'Significant change' in bombardment timing

Among the things complicating the definitive dating of the familiar nearside basins is each shows signs of having been resurfaced more than once after their violent formation. Researchers progressed rapidly with secondary and primary crater counting and by retracing contours of topography based on the principle of superposition, that newer craters disrupt the old. Direct sampling allowed further for radio-isotope dating. Now high-resolution photography from LRO is allowing the reading of topography under nearly all lighting conditions. Painstaking analysis in years past has recently been renewed, suggesting a need for revision to the age of Serenitatis basin.

"A Significant change in our view of the impact process, and the history of the Earth-Moon system" is offered by three leading planetary scientists following a pain-staking analysis of LROC images of the eastern side of Mare Serenitatis.

Research by three eminent planetary scientists in the American Geophysical Union's Journal of Geophysical Universe will almost certainly cause a revision in generally accepted lunar timescale and ages for the Moon's most familiar basins. This is so primarily because the authors have had much to do with gathering the original evidence for the accepted dating over the past four decades. Based on high-resolution photography returned by the Lunar Reconnaissance Orbiter Camera their most recent work is filling gaps in tried and true methods for reading the story of the Moon (and the Solar System) engraved on the lunar surface.

The wide-ranging effect of the impact that formed Mare Imbrium has been obvious since the invention of the telescope. Just how widespread has been more difficult to determine. This LROC Wide Angle Camera (WAC) mosaic shows the mixed terrain of the Sulpicius Gallus area within and adjacent to the southwest corner of Mare Serenitatis basin. Radial grooving from Mare Imbrium (not shown), testifies clearly as to the violence unleashed by that basin-forming impact. Until very recently it was thought Serenitatis basin must have formed after the Imbrium event.  [NASA/GSFC.Arizona State University].

LROC WAC monochrome (643nm) observation M119645947ME, LRO orbit 2766, February 1, 2010. Astronauts Gene Cernan and Jack Schmitt explored the Taurus Littrow valley, in the hills southeast of Serenitatis in 1972. The forces that shaped South Massif (SM), North Massif (NM) and the Sculptured Hills (SH) were thought to have originated with the Serenitatis impact event. More recent study of LROC imagery, however, appears to show their near final form resulted from the Imbrium basin-forming impact [NASA/GSFC/Arizona State University].

The Taurus Littrow Valley, explored by Cernan and Schmitt of Apollo 17 (White Arrow, 1972) is a crossroads of lunar morphology immediately adjacent to the Serenitatis basin. Geologist astronaut Harrison "Jack" Schmitt, for example, confirmed his theory that the "Tortilla Flat" ray of material he and Capt. Cernan explored during their second EVA was radial to the 109 million year old "recent" Tycho crater.

At Shorty crater an abundance of orange regolith had been naturally excavated, offering evidence of ancient fire fountains deep in in the Moon's primeval past. Still, snuggled near the shore of Mare Serenitatis, it was far from certain if the Sculptured Hills and other mountains around the valley, indeed whether the valley itself, had been sculpted out originally by the force of the Serenitatis or the more distant Imbrium basin-forming impact.

During their third and final EVA, the last walk the Moon on December 13, 1972, Cernan and Schmitt had the opportunity to sample "Tracy's Rock," or 'Split Rock', a hefty boulder that had, at some point in the relatively recent past, rolled down the south-facing wall of North Massif where it partly broke apart near the valley floor. It offered an opportunity to analyze and sample part of the high mountains imaged almost four decades later from LRO.

Tracy's Rock - the split boulder that brought a significant sample of the Sculptured Hills-type mountains, in this case the North Massif down to the Taurus Littrow valley floor, where geologist astronaut Jack Schmitt and Apollo 17 commander Capt. Gene Cernan could sample it during the last walk on the Moon, December 13, 1972. At top, the same boulder heap is seen in LROC NAC observation M165645700RE, orbit 9545, July 18, 2011; resolution 47.7 cm per pixel from 40.6 kilometers [NASA/GSFC/Arizona State University].

Distinguished planetary geologist Don E. Wilhelms, retired from the U.S. Geological Service, Paul D. Spudis of the Lunar and Planetary Institute and LROC principal investigator Mark Robinson of Arizona State co-wrote the study published in late December. They conclude LRO imagery show the Serenitatis basin is relatively old, not young. 

Additionally, "an old Serenitatis means Apollo 17 impact melts may not date the Serenitatis basin," and either the late bombardment theory was less likely or the Moon's morphology is more poorly understood than is generally believed.

"New images from the Lunar Reconnaissance Orbiter Camera show the distribution and geological relations of the Sculptured Hills, a geological unit widespread in the highlands between the Serenitatis and Crisium basins. The Sculptured Hills shows knobby, undulating, radially textured and plains-like morphologies, and in many places is indistinguishable from the similarly knobby Valles Alpes formation, a facies of ejecta from the Imbrium basin.

"The new LROC image data show the Sculptured Hills in the Taurus highlands is Imbrium ejecta, not directly related to the formation of the Serenitatis basin. This occurrence and the geological relations of this unit suggest the Apollo 17 impact melt samples may not be not samples of the Serenitatis basin-forming impact, leaving their provenance undetermined and origin unexplained. If the Apollo 17 melt rocks are Serenitatis impact melt, then up to half the basin and a large crater population on the Moon was created within 30 million year interval around 3.8 billion years ago, in a global impact “cataclysm.”

"Either interpretation significantly changes our view of the impact process and history of the Earth-Moon system."

Abstract and Text (Subscription), HERE.
The Sculptured Hills of the Taurus Highlands:
Implications for the relative age of Serenitatis,
basin chronologies and the cratering history of the Moon.

JOURNAL OF GEOPHYSICAL RESEARCH

VOL. 116, E00H03, 9 PP., 2011

doi:10.1029/2011JE003903