Thursday, March 28, 2013

New Views of the Hollows of Rimae Sosigenes

The Rimae Sosigenes region of northwest Mare Tranquillitatis, from a mosaic of newly released LROC Narrow Angle Camera (NAC) oblique (slew -55°) observations M1108117962LR captured in orbit 15584, November 12, 2012. The field of view is roughly 43 km from west to east (left to right) and scaled up considerably from an original 2.5 meter resolution, imaged from 146 km distance (114.87 km over 8.63°N, 24.9°E, angle of incidence 70.37°).   The largest crater above, Sosigenes A at lower left, is 11.3 km in diameter [NASA/GSFC/Arizona State University].
Joel Raupe
Lunar Pioneer

Running behind this holiday week, we're still busy digging through the latest, 13th batch of Lunar Reconnaissance Orbiter (LRO) data uplinked to the Planetary Data System (PDS) last week. The latest 90 day batch covers September 15 - December 15, 2012, and we're still hopscotching through it like a bee in a springtime garden.

But I wanted to take a moment to talk about hollows. The subject came up at the just-completed 44th Lunar and Planetary Science Conference, last week, in formal discussions about hollows on Mars and Mercury, and that fact reminded me it's been almost exactly a year since the fascinating topic of "meniscus hollows" on the Moon was discussed here.

A new LROC NAC observation of the Rimae Sosigenes area, captured last November, stood out among the few newly-released side-glance, or "oblique," NAC footprints in the latest LROC PDS batch, providing a new perspective on lunar hollows that did not come up in the conversation last year.

Phil Stooke of Western Ontario University collected an excellent inventory of these apparent remnants of gaseous blow-outs and presented it to the 43rd LPSC, last year. He's come up with the designation 'meniscus hollows," as fine description as may be. Among those in his catalog is "No. 8," on the "floor of a depression." 

The new oblique LROC NAC observation assembled at the head of this post shows us how deep that depression really is. Like a necklace encircling the floor of that depression (which resembles a vent, as seen elsewhere on the Moon) where it joins the wall.

A closer look at what was once thought to be a 'chain of craterlets,' a minor catena, slicing perpendicularly through one of a system of parallel north to south rilles - likely faults, that run from Maclear to Sosigenes A. Closer examination in more recent surveys by the LROC Narrow Angle Camera seem to show this structure to be a vent, perhaps related to the faulting. The above is medium resolution sample from  LROC NAC  M1108117962LR [NASA/GSFC/Arizona State University].
Elsewhere we've seen these hollows as hints of relatively recent geological activity, on ancient plains or on small extrusive domes - yet here we seem to see hollows placed in context with a visible complexity of formations with which they may be related. Perhaps some of the other hollows, like those situated alone on the Tranquillitatis plains south of Ross crater and east of Sosigenes, north of Arago and its domes, are all related to conditions under the surface that will turn out at least as complex. 

We don't know enough about Mare Tranquillitatis. It's not clearly an impact basin, like Imbrium and neighboring Serenitatis. It should be interesting to eventually examine the finer detail of these formations in the deeper granularity of the GRAIL mission data.

At Sosigenes faults run south to north parallel with the northwest edge of the mare. Classified as linear rilles, a kilometer or more wide, one is bisected perpendicularly by an 18 km long "gash," hinting at the collapse or shifting under the surface. It is probably immensely old, though at some point the floor of the depression may have been intruded upon from below, perhaps by gases venting at what may have been a weak zone, where the steep 380 meter walls of the depression meet the floor.

In depressed zones east and west of the central "depression of interest," the contact zone joining wall and floor has been blurred by mass wasting. Perhaps in the central depression, however, the distinction was made by material being blown out around the central floor. If so, it's not readily apparent around the outer edge of the formation (which is not the same as saying it isn't there.) Where did it go?

This 580 by 800 pixel sample of the full resolution mosaic (LROC NAC M1108117962LR) shows the 380 meter depths of the elongated formation is punctuated by 'meniscus hollows' very similar to others found just to the east, at the surface of Mare Tranquillitatis south of Ross crater, and more famously at the Ina structure, much further west. Perhaps they most resemble 'hollows' on the floor of the central depression of Rima Hyginus. Found here at the contact zone between the structure's floor and walls, the primary surface seems characterized by a low crater count. Are these hollows points where gases have explosively uncovered the structure's original floor?  [NASA/GSFC/Arizona State University].
 Closer still, in the full-resolution close-up above, something stands out, much as it does at that most famous of the 'meniscus hollows,' Ina. You can make your own comparisons with another new LROC NAC oblique mosaic detailed further below.

In short, the material covering the central depression's floor at its interior, away from the encircling hollows, doesn't appear to share the saturation cratering superpositioned upon it that is more characteristic of the surrounding plain. In fact, like the beaded "frozen liquid' appearance of the material at Ina, it looks relatively new.

A look more or less straight down (and at much higher resolution) at the eastern interior of the 380 meter deep central depression, at its walls together with a small part of the surrounding Rimae Sosigenes plain. Under higher illumination angles the depths of the topography defies our intuition. It doesn't look as deep as it in the oblique view further above, illustrating once again how relief seems to disappear on the Moon in the absence of shadows. From earlier LROC NAC observation M152750200LR, orbit 7645, February 19, 2011; resampled from 0.47 cm per pixel resolution  - angle of incidence 35° - from 39.56 km [NASA/GSFC/Arizona State University].
And an even closer look at one of the hollows along the contact between floor and the steep walls of the central depression at Rimae Sosigenes. Small boulders - apparently - shed from the wall  (and a slope of collapsed talus is piled at right (easier to see in the oblique view above). This field of view, 335 meters wide, comes from LROC NAC observation M177508146LE, LRO orbit 11295, December 2, 2011; angle of incidence 69.92° (slew -14.8°) resolution 0.48 cm per pixel from 37.86 km [NASA/GSFC/Arizona State University].
Optical maturity is no help here, hinting that the newest material is either more than a billion years old or does not result from the kind of kinetic energies released by cratering. In the image above, at bottom center, runs another contact line between the slope of "older" debris from collapsed walls at bottom right and the "newer" material "beaded" at the depression's center.

Is there a difference in the crater count? The material on the slope at bottom right seems darker, but that is likely an illusion from shadow and the elephant skin on nearly every lunar sloped terrain. It hardly counts as a control sample for counting craters and their size and erosion, etc., but no obvious difference in ages stands out between the two zones - nor are there any boulders or trails on either patch of ground. Those seem almost exclusive to the interior of the "blown out" hollows.

Perhaps the best large scale comparison with the meniscus hollows of the Sosigenes depressions is the endlessly compelling Ina formation of Lacus Felicitatis. And that's as good a segue as any to another newly-released LROC NAC oblique observation of that much studied and barely understood feature just below.

A fresh LROC NAC oblique view the Ina formation and its surroundings, a challenge for telescopes on Earth. A closer look at this mosaic's rendition of Ina's distinctive "D" is shown immediately below. Note the apparent rising cone of Mount Agnes, to Ina's northwest. This mosaic is assembled from LROC NAC M1108203502LR, exposing a field of view also roughly 43 km across, with both spacecraft and camera slewed -53°), looking west from 127.29 km over 18.77°N, 11.64°E, Sulpicius Gallus and southern Mare Serenitatis were directly below LRO, outside this side-looking view [NASA/GSFC/Arizona State University].
Perhaps it's a toss-up whether Ina or the hollows on the floor of the central depression of Rima Hyginus more closely resemble the hollows at Rimae Sosigenes. Perhaps the latter is something in-between. 

The view of Ina at an oblique angle, above and below, captured more than 140 km away from the LROC cameras, is not our most detailed look at that formation. Some unscaled examples of the very closest views of Ina surfaced with the LROC PDS release in December 2011. There doesn't seem to be much in the way of a depression resulting from the forces that created Ina, though the beaded remnants or surfacing seems to closely resemble the floor of the Rimae Sosigenes depression.

A new angle on the Ina (3 km across, 18.65°N, 5.3°E) from the immediately preceding mosaic sampled at its full 2.6 meter per pixel resolution [NASA/GSFC/Arizona State University].
Over the next decade or so, when researchers have time to weave together data collected by LROC, Mini-RF and from the GRAIL twins, a better picture of these 'meniscus hollows' will likely emerge. In the end, the formations themselves may turn out to be the surface manifestation of something deeper that we can hardly imagine today.

Related Posts:
Inside Rima Hyginus (June 12, 2012)
Whale of a Hollow (March 20, 2012)
Ina of the Meniscus Hollows (March 21, 2012)
The closest of lunar close-ups (December 16, 2011)
It's a gas, man (October 8, 2011)
Spectral Properties of Ina (February 7, 2011)

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