The Planets. Professor Cox Brian

The Planets

NASA/JPL-Caltech

READING THE MAPS OF MARS

A map of Mars can be read like a history book. Unlike Earth, where constant weathering, tectonic activity and volcanism have erased the deep geological past, Mars has been relatively quiescent for most of its life. The scars of collisions from the first turbulent billion years after the formation of the Solar System can still be seen from orbit; ancient cataclysms documented below a thin film of dust.

NASA’s Mars Global Surveyor spacecraft spent four and a half years mapping Mars in the late 1990s and provided detailed maps such as the one shown on the previous page, with colours corresponding to differences in altitude. Just as on Earth, there is significant variation, but the geological features on our smaller sister world are much bigger and bolder.

The highest elevations on Mars are found on the Tharsis Rise, a great volcanic plateau and home to the largest volcano in the Solar System, Olympus Mons. At over twice the height of Everest, Olympus Mons towers 25 kilometres above the lowlands of Amazonis Planitia to the west, and its base would fit inside France, just about. Cutting a deep scar across Tharsis to the south-east of Olympus Mons is Valles Marineris, named after the Mariner 9 spacecraft that discovered it, a canyon that dwarfs anything on Earth; the Grand Canyon would fit into one of its side channels.

The lowest points on Mars are found in the Hellas impact basin, the largest clearly visible impact crater in the Solar System. From the highest points on the crater rim to the floor, Hellas is over 9 kilometres deep; it could contain Mount Everest. The atmospheric pressure at the floor is twice that at the rim; high enough for liquid water to exist on the surface in a narrow range of temperatures.

These are extreme altitude differences for a small world; over 30 kilometres from the summit of Olympus Mons to the floor of Hellas. On much-larger Earth, for comparison, there is only 20 kilometres difference between the summit of Everest and the Challenger Deep in the depths of the Mariana Trench.

The most striking and ancient elevation difference on Mars is that between the Northern and Southern Hemispheres of the planet, known as the global dichotomy; Mars is an asymmetric world. The Northern Hemisphere is on average 5.5 kilometres lower in altitude than the Southern. There is no consensus as to how the dichotomy formed, other than that it was early in the planet’s history and before the large impacts which created the Utopia and Chryse Basins around 4 billion years ago. At some later time, the Northern lowlands were resurfaced by volcanic activity in a similar fashion to the smooth lunar seas, which accounts for their lack of cratering relative to the much more ancient terrain to the south.

The oldest terrain on Mars is found in the Noachis Terra region of the Southern Highlands. It is characterised by heavy cratering reminiscent of the far side of the Moon. Even small craters in the Noachian Highlands are heavily eroded, which suggests the regular, if not persistent, presence of liquid water. There are dry river valleys and deltas and evidence of water pooling in the craters and overflowing their walls, forming interconnected networks of lakes. This is how we know Mars was once a warmer and wetter world, at least occasionally; the evidence is written across the Land of Noah.

‘We found three-and-a-half-billion-year-old lake sediments that contained a diversity of organic materials. It tells us there’s actually organic matter present. What we found in those rocks, is what we expected of natural organic matter. It’s what you would expect to find on Earth.’

Jennifer Eigenbrode, astrobiologist

In contrast, the younger terrain of Hesperia Planum displays much less evidence of regular erosion by water, but bears the scars of occasional catastrophic floods that cut deep valleys over very short periods of time and may have formed temporary large lakes or seas.

Winter frosts at the North Pole of Mars are disappearing, revealing the surface features of the ice cap.

Alluvial fans are gently sloping wedges of sediment deposited by flowing water. Some of the best-preserved examples on Mars are in Saheki crater. On Earth, they are found in deserts, for example, in Death Valley, California.

Geological faults have disrupted layered deposits, creating a striking landscape in the northern Meridiani Planum region.

The Grand Canyon, in Arizona, began to form about 1,200 million years ago in the late Proterozoic period. Mars was already long frozen by this point.

The Amazonis Planitia region shows little sign of flowing water, fewer impact craters and less evidence of active volcanism, suggesting it was formed more recently when Mars was significantly less geologically active.

The persistence of surface features over many billions of years in the Noachian, Hesperian and Amazonian regions has led to the historical epochs of Mars being named after the distinctive terrains that still bear the characteristic marks of the climate and geological activity that formed and sculpted them.

The Noachian Period was the earliest and wettest, and coincided with the origin of life on Earth around 4 billion years ago, when conditions on both worlds appear to have been very similar. The Martian atmosphere may have been denser than Earth’s, and dominated by carbon dioxide, but significant questions remain about how such an atmosphere could have warmed Mars sufficiently to deliver the warm, wet climate and how that atmosphere was lost. The MAVEN spacecraft currently in orbit around Mars aims to answer this question, as we’ll discuss later in this chapter. The Noachian Period ended as Mars became increasingly cold and arid around 3.5 billion years ago, just as life was gaining a foothold on Earth.

The Hesperian Period, the time of catastrophic floods, ran from the end of the Noachian to around 3 billion years ago, when Mars entered its current frozen, arid phase, punctuated by occasional volcanic activity and the large-scale movement of ice, but with very little evidence of flowing water. The long 3-billion-year freeze from the end of the Hesperian to the present day is known as the Amazonian.

This is a summary of what we know about Mars; the whys pose a significant challenge to planetary scientists. Given a warm, wet and seemingly stable world early in its history, what triggered the loss of atmosphere and descent into modern-day aridity? What happened to the water on Mars? Was it lost to space or does it persist today as surface ice or in subsurface rocks or reservoirs? If so, how much water is still accessible? Could we exploit the ancient reservoirs of Mars to support a human colony? And perhaps most significantly of all, did life arise on the planet during the Noachian Period, coincident with the origin of life on Earth, and could that life still be present on Mars today?

The current fleet of spacecraft in orbit around Mars and roving across its surface has been designed to answer these questions.

The history of Mars from formation to the present, including major geological events, is shown in comparison to Earth’s timeline. Time is measured in Ga (giga annum): billions of years before the present. The Phanerozoic and Amazonian eons extend to and include the present day on the two planets.

An artist’s illustration of NASA’s Mars Reconnaissance Orbiter (MRO) passing above Nilosyrtis Mensae, a portion of the planet.

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