Nestled in a mare region known as Lacus Felicitatis, Ina is one of the Moon’s most puzzling surface features. Though often labeled a “crater,” its shape, texture, and geologic implications defy simple explanation. To many lunar scientists, Ina is a window into how the Moon might still quietly change—and why our assumptions about lunar dormancy may need revisiting.
What Is Ina?
Ina is a shallow, D-shaped depression roughly 2.9 × 1.9 km across, with a depth from its lowest point to the rim of about 64 meters. Surrounding its interior are dozens of small hills or mounds—rounded, flat-topped, and sharply defined—set within a smoother, rougher “valley” floor. The contrast is striking: the hills look like ordinary lunar surface, while the space between them is markedly different—rougher, lighter in tone, and less cratered.
The most prominent hill inside Ina is Mons Agnes, a relatively small raise within the depression, named and measured from imagery.
What makes Ina stand out is not just its shape, but how “young” its terrain appears. The lower areas show a surprising lack of cratering compared to neighboring lunar surfaces. The wide rim and shallow profile also distinguish it from typical impact craters.
Ina is the most well-known of a class of features called Irregular Mare Patches (IMPs)—patches in mare areas that appear anomalously fresh and possibly volcanic in origin.
The Puzzles & Mysterious Properties
1. Apparent Youth & Crater Scarcity
The lowland interior of Ina shows few craters, which indicates the surface has not endured impacts for long. That suggests the surface may be geologically young—aging on the order of tens of millions of years or even less. Yet conventional models of the Moon hold that active volcanism largely ceased long ago, billions of years in the past.
This apparent youth has prompted suggestions that Ina might represent one of the latest volcanic or outgassing events on the Moon—if not truly active today, then relatively late in lunar history.
2. Drastic Surface Contrast
The hills inside Ina appear smoother and darker, with crater densities intermediate between ancient terrain and the near-pristine lowlands. The boundaries between hilltops and valley floor are sharply defined and undercut in some spots (forming “moats”). The valley floor is rough and irregular, full of small bumps and perhaps exposed rocks or immature regolith.
This discontinuity implies a process that exposed or constructed the valley surface more recently than the hills. Perhaps the valley was cleared of regolith, or the hills represent remnants from an older surface.
3. Porosity, Foam, and Undermining
One intriguing hypothesis proposes that the materials in Ina—especially the valley floor deposits—are highly porous, like magmatic foam. In this model, as late-stage lava or gas-rich flows cooled, they left behind a sponge-like structure. Because of the Moon’s weak gravity and lack of atmosphere, that foam would be especially fluffy (highly porous).
This porosity could help explain why regolith (lunar soil) accumulation appears minimal: loose dust and small particles might filter into pore space rather than building a surface dust layer. Impacts into porous material also produce smaller craters, which would obscure crater counts and overestimate youth. In sum, Ina might appear younger than it is because its structure hides or absorbs signs of aging.
4. Volcanic or Outgassing Origin?
Several origin ideas compete:
- Volcanic caldera / collapse model: Ina may be the remnant of a shallow volcanic collapse, with the hills representing collapse blocks or remnant mounds.
- Regolith removal: Some propose gas release or seismic processes ripped away the overlying regolith, exposing fresh rock or shallow lava flows in the valley, leaving behind islands (hills) of older material.
- Inflated lava flows: Another idea is that the hill mounds were thicker lava flows inflated beneath a crust, leaving the valleys where overlying material collapsed or eroded.
- Continued activity: Though speculative, some interpretations suggest Ina might still be slowly evolving, perhaps by degassing or micro-reshaping.
No single hypothesis satisfies all constraints—especially how to explain the minimal crater density, the sharp boundaries, and the contrast between hill and valley.
Why Ina Matters
Ina’s mysteries challenge our assumptions about lunar quiescence. If Ina’s features are indeed relatively recent or maintained, then the Moon might not be as inert as once thought. That has profound implications:
- Lava or gas activity may have lasted much later than expected.
- Heat retention or localized magmatic processes could persist in the lunar crust.
- Lunar stratigraphy and surface dating (based heavily on crater counting) might need adjustments for porous, low-crater terrains.
Moreover, Ina is a compelling target for future missions. Landers or rovers exploring the hills vs. valleys could sample age, composition, porosity, and structure—possibly revealing whether lunar “extinction” of volcanism is as complete as commonly believed.
Ina remains one of the Moon’s most enigmatic features. The stark “hills in a hollow,” the apparent youth, and the architectural contrast make it more than a curiosity—it might be a key to unlocking hidden chapters of lunar geology.