7. The Habitable Zone — Venus, Earth, and Mars

Overview

Compared to the Moon and Mercury, you will find Venus, Earth and Mars to be far more dynamic and complex. The Moon and Mercury are airless, lifeless worlds where there is little hope that life could ever have emerged. The Earth, on the other hand, is full of life. Earth is the standard to which we compare all other planets in our search for possibilities of life or potential habitability elsewhere in the Universe—and as you’ll see in this module, scientists now think that long ago both Venus and Mars were far more similar to Earth than they are today. Spacecraft and rover observations have confirmed that both planets once hosted flowing surface water and, likely, more temperate climates.

Life on Earth exists in hydrothermal vents deep in oceans where light is never seen. Organisms have been discovered which need no oxygen for growth. Studies tell us that there are just three essential ingredients for life: an energy source, organic material, and liquid water. Modern astrobiology also recognizes the importance of chemical energy gradients—sources of disequilibrium that can drive metabolism. Given the relative abundances of energy sources (e.g. stars, radioactive elements, hydrothermal vents) and the elements that form organic molecules (viz. carbon, though generally we’re interested in compounds containing hydrogen, nitrogen, and oxygen, elements which are now abundant due to fusion processes that happen in every star throughout the Universe), by far the rarest of these seems to be liquid water.

A comparison of Venus, Earth and Mars, provides a useful basis from which to determine the factors that are required for a planet to sustain liquid water, since it was thought to be present on all three planets early in the Solar System’s history. Comparing the physical characteristics of these three planets brings to mind the story of Goldilocks and the Three Bears. Venus, with a surface temperature of about 737 K (464 °C) and a pressure ≈ 92 times Earth’s, is much too hot and dense to support life as we know it. Mars has a very thin atmosphere, and its average temperature is too cold—well below the freezing point of water. Earth is just right. If all three planets were once similar, what happened to Venus and Mars?

In essence, the thinking is that Venus is too close to the Sun and fell victim to a runaway greenhouse effect early in its history which boiled off all its water and carbon dioxide into a thick cloud layer that maintains a very high temperature by trapping what little light and heat it actually lets through (Venus reflects about 75–80 % of incoming sunlight). Recent observations from orbiters such as Venus Express and Magellan suggest ongoing or geologically recent volcanic activity, helping maintain its thick CO₂ atmosphere. If Venus had been further from the Sun, in a region known as the Sun’s Goldilocks—or, Habitable—Zone, scientists believe its atmosphere could have rained out sufficient amounts of greenhouse gases (e.g. H2O, CO2) to maintain lower temperatures and a healthy hydrologic cycle, and its evolutionary history would have been similar to Earth’s. Mars lies within the Sun’s habitable zone, but with a mean global temperature near 210 K (−63 °C) and a thin CO₂ atmosphere, liquid water can now exist only transiently or underground. Therefore, the fact that Mars has mostly lost its atmosphere, so that water no longer flows on its surface, indicates that there are other important factors required to sustain an Earth-like planet.

In this module, you will explore what is currently known about the factors that come together to sustain our planet’s hydrologic cycle. You will begin by looking at just the Earth on its own, exploring how it maintains its fine balance of enough, but not too much atmosphere. You will then move on to investigate the ways in which the mechanism that supports things on Earth is thought to have failed on both Venus and Mars, resulting in their loss of permanent liquid water and, we believe, their potential to support life as we know it.

Ongoing missions such as NASA’s Perseverance rover, ESA’s ExoMars Trace Gas Orbiter, and JAXA’s Akatsuki orbiter at Venus continue to refine our understanding of planetary habitability.

Learning Objectives

When you have finished this module, you should be able to do the following:

  1. Examine the observational histories of Mars and Venus.
  2. Investigate the conditions necessary for sustained liquid water on a planet’s surface.
  3. Contrast Earth’s atmosphere with the atmospheres of Venus and Mars, and evaluate the evidence that Venus and Mars once had atmospheres similar to Earth, including evidence from recent orbital and surface missions.
  4. Explore the geological and atmospheric evolution of Venus, Earth, and Mars, and assess the roles of volcanism, magnetic fields, and plate tectonics in planetary habitability.
  5. Explain how the concept of the habitable zone defines the limits for surface liquid water around stars.

Key Terms and Concepts

  • albedo
  • global magnetic field
  • plate tectonics
  • volcanism
  • greenhouse effect
  • habitable zone
  • runaway greenhouse effect
  • carbon cycle
  • dynamo effect
  • atmospheric escape
  • hydrologic cycle
  • Perseverance rover
  • ExoMars Trace Gas Orbiter
  • Akatsuki orbiter
  • Venus Express mission