50 YEARS OF IMAGING FLUVIAL FEATURES ON MARS: A CONTINUING HISTORY OF THRILLING DISCOVERY (Invited Presentation)

The HiRISE instrument, conceived by 2019 Gilbert Award recipient Alfred McEwen, continues a 50-year history of imaging fluvial features on Mars. Ironically, though fluvial landforms on Mars were first imaged in 1969 by Mariner 6 and 7, there was no initial recognition of their significance. The thrill of discovering the fluvial legacy of Mars began with the 1971 orbital insertion of Mariner 9. After a planetary dust storm dissipated in early 1972, Mariner 9’s vidicon imagery revealed the global context of fluvial dissection. With Daniel Milton I was privileged to contribute to the earliest investigations of Martian fluvial landforms, particularly those features that came to be known as “outflow channels.”

Following Mariner 9, a succession of missions added many more discoveries relevant to the fluvial evolution of Mars. Results from these missions answered important questions, but some of the biggest mysteries remain. How and why, over billions of years, did today’s cold, desert planetary surface evolve from the very ancient times when the Earth-like fluvial landscapes were generated? What was the role of surface water bodies in facilitating the active hydrological cycling needed to produce the fluvial runoff that shaped the early Mars surface? What happened to the huge volumes of water that had to be associated with producing the Earth-like environments on early Mars, and how is all this related to the possibility of a Martian biosphere?

Mars undoubtedly has the answers, but Mars is also very stubborn in revealing its secrets. Those who would investigate need to probe with the right instruments and make the right kinds of interpretations. In the follow-on period to the 1976 Viking Mission a high-resolution imaging system was nearly eliminated from mission planning. It took nearly two decades before the Mars Observer Camera (MOC) was deployed, leading to discoveries of deltas, alluvial rivers, gullies, and sedimentary rocks. A similar delay occurred with deployment of a high-resolution visible near/IR spectrometer—a failure made apparent by the later discoveries of Mars’s aqueous mineral evolution via the OMEGA and CRISM instruments. Given this history we are indeed fortunate to have Alfred McEwen and like-minded visionaries, who know how to probe with the right instruments to make the right kinds of interpretations.