L31 Taruntius
A floor-fractured crater probably caused by magma intruding beneath the crater floor, bowing the surface upward — this process created a distinctive concentric inner ring and a crater floor cut by a complex network of tectonic fractures.
L31 Taruntius
Northwestern Mare Fecunditatis📉 Vital Statistics
🔭 Field Notes
Taruntius sits on the northwestern floor of Mare Fecunditatis — a 57 km impact crater so thoroughly modified by mare volcanism that its most distinctive features are not from the original impact at all, but from everything that happened to it afterward. The rim is low and subdued compared with craters of similar size elsewhere on the near side, worn down over billions of years of secondary bombardment and broadly rounded rather than sharp. The floor is remarkably flat, flooded by mare basalt that pooled inside the crater after it formed. And crossing that floor is the feature that earns Taruntius its Lunar 100 designation: a system of concentric rilles — curved fractures that roughly parallel the inner wall, generally attributed to volcanic and tectonic modification of the crater floor as magma intruded beneath it and the surface adjusted under loading and cooling. A small central peak survives but is largely buried. Under low terminator light the rilles are the reward; under high Sun, the unusually low and soft rim profile is most apparent, giving Taruntius a flattened, muted appearance that stands in visible contrast to younger craters in the same field.
- ▶ Concentric Floor Fractures: The floor fractures of Taruntius curve roughly parallel to the inner wall rather than crossing it in straight lines — a geometry consistent with floor uplift and subsidence driven by sub-surface magmatic intrusion after the crater was flooded. Often described by observers as rilles, these features are more precisely concentric floor fractures or graben. The system is subtle and requires good seeing at moderate to high magnification to trace clearly. It is among the more accessible examples of floor-fractured crater geometry on the near side, partly because Taruntius is well-placed near the equator and at a longitude that keeps it well clear of limb foreshortening.
- ▶ Low, Subdued Rim: Taruntius possesses a noticeably degraded rim compared with younger craters of similar diameter. Rather than the crisp, elevated wall typical of a geologically young crater, the rim has been reduced to a gently rolling, rounded ring that barely rises above the surrounding mare. LROC imagery shows the rim remains intact in outline while exhibiting strongly subdued relief — a degree of modification more consistent with an Eratosthenian-age surface long exposed to secondary impacts and regolith gardening than with the sharp rims typical of true Copernican-age craters.
- ▶ A Deceptive Ray System: Taruntius carries a bright ray system extending over 300 km, which originally led to a Copernican-age classification. Later analysis (Hawke et al., 2005) showed the rays are compositional — optically matured highland material excavated by the impact rather than fresh, high-albedo ejecta — meaning Taruntius is substantially older than its rays suggest, and is now considered Eratosthenian in age.
- ▶ Setting on Mare Fecunditatis: Taruntius occupies the northwestern corner of Mare Fecunditatis, where the mare meets the older highland terrain to the north and west. The surrounding lava plain is relatively dark and smooth, making Taruntius easy to identify at low power once the mare itself is located. Several wrinkle ridges occur elsewhere within Mare Fecunditatis and may be visible under favorable illumination, adding context to the regional setting.
📍 Nearby L100 Targets
- L25 Messier & Messier A: The most studied oblique-impact crater pair on the Moon, sitting roughly 200 km southeast of Taruntius on the floor of Mare Fecunditatis. Messier is markedly elongated east-west; Messier A, roughly 20 km to its west, has an irregular outline. A pair of nearly parallel bright rays extends westward from Messier A for over 100 km across the dark mare — one of the most geometrically distinctive ray systems on the near side, best seen near Full Moon when albedo contrast is highest. The Messier pair and Taruntius together bracket the western portion of the mare and can be studied in the same session under different lighting conditions.
- L12 Proclus Crater: A 28 km Copernican-age impact crater on the western rim of Mare Crisium, roughly 350 km to the north-northeast of Taruntius. Proclus is one of the brightest craters on the near side and one of the finest examples of an asymmetric ray system produced by an oblique impact — the rays spread in a broad fan to the west, north, and east, while a conspicuous dark wedge of ray-free terrain to the southwest marks the direction from which the impactor arrived. The comparison with Taruntius is instructive: Proclus is a genuinely young, sharp-rimmed, ray-bearing crater whose fresh ejecta signature is still fully legible, while Taruntius shows what a similar ray pattern looks like once the rays themselves have optically matured and the crater beneath has been reworked by billions of years of mare-adjacent modification.
- L10 Mare Crisium: The most isolated and compact of the major near-side maria, occupying an ancient impact basin roughly 550 km across near the northeastern limb. Unlike Mare Fecunditatis, whose borders blend gradually into the surrounding terrain, Crisium is sharply bounded by a nearly complete ring of uplifted highland terrain that gives it the appearance of a dark oval lake ringed by mountains — readily visible to the naked eye and striking in binoculars. Luna 24, the last Soviet lunar sample return mission, landed on Crisium’s floor in 1976 and returned 170 grams of basalt drilled from nearly 2 m depth, including material dated to approximately 3.3 billion years ago.
🚀 Mission Log
Target Acquisition
Find Mare Fecunditatis, then work to its northwestern shore
Mare Fecunditatis is easy to pick out at low power — a dark, roughly oval basin on the eastern half of the disc. Taruntius sits right where the mare meets the older highland terrain to the north and west, making it one of the easier floor-fractured craters to relocate session after session. At 57.32 km across it holds the field comfortably even at modest magnification; nearby you’ll find the lava-flooded Lawrence to the northwest and Watts and da Vinci to the north, all useful confirmation that you’ve got the right corner of the mare.
Two lighting conditions, two different crater — plan for both
Taruntius rewards opposite ends of the lunar day. Under a rising or setting Sun, with the terminator near Taruntius’s own longitude (roughly Moon Day 3–4 waxing or Day 17–18 waning, as an approximate guide — actual timing shifts a bit with libration), the low, subdued rim throws just enough shadow to trace its rounded, worn outline, and the concentric floor rilles become visible as faint shadowed lines. Under high Sun, the rilles wash out completely, but that’s when the crater’s unusually flat, muted profile — flooded and softened compared to sharper craters nearby — is most apparent. If you only have one session, favor the terminator; if you can return later in the same lunation under high Sun, the contrast is worth it.
Work up in power to trace the fracture system
At 75x–100x, confirm the shallow, rounded rim — broken in the northwest by the small crater Cameron — and the flat, lava-flooded floor with its low, largely buried central peak. Push to 150x+ under good seeing near the terminator to trace the concentric floor fractures: curved graben running roughly parallel to the inner wall rather than crossing it, widely interpreted as the result of magmatic intrusion beneath the crater floor after it formed. This is a subtle target — Taruntius is one of the more accessible floor-fractured craters on the near side specifically because it sits well clear of limb foreshortening, so take the extra time here.
Compare against the ray craters and basin scenery nearby
Roughly 200 km southeast, on the same mare floor, the Messier and Messier A pair (L25) is one of the most-studied oblique-impact sites on the Moon — sharp, elongated, and still ray-bearing, a useful contrast to Taruntius’s soft, degraded profile. About 350 km north-northeast, Proclus (L12) is a young, Copernican-age ray crater with a textbook asymmetric ejecta fan. Taruntius also displays bright rays, but they are not considered evidence of a comparably young Copernican age — a useful reminder that ray brightness alone can mislead. And to the north, Mare Crisium (L10) offers a completely different basin character: sharply walled and isolated, rather than blending gradually into highland terrain the way Fecunditatis does around Taruntius.
📝 Observation Log — L31 Taruntius Crater
0/4 CompleteIs Taruntius visible tonight?
The concentric floor fractures of Taruntius require low terminator light to show clearly — aim for Waxing Gibbous (Day 10–11) or Waning Gibbous (Day 23–24) when the terminator crosses western Mare Fecunditatis at Taruntius’s longitude (~47°E). Under high Sun the low, subdued rim profile is most apparent, but the floor detail disappears. The floor fractures are most rewarding near the terminator.
Check Moon Phase TodayReturn to the List
Ready to find the next target? Go back to the full map to see what else is visible tonight.
← Back to Lunar 100 Map
When to Observe Taruntius
Taruntius sits right on the boundary between Mare Fecunditatis and the highlands to the north, and its most distinctive trait — a floor that looks almost like a smaller crater nested inside a larger one — is most striking under low-Sun illumination. Under high illumination the floor detail flattens out and the crater reads as fairly unremarkable; near the terminator, the concentric structure and fracture pattern become one of the better-known floor displays on the near side.
- Best Window: Roughly Day 3–5 after New Moon, when the morning terminator crosses the western edge of Mare Fecunditatis. Because Taruntius sits fairly far east, this early-lunation window is when it first clears the terminator with useful shadow.
- Second Window: Around Day 18–20, when the evening terminator returns from the opposite direction. The floor shadows fall the reverse way, which is a useful check on any structure you think you’ve identified in the first window.
- Also Worth a Look: Near Full Moon. Taruntius carries a modest bright ray system, and under high illumination the rays and the brightness of the rim stand out against the darker mare — a completely different, and complementary, view to the terminator sessions.
What to Look For
At 100x–150x near the terminator, look at the floor of Taruntius for a raised, roughly concentric ridge or broken ring of uplift, set inside the main rim and offset from a conventional central peak. The effect at the eyepiece is genuinely of a smaller, subdued crater sitting inside the larger one — this concentric arrangement is the single feature Taruntius is most often observed for, and it’s rare enough among lunar craters to make it worth deliberate attention rather than a quick glance.
Push to 150x–200x on a good night and look for fine linear rilles running across the floor, generally following the same rough concentric and radial pattern as the inner ring. These are subtle — thin, low-relief cracks rather than bold rilles — and asking for a steady atmosphere is not an exaggeration here. They’re one of the better-known examples of the “floor-fractured crater” pattern used to classify a whole family of similar craters across the Moon.
At low-to-moderate power, examine the rim itself: Taruntius has a relatively fresh-looking, terraced inner wall — features generally associated with a relatively fresh impact crater. Near Full Moon, switch to naked-eye or low-power binocular viewing and look for the bright ray material extending out from the rim across the neighbouring mare and highland terrain, another trait typical of a comparatively young crater.
If your session allows, compare Taruntius with other well-known floor-fractured craters such as Posidonius (on the Serenitatis shore) or Gassendi (on the Humorum shore), if either is favourably placed the same night or on a nearby date. Each shows some combination of a raised or domed floor and fracture patterns, but the details — how prominent the fractures are, whether an inner ring is present, how domed the floor looks — differ from crater to crater.
The Science: A Floor Pushed Up From Below
Taruntius belongs to a recognized category of lunar craters — floor-fractured craters — whose floors have been modified after the original impact, most often by uplift and cracking rather than simple lava flooding. The general mechanism is reasonably well accepted; the specifics for any individual crater, including Taruntius, are less settled.
Magmatic Intrusion (Laccolith-Style Uplift)
The leading explanation for floor-fractured craters like Taruntius is that magma rose beneath the crater floor after the impact and is generally thought to have stalled beneath the floor rather than erupting extensively onto the surface, intruding instead as a shallow sill or laccolith-like body. As this intrusion pushed upward, it domed and fractured the overlying floor, producing the concentric and radial crack patterns visible today. This model is widely applied across the floor-fractured crater class, of which Taruntius is a commonly cited example, though the exact depth, volume, and timing of the intrusion beneath any specific crater floor is difficult to pin down from surface observations alone.
Viscous Relaxation and Structural Adjustment
A less widely supported idea proposed for some floor-fractured craters is that the floor deformed through slow structural relaxation of the crust beneath the crater, rather than active magmatic doming — essentially the floor settling or adjusting under its own weight and local stresses over time. This explanation is generally regarded as inadequate on its own for classic floor-fractured craters, and it’s not clear how much, if at all, it applies specifically to Taruntius rather than to the class as a whole.
What Gravity and Imaging Data Have Added — and Not Resolved
Studies of several floor-fractured craters using GRAIL gravity data have identified localized anomalies consistent with dense intrusive material beneath some crater floors, broadly supporting intrusive models for the class as a whole. High-resolution imagery has meanwhile refined the mapping of fracture patterns at Taruntius and similar craters in detail. What hasn’t been specifically established is a confirmed subsurface gravity anomaly beneath Taruntius itself — its intrusive structure is still inferred primarily from its surface morphology rather than demonstrated directly. Nor has the broader data settled the precise sequence of events: how the intrusion, the floor uplift, and the fracturing relate in time, and why some floor-fractured craters show a prominent inner ring like Taruntius’s while others show a smoother dome instead.
What makes Taruntius worth returning to at the eyepiece is that the crater-within-a-crater impression, oddly, points at real geology: it’s the surface expression of something rising from underneath rather than settling from above. Reading the floor for its ring, its fractures, and its freshness is close to reading the same evidence a geologist would use to argue about what happened beneath it.