The Red Planet: A Natural History of Mars by Simon Morden

Author:Simon Morden [Morden, Simon]
Language: eng
Format: epub
ISBN: 9781783965618
Google: XbFgzgEACAAJ
Publisher: Elliott & Thompson, Limited
Published: 2021-09-15T23:29:40.688800+00:00

THE HESPERIAN CLIMATE CHANGE

The beginning of the Hesperian era is defined by a dramatic change in Mars’s climate. The late Noachian had been mild and generous – rich with water and covered by a thick blanket of atmosphere. It would have been still, obviously, uninhabitable by us, but it had potential.

Sustained volcanism changed everything. Once magma broke through to the surface in sufficient quantities, gases poured into the atmosphere and the quality of the air altered. One single volcanic eruption might have had a significant local effect on air quality – in terms of not only dust and ash, water and carbon dioxide, but also sulphur dioxide and other sulphur compounds, hydrogen fluoride, hydrogen chloride and carbon monoxide. But more eruptions, either all at once or intermittently and repeatedly over tens of millions of years, fundamentally altered the global balance.

The ocean, up until this point, was mildly alkaline, and conditions were conducive to the production and laying down of clays. While extra carbon dioxide and water vapour bolstered the thinning Martian atmosphere, the other volcanic gases readily dissolved in rain to form acid solutions. The slow drip-drip of acid then fell on the highlands and found its way into the northern ocean and the crater-lakes. It shifted the balance from clay to the laying down of sulphates – gypsum and other salts.

Mars’s water gradually became more acidic, and it stayed that way from then on. Whether or not that ended any nascent Martian life is unknown, but extremophiles are usually highly specialised organisms. Those that are resistant to heat are not usually also resistant to acid, and vice versa.

It’s thought that Hesperian-age volcanic rock eventually covered almost a third of Mars – much of it in the form of flood basalts in the northern lowlands, as well as on Tharsis and on the floors of highland craters, notably Hellas. Much of this seems to have started in the late Noachian and intensified in the early Hesperian.

On its own, volcanism would have raised the temperature of Mars a little, but there was a greater and more dramatic change happening far above sea level. The volcanic sulphur dioxide aerosols high in the atmosphere reflected sunlight back out into space, causing the air below, and therefore the ground, to cool rapidly. The gases tended to wash out quickly – within decades – and turn into acid rain, but because the volcanoes constantly belched out more sulphur, there was always enough to keep the levels up. The immediate effect was a global chilling. Frost formed where it had never formed before. High-latitude lakes iced over. The longer those conditions persisted, the deeper the cold bit. The first frosts on the equator signalled that the climate had tipped in favour of winter.

Water-saturated ground turned from being merely ice-covered into permafrost. Ice penetrated tens of metres deep. Lakes froze from top to bottom. Ice caps formed, grew larger and crept towards the lower latitudes. In the highlands, snow fell, accumulated and didn’t melt in summer. Persistent, year-round ice turned into glaciers.

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