A moonwalker’s perspective 30 years later
The date was December 11, 1972, and the occasion was the last Apollo mission to the Moon. Apollo 17 astronaut Harrison Schmitt landed in the Valley of Taurus-Littrow, the only scientist and the last of 12 men to step onto the lunar surface. Standing in a brilliantly sun-lit valley deeper than the Grand Canyon and gazing at a nearly full Earth in a deep black sky, Schmitt’s questions about the origins of the Moon and terrestrial planets and their subsequent history took on very personal significance.
Today, geoscientist Schmitt, literally one in six billion human beings to combine science with actual lunar exploration, continues to ponder those big questions. He’ll share some of the results of his synthesis of the research of many others on Tuesday, Oct. 29, at the annual meeting of the Geological Society of America in Denver, CO. At the GSA Planetary Geology division’s Gilbert Lecture and Award Ceremony, Schmitt will discuss “A Lunar Field Geologist’s Perspective 30 Years Later: Shocking Revelations about the Moon, Mars, and Earth.”
Shocking? The orange “soil” or pyroclastic glass that Schmitt found on the Moon, for example, continues to provide clues about the origin of the Moon. In Schmitt’s view, it also reveals why the prevailing Giant Impact hypothesis of the Moon’s origins doesn’t work.
“The major problem with this hypothesis,” says Schmitt, “is that the interior of the Moon is not cooperating. Most importantly, the lower lunar mantle, based on analyses of the Apollo 17 orange pyroclastic glass, has a chondritic, that is, primordial elemental and isotopic imprint. This primordial imprint would have disappeared or have been significantly modified if the mantles of the Earth and the impactor had already formed as required by the current Giant Impact hypothesis.
According to Schmitt, “If the Giant Impact hypothesis is not compatible with this evidence, alternatives to it should be considered, including capture of a small, independent planet from a solar orbit near that of the Earth’s.”
Similarly, many scientists agree that the Moon’s 50 or so basins greater than 300km in diameter, as well as most other ancient lunar craters, were formed at about the same time by an apparent “cataclysm” 3.9 billion years ago. According to Schmitt, “the primary argument against this hypothesis is found in the sampling sites for Apollo and lunar meteorite samples of impact-created glass for which formation ages have been determined. These samples have come largely from the surface of the Moon most affected by the 14 youngest large basin-forming impacts and debris thrown from them. These 14 youngest impacts are, indeed, 3.9-3.8 billion years old based on the dating of Apollo samples. A variety of volcanic and impact evidence indicates that it is highly unlikely that all the 35 or more older impact basins formed during the same interval.
“One of the most exciting aspects of studying lunar origin and evolution is applying that understanding to the early Earth and Mars,” says Schmitt. “And herein lies a ‘shocking’ revelation about the possible origin of Earth’s first continents.”
The 2500km diameter basin on the far-side of the Moon, known as South Pole-Aitken, records an impact of an extraordinarily energetic object near the end of the period of smaller scale saturation cratering that followed the solidification of the lunar crust. South Pole-Aitken is just the most obvious manifestation of possibly three or four other such huge early impacts, including the 3200km diameter front-side basin called Procellarum.
Schmitt estimates that the Procellarum basin formed at about 4.3 b.y and South Pole-Aitken at about 4.2 b.y. If these formation ages are in the ballpark, they suggest an explanation for detrital zircon (ZrSiO4) crystals of about the same ages in very old sedimentary rocks on Earth. Early impacts of the scale of South Pole-Aitken and Procellarum, occurring in water-rich environments such as the Earth and Mars, would create thick sheets of impact generated rock melt on a continental scale. As these magma sheets crystallized, zirconium concentrations may have reached levels that produced the very old zircons.
And what about Mars? Schmitt also suggests that there is evidence for and reason to believe that Mars had both early (older than 4.2 billion years) and late (younger than 3.8 billion years) oceans due to separate periods of intense volcanic eruptions that included abundant water. The shores of these two oceans appear to have been identified in the data returned by the Mars Surveyor spacecraft now in orbit around that planet. Further, he speculates that the most stable ecological niche for Martian life has been the boundary between the subsurface water ice zone and liquid water expected beneath that zone. If simple, one cell life forms evolved on Mars in parallel with their evolution on the Earth prior to 3.8 billion years ago, they may have adapted to survive in this global niche as the surface of Mars became hostile to any life.
“Extrapolating what we now know about the Moon and applying it to Earth, Venus, Mars, and Mercury – the terrestrial planets – is one of the primary scientific returns of lunar research. But looking ahead, the Moon will also mature our thinking about the gas giants and other parts of the solar system,” says Schmitt. For example, whether as a result of a cataclysm or not, where did the objects originate that created the 50 or more large basins on the Moon? He’ll continue to contribute to that work, this time with his feet firmly planted on Earth, while developing a business rationale to return to the Moon for its energy resources.
“A Lunar Field Geologist’s Perspective 30 Years Later: Shocking Revelations about the Moon, Mars, and Earth”
Harrison H. Schmitt
Gilbert Lecture ñ GSA Planetary Geology Division
Tuesday, Oct. 29, 6:00-7:00 p.m.
CONTACT INFORMATION
During the GSA Annual Meeting, Oct. 27-30, contact Christa Stratton at the GSA Newsroom in the Colorado Convention Center, Denver, Colorado, for assistance and to arrange for interviews: 303-228-8565.
Post-meeting contact information:
Harrison H. Schmitt
schmitt@engr.wisc.edu
505-823-2616
Ann Cairns
Director of Communications
Geological Society of America
acairns@geosociety.org
303-357-1056
