Targeting a Lunar Bulls-Eye

This oblique view of Orientale emphasizes the distribution of ejecta material on the southeast side of the basin. Portions of all three major basin rings are visible. Basin ejecta begins just outside the Cordillera Mountains and extends up to 500 kilometers beyond the base of the mountains. This ejecta has a rough, hummocky texture and contains linear patterns that point back at the center of Orientale. Credit: NASA/Lunar Orbiter image IV-172M

Researchers have used Earth-based radar to examine ejecta material from the massive impact that formed the moon’s Orientale impact basin. Orientale basin is located at the very edge of the moon’s visible face. Previously, this crater was difficult to study because it is only partially visible to telescopes on Earth. Newly developed techniques now have been used to make high-resolution radar observations of this important region.

The Orientale basin was formed early in the moon’s history. The crater is surrounded by three concentric circles and has the appearance of a giant ‘bulls-eye’. Studying ejecta from the impact that created this massive lunar bulls-eye can teach us about the history of the moon and how it developed over time.

Impact features on the lunar surface can also help scientists understand the impact history of Earth. Ancient impacts on Earth have played a major role in the evolution of our planet. They’re thought to have caused a number of mass extinction events. Asteroids and comets may also have delivered important precursor molecules for the development of life.

Impact Evidence

Studying ancient impacts on Earth is difficult. Volcanism, plate tectonics and weather have erased the evidence of old craters on our planet. Unlike the Earth, the surface of the moon isn’t altered by wind, erosion or volcanoes. This is why the lunar surface is riddled with craters. Studying these impact features on the moon can help scientists determine what types of impacts may have occurred on Earth and when they most likely happened.

Harrison Schmitt investigating the unique geology of the lunar surface during Apollo 17, the last manned mission to the moon. Today studies of the moon’s surface must be made from a distance.

Craters on the moon are associated with ejecta materials — chunks of rock and dust that are tossed into the air by impacts. The dusty lunar regolith is mostly pulverized ejecta, and it can be several kilometers thick in places. Previously, scientists had difficulty figuring out which ejecta deposits were associated with specific impact basins.

Digging Deep

The new Earth-based radar observations allow scientists to look deeper into the regolith, and to put a number to the rocks associated with Orientale basin. The research team has determined what the different kinds of ejecta material look like and just how far they spread over the lunar surface. Surprisingly, ejecta from the Orientale basin can be found over much of the moon’s south polar highlands. In fact, material ejected by the impact contributes significantly to the composition of the lunar regolith in this region.

The recent study will help define the scientifically interesting places for future human missions to explore, and what scientists should be looking for when they land.

The research team, led by Rebecca Ghent of the University of Toronto, feels that the findings have "implications for future exploration of the south polar region and the South Pole–Aitken basin," both of which are "likely targets for future landed and sample return missions." The paper was published in the May edition of the journal Geology.