The Hadean Eon is the first geologic eon of the Earth and lies before the Archean Eon. It began with the formation of the Earth about 4.56 billion years ago and ended roughly 3.8 billion years ago, though the latter date varies according to different sources. The name "Hadean" derives from Hades, the Greek name for the underworld. The name is in reference to the "hellish" conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies. The geologist Preston Cloud coined the term in 1972, originally to label the period before the earliest-known rocks on Earth.
The Moon is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having 27% the diameter and 60% the density of Earth, resulting in 1⁄81 its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter.
The Moon is in synchronous rotation with the Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a reflectance similar to that of coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses. This matching of apparent visual size is a coincidence. Earlier in the Earth's history, the Moon was closer to the Earth, and had an apparent visual size greater than that of the Sun.
The Moon is thought to have formed nearly 4.5 billion years ago, not long after the Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body. The Moon is the only celestial body other than the Earth on which humans have set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo program achieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg of lunar rocks, which have been used to develop a geological understanding of the Moon's origins, the formation of its internal structure, and its subsequent history.
Although the Moon itself is devoid of air, water, and life, its creation and existence has proven to be essential to the origin of life on Earth. It is, after all, our unusually large moon which exerts a stabilizing influence on the Earth's spin axis that has prevented the planet from wobbling like an out-of-control top -- and has thereby saved the Earth from wild climate fluctuations that would have been hostile to life. Indeed, it is generally conceded that we, the inhabitants of the Earth, are in an exceptional state. We owe our very existence to the terrestrial stability caused by the nightly present Moon.
The tilt of the Earth's spin axis, called the obliquity, is what gives us seasons. If the Earth were spinning like a perfectly upright top, perpendicular to the plane of its orbit around the Sun (an obliquity of zero), there would hardly be any seasons at all because every point on the planet would receive a constant amount of sunlight all year long. However, if the Earth were rolling on its side (an obliquity of 90 degrees), each pole would swing between extreme heat and total darkness every year. Because the Earth's spin axis is only slightly tilted, by about 23.5 degrees, the planet enjoys moderate seasonal variations.
This slight tilt of the Earth's spin axis has been the case for eons and it has been so because of the Moon. Because the Earth spins, the planet bulges at the equator. The Sun and the other planets exert a gravitational pull on this bulge, causing the Earth's axis to rock slowly. As the planets move in their orbits -- and as they deform one another's orbits through their gravitational interactions -- the overall strength of the various forces would cause the Earth's spin axis to oscillate in an inherently unpredictable, chaotic way. Computer modeling indicates that the oscillation could vary anywhere from 0 to 85 degrees -- an amount that would make the possibility of life on Earth very unlikely.
However, with the Moon, chaos is averted and stability is imposed. The Moon packs enough gravitational pull to effectively cancel most of the other forces on the Earth's spin axis. With just a little help from the Sun, the Moon makes the Earth's axis settle into a regular motion, causing it to precess in a small circle every 26,000 years. The tilt of the axis does change over time, but only by 1.3 degrees instead of 85.
It is important to note that a mere one degree change in obliquity is not entirely harmless. Indeed, many scientists think that such a change could trigger an ice age. Given that such a traumatic climatic change could accompany only a minor oscillation, an obliquity that fluctuated by tens of degrees would have played havoc with the world's environment. The climate would have swung between epochs with extreme seasonal variations and epochs with none at all. Eco-systems would not have been able to stabilize long enough for advanced life forms such as humans to evolve.
It is now a generally accepted theory that the stability of the Earth's spin axis is a key to life. And as further proof of the validity of this theory, scientists point to Mars. Even though Mars currently tilts at a comfortable 25 degrees, that situation is probably temporary. Mars has only two Manhattan size satellites. Even together, these moons exert a puny gravitational force -- hardly enough to counteract other planetary influences.
It is estimated that the obliquity of Mars has probably varied between 0 and 60 degrees. That suggests a Martian history (and future) of wild climate fluctuations. Indeed, photographs of Mars' polar regions from the twin Viking orbiters reveal layers of ice and dust stacked unevenly like hotcakes -- a possible geologic record of the advance and retreat of the polar ice caps over hundreds of millions of years.
There are two implications of the association of the Moon with life on Earth that are quite profound. While planets like the Earth may well be abundant in the Universe, astronomers now concede that Earth-size planets with moons as large as ours are likely to be extremely rare. Thus, if a large moon is a prerequisite for the evolution of life, beings like us might turn out to be exceedingly rare as well.
The other implication is that the Earth can count on its stable climate only as long as it has a large moon nearby. However, the Moon, as a result of its gravitational interaction with the Earth, is gradually being accelerated into a higher orbit. The Moon is receding from the Earth at a rate of about one inch per year. In about one billion years, the pull of the Moon will be so weak that the Earth's obliquity will begin to fluctuate chaotically. When that happens the Earth's climate will surely undergo wild variations making life on the planet extremely difficult and probably impossible -- at least based upon our current perspective and upon the currently known abilities of the beings known as Homo sapiens.
There are two implications of the association of the Moon with life on Earth that are quite profound. While planets like the Earth may well be abundant in the Universe, astronomers now concede that Earth-size planets with moons as large as ours are likely to be extremely rare. Thus, if a large moon is a prerequisite for the evolution of life, beings like us might turn out to be exceedingly rare as well.
The other implication is that the Earth can count on its stable climate only as long as it has a large moon nearby. However, the Moon, as a result of its gravitational interaction with the Earth, is gradually being accelerated into a higher orbit. The Moon is receding from the Earth at a rate of about one inch per year. In about one billion years, the pull of the Moon will be so weak that the Earth's obliquity will begin to fluctuate chaotically. When that happens the Earth's climate will surely undergo wild variations making life on the planet extremely difficult and probably impossible -- at least based upon our current perspective and upon the currently known abilities of the beings known as Homo sapiens.
4.560 - 4.550 Billion Years B.C.T. – A Proto-Earth formed at the outer (cooler) edge of the habitable zone of the Solar System. At this stage the solar constant of the Sun was only about 73% of its current value, but liquid water may have existed on the surface of the Proto-Earth, probably due to the greenhouse warming of high levels of methane and carbon dioxide present in the atmosphere. The early bombardment began. Because the solar neighborhood is rife with large planetoids and debris, Earth experiences a number of giant impacts that help to increase its overall size.
4.533 Billion Years B.C.T. – The Precambrian, now termed a "supereon" but formerly an era, is split into three geological time intervals called eons: Hadean, Archaean and Proterozoic. The latter two are sub-divided into several eras as currently defined. In total, the Precambrian comprises some 85% of geological time from the formation of Earth to the time when creatures first developed exoskeletons (i.e., hard outer parts) and thereby left abundant fossil remains.
4.533 Billion Years B.C.T. – The Hadean Eon, Precambrian Supereon, and unofficial Cryptic era start as the Earth-Moon system forms, possibly as a result of a glancing collision between proto-Earth and the hypothetical protoplanet Theia (the Earth was considerably smaller than now, before this impact). This impact vaporized a large amount of the crust, and sent material into orbit around Earth, which lingered as rings, similar to those of Saturn, for a few million years, until they coalesced to become the Moon. The Moon geology pre-Nectarian period starts. Earth was covered by a magmatic ocean 200 kilometers (120 miles) deep resulting from the impact energy from this and other planetesimals during the early bombardment phase, and energy released by the planetary core forming. Outgassing from crustal rocks gives Earth a reducing atmosphere of methane, nitrogen, hydrogen, ammonia, and water vapor, with lesser amounts of hydrogen sulfide, carbon monoxide, then carbon dioxide. With further full outgassing over 1000–1500 K, nitrogen and ammonia become lesser constituents, and comparable amounts of methane, carbon monoxide, carbon dioxide, water vapor, and hydrogen are released.
4.5 Billion Years B.C.T. – The Sun entered a main sequence: a solar wind swept the Earth-Moon system clear of debris (mainly dust and gas). The end of the Early Bombardment Phase. The Basin Groups Era begins on Earth.
Basin Groups refers to 9 subdivisions of the lunar Pre-Nectarian geologic period. It is the second era of the Hadean.
The motivation for creating the Basin Groups subdivisions was to place 30 pre-Nectarian impact basins into 9 relative age groups. The relative age of the first basin in each group is based on crater densities and superposition relationships, whereas the other basins are included based on weaker grounds.[1] Basin Group 1 has no official age for its base, and the boundary between Basin Group 9 and the Nectarian period is defined by the formation of the Nectaris impact basin.
The age of the Nectaris basin is somewhat contentious, with the most frequently cited numbers placing it at 3.92 Ga, or more infrequently at 3.85 Ga.[2] Recently, however, it has been suggested that the Nectaris basin could be, in fact, much older and might have formed at ~4.1 Ga.[3] Basin Groups are not used as a geologic period on any of the United States Geological Survey lunar geologic maps. Basin Groups 1-9 and the earlier (informal) Cryptic era together make up the totality of the Pre-Nectarian period.
Since little or no geological evidence on Earth exists from the time spanned by the Pre-Nectarian period of the Moon, the Pre-Nectarian has been used as a guide by at least one notable scientific work[4] to subdivide the unofficial terrestrial Hadean eon. In particular, it is sometimes found that the Hadean eon is subdivided into the Cryptic era and Basin Groups 1-9 (which collectively make up the Pre-Nectarian), and the Nectarian and Lower Imbrian. The first lifeforms (self replicating RNA molecules, see RNA world hypothesis) may have evolved on earth around 4 bya during this era.
- c. 4,450 Ma – 100 million years after the Moon formed, the first lunar crust, formed of lunar anorthosite, differentiates from lower magmas. The earliest Earth crust probably forms similarly out of similar material. On Earth the pluvial period starts, in which the Earth's crust cools enough to let oceans form.
- c. 4,404 Ma – First known mineral, found at Jack Hills in Western Australia. Detrital zircons show presence of a solid crust and liquid water. Latest possible date for a secondary atmosphere to form, produced by the Earth's crust outgassing, reinforced by water and possibly organic molecules delivered by comet impacts and carbonaceous chondrites (including type CI shown to be high in a number of amino acids and polycyclic aromatic hydrocarbons (PAH)).
- c. 4,300 Ma – Nectarian Era begins on Earth.
- c. 4,250 Ma – Earliest evidence for life, based on unusually high amounts of light isotopes of carbon, a common sign of life, found in Earth's oldest mineral deposits located in the Jack Hills of Western Australia.[4]
- c. 4,100 Ma – Early Imbrian Era begins on Earth. Late heavy bombardment of the Moon (and probably of the Earth as well) by bolides and asteroids, produced possibly by the planetary migration of Neptune into the Kuiper belt as a result of orbital resonances between Jupiter and Saturn.[5] "Remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.[6][7] According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."[6]
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