May 25, 2020

AstroEvent: Exploring the Lunar Apennines & Ina Caldera.

Ina Caldera as seen from LRO. (Credit: NASA).

Whew! With yet another “Super-Moon” weekend has come and gone, and lunar exploration can now be safely left up to those who know and love the Moon. This week, as said Full Moon gives way to waning gibbous, I’d like to draw your attention to a famous mountain range on the edge of the Mare of Imbrium. The Lunar Apennines or Montes Appenninus lie at the edge of a volcanic scarp and feature some of the largest peaks on the Moon rising as high a 5 kilometers above the surrounding terrain (there’s no sea-level on the Moon). The Apennines are flanked by the impact craters Eratosthenes and Copernicus to the west and the older craters Autolycus and Archimedes to the north. Luna 2, the first manmade object to impact the Moon, struck near Autolycus crater in 1959, complete with ideologically appropriate Soviet-era pennants. The entire area has a storied history both in geological and modern lunar exploration, but it’s a tiny feature on the slopes of the lunar Apennines that I’d like to draw your attention to (pictured above) known as Ina Caldera.

The Mare Imbrium/Lunar Apennines region. (Photos by Author).

Looking like a dried out sponge, this 3 kilometer long feature does not look like the typical lunar terrain that we’re used to. In fact, it was first only noticed by Apollo 15 astronauts in 1971. If that mission sounds familiar, it’s because Al Worden performed repeated orbital passes over the area while his crewmates were exploring Hadley Rille, also located in the Lunar Apennine region. NASA’s Lunar Reconnaissance Orbiter revealed Ina Caldara in all of its glory, but don’t expect it to look like much in a telescope. In fact, this is one of the tiniest (less than two arc seconds across!) and toughest objects on Charles Woods’ “Lunar 100” list of objects for visual observers on the nearside of the Moon. #99 on the list, this D-shaped volcanic caldera stands as a testament to recent volcanic activity on the Moon, with out-gassing and subsidence leaving relatively bright areas of lunar regolith exposed. And although those features look flat, they tower 80 meters over the recessed areas, with scarps as steep as 40° degrees! The region has to be young in terms of lunar features because of the conspicuous lack of impact craters mottling its surface like elsewhere on the Moon. In fact, such a collapsed volcanic caldera would be more at home on the surface of the Earth than the Moon.

Exploring the Lunar Apennines! (Photo by Author).

But nabbing this feature is no mean feat; in fact, skilled astrophotographers have only recently managed to image the feature, and even then only as a tiny dot. The nearby Conon crater (named for the ancient astronomer Conon of Alexandria) serves as a good guidepost to the region, and don’t be afraid to “crank-up” the magnification on this one. The region is favorably illuminated a day before First Quarter phase or a day after Last Quarter (which occurs this coming weekend on May 13th) when the Moon is a thick crescent. Keep in mind that the illumination angle of the Moon changes from one lunation to another due to the rocking motion of the Moon known as libration; features that are apparent tonight may vanish one lunar cycle later, even when the Moon has reached an identical phase! This cycle of illumination angles only repeats once every 18 years, and hence it’s worth checking out the Moon to hunt for this elusive feature and explore the exciting region of the Lunar Apennines at every opportunity.

Next week; we head back to Earth for the eclipse highlight of the year as we cover the annular eclipse of May 20th with our comprehensive observing guide and features that you’ll only see here!


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