Whale of a hollow

"Another Ina," a 'hollow on the Moon resembling a cave-dweller's representation of a whale, located on the western floor of Mare Tranquillitatis (8.89147°N, 21.48729°E) near the February 1964 impact of Ranger 6. LROC Narrow Angle Camera (NAC) observation M177494593R, orbit 11293, December 2, 2011; incidence angle 62.5° and at a resolution of 0.46 meters per pixel from 38 kilometers [NASA/GSFC/Arizona State University].

Joel Raupe
Lunar Pioneer

Hollows, as distinct from pit craters, have been discovered during the course of the Messenger survey of Mercury. As such, these new discoveries heightened awareness of similar features on the Moon, some of these well-known and others also newly discovered by LRO science teams.

Two hollows, or hollow clusters, for example, have been confirmed in LROC high-resolution images not far from the Moon's most famous example, "Ina," the "D Caldera" well-known to telescope observers looking for the challenging feature from Earth.

All three of these features are presumed to result from outgassing, though details of the dynamic remain elusive. Ina is the most studied, and a small cluster of hollows to its north, situated on an extrusion dome on the edge of the Serenitatis basin is the next most well known. The Ina formation had been thought to be unique, but another smaller version has turned up in three LROC Narrow Angle Camera frames showing the area in Mare Tranquillitatis where Ranger 6 made its impact in 1964.

The whale in the Sea of Tranquility (yellow oval) doesn't stand out like Ina, but even in this simulated oblique view of the LROC WAC 100 meter Global monochrome mosaic, overlaid on am elevation model assembled from LOLA laser altimetry, displayed in the NASA ILIADS application reveals how easy it is to find, if you know where to look [NASA/LMMP/GSFC/Arizona State University].

"Ina," (18.65°N, 5.3°E) an extremely young and unusual 3 by 2 km depression that may represent a gas eruption site on the Moon. LROC Narrow Angle Camera (NAC) observation M119815703, LRO orbit 2791, February 3, 2010 [NASA/GSFC/Arizona State University]

Three examples, each in very different areas of the Moon, seem to represent a range of possibilities. Ina appears very young, and it rests on a wide and flat zone at a relatively high elevation above the Serenitatis basin to its north, in the midst of hills etched deep by the primeval blast that formed Mare Imbrium. There seems to be little sign of an explosive debris field though a rivulet of melt may have run from Ina downslope to the east.

Ina, , north of Mare Vaporum, before local sunset in a roughly 46 kilometer-wide LROC Wide Angle Camera (WAC) color (689 nm) mosaic stitched from sequential observation opportunities; January 6, 2010. The feature is situated in on a high, wide and flat mesa still carrying the scars of the Imbrium impact, eons before Ina took shape. Down slope from the feature, to the east by southeast, younger surface material may be a hint of pyroclastic flow [NASA/GSFC/Arizona State University].

In contrast, a smaller version of Ina, shaped more irregularly and resembling a whale, photographed by the LROC Narrow Angle Camera (NAC) at least three times, is located in western Mare Tranquillitatis. The "Whale," about twelve kilometers south of the Ranger 6 impact (and easier to locate). Though situated on a vast mare plain and at a lower elevation the "Whale" has the tapered edges and "beads" characteristic of Ina.

The "whale" formation under a high local sun, LROC NAC
M139768545R, orbit 5731, September 22, 2010, incidence angle
13.6°, res. 0.5 meters from 44.3 km [NASA/GSFC/Arizona State
University].

Spectral analysis of Ina, along with crater counts and analysis of space weathering has led to speculation that the feature may be less than 10 million years old, and may enen be reforming periodically. The explosive nature of most lunar morphology does not lend itself well to imagining anything forming on the Moon from something like a slow leak in a tire, but this may be just what has occurred. Despite the apparent youth of its relief Ina (and perhaps the "Whale" in Tranquility, also) don't show much sign of "optical maturity" beyond their borders, a contrasting bright and reflective debris fields we associate with craters. Ina's interior surface does show immaturity, much less of the fusing with nanophase iron from eons of bombardment by highly kinetic atomic nuclei typical of the Moon's exposed surface most everywhere else.

The inevitable reddening, the darkening, of the outer 3 cm. of the lunar surface, from relentless bombardment of solar and extra-solar radiation (particularly cosmic rays) should cause brilliant 109 million year-old Tycho, for example, to fade into the background in just shy of a billion years. But if outgassing formed, or continues to form, Ina or the "whale," both of which show compelling signs of sprightly youth in their exposed interior, where is a fallout field of ejected material beyond?

Though these features may be the result of sporadic or even continuous "slow leaks," this outgassing probably occurred at some pressure. It wouldn't take much for nearly all of this evacuated material to reach escape velocity. And yet, though it's more obvious to the human eye just beyond the lip of the "whale," there actually is a fine "spray" of accumulated, more reflective (less optically mature) material in their immediate vicinity. Just not the macro-jumble of shocked rocks and blocks of every size we are used to seeing around impact craters.

Close up of the lunar hollows that gained the most immediate interest after the discovery of similar features on Mercury, perhaps because they most closely resembled those first located there, though these vents near the apex of a shallow dome on the southwestern edge of Mare Serenitatis (24.48°N, 7.99°E) are considerable smaller.  LROC NAC M104469044R, orbit 555, August 9, 2009; incidence angle 57.65° resolution 1.45 meters per pixel from 145.5 kilometers [NASA/GSFC/Arizona State University].

A third hollow in the lunar catalog is, again, different from Ina or "the whale." The closest view we presently have of the cluster of vent associated with an extrusion dome on the southwestern edge of Mare Serenitatis may not allow us the kind of spectral analysis of their interiors now available for Ina. Disappointingly, the only high-resolution LROC NAC observation in the Planetary Data System (PDS) was captured very early in the LRO's Commissioning Phase, from 145 kilometers overhead.

How old is this extrusion dome, just inside Mare Serenitatis (right)? The southwestern part of the larger basin exhibits a lot of interesting features. The well-known basins of the nearside tend to be lower in elevation than their circumferences. For some reason, however, moving from the interior toward the southwestern edge, elevations slope in the opposite direction. What does this cluster of hollows have in common with the "open" hollows, Ina and "the whale, if anything? [NASA/JAXA/SELENE/LMMP].

It's tempting, anyway, to "see" a fine haze of less optically mature material in wisps outside these hollows, but such a leap would definitely be immature.

The "trough" on the immediate edge of southwestern Mare Serenitatis. The hollows on the apex of an extrusion dome (yellow arrow) are invisible at this scale, though the area boasts a wide anatomical variety of features testifying to the activity that happened here, probably beginning with the Imbrium impact event (over the Apennine front, upper left). A very close examination of this area's surface is needed to see if those hollows, and perhaps other features like Aratus CA, are of a more recent origin. LROC WAC monochrome (604nm) mosaic [NASA/GSFC/Arizona State University].

Related Posts:
Spectral Properties of Ina (February 7, 2011)
It's a gas, man - (October 8, 2011)
The closest of lunar close-ups, now available (December 16, 2011)
Some LROC Highlights, M. Robinson and the LROC Team (.pdf)
LEAG Conference, December 3, 2009