FIGURE 5.1 Mercury (left), Venus (middle), and the Moon (right) are essential to understanding how terrestrial planets form and change with time. SOURCE: Mercury, NASA/JPL; Venus, NASA/JPL/USGS; Moon, NASA/JPL.

their composition, and about the processes by which they have evolved are a major part of the question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time?


The overarching concept that drives the study and exploration of Mercury, Venus, and the Moon is comparative planetology—the idea that learning about the processes and history of one planet (including Earth) is enabled by an understanding of and comparison to other planets. An understanding of any individual planet relies on knowledge of the whole solar system, which in turn relies on an in-depth exploration of every component of the system: from dust to planets, from Mercury to the outermost comets, from the Sun’s deep interior to the far reaches of the interstellar medium. Comets and asteroids (and meteorites and dust from them) preserve clues to the formation of the solar system and its planets; now-quiescent bodies like the Moon and Mercury preserve evidence of the early histories of the terrestrial planets; large, active planets like Venus and Mars show some of the variety of geologic and climatic processes; all help in understanding Earth’s past, present, and possible futures. And, as the number of known extrasolar planets continues to grow, the goal of understanding Earth and its life takes on the broader dimension of the search for habitable bodies around other stars.

The goals for research concerning the inner planets for the next decade are threefold:

• Understand the origin and diversity of terrestrial planets. How are Earth and its sister terrestrial planets unique in the solar system, and how common are Earth-like planets around other stars? Addressing this goal will require constraining the range of terrestrial planet characteristics, from their compositions to their internal structure to their atmospheres, to refine ideas of planet origin and evolution.

• Understand how the evolution of terrestrial planets enables and limits the origin and evolution of life. What conditions enabled life to evolve and thrive on early Earth? The Moon and Mercury preserve early solar system history that is a prelude to life. Venus is a planet that was probably much like Earth but is now not habitable. Together, the inner planets frame the question, Why is Earth habitable, and what is required of a habitable planet?

• Understand the processes that control climate on Earth-like planets. What determines the climate balance and climate change on Earth-like planets? Earth’s climate system is extraordinarily complex, with many interrelated feedback loops. To refine concepts of climate and its change, it is important to study other climate systems, like those of Venus, Mars, and Titan, which permit us to isolate some climate processes and quantify their importance.

Subsequent sections examine each of these goals in turn.

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