetic field around Mars today suggests that the core no longer contains liquid metal. However, the presence of sulfur could have lowered its freezing point, allowing the core to remain molten for longer and generating a field for much of the planet’s history that would have helped to protect a once-thicker atmosphere from the stripping effect of the solar wind. Heat rising through the silicate minerals of the overlying mantle powered volcanic activity for much of Martian history. A fair amount of this rising heat seems to have concentrated in a single column, creating a ‘hot spot’ beneath a region called Tharsis, giving rise to huge volcanoes and a vast bulge of accumulated lava flows – the ‘Tharsis Rise’ – that stands up to 7 km (4.4 miles) above its surroundings.
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Mantle of silicate rocks
Crust, perhaps formed from two tectonic plates
Core of iron, nickel and sulfur
Martian geography
The Martian landscape can be divided into two distinct terrains – relatively smooth lowlands in the north (with
the huge bulge of the Tharsis Rise emerging from them), and heavily cratered southern highlands. Seen from space, two of the most prominent features are Syrtis Major, a dark, wedge-shaped plain just north of the equator, and Hellas Planitia, an ancient dust-filled impact basin in the mid-southern latitudes. The boundary between highlands and plains is marked in many places by ‘fretted terrain’, a chaotic jumble of elevated flat-topped mesas with valleys running between them.
The origins of this ‘Martian surface dichotomy’ are much debated, but it’s clear that some major event either erased the record of early cratering across the planet’s northern hemisphere, or prevented this hemisphere’s crust from solidifying at all until the heaviest cratering had subsided. Possible explanations include one or more major impacts from space, or large-scale tectonic processes in early Martian history.
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Olympus Mons
The largest volcano on Mars, Olympus Mons is also the biggest mountain in the solar system, with a shallow dome shape more than 620 km (385 miles) across, rising to 21.3 km
(13.2 miles) above the average Martian surface level. It lies
just off the northwestern side of the Tharsis Rise, and is an enormous shield volcano, created by lava that erupted from multiple fissures along its flanks. Like Earth’s Hawaiian island volcano chain, Olympus Mons sits above a ‘hot spot’ in the underlying mantle. However, because the red planet lacks plate tectonics, the eruptions powered by this source have remained in one place, building layer upon layer of lava over an estimated three billion years. The 85-km (53-mile) crater complex at the peak is a ‘caldera’ – a series of pits created by subsidence when the supporting pressure of molten magma in the underlying mantle was withdrawn at various times. Although the volcano appears dormant today, some of its youngest lava flows have been dated to as recently as two million years ago, so Olympus Mons may not yet be entirely extinct.
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Valles Marineris
One of the most spectacular features in the solar system, the Valles Marineris (Mariner Valleys, named after the Mariner 9 space probe that discovered them in 1972) form a continent-sized scar across the Martian surface. More than 4,000 km (2,500 miles) long, and up to 10 km (6 miles) deep in places, they dwarf Earth’s own Grand Canyon.
Unlike the Grand Canyon, however, this valley system is not the result of water erosion. Running along the southern edge of the volcanic Tharsis Rise, it is thought to have been cracked open by stresses in the Martian crust due to the enormous weight of accumulated lavas. This is not to say that the valleys have always been entirely dry, however – as the lowest-lying part of the surface, its higher air pressure would have made this a favourable location for surface water in the distant past. Confirming this, analysis of minerals exposed by landslips in the vast central depression known as Melas Chasma suggests that the area was once covered by a large lake.
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This 3D view shows the broad ‘Candor Chasma’ portion of the Valles Marineris complex.
Chryse Planitia
Lying to the west of the Tharsis Rise and north of the Martian equator, Chryse Planitia is a low-lying circular plain covered in broad channels that wind between teardrop-shaped mounds. Occupying an ancient impact basin that formed early in Martian history, the overall terrain is reminiscent of the ‘channelled scablands’ found on Earth in the northwestern United States and other areas. In our own planet’s case, such features are linked to the melting of glaciers at the end of
the last ice age – vast lakes, held back for centuries by walls of ice, were suddenly released to cause catastrophic floods that scoured the landscape. On Mars, it seems something similar happened – billions of years ago, a large canyon system called Kasei Valles, which empties into its western side, formed one of the main outflows for water draining off the vast Tharsis Rise. In 1997, NASA’s Mars Pathfinder lander arrived in Ares Vallis, a similar outflow channel that runs into the southeast edge
of Chryse, where its Sojourner rover investigated a variety of highland rocks deposited by ancient floods.
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Polar caps
Seen from Earth, the most prominent Martian features are two distinctive ice caps that grow and shrink with the changing seasons. Each cap consists of a permanent body of deep-frozen water overlaid with a changeable layer of frozen carbon dioxide (CO2 or ‘dry ice’). About 25 per cent of all the atmosphere’s CO2 settles as frost or snow during a particular hemisphere’s autumn, and returns to the air by a process called sublimation (transition directly from solid to gas) in spring. This activity drives prevailing winds.
As small amounts of ice mixed with the ubiquitous Martian dust are left behind in each successive cycle, the polar caps have built up a complex layered structure kilometres deep. Spiral ravines that cut into the ice caps show where prevailing winds carry the sublimated carbon dioxide away. However, water ice extends far beyond the visible limits of the polar caps. Mixed with the soil it forms permafrost beneath the northern plains, while in purer form it produces slow-flowing glacial features, disguised by a thin coating of red dust, amid the craters of the southern highlands.
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A perspective view of the Martian north polar ice cap
The wet Martian past
Evidence that Mars had water flowing on its surface in its ancient past is abundant. Features that resemble the work of water on Earth, including winding river valleys and catastrophic flood features, such as Chryse (see page 146), were discovered
in satellite images during the 1970s. However, alternative explanations remained on the table until the 2000s, when a new wave of orbiters and landers confirmed the widespread presence of hydrated minerals formed in the presence of liquid wat
er.
At first, many scientists believed that the Martian water had been lost forever – evaporated into the atmosphere by ancient climate change and then blown away into space on the solar wind. However, the discovery that much of the Martian soil is ice-rich permafrost suggests a different story. It’s now recognized that the red planet goes through long-term climate cycles thanks to slow variations in its orbit. Mars may even now be emerging from a long ice age, and could develop a thicker atmosphere and surface water once again in the future.
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page 146
Artist’s impression of the shoreline of an ancient Martian sea
Present-day water
Evidence for liquid water running on the surface of Mars today (or concealed in underground watercourses) would be a huge scientific breakthrough, but despite several significant discoveries, the clinching proof remains frustratingly elusive. During the 1990s, images sent back from NASA’s Mars Global Surveyor satellite revealed geologically recent gullies along the walls of several Martian valleys, apparently created by liquid seeping out from a layer just beneath the surface. However, while such features on Earth would immediately be interpreted as evidence of water, alternative explanations, perhaps involving the escape of frozen carbon dioxide, cannot be ruled out in
the case of Mars. In 2011, spectacular pictures from the Mars Reconnaissance Orbiter showed dark streaks on the slopes of a crater called Newton, changing with the Martian seasons and apparently running downhill. These, too, have been explained
in terms of subsurface water but, frustratingly, attempts to detect the water directly (using satellite spectrographs) have so far proved inconclusive.
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The rim of Newton crater shows seasonal dark streaks, perhaps linked to water, running down the crater wall at bottom.
Life on Mars?
Although the idea of plants and advanced life forms on Mars was sadly abandoned as the planet’s true barren nature became clear in the 1960s and 1970s, new discoveries about life in extreme environments, and conditions on Mars itself, have kept open the possibility of simpler past or present life. Claims of ‘microfossils’ and biochemicals in a sample of Mars rock that crashed to Earth
in Antarctica first grabbed headlines around the world in 1996. However, alternative explanations have since been put forward and, though the discoverers stand by their findings, the debate seems unlikely to be settled without new evidence. So far, the only probes to search directly for signs of life were NASA’s 1970s Viking landers (see page 384), although their results were frustratingly inconclusive. More recently, scientists have discovered intriguing seasonal emissions of methane – a rapidly degrading chemical generated by only a few processes, including volcanism and life. In 2018, NASA’s Curiosity rover (see page 394) discovered traces of carbon-based organic chemicals in the soil – not conclusive proof of life itself, but potential traces of its presence.
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page 384
page 394
Interior of the famous Martian meteorite ALH 84001, showing alleged ‘microfossil’ bacteria
Phobos
Mars has two small moons, named after the sons of the warrior god Ares in ancient Greek mythology. Phobos
is the larger of the two – a potato-shaped lump of rock with dimensions of 27 × 22 × 18 km (17 × 14 × 11 miles). Its surface is dominated by a large crater called Stickney, from which curious parallel furrows radiate across the surface. One possibility is that these scrapes were made by ejecta that tumbled across the surface in the weak gravity following the initial Stickney impact, but later escaped into space.
Phobos is also the closer of the two satellites to Mars. It orbits at an altitude of just 6,000 km (3,700 miles) above the surface (60 times closer to Mars than the Moon is to Earth), and circles the planet in 7 hours 39 minutes, clipping the thin upper atmosphere as it does so. In around 40 million years, this orbit will decay to instability and Phobos will begin to tumble out of control, probably breaking up before its fragments crash into Mars itself, forming a new family of craters.
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Deimos
he smaller and more distant of the two Martian moons, Deimos orbits its parent planet in a relatively sedate 30 hours 18 minutes. In contrast to the pockmarked Phobos, its surface is relatively smooth, suggesting that impact craters on its surface have been filled and softened by large amounts of dust.
Astronomers used to assume that these two small Martian moons must be asteroids captured into Martian orbit, although their peculiarly unique surface chemistry and reddish colour led to the suggestion that they might be stray Trojans from around the orbit of Jupiter (see page 192). Recent computer models, however, suggest that a precise capture of even one asteroid by the weak Martian gravity, let alone two, would require a very precise set of circumstances that are unlikely to have occurred in the entire history of the solar system. An increasingly attractive idea, therefore, is that Phobos and Deimos formed in a similar way to Earth’s Moon, coalescing out of debris ejected after a major impact on the parent planet’s surface.
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page 192
The asteroid belt
Although the inner solar system is scattered with asteroids, the vast majority lie within a broad, doughnut-shaped belt between the orbits of Mars and Jupiter. With an inner edge around 2.1 AU and an outer cut-off 3.3 AU from the Sun, the belt contains several hundred million objects of varying size, scattered in mostly empty space (though collisions and close encounters are relatively frequent on astronomical timescales). The first objects to be found here were discovered in the early 19th century, as astronomers looked for traces of a planet in the apparent ‘gap’ between the orbits of Mars and Jupiter.
Only around 200 asteroids have diameters greater than 100 km (60 miles) and the entire belt’s combined mass amounts to just four per cent of Earth’s Moon. Nevertheless, the asteroid belt is commonly held to mark a region where Jupiter’s powerful gravity prevented the formation of a more substantial fifth rocky planet – most of its potential material was either flung towards the Sun or completely ejected from the inner solar system.
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Orbit of Mars
Main asteroid belt
Orbit of Jupiter
Kirkwood gaps
By 1866, enough asteroids had been discovered for US astronomer Daniel Kirkwood to note the existence of a number of ‘gaps’ in the asteroid belt. These asteroid-free regions exist because the orbital periods of any object orbiting here would bring it into repeated close alignment with Jupiter. Therefore, any asteroids that fall by chance into such ‘resonant’ orbits are soon kicked out onto a different path. In 1898,
the first refugee from these regions, a so-called Near Earth Asteroid catalogued as 433 Eros (see pages 164 and 190), was discovered by German astronomer Gustav Witt.
The major Kirkwood gaps are located around 2.5, 2.82 and 2.92 AU from the Sun, in regions where orbital periods are one-third, two-fifths and three-sevenths of Jupiter’s, respectively. Two further ‘gaps’, at 2.06 and 3.27 AU, are generally taken as the inner and outer edges of the main asteroid belt itself. Curiously, some other simple resonances seem to concentrate asteroids rather than ejecting them, producing asteroid ‘families’.
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pages 164 and 190
Kirkwood gaps and orbital resonances with Jupiter
3:1 resonance 5:2 7:3 2:1
Number of known asteroids
300
250
200
150
100
50
0
2.0 AU
2.5 AU
3.0 AU
3.5 AU
Average distance from Sun
Near-Earth Objects
The discovery of asteroids orbiting within the main belt, on elliptical paths that bring them close to the strong gravity of the rocky planets, seems surprising at first. Any object in such an orbit is living on borrowed time and will inevitably experience close planetary encounters that will alter its orbit, perhaps ejecting
it from the solar system, hurling it into the Sun, or ending with a spectacular planetary collision.
The most likely explanation for the significant number of such bodies, then, is that they are recent arrivals from elsewhere
in the solar system. Most probably originated in the main asteroid belt and fell into their present orbits after straying into a resonant orbit with Jupiter (see page 162). Known
as either Near-Earth Asteroids or, more inclusively, Near-Earth Objects (NEOs), their orbits are typically unstable over timescales of millions of years, so while we might predict their paths and the likelihood of close approaches to Earth in the short term, the more distant future is far harder to determine.
Solar System in Minutes Page 6