• What is the history of volatile compounds across the solar system?

• What is the nature of organic material in the solar system, and how has this matter evolved?

• What global mechanisms affect the evolution of volatiles on planetary bodies?

This crosscutting theme recognizes that our concept of the “habitable zone” has been overturned, and greatly broadened, by recent findings on Earth and elsewhere throughout our galaxy. Key SSE decadal survey questions associated with this theme that are addressed by exploring Mars include the following:

• What are the habitable zones for life in the solar system, and what planetary processes are responsible for generating and sustaining habitable worlds?

• Does (or did) life exist beyond Earth?

• Why did the terrestrial planets differ so dramatically in their evolutions?

This crosscutting theme concerns the search for a deeper understanding of the fundamental mechanisms operating in the solar system today. A key SSE decadal survey question associated with this theme that is addressed by exploring Mars is the following:

• How do the processes that shape the contemporary character of planetary bodies operate and interact?

In response to the question, How does the Mars Exploration Program fit into the overall goals for the solar system as defined by the SSE decadal survey?, the committee is in agreement that because the exploration of Mars addresses all of the crosscutting themes and many of the key questions identified in the SSE decadal survey, it continues to be a significant priority in a balanced program for exploring the solar system.

What major new discoveries have occurred since the SSE decadal survey report was issued that might call into question the recommendations of that report? The solar system exploration decadal survey, New Frontiers in the Solar System, captured the state of Mars exploration in early 2002, and the recommendations of that study were based on that state of knowledge. A range of seminal observations have been made since that time, and these properly influence the view of this committee. These observations address two equally fundamental issues about the history and evolution of Mars.

• Evidence for the past presence of water; and

• Evidence of a diverse igneous history, possibly extending to the present.

Key recent discoveries relating to the presence of water in the martian past include the following:

• Observations from Mars Odyssey for the presence of abundant near-surface hydrogen (possibly indicative of water ice) at mid- to high latitudes;¹

• The inference, from Mars Global Surveyor observations of layered sedimentary rocks in craters, that large areas of the martian surface have periodically been buried and exhumed;²

• The positive identification of clay minerals in ancient terrains by Mars Express;³