Suppose we detect a planet half the size of Venus orbiting a 5 billion year old M-type star at 0.5 AU. To our surprise the planet has detectable radiation belts. How might the planet's climate and surface habitability differ from that of Venus?
The prospect that the scientific community might be faced in the next decade or two with questions like the one above is exciting. It is also daunting, because we will likely be required to make inferences about distant planets based on partial information about their environment, orbit, and characteristics. Fortunately, we have at our disposal abundant information about the planets in our own solar system, and more than a half century of practice studying how very complex climate systems function.
This chapter provides the interested reader with an overview of the likely links between heliophysics and climate. Climate is typically defined as the long-term (multi-decade or longer) average of weather. For example, Merriam Webster defines climate as “the average course or condition of the weather at a place usually over a period of years as exhibited by temperature, wind velocity, and precipitation”, while Wikipedia currently describes climate as “a measure of the average pattern of variation in temperature, humidity, atmospheric pressure, wind, precipitation”, atmospheric particle count, and other meteorological variables in a given region over long periods of time. Climate studies therefore investigate properties of planetary atmospheres – properties that are influenced by both intrinsic characteristics of the planet and by interactions with the host star. To determine whether a planet is or has been habitable at its surface, therefore, it is helpful to understand how a variety of processes act together to influence climate over time. Heliophysical processes are important components of this understanding.
Current climates of terrestrial planets
This chapter focuses on the global climates of terrestrial (rocky) planets Venus, Earth, and Mars. Because they have atmospheres, gas and ice giant planets such as Jupiter, Saturn, Uranus, and Neptune have climates, as do planetary moons with gravitationally bound atmospheres ranging from very thick (e.g., Saturn's moon Titan) to considerably more tenuous (e.g., Jupiter's moons Io and Europa). Dwarf planets (e.g., Pluto) also have climates, though we know relatively little about them at present. The terrestrial planets are of special interest because they are thought to have been habitable at their surfaces at some point during solar system history.