The Mars sessions at the Lunar and Planetary Science Conference (LPSC) are both plentiful and complex, covering a variety of topics in martian history from its beginnings to the present day. Here I'll break down just one of these sessions: "Early Mars Surface Processes: Alluvial Fans and Paleolakes," a mini-session early Tuesday afternoon that engaged martian geomorphologists in a discussion on the ways in which we currently understand early martian climatic and geologic history through preserved fluvial features.

This session of LPSC focused on, as the name implies, the variety of alluvial fans and paleolakes that cover the martian surface. In terms of fluvial features, the southern highlands of Mars are most known for hosting riverlike valley networks, the telltale sign of flowing liquid water billions of years ago. However, Mars also possesses a number of other geologic features associated with the flow of liquid water that allow martian geomorphologists to explore the possibility of a much more complex martian hydrologic history.

Widely held views of Mars' geologic and climatic history in the community state that most of the fluvial activity on the martian surface was concentrated in the Noachian (over 3.7 billion years ago), with very limited fluvial activity in the later Hesperian and Amazonian epochs. In these later periods, chances for the stability of liquid water on the surface became few and far between as Mars transitioned to the hyperarid, hypothermal desert we observe today. However, this session demonstrated the growing inventory of fluvial landforms that formed during and after Noachian – thus, indicating later episodes of aqueous activity. These features have been recently identified in the higher resolution image data sent back from the Mars Reconnaissance Orbiter, and a flurry of mapping has followed; as Alex Morgan said in his talk, "mapping a landform's distribution is the first step in understanding its formation." In this session, we explored the findings of these mapping efforts and other associated investigations.

The first two talks, by Sharon Wilson and Alex Morgan, focused on the formation of alluvial fans, which represent fluvially eroded sediment deposited at a slope break; on earth, these occur at the edges of mountain ranges and hills. Most fan deposits on Mars are located at the slope break formed by the intersection of crater walls and crater floors. While their depositional mechanisms are quite different, alluvial fans align with deltas in that they require water to transport and deposit sediment, and are thus also indicative of regional fluvial activity.

Wilson performed detailed mapping of the large, well-exposed alluvial fans in Roddy Crater to determine the age and sequence of their development as well as possible sources of the water that assisted in their formation. Such in-depth investigations are crucial, as alluvial fan deposition remains a complex process and often occurs in multiple stages. Wilson found that fan morphology varies across the crater, and that a long interval may have passed between initial crater formation and formation of the alluvial fans, suggesting these fans formed in association with fluvial activity in the early Hesperian or later.