Four billion years ago, an interstellar traveler approaching the now-red planet with his spaceship would probably witness beautiful blue skies with fluffy clouds delivering rain and snow, ice-capped mountain ridges, rivers flowing through incised valleys, lakes, and even an ocean covering most of one hemisphere.
A pretty much favorable condition for the birth of life on Mars which, at the current state of knowledge, doesn’t really seem so unlikely. And it doesn’t seem unlikely either, that simple life forms originating from Mars may have reached our Earth traveling on pieces of Martian rocks, ejected at impressive speeds during the frequent impacts of those meteorites that shaped the early surface of Mars and the other inner planets.
Signs of heavy bombarding by meteorites are evident on Mercury; they are concealed on Venus by its thick atmosphere; they are apparent on the Moon, especially on the hidden face – much more exposed than the one facing us – but were obliterated on the Earth by the combined action of tectonics, weathering, and erosion by water, glaciers, and atmospheric processes. And on Mars? Its tectonics and volcanism vanished rather early in its life. Nonetheless, the meteorite craters have been largely reworked by surface processes, most probably by rivers of liquid water that reached the ridges in the form of rain or snow and then flew downslope, producing ravines and incised valleys pretty similar to those found on the Earth.
But how could the Martian atmosphere become warm enough to allow above-freezing temperatures during at least part of the year despite a faint Sun, that was even fainter during its early times? Scientists are still puzzled by the outputs of climate models that can hardly reproduce the necessary conditions. On the other hand, the abundant evidence of stream erosion recorded by our satellites and landers seem incontrovertible.
They definitely point to an early Mars with landscapes somehow familiar to those we are used to on today’s Earth, with mountains, valleys, highlands, plains, and sea, and the sparkle of a gentle sun taking turns with the gloom of rainy days. However, at some point, the present-day Martian desolation took over and continued uninterrupted. Besides the nature and timing of the processes that warmed the early Mars, we also haven’t understood yet for how long and how this ancient warm scenery managed to persist before it somehow cooled and vanished.
Ramses Ramirez from the Tokyo Institute of Technology and Robert Craddock from the National Air and Space Museum in Washington, D.C., shed light on these outstanding questions, taking us on a fascinating journey through the geologic evidence and the struggle with unfitting climatic simulations. In their recent, elegant and enjoyable review on the prestigious Nature Geoscience they examined strengths and weaknesses of various models that have been proposed, which depict either a cold and icy planet with episodic warming – triggered, for instance, by meteoritic impacts or volcanic eruptions – or a more persistent warmer and wetter condition fueled by a mix of greenhouse gases comprising sulfur and methane in addition to water and carbon dioxide.
Both scenarios would result in the presence of liquid water on the planet, enough even to fill an ocean several hundreds of meters deep. However, they differ on the resulting duration and recurrence of precipitations feeding an Earth-like hydrologic cycle, with rain and snow, hillslope erosion and sediment transport by rivers flowing from the mountains into the ocean through incised valleys, alluvial beds and finally meanders, deltas and estuaries.
Ramirez and Craddock systematically find flaws in the cold, icy and arid scenarios, demonstrating crucial inconsistencies with recorded geological and geophysical data and ultimately argue in favor of a warm and wetter scenario, which they prove to be the most capable of reconciling the observed geologic features with the climatic processes that may have generated them.
These findings are described in the article entitled The geological and climatological case for a warmer and wetter early Mars, recently published in the journal Nature Geoscience. This work was conducted by Ramses Ramirez from the Tokyo Institute of Technology and Robert Craddock from the National Air and Space Museum.
A warmly suggested reading: Ramirez R. M. & Craddock R. A. (2018) The geological and climatological case for a warmer and wetter early Mars. Nature Geoscience 11, 230-237. https://doi.org/10.1038/s41561-018-0093-9