Mystery Maria

Home , Lacus Autumni, Lacus Veris, Mare Anguis, Mare Crisium, Mare Frigoris, Mare Insularum

exploring the moon by charles a. wood

Mystery Maria

nearly all the Moon’s big maria (“seas”) are circular because they fi ll ancient impact basins. But Mare Frigoris The Lunar 100 isn’t. Frigoris (L26 in the Lunar 100) is a 1,500-kilometer- L Feature name Significance long (930-mile), 200-km-wide arc of lava that spans from 10 Mare Crisium Mare contained in large circular basin Oceanus Procellarum in the west to the craters Atlas and Hercules in the east. 26 Mare Frigoris Arcuate mare of uncertain origin One characteristic of Mare Frigoris that See Sky & Telescope: April 2004, page 113, or point your Web browser to SkyTonight A few of the .com/lunar100. may help explain its origin is the fact Moon’s biggest that it’s mostly concentric with the Im- features are brium impact basin. But Frigoris isn’t the “basins” typically have two to six concentric rings and are only mare hugging Imbrium’s rim. Mare more accurately referred to as multiring impact basins. also its most Vaporum, Sinus Medii, Sinus Aestuum, This aspect is best illustrated by the far-side basin Orientale puzzling. and Mare Insularum help defi ne a moat — the youngest and least modifi ed lunar basin. of lava that encircles much of Imbrium. Although six rings have been mapped around Orientale, And Imbrium isn’t the only basin surrounded by maria. only three are clearly visible. The most dramatic is the If you look closely outside the main rim of Mare Crisium 930-km-wide Cordillera Mountains ring, which is partially (L10), you’ll notice that it’s surrounded by a similar, but visible when the western edge of the Moon is tilted to- less conspicuous, lava arc. Arrayed around Crisium are ward us by libration. Inside the Cordillera range are three Lacus Bonitatis, Mare Anguis, and Mare Undarum. These smaller, hilly rings. Some lunar scientists speculate that lesser mare regions are low zones, and the key to under- two indistinct rings also exist outside the Cordillera rim. standing them is realizing that, unlike typical impact cra- In the low zones between Orientale’s rings are the ters, large basins have more than one rim. For reasons that puddles of mare lava named Lacus Autumni and Lacus we don’t fully understand, the highly energetic impacts Veris. The lavas forming these “lakes” apparently rose up that produce depressions larger than about 300 km across through fractures associated with the rings and ponded in create a series of concentric rings inside the main basin the spaces between them. rim and fault scarps outside it. Indeed, the features we call Orientale’s nearly pristine nature provides a model of basin topography that helps explain some other- Mare Frigoris appears concentric 26 wise mysterious features found at older basins. with the northern shore of Mare For example, the Imbrium basin’s equivalent of Imbrium — a relationship that may the Cordillera rim is the Apennine range. The be more than coincidental. MARE FRIGORIS Apennine rim has a diameter of 1,160 km, and

When to View Them 26 Mare Crisium: MARE IMBRIUM March 5th and 22nd; April 4th and 21st. 10 Mare Frigoris: March 1st to W E 8th, March 30th to April 7th, and MARE April 28th to SINUS MARE INSULARUM May 6th. AESTUUM VAPORUM

Universal dates indicate when these features are favorably illuminated. SINUS S MEDII RÜKL ANTONÍN GARY SERONIK

beyond that is a 1,700-km-diameter rim 3,200-km-wide circular basin. This puta- that’s defi ned partially by the northern tive basin has been mapped with several shore of Mare Frigoris. Perhaps the lava in boundaries and has been known by differ- Frigoris fi lls a broad depression between ent names: the Procellarum basin and the two basin rims, just as in Lacus Veris and Gargantuan basin. Lacus Autumni outside Orientale. The According to one interpretation, the only problem with this explanation is that giant basin’s main rim involves the west- except for the edge of Frigoris and a few ern shore of Procellarum and the north- scattered peaks, there’s very little topo- ern edge of Frigoris but is poorly defi ned graphic evidence for a Frigoris rim! along its southern margins. In another At the Crisium impact basin, the best- interpretation, the northern edge of Frigo- preserved outer ring passes through the ris is the boundary of an inner ring of a crater Geminus and traces its way along basin that includes nearly two-thirds of the southern edge of the Taurus Moun- the lunar near side. The existence of a gi- tains. The space between the 1,075-km- gantic Procellarum basin is controversial, diameter Geminus ring and Crisium’s but if it’s real in any of its guises, Frigoris mountainous 500-km-diameter main ring is probably somehow related to it. is relatively low and fl at. Within this zone Regardless of which explanation is cor- we fi nd the small mare patches of Bonita- rect, Frigoris seems tied to impact basins tis, Anguis, and Undarum. after all — just like most other mare de- Can rings and ponds really explain posits on the Moon. † Mare Frigoris? Perhaps. Nearby Oceanus Charles A. Wood Procellarum is another arcuate mare. is the author of The Mod- It stretches about 2,000 km along the ern Moon: A Personal View and the Lunar Moon’s western limb. Procellarum was 100 Card (both available from Sky Publishing). long considered a different class of mare His Lunar Photo of the Day website (www.lpod from the circular maria. But perhaps .org) recently passed the 1,000-day mark, and he Procellarum is the western third of a still fi nds new lunar delights to write about.

LACUS MARE BONITATIS ANGUIS

MARE CRISIUM

MARE UNDARUM GARY SERONIK