Chronicles of Mars

A few weeks ago, I gave an overview of the geologic and human history of the Moon. Now, I shall do the same with Mars. The red planet has been an object of fascination for ever since Christiaan Huygens sketched the first primitive map of Mars’s surface in 1659. Since then, more than a dozen orbiter and rover missions have filled in many of the details.

Over the past 4.5 Billion years since its formation, Mars has been scarred by craters, covered in lava from giant volcanic eruptions, covered in water by floods and possible transient oceans, and rivers, and covered in dust by eons of being a windswept desert. Recently, Mars has also entered the Anthropocene with arrival of human-built rovers, landers and orbiting spacecraft. Since Mars is the only planet other than Earth to have its own satellite array, you could say it is one of the most technologically advanced worlds in the solar system, second only to Earth, or the most advanced if you consider that it is entirely populated by alien robots, aliens from Earth that is.

Also, like Earth, Mars is experiencing an ecological transformation as it is likely that humans have contaminated Mars with microbial Earth life or will soon despite our best efforts to avoid it. Instead of Martians invading Earth, it is a case of Earthlings invading Mars.

The Martian geologic timescale is divided into four epochs: the Pre-Noachian, the Noachian, the Hesperian, and the Amazonian. On Earth, the boundaries between geologic time periods are determined by mass extinctions. On Mars and the other planets of the solar system, geologic time period boundaries are determined by volcanic eruptions and major impact events.

The Pre-Noachian begins with the formation of the planet Mars ~4.6 billion years ago (Ga) to ~4.1 billon Ga During the Pre-Noachian, Mars was likely still cooling. vey early Mars may have had a global magma ocean which cooled to form the Martian crust. The Pre-Noachian also was likely a time of frequent impact events from collisions between early Mars and protoplanets, the building blocks of the inner planets. By the end of the Pre-Noachian the surface of Mars had cooled enough that liquid water could exist at its surface, part of the transition to the Noachian Epoch.

Figure 1. Mars during the Noachian? Image credit: NASA/GSFC

The Noachian (~4.1-3.7 Ga) is not far removed from the Pre-Noachian. Named for the Noachis Terra region on Mars, the Noachian epoch was dominated frequent impact events fom colliding asteroids or protoplanets. The major Martian impact basins, like Argyre Basin and Hellas Basin, likely formed around this time. It is even possible that the northern lowlands (Vastitas Borealis) in the Martian northern hemisphere are the result of giant impact by an asteroid ~500 km or ~300 miles across. The two moons of Mars, Phobos and Deimos, may be fragments from this collision. A sample return mission could help confirm whether the Martian moons are fragments of Mars or captured asteroids. The Russian space agency, Roscomos, attempted a sample return mission (Fobos-Grunt) in 2011. It unfortunately failed to leave Earth’s orbit.

Figure 2. A helpful global elevation map of Mars for context. Image credit: Emily Lakdawalla at the Planetary Society originally adapted from a NASA map.

The Noachian is also a time when Mars likely had a thicker atmosphere. There is geochemical evidence from the Mars Curiosity and Perseverance rover missions that lakes of liquid water once existed on the surface of Mas and orbiter image data reveal evidence of possible paleo-shorelines of Martian oceans and valley networks, which may have been rivers. If there ever was a time when Mars could have hosted life, it would have been around this time. This all changed when Mars lost its thicker atmosphere. This was likely due to solar wind stripping the atmosphere away due to a weak or absent magnetic field. This transformed a relatively clement ancient Mars to the barren, desiccated wasteland it is today.

The Hesperian epoch (3.7-3.0 Ga) is named for Hesperia Planum, an ancient lava plain on Mars. Hesperia Planum form during the Hesperian epoch, but the Hesperian is no longer tied to the volcanic plain. The Hesperian was a time of climate shifts. The valley networks disappeared and ice sheets covered Mars at time. These ice sheets occasionally melted causing great floods that created transient lakes. The Hesperian is also when the giant Arizona-sized volcano, Olympus Mons, in the Tharsis region of Mars began to erupt, accumulating so much lava over millions of years.

The next and final epoch in Martian geologic history is the Amazonian epoch (3.0 Ga-Present). It is also longer than the three previous epochs, combined, ~3 billion years. The Amazonian is marked by a gradual reduction in volcanic activity. Mars also experienced much fewer impacts than in previous epochs. By the Amazonian, the atmosphere was worn thin by solar wind and eventually liquid water could no longer be sustained on Mars except for brief periods at the equator or in deep basins like Hellas basin, where the atmospheric pressure is slightly higher. It is during the Amazonian that Mars became the planet it is today, dry, barren, windswept, and almost no atmosphere.

Figure 3. Olympus Mons, the largest volcano in the solar system. Image credit: NASA/JPL-Caltech.

Nonetheless, even Amazonian Mars has its surprises. Volcanic activity, although less common in the Amazonian, may have persisted until 180-200 million years ago, the recent past in geologic terms. Mars was long thought to be geologically dead, but it may just be dormant. It is also possible that transient puddles of liquid water can be sustained for a few hours on the floor of Hellas basin during the warmest day of the Martian year. This does not mean life could exist at the surface, but it does make the case for deep subsurface layers of brine where life could still linger on Mars more promising.

A new era history of Mars began when on December 2, 1971, the Soviet Mars 3 lander touched down on red planet. Communication with the probe abruptly ended seconds later, but it was beginning of series of robotic visitors to the red planet. The number of robots on Mars continues to grow as the United States, ESA, China, and India have all sent orbiters, and landers and rovers in the case of China and the United States. These robotic explorers are expanding our knowledge of Mars and may be paving the way for humans to settle Mars.

Until the 20^th century Mars had an atmosphere, albeit not much of one, and a geosphere. It did not have a biosphere let alone a technosphere. Mars, like Earth and the Moon, now has a technosphere because of the many active orbiters and rovers. Early in solar system history, Earth and Mars both experienced catastrophes, meaning they both experienced dramatic, functionally irreversible changes to their planetary environments. For Mars, it was the loss of its atmosphere. For Earth, it was the emergence of life. In the next century, Mars may experience a similar catastrophe.

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