Mars once had an atmosphere capable of supporting vast reserves of liquid water until about billion years ago. lttiz, CC-BY-SA 3.0

The Greenhouse Effect, Martian-style

In order to properly debate our potential future on Mars, it makes sense to take the long view, considering the broader geological history of the world beyond ours.

On Earth, the greenhouse effect is familiar: a certain arrangement of gasses in our atmosphere (including carbon dioxide, water vapor, and methane) traps much of the incoming radiation from the sun, converting it to heat and warming our planet. Over the past few centuries, human-driven industrial activity has released untold gigatons of greenhouse gasses into the atmosphere, speeding the warming of our planet. Yet Earth is not the only planet in our solar system with a greenhouse effect in its atmosphere.

Venus, our nearest interplanetary neighbor, experienced what is known as a runaway greenhouse effect some time in the last billion years. In this runaway greenhouse effect, Venus went from a planet that may have had a habitable atmosphere & vast oceans of liquid water to one with a toxic atmosphere & dry barren land, inhospitable to even the most extremophilic forms of known life. The water that may have made up Venus’ oceans began to evaporate into its atmosphere, trapping more heat and leading to a death spiral that eventually boiled up any remaining pockets of liquid water on the planet’s surface. Venus, however, is not the only one of our solar system neighbors that may have experienced a dramatic change in its greenhouse effect in its distant past.

A new study in Science Advances explores the climatic development of Mars, pointing to a key shift in the Red Planet’s history more than 3 billion years ago that led to the end of the planet’s hot, wet, and habitable era. Scientists have puzzled over the stark difference between the deep past of Mars and its present. The scars left on the Martian surface by ancient rivers are visible from orbit, but even the best observations of our robotic Mars missions have not provided much insight into how the planet became the vast and dusty desert it is today.

The study modeled the change in temperature between the Mars of 3.6 Billion Years Ago (Top) and the noticeably colder Mars of 3.0 Billion Years Ago

The study, which was funded by NASA and led by researchers at the University of Chicago, the Jet Propulsion Laboratory, and other institutions, details one possible model for the Martian climatic shift. The study first analyzes the differences between Martian rivers that formed prior to 3.6 billion years ago, in what is known as the Noachian era, and those formed between 3.6 and 3 billion years ago. The later river systems were found to be patchier in their distribution and lower in elevation, indicating a more intermittent period of wetness relative to the Noachian era’s more humid climate. In short, Mars was warm and wet, then cold and wet, before it reached its current cold and dry state. The study’s authors point to “a waning greenhouse effect” as the likely explanation of this change, finding it probable that Mars’ already-thin atmosphere further lost heat-trapping abilities by 3 billion years ago.

Most surprising, though, is the root cause of the decay of Mars’ greenhouse effect. While many scientists have suggested that a loss of atmospheric carbon dioxide (CO2) made Mars uninhabitable, the study in Science Advances finds instead that changes in the non-CO2 greenhouse gas concentrations spelled doom for the prospects of life on the Red Planet. The global climate model used in the study found that changing the concentration of carbon dioxide in the atmosphere over time had little effect on the overall climatic scheme, while other atmospheric substances like methane and hydrogen gas more strongly drove changes in radiative forcing.

The results of the study provide a fascinating insight into both the history of one of our solar system neighbors and into humanity’s futures on Earth and in the broader cosmos. In order to properly debate our potential future on Mars, it makes sense to take the long view, considering the broader geological history of the world beyond ours.

Learn More:

  • Read the full study in Science Advances
  • Watch Long Now Executive Director Alexander Rose and geologist Marcia Bjonerund discuss whether or not Mars could serve as a solution to Earth’s problems in the Q&A to Bjonerund’s Long Now Talk:
  • Watch Andy Weir’s Long Now Talk on his novel The Martian and what a real-world mission to Mars would look like.
  • Watch NASA engineer Adam Steltzner’s Long Now Talk discussing his experiences as lead engineer on the Mars Science Laboratory’s Entry, Landing, and Descent phase.
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