Alright, let's dissect this lunar timeline. It’s less a charming story and more a meticulous, and frankly, grim accounting of how this desolate rock got its scars. Don't expect any waxing poetic about moonbeams; this is about impact events and the relentless march of geological time.
Geological Dating System of the Moon
This is how we attempt to impose order on the Moon's chaotic past. It's a system built on craters, on the scars left by cosmic violence, and on what little tangible evidence we’ve managed to pry from its surface. It’s a chronology, yes, but one etched in dust and pulverized rock.
Chronology
Imagine a timeline, stark and unforgiving, stretching back millennia.
- 4500 – 4000 Ma: A vast, undefined expanse. The early Moon, a molten mess, solidifying. The raw material of what’s to come, but not yet defined by the violence that would shape it.
- 4000 – 3500 Ma: The nascent stages of bombardment. The cracks begin to show, the first significant impacts.
- 3500 – 3000 Ma: A period of intense activity. The basin-forming impacts really get going.
- 3000 – 2500 Ma: The bombardment continues, though perhaps with slightly less ferocity. The landscape is being irrevocably altered.
- 2500 – 2000 Ma: More impacts, more modification. The Moon is slowly, painfully, taking shape.
- 2000 – 1500 Ma: The era of the great basins solidifies. The foundations of the major features are laid.
- 1500 – 1000 Ma: A significant phase of volcanism, filling in some of those basins. The dark seas begin to form.
- 1000 – 500 Ma: Continued, though perhaps less dramatic, geological activity. The surface continues to evolve.
- 500 – 0 Ma: The more recent history. The Copernican period, where the most recognizable features, like the crater Copernicus, dominate.
This is a rough sketch, mind you. The actual boundaries are far more precise, defined by specific, catastrophic events.
Usage Information
- Celestial Body: Earth's Moon. Naturally.
- Time Scale(s) Used: The Lunar Geologic Timescale. It’s the only one that matters for this particular piece of cosmic real estate.
- Geological Map of the Near Side of the Moon: This is your visual aid. Click to zoom, if you have the patience. It shows the strata, the layers of impact and eruption.
The lunar geological timescale, or rather, the selenological timescale, is a rather stark division of the Moon's history. It’s broken down into five periods that are generally, and I stress generally, agreed upon: the Copernican, Eratosthenian, Imbrian (which itself is split into Late and Early epochs), Nectarian, and the primordial Pre-Nectarian. These aren’t arbitrary divisions; they’re pinned to the massive impact events that irrevocably altered the lunar surface. Think of them as the moments the Moon truly changed. The formation and subsequent modification of craters are key. The size and distribution of these craters, superposed on different geological units, tell us about the rate of bombardment, and thus, the age of those units. The absolute ages? Those are pieced together through radiometric dating of samples, a process made maddeningly difficult by the fact that most of our samples come from a surface that's been pummeled relentlessly. Correlating a bit of lunar regolith to a specific geological formation is a detective job, and one often hampered by the sheer violence of the Moon's history.
Lunar Stratigraphy
This is where we try to make sense of the layers. It’s a geological narrative, written in dust and ejecta.
- Millions of years before present: The axis of our timeline. Unforgiving, absolute.
The primary sculptors of the lunar surface are, predictably, impact cratering and volcanism. Using fundamental stratigraphic principles, like the law of superposition – the oldest layers are at the bottom, the youngest on top – we can begin to order these events. It was once thought that the mare basalts were all laid down at one singular point in time. How naive. We now understand that mare volcanism was a protracted affair, starting as early as 4.2 billion years ago (Ga) and potentially sputtering on until 1.2 Ga. [1] [2] Impact events, however, are the real workhorses for defining stratigraphy. They happen fast, they’re numerous, and they leave undeniable marks. [4] The constant barrage of impacts over eons erodes and modifies lunar landforms in a quantifiable way. The degree of this erosion, the "state of erosion," is another clue to assigning relative ages. [5] [6]
Our lunar geological timescale is divided into those five periods – Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican – with the Imbrian period further split. These aren't necessarily moments of fundamental geological upheaval; they’re convenient markers, geomorphological signposts. The Moon, thankfully, is unique in the Solar System in that we possess rock samples with a known geological context. The Apollo missions gave us that. By linking the ages of these samples to specific geological units, we can attach absolute ages to some of these periods. But it’s not a perfect science. There’s still significant debate, especially in the lunar highland regions where distinguishing between Nectarian and Pre-Nectarian materials can be a nightmare. Often, these deposits are simply lumped together as "Pre-Imbrian."
Pre-Nectarian
- Main article: Pre-Nectarian
This period begins with the formation of the lunar crust itself and concludes with the Nectaris impact event. Mare Nectaris is a colossal multi-ring impact basin on the near side of the Moon, and its ejected material acts as a crucial stratigraphic marker. We recognize about 30 impact basins from this era, the most ancient being the monumental South Pole–Aitken basin. This period is informally broken down into the Cryptic and Basin Groups 1 through 9, [1] though these subdivisions are rarely seen on actual geological maps. They’re more like whispered secrets among geologists.
Nectarian
- Main article: Nectarian
The Nectarian period covers the events between the formation of the Nectaris basin and the Imbrium impact basin. Twelve multi-ring impact basins are identified within this period, including significant ones like Mare Serenitatis and Mare Crisium. One of the key goals of the Apollo 16 mission was to date material ejected by the Nectaris basin. Even so, the exact age of the Nectaris basin remains a point of contention, with the most commonly cited figures hovering around 3.92 Ga, though sometimes 3.85 Ga is suggested. More recently, some have proposed that the Nectaris basin could be considerably older, perhaps as far back as ~4.1 Ga. [7]
Imbrian
- Main articles: Late Imbrian and Early Imbrian
The Imbrian period is bifurcated into Late and Early epochs. The Early Imbrian is defined by the span between the formation of the Imbrium basin and the Orientale impact basin. The Imbrium basin is generally thought to have formed around 3.85 Ga, although a less common theory places this event at 3.77 Ga. The Schrödinger basin is the only other identified multi-ring basin dating to the Lower Imbrian epoch. No large multi-ring basins are known to have formed after this point.
The Late Imbrian is defined by the time between the formation of the Orientale basin and when craters of a specific size (D L ) become so degraded by erosional processes that they are virtually obliterated. The age of the Orientale basin hasn’t been directly pinned down, but it must predate 3.72 Ga (based on Upper Imbrian mare basalt ages) and could potentially be as old as 3.84 Ga, if we consider the size-frequency distributions of craters superposed on its ejecta. A staggering two-thirds of the Moon's mare basalts erupted during the Upper Imbrian Series, with many of these lava flows filling the vast depressions left by earlier impact basins.
Eratosthenian
- Main article: Eratosthenian
The base of the Eratosthenian period is marked by the time when craters of a specific size (D L ) on a geological unit are almost entirely erased by erosional processes. The primary agent of erosion on the Moon? Impact cratering itself. Seismic modification might play a minor role, but it's the constant hammering that truly wears things down. The absolute age of this boundary isn't precisely known, but it's commonly placed around 3.2 Ga. The younger boundary of this period is determined by observing how freshly excavated lunar materials, which are typically bright, darken over time due to space weathering. Operationally, this period was initially defined by the time when impact craters lost their bright ray systems. However, this definition has faced scrutiny. Some rays remain bright for compositional reasons, independent of space weathering. For instance, if ejecta from a highland crater, composed of bright anorthositic materials, lands on the darker mare, it will retain its brightness even after weathering.
Copernican
- Main article: Copernican period
The Copernican period is the most recent chapter in the Moon's geological history. Initially, the presence of a bright ray system around a crater was the defining characteristic of Copernican units. But, as noted, this can be misleading due to compositional variations. The boundary at the base of the Copernican period does not coincide with the formation of the crater Copernicus itself. The age of this boundary is not well-constrained, but a frequently cited figure is 1.1 Ga. The Copernican period extends right up to the present day.
Relationship to Earth's Geologic Time Scale
The divisions of the lunar geologic time scale are based on the identification of a few distinct geomorphological markers. While incredibly useful for establishing the relative order of geological events, it’s crucial to remember these boundaries don't necessarily signify fundamental shifts in geological processes. Furthermore, since the oldest lunar geological periods are exclusively defined by the timing of specific impact events – notably Nectaris, Imbrium, and Orientale – these singular events are unlikely to align with any particular geological event on other terrestrial planets like Mercury, Venus, Earth, or [Mars].
Despite this, at least one significant scientific work [8] has proposed using the lunar geological time scale to subdivide Earth's Hadean eon. Specifically, the Hadean is sometimes divided into the Cryptic, Basin Groups 1–9, Nectarian, and Early Imbrian. This nomenclature isn't entirely consistent with the lunar geologic time scale described above, as the Cryptic and Basin Groups 1–9 (both informal terms not used on geological maps) actually constitute the Pre-Nectarian period.