It is really hard to figure our from where to start when it comes to Earth as we know so much about it. We still keep learning and there so much to be learned. Having privilege to live here, makes planet Earth celestial body we know the best. And for many things we take it as reference. While many things about Earth are different when compared to other planets, we do share same genes and history within our solar system.  Therefore, ladies and gentlemen, I present you planet Earth and its companion Moon.




Earth is the only planet in our solar system known to harbor life as we know it. All of the things we need to survive are provided under a thin layer of atmosphere that separates us from the uninhabitable void of space. Earth is made up of complex, interactive systems that are often unpredictable: air, water, land and life combine forces to create a constantly changing world that we are striving to understand. Viewing Earth from the unique perspective of space provides the opportunity to see Earth as a whole. Scientists around the world have discovered many things about our planet by working together and sharing their findings. Some facts are well known. For instance, Earth is the third planet from the sun and the fifth largest in the solar system. Earth's diameter is just a few hundred kilometers larger than that of Venus. The four seasons are a result of Earth's axis of rotation being tilted more than 23 degrees. How does that work?


The Earth travels around the Sun at speed of 30 km/s, making a gigantic circle nearly a billion kilometers in circumference once per year.  The journey isn't actually a perfect circle. It's off by a tiny amount, so slight that you'd never notice if someone didn't tell you. But in fact, this minor deviation can make a big difference: it means that the Earth's distance from the Sun changes over its orbit to the tune of over 5 million kilometers. That difference amounts to about 3.4% of the average distance to the Sun, which astronomers, for convenience, call an Astronomical Unit, or AU. In real terms, an AU is 149597870 km plus a bit. When the Earth is at aphelion - its farthest point from the Sun - it's about 152141000 kilometers from our star. At perihelion, that distance shrinks to 147055000 kilometers. Aphelion occurs on one side of the Earth's orbit, and perihelion on the other. It takes the Earth half a year to travel that distance, of course, so in a sense it drops toward the Sun by 5 million kilometers in about 182.5 days - an average velocity of well over 1100 km/hr. This change in distance to the Sun has an effect on the Earth's temperature, but it's pretty small, only a couple of degrees Celsius. That's swamped by the seasonal change in temperature due to the Earth's tilt, which is what really drives the seasons. Think of it this way: the Earth reaches perihelion in January, which is the dead of winter for the northern hemisphere. While this does make northern winters a tad more clement, the amount is too small to make an appreciable difference.


earth03.jpgOddly, for those in the antipodes of the southern hemisphere, we're closest to the Sun in their summer. You'd expect temperatures then to be higher than average, but in reality they're about the same as their boreal neighbors. Why is that? It's because the southern hemisphere is dominated by the Pacific Ocean, and water is an excellent heat sink. It absorbs the extra heat in the summer, and releases it in the winter, mitigating temperature extremes. Weather is complicated. The size of the Sun changes by that same 3.4% over the course of half a year. You'd never suspect that by eye since the change is slow day-by-day so it's beneath our notice, but the telescope doesn't lie.


Oceans at least 4 kilometers deep cover nearly 70 percent of Earth's surface. Fresh water exists in the liquid phase only within a narrow temperature span (0 to 100 degrees Celsius). This temperature span is especially narrow when contrasted with the full range of temperatures found within the solar system. The presence and distribution of water vapor in the atmosphere is responsible for much of Earth's weather. Near the surface, an ocean of air that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other ingredients envelops us. This atmosphere affects Earth's long-term climate and short-term local weather; shields us from nearly all harmful radiation coming from the sun; and protects us from meteors as well. Satellites have revealed that the upper atmosphere actually swells by day and contracts by night due to solar activity. You may find cool to check this video for ocean currents. And while on the subject of water, you will find following interesting. We know that water covers 70% or a bit more of Earth surface. So if you had to imagine sphere made of this water one could think that would be 70% of Earth, but of course that would be wrong since surface does not equal volume.  Image below shows sphere made of all the water on Earth - freshwater, saltwater, water vapor, water in plants and animals. Jay Kimball of 8020Vision went ahead and made another sphere, but this time only of fresh water - the one we can use. For math click here and for picture look below - all of sudden idea of water being brought to our planet by comets does not seem so wild, does it?




While on subject of water, let's stay there for few more moments. Scientists have long believed that comets and, or a type of very primitive meteorite called carbonaceous chondrites were the sources of early Earth's volatile elements - which include hydrogen, nitrogen, and carbon - and possibly organic material, too. Understanding where these volatiles came from is crucial for determining the origins of both water and life on the planet. New research led by Carnegie's Conel Alexander focuses on frozen water that was distributed throughout much of the early Solar System, but probably not in the materials that aggregated to initially form Earth. The evidence for this ice is now preserved in objects like comets and water-bearing carbonaceous chondrites. The team's findings contradict prevailing theories about the relationship between these two types of bodies and suggest that meteorites, and their parent asteroids, are the most-likely sources of Earth's water. Looking at the ratio of hydrogen to its heavy isotope deuterium in frozen water, scientists can get an idea of the relative distance from the Sun at which objects containing the water were formed. Objects that formed farther out should generally have higher deuterium content in their ice than objects that formed closer to the Sun, and objects that formed in the same regions should have similar hydrogen isotopic compositions. Therefore, by comparing the deuterium content of water in carbonaceous chondrites to the deuterium content of comets, it is possible to tell if they formed in similar reaches of the Solar System. It has been suggested that both comets and carbonaceous chondrites formed beyond the orbit of Jupiter, perhaps even at the edges of our Solar System, and then moved inward, eventually bringing their bounty of volatiles and organic material to Earth. If this were true, then the ice found in comets and the remnants of ice preserved in carbonaceous chondrites in the form of hydrated silicates, such as clays, would have similar isotopic compositions. Researchers analyzed samples from 85 carbonaceous chondrites, and were able to show that carbonaceous chondrites likely did not form in the same regions of the Solar System as comets because they have much lower deuterium content. If so, this result directly contradicts the two most-prominent models for how the Solar System developed its current architecture. This suggests that carbonaceous chondrites formed instead in the asteroid belt that exists between the orbits of Mars and Jupiter. What's more, most of the volatile elements on Earth arrived from a variety of chondrites, not from comets.




Astronomers have been puzzled by Earth's water deficiency though. The standard model explaining how the solar system formed from a protoplanetary disk, a swirling disk of gas and dust surrounding our Sun, billions of years ago suggests that our planet should be a water world. Earth should have formed from icy material in a zone around the Sun where temperatures were cold enough for ices to condense out of the disk. Therefore, Earth should have formed from material rich in water. So why is our planet comparatively dry? A new analysis of the common accretion-disk model explaining how planets form in a debris disk around our Sun uncovered a possible reason for Earth's comparative dryness. Led by Space Telescope Science Institute in Baltimore, the study found that our planet formed from rocky debris in a dry, hotter region, inside of the so-called "snow line". The snow line in our solar system currently lies in the middle of the asteroid belt, a reservoir of rubble between Mars and Jupiter; beyond this point, the Sun's light is too weak to melt the icy debris left over from the protoplanetary disk. Previous accretion-disk models suggested that the snow line was much closer to the Sun 4.5 billion years ago, when Earth formed. Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth's orbit. Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry.




Above illustration of two different disk models shows overhead views of the structure of the protoplanetary disk that encircled the newborn Sun 4.6 billion years ago. The Sun's family of planets agglomerated from dust and ices within the disk. The major difference between the two models is the location of the so-called snow line, which divides a warm, dry area of the disk from an icy, turbulent region. In the standard disk model, shown at left, Earth formed beyond the snow line, in an icy region. Our planet should, therefore, contain lots of water because it formed from ices that would have been a major fraction of its composition. However, it's estimated that less than 1 percent of Earth's mass is locked up in water, which has puzzled scientists. In the new disk model, shown at right, Earth formed in a warmer, dry region, outside the snow line, which is much farther away from the Sun. This model explains why Earth is comparatively dry. It provides new insights into estimates of the abundance of Earth-like planets in the galaxy.


Scientists have long speculated about why there is a large change in the strength of rocks that lie at the boundary between two layers immediately under Earth's crust: the lithosphere and underlying asthenosphere. Understanding this boundary is central to our knowledge of plate tectonics and thus the formation and evolution of our planet as we know it today. The lithosphere-asthenosphere boundary, or LAB, represents the transition from hot, convecting mantle asthenosphere to overlying cold and rigid lithosphere. The oceanic lithosphere thickens as it cools over time, and eventually sinks back into the mantle at Earth's so-called subduction zones. Studies of seismic waves traveling across the LAB show higher wave speeds in the lithosphere and lower speeds in the asthenosphere. In some regions, seismic waves indicate an abrupt 5 to 10% decrease in wave speeds between 35 and 120 km depth, forming a boundary known as the Gutenberg discontinuity. In many cases, the depth of the Gutenberg discontinuity is roughly coincident with the expected depth of the LAB, leading to the suggestion that the two boundaries are closely inter-related. However, temperature alone cannot fully explain the abrupt change in the mechanical and seismic properties that have been observed at the Gutenberg discontinuity. This has led many scientists to suggest that other factors, such as the presence of molten rock, water, and/or a decrease in the grain size of minerals may also play important roles.


Older techniques made imaging seismic discontinuities shallower than 100 kilometers quite difficult, and regions beneath the oceans could only be accessed where seismic stations were installed on ocean islands or by deploying ocean bottom seismometers, giving an incomplete picture of where the Gutenberg occurs beneath the Pacific Ocean. But an innovative observation technique (seismic waves that sample beneath remote regions of Earth at higher frequencies) and new signal processing techniques - changed this. earth05.jpg


That way we discovered that the seismic discontinuity is not a Pacific-wide phenomenon, but rather only detectable beneath regions with recent surface volcanism. We also found the Gutenberg appears to become deeper beneath older crust, confirming the discontinuity is, indeed, related to the LAB. That may indicate that the Gutenberg is formed by partially molten rock produced in the asthenosphere that collects and ponds at the base of the lithosphere. Decompression of hot rock at small-scale upwellings or hot mantle plumes is responsible for generating the melt. Plumes will thermally reheat the lithosphere, making it shallower than would be expected underneath older crust. This means plate tectonics are enabled on Earth because of mantle composition and or grain size, not necessarily the presence of melt.


Our planet's rapid spin and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape. The solar wind is a stream of charged particles continuously ejected from the sun. The magnetic field does not fade off into space, but has definite boundaries. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow and are known as the aurorae, or the Northern and Southern Lights.




It was believed that Earth grew to its current size by collisions of bodies of increasing size, over what may have been as much as tens of millions of years. Some recent research results suggest that some portions of the Earth formed within 10 to 20 million years of the creation of the Solar System and that parts of the planet created during this early stage of construction remained distinct within the mantle until at least 2.8 billion years ago. Prior to that finding, scientific consensus held that the internal heat of the early Earth, in part generated by a massive impact between the proto-Earth and a planetoid approximately half its size (for example, the size of Mars), would have led to vigorous mixing and perhaps even complete melting of Earth. This, in turn, would have homogenized the early mantle, making it unlikely that any vestiges of the earliest-period of Earth history could be preserved and identified in volcanic rocks that erupted onto the surface more than one and a half billion years after Earth formed. However, researchers examined volcanic rocks that flourished in the first half of Earth's history, called komatiites, and found that these have a different type of composition than what they, or anyone, would have, expected.  They discovered 2.8 billion year old volcanic rocks from Russia that have a combination of isotopes of the chemical element tungsten that is different from the combination seen in most rocks - different even from the tungsten filaments in incandescent light bulbs. So this may be isotopic signature of one of the earliest-formed portions of the Earth, a building block that may have been created when the Earth was half of its current mass. These findings indicate that the Earth's mantle has never been completely melted and homogenized, and that convective mixing of the mantle, even while Earth was growing, was evidently very sluggish. Many questions remain. The rocks studied are 2.8 billion years old, but we don't know whether the portion of the Earth with this unusual isotopic composition or signature can be found in much younger rocks for example. But what about oxygene?





We live in a unique environment and Earth is the only planet we know that has an oxygen-rich atmosphere, as well as hydrosphere that is vital for complex life. But the Earth's early atmosphere was oxygen-poor in the Archaean prior to the Great Oxidation Event, which happened between 2.5 and 2.3 billion years ago, so it's vital that we understand how oxygen rose. Recente study by Mark Barley has identified how links between tectonics and ocean and land chemistry combined to give rise to life on earth about 2.5 billion years ago, during a period known as the Great Oxidation Event (GOE). The GOE changed surface environments on Earth and ultimately made advanced life possible. Research team found a rising abundance of chromium in the banded iron formations starting 2.48 billion years ago, which was an important indication of links between life and the growth of continents. Using this data, they were able to see cyanobacteria had started to produce oxygen, and that aerobic respiring chemolithotrophic bacteria oxidised pyrite linked to acid rock drainage that dissolved chromium on land and added chromium and sulphate to the ocean. This confirms that the 2.48 to 2.32 billion year period was the protracted GOE which eventually helped the formation of complex life.


More than 2 billion years ago, a much fainter sun should have left Earth as an orbiting ice ball, unfit to develop life as we know it today. Why Earth avoided the deep freeze is a question that has puzzled scientists. If you go back in time to about 2 billion years ago, the Earth should have been frozen over. There's a lot of geological evidence that the Earth wasn't frozen over. So, what is not equal? That is the so called Faint Young Sun Paradox. Purdue University's David Minton has offered a new explanation of why Earth avoided freezing over during a period when, according to geological and astrophysical observations, the sun burned at about only 70% of its current brightness. In short, he believes our planet might have been in a warmer place. To keep the Earth from being frozen over at the beginning of its history, it would have to be 6% or 7%closer to the Sun than it is now. It's a few million km, but from an orbital mechanics standpoint, it's not that far. The question is what could make a planet move from one location to another? Minton proposes Earth may have migrated from the sun over time through a process called planet-planet scattering, which occurs when one planet or more is ejected from its orbit, an increase in orbital separation occurs, or when planets collide. There are many possible ways a planet could move, but Minton said most alternatives could be ruled out because of the timeline involved. And while most theories can be ruled out, planet-planet scattering is not ruled out. When a planet system or solar system forms there is no knowledge of how long they will be stable. They form and then they can go unstable in some time scale, and that time scale is set arbitrarily. Most of the instabilities happen early, and the longer you go in history, the more rare instabilities become. But rare does not mean never, and rare events can happen. Minton speculates two proto-Venus planets existed at one point and went into a chaotic and unstable phase, crossing Earth's path and boosting us to our familiar orbit. The two proto-Venus planets then collided, forming the planet Venus that exists today. One way we could have ruled this out would be if Venus had a geological history older than 2 billion years ago. We know, though, Venus is a relatively young planet. The oldest surface on Venus is estimated to be 500 million to 700 million years old, a relatively young surface by planetary science standards. Impact craters on Earth can stretch back 1 billion to 2 billion years old, with a variety of ages on the surface. This hypothesis of the Faint Young Sun Paradox fits the evolution of Venus.


It is very hard to determine when life begun on Earth. Earlier this year an international team, including a Cardiff University researcher who previously found evidence of Earth's earliest tree, has gone one step further - they have unearthed and investigated an entire fossil forest dating back 385 million years. The Gilboa fossil forest, in the Catskill Mountains in upstate New York, is generally referred to as "the oldest fossil forest". One of the biggest surprises was that the researchers found many woody horizontally-lying stems, up to about 15cm thick, which they have demonstrated to be the ground-running trunks of another type of plant, only previously known from its upright branches. They also found one large example of a tree-shaped club moss, the type of tree that commonly forms coal seams in younger rocks across Europe and North America.earth04.jpg
earth06.jpgThe oceans teemed with life 600 million years ago, but the simple, soft-bodied creatures would have been hardly recognizable as the ancestors of nearly all animals on Earth today. Then something happened. Over several tens of millions of years - a relative blink of an eye in geologic terms - a burst of evolution led to a flurry of diversification and increasing complexity, including the expansion of multicellular organisms and the appearance of the first shells and skeletons. The results of this Cambrian explosion are well documented in the fossil record, but its cause - why and when it happened, and perhaps why nothing similar has happened since - has been a mystery.


New research shows that the answer may lie in a second geological curiosity - a dramatic boundary, known as the Great Unconformity, between ancient igneous and metamorphic rocks and younger sediments. The Great Unconformity is a very prominent geomorphic surface and there's nothing else like it in the entire rock record. Occurring worldwide, the Great Unconformity juxtaposes old rocks, formed billions of years ago deep within Earth's crust, with relatively young Cambrian sedimentary rock formed from deposits left by shallow ancient seas that covered the continents just a half billion years ago. Named in 1869 by explorer and geologist John Wesley Powell during the first documented trip through the Grand Canyon, the Great Unconformity has posed a longstanding puzzle and has been viewed (by Charles Darwin, among others) as a huge gap in the rock record and in our understanding of Earth's history. But the gap itself - the missing time in the geologic record - may hold the key to understanding what happened. At least one theory states that the same geological forces that formed the Great Unconformity may have also provided the impetus for the burst of biodiversity during the early Cambrian. Hypothesis is that biomineralization evolved as a biogeochemical response to an increased influx of continental weathering products during the last stages in the formation of the Great Unconformity. During the early Cambrian, shallow seas repeatedly advanced and retreated across the North American continent, gradually eroding away surface rock to uncover fresh basement rock from within the crust. Exposed to the surface environment for the first time, those crustal rocks reacted with air and water in a chemical weathering process that released ions such as calcium, iron, potassium, and silica into the oceans, changing the seawater chemistry. The basement rocks were later covered with sedimentary deposits from those Cambrian seas, creating the boundary now recognized as the Great Unconformity.




Evidence of changes in the seawater chemistry is captured in the rock record by high rates of carbonate mineral formation early in the Cambrian, as well as the occurrence of extensive beds of glauconite, a potassium-, silica-, and iron-rich mineral that is much rarer today. The influx of ions to the oceans also likely posed a challenge to the organisms living there. Your body has to keep a balance of these ions in order to function properly - if you have too much of one you have to get rid of it, and one way to get rid of it is to make a mineral. The fossil record shows that the three major biominerals - calcium phosphate, now found in bones and teeth; calcium carbonate, in invertebrate shells; and silicon dioxide, in radiolarians - appeared more or less simultaneously around this time and in a diverse array of distantly related organisms. The time lag between the first appearance of animals and their subsequent acquisition of biominerals in the Cambrian is notable. It's likely biomineralization didn't evolve for something, it evolved in response to something - in this case, changing seawater chemistry during the formation of the Great Unconformity. Then once that happened, evolution took it in another direction. Together, the results suggest that the formation of the Great Unconformity may have triggered the Cambrian explosion. A dozen different species in interstellar dust particles that formed the solar system have evolved to more than 4500 species today. Previous work from Carnegie's Bob Hazen demonstrated that up to two thirds of the known types of minerals on Earth can be directly or indirectly linked to biological activity.


In June 2012, University of Alberta researchers have uncovered physical proof that animals existed 585 million years ago - 30 million years earlier than previous records showed. They found fossilized tracks a centimeter-long, slug-like animal left behind 585 million years ago in silty, shallow-water sediment. Fossilized tracks indicate that the soft-bodied animal's musculature enabled it to move through the sediment on the shallow ocean floor. The pattern of movement indicates an evolutionary adaptation to search for food, which would have been organic material in the sediment. There were no fossilized remains of a bilaterian's body, just its tracks. Generally when researchers find tracks of a soft-bodied animal, it means there's no trace of the body because they fossilize under different conditions. It's usually just the body or just the tracks, not both. Before this, the oldest sign of animal life was dated at 555 million years ago, from a find made in Russia.




Most of the world's species are still unknown to science although many researchers grappled to address the question of how many species there are on Earth over the recent decades. Estimates of non-microbial diversity on Earth provided by researchers range from 2 million to over 50 million species, with great uncertainties in numbers of insects, fungi, nematodes, and deep-sea organisms. Novel techniques, such as DNA barcoding, new databases and crowd-sourcing, could greatly accelerate the rate of species discovery. New technologies such as environmental DNA analyses now exist and can detect a species' presence from mere water samples without ever visually observing it. Data sharing technologies over the Internet about species locations and discoveries are also expediting and expanding the catalogue of life.


During the geological era known as the Mesozoic, the continental crust was concentrated in a single huge landmass, the supercontinent Pangea. Pangea began to break up about 150 million years ago, and the fragments drifted apart, eventually giving rise to the disposition of continents we know today. The progressive break-up of such a large landmass meant that existing groups of plant and animal species were split apart, and the descendant lineages then evolved in isolation from each other. Plate tectonics is a theory of continental drift and sea floor spreading. A wide range of phenomena from volcanism, earthquakes and undersea earthquakes (and pursuant tsunamis) to variations in climate and species development on Earth can be explained by the plate tectonics model, globally recognized during the 1960's. Plate tectonics theory can be applied to about 3 billion years of the Earth's history. However, the Earth is older, up to 4.567 billion years old. We can now demonstrate that there has been a significant shift in the Earth's dynamics. Thus, the Earth, under the first third of its history, developed under conditions other than what can be explained using the plate tectonics model.




Earth has companion; just as we circle Sun does our moon circle us and we call it simply - Moon. The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. The natural satellites orbiting other planets are called "moons" after Earth's Moon. The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator. You may wish to click here and see interaction for any day you wish. Phil Plait, based on data provided in that link, made a video below (with beautiful music by Kevin MacLeod). Because of their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications add up to significant changes. During the Devonian period, for example, (approximately 410 million years ago) there were 400 days in a year, with each day lasting 21.8 hours.





As Phil explains, weird rocking and tilting motion in video above is real. It’s called libration. The Moon’s orbit around the Earth is elliptical, so sometimes it moves faster in its orbit than other times. However, the Moon’s spin is constant. The geometry of these two things add together, allowing us to sometimes peek a little bit over the eastern and western horizons. Not only that, the Moon’s orbit is tilted a bit with respect to our Equator, so we sometimes get a little peak over the north and south poles too.


The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon. Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars. Viewed from Earth, the Moon is just far enough away to have very nearly the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant. This allows total and annular solar eclipses to occur on Earth. The most widely accepted theory of the Moon's origin, the giant impact hypothesis, states that it formed from the collision of a Mars-size protoplanet. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.




The giant impact hypothesis states that the Moon was formed out of the debris left over from a collision between the Earth and a body the size of Mars, approximately 4.5 Gya (four and a half billion years ago). The colliding body is sometimes called Theia, for the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. The giant impact hypothesis is the currently-favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes: the identical direction of the Earth's spin and the Moon's orbit, Moon samples which indicate the surface of the Moon was once molten, the Moon's relatively small iron core, lower density compared to the Earth, evidence of similar collisions in other star systems (which result in debris disks), and that giant collisions are consistent with the leading theories of the formation of the solar system. There remain several questions concerning the best current models of the giant impact hypothesis, however. The energy of such a giant impact is predicted to heat Earth to produce a global 'ocean' of magma; yet there is no evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle. At present, there is no self-consistent model that starts with the giant impact event and follows the evolution of the debris into a single moon. Other remaining questions include: when did the Moon lose its share of volatile elements; and why Venus, which also experienced giant impacts during its formation, does not host a similar moon.




Of course, not everyone agrees on certain details. In the giant-collision scenario, computer simulations suggest that the moon had two parents: Earth and a hypothetical Theia. But a comparative analysis of titanium from the moon, Earth and meteorites, published  in March 2012 indicates the moon's material came from Earth alone. If two objects had given rise to the moon, just like in humans, the moon would have inherited some of the material from Earth and some of the material from the impactor, approximately half and half. New data suggests Moon is a child with only one parent, as far as we can tell. The research team based their analysis on titanium isotopes - forms of titanium that contain only slight subatomic variations. The researchers selected titanium for their study because the element is highly refractory. This means that titanium tends to remain in a solid or molten state rather than becoming a gas when exposed to tremendous heat. The resistance of titanium isotopes to vaporization makes it less likely that they would become incorporated by Earth and the developing moon in equal amounts. Titanium also contains different isotopic signatures forged in countless stellar explosions that occurred before the sun's birth. These explosions flung subtly different titanium isotopes into interstellar space. Different objects in the newly forming solar system gobbled up those isotopes in different ways through collisions, leaving clues that let scientists infer where the solar materials including the moon came from. When we look at different bodies, different asteroids, there are different isotopic signatures. It's like their different DNAs. Meteorites, which are pieces of asteroids that have fallen to Earth, contain large variations in titanium isotopes. Measurements of terrestrial and lunar samples show that the moon has a strictly identical isotopic composition to the Earth. Researchers initially found variations in the titanium isotopic composition between the lunar and terrestrial samples. They then corrected the results for the effects of cosmic rays, which could have changed the titanium isotopic composition of the lunar samples. Earth and the moon are constantly bombarded by cosmic rays from the sun and from more distant sources in the galaxy. Earth's atmosphere and magnetic field prevents most of these rays from reaching its surface, but the moon has no such protection. Once correction is made, result was 1+1=1. This analyses greatly reinforce previous work by other researchers who came to the same conclusion after comparing terrestrial and lunar oxygen isotopes, which are less refractory and thus more likely to gasify during a giant impact than titanium. Solving the conundrum of the moon's origin probably will prove challenging because all of the alternative scenarios for the moon's formation have drawbacks. For example, it is possible that even though titanium is refractory, it might still have gasified in the giant impact and then became incorporated into the disk of Earth-orbiting material that developed into the moon. This might have erased the signature of the titanium from Theia. The problem with this scenario is that the disk may have fallen back to Earth if too much material was exchanged between the two bodies. An old idea, long abandoned, is that the moon arose via fission from a molten, rapidly rotating Earth following a giant impact. This idea explains the similarity between Earth and moon, but how such a large, concentrated mass could spin fast enough to split in two remains problematical. According to a third scenario, Earth collided with an icy body lacking entirely in titanium. There are no bodies made purely of ice in the solar system, however. It's also possible that Theia had the same composition as Earth. This is unlikely, however, because of the widely accepted view that Earth incorporated material over tens of millions of years in collisions with smaller bodies that flew in from different regions of the developing solar system.




When lunar rocks were first analyzed in the 1970s after having been brought to Earth by US astronauts Neil Armstrong, Edwin "Buzz" Aldrin and Michael Collins, scientists identified three minerals - armalcolite (after Armstrong, Aldrin and Collins), pyroxferroite and tranquillityite - that they believed were unique to the Moon. Armalcolite and pyroxferroite were later found on Earth. In January 2012 it was tranquillityite, named after the Sea of Tranquillity where the Apollo 11 moon-walkers landed in July 1969, identified by Birger Rasmussen from Curtin University. The researchers believe tranquilliltyite is the final 'lunar' mineral to be found on Earth because it is rare, small and prone to change. The Moon lacks water and its minerals are pristine, but even a small amount of water in terrestrial magmas will cause minerals to be altered and difficult to identify.


Unlike Earth, our Moon has no active volcanoes and the traces of its past volcanic activity date from billions of years ago. This is surprising because recent Moonquake data suggest that there is plenty of liquid magma deep within the Moon and part of the rocks residing there are thought to be molten. Scientists recently have identified a likely reason for this peaceful surface life: the hot, molten rock in the Moon's deep interior could be so dense that it is simply too heavy to rise to the surface like a bubble in water. The driving force for vertical movement of magma is the density difference between the magma and the surrounding solid material, making the liquid magma move slowly upwards like a bubble. The lighter the liquid magma is, the more violent the upward movement will be. Nearly all the lunar magmas were found to be less dense than their solid surroundings, similar to the situation on Earth. There is one important exception: small droplets of titanium-rich glass first found in Apollo 14 mission samples produce liquid magma as dense as the rocks found in the deepest parts of the lunar mantle today. This magma would not move towards the surface. Such titanium-rich magma can only be formed by melting titanium rich solid rocks. Previous experiments have shown that such rocks were formed soon after the formation of the Moon at shallow levels, close to the surface. How did they get deep into the mantle? The scientists conclude that large vertical movements must have occurred early in the history of the Moon, during which titanium-rich rocks descended from near the surface all the way to the core-mantle boundary. After descending, magma formed from these near-surface rocks, very rich in titanium, and accumulated at the bottom of the mantle - a bit like an upside-down volcano. Today, the Moon is still cooling down, as are the melts in its interior. In the distant future, the cooler and therefore solidifying melt will change in composition, likely making it less dense than its surroundings. This lighter magma could make its way again up to the surface forming an active volcano on the Moon. What a sight would that be!?  NASA earlier this year release following video showing violent history Moon had according to our understanding:





What about water itself - is there any?  Yes, and not only that. New maps produced by the Lyman Alpha Mapping Project (LAMP) aboard NASA's Lunar Reconnaissance Orbiter reveal features at the Moon's northern and southern poles in regions that lie in perpetual darkness. LAMP uses a novel method to peer into these so-called permanently shadowed regions (PSRs), making visible the invisible. Results suggest there could be as much as 1 to 2 percent water frost in some permanently shadowed soils.




The LAMP team estimates that the loss of water frost is about 16 times slower than previously believed. In addition, the accumulation of water frost is also likely to be highly dependent on local conditions, such as temperature, thermal cycling and even geologically recent "impact gardening" in which micrometeoroid impacts redistribute the location and depth of volatile compounds. Finding water frost at these new locations adds to a rapidly improving understanding of the Moon's water and related species, as discovered by three other space missions through near-infrared emissions observations and found buried within the Cabeus.


In May 2012, additional information become available - scientists studying samples of volcanic glass from the Moon announced there’s more water in them than was once thought. A lot more water. Not enough to go swimming or anything like that, but certainly enough to have affected the Moon’s geologic history. The scientists looked at glass created in volcanic fire fountains, eruptions billions of years ago that left tiny (roughly the diameter of a human hair) grains of colored glass on the surface. These lay there for quite some time until 1972, when they were spotted by geologist Harrison Schmitt, who happened to be standing on the Moon at the time as part of Apollo 17. He brought them back to Earth for study. In the ensuing decades technology improved quite a bit, and figuring out the contents of the glass beads has become a lot more accurate. In this new research, the scientists found (in 2008, actually, but their results are now confirmed) that the beads have a water content of about 750 parts per million, roughly equivalent to what you’d find in magma in the Earth’s upper mantle. That’s very surprising; many of the rocks on the Moon’s surface are very dry, which for years has led scientists to assume the Moon itself was very dry. Dig down and the rocks are expected to be drier. This is turning out not to be the case, since these glass beads come from deep below the lunar surface and have so much water in them. Important part here is that this has a huge effect on how we think the Moon formed. The current thinking is that shortly after the Earth itself formed, maybe 50 - 100 million years or so later, something the size of Mars came careening in and smacked into our new planet. It was a glancing blow, so a vast amount of material blasted out, away from the Earth, and into orbit around us. This stuff formed the Moon. This idea has a lot going for it: for example, it explains why the Moon is less dense than the Earth on average (by the time of impact, a lot of heavier stuff had settled to the Earth’s core, leaving the lower density materials near the surface to become the Moon). It was also thought to take care of why the Moon is so dry compared to Earth: all the water boiled away in the impact. In fact, a lot of the more delicate (technically called volatile) materials would have been destroyed, explaining why we see so few volatiles on the Moon. Except now we see more water.  In another study, using similar samples from Apollo 15, these same authors found that the ratio of deuterium to hydrogen in the lunar samples was indistinguishable from that of terrestrial water. That implies very strongly that the water on the Moon came from the Earth, or they both came from the same source. The D to H ratio changes across the solar system, meaning conditions in the early stages of the planets’ formations were different depending on their distance from the Sun. So this is yet another monkey in the wrench that must be dealt with.


The scientists on LRO used collected information to put together a wild topographic map of the Moon’s far side:




In above map, red represents stuff higher up, blue lower down. The resolution is decent: 100 meters across the surface (NSEW) and 20 meters vertically. To learn more about science behind above picture, click here. For pan-and-zoom versions of it, click here.


Today is almost 40 years since the last humans visited the lunar surface during the Apollo 17 mission of December 1972. The last controlled landing on the moon was just four years later when the Soviet Union's sample-return mission, Luna 24, touched down in July 1976. Since then, nothing (although in recent years both the US and India have crashed probes into the lunar surface). There are reasons to go there back. For a start, the Moon is a good place to learn about the Earth obviously. Throughout its history, our planet has been hit by a multitude of asteroids and comets that ejected countless billions of Earth rocks into space. Some of this stuff will have landed on the Moon where it almost certainly still sits today, pristine and untouched. By some estimates there could be as much as 200 kilograms of Earth per square lunar kilometre. That means the best place to study early Earth rock, its chemical composition and perhaps even the prebiotic cocktail that led to the origin of life is on the surface of the Moon. Then there is the argument that the Moon is the only place that certain types of astronomical observations are possible. Astronomers have studied the universe across the entire electromagnetic spectrum but there is one small corner of the rainbow that is still inaccessible to Earth-based instruments - ultra-low frequency radio waves. Below about 30 MHz, the ionosphere does a pretty good job of absorbing or reflecting more or less everything the Universe throws at us. Consequently, the cosmos is essentially uncharted at these frequencies. The far side of the Moon, on the other hand, is the perfect radio-silent place to observe them. In addition to these goals, there are the well worn arguments about better understanding the Moon itself and the resources it may hold for future exploration, such as water and other volatiles. Additionally, study of humans on the Moon would give an important insight into effects of low gravity on human health. Will we ever live on Moon? I doubt - at least not without atmosphere being built to shield us in the first place. Phil Plait wrote an article for BBC on this subject - check it out.


By looking at the pictures and videos of the Moon (or by just looking at it at night using zoom vision) you may wonder why Moon's surface is so bombarded while Earth's is not - the answer is simple; Moon does not have atmosphere to shield surface nor does it have weather to erode craters. And these craters can tells us stories. The lunar surface is marred by impact craters, remnants of the collisions that have occurred over the past 4.5 billion years.  Check the following video first about crater forming in general:





Now let's go back to the Moon. Following picture below. shows part of crater called Copernicus. Image was taken by LRO. Copernicus is a big crater, and easy to spot even with binoculars since it sits in a vast lava plain; the surrounding material is darkish grey, while the crater is far brighter. It’s also surrounded by a gorgeous system of rays: linear streaks caused by the collapsed plumes of material after the asteroid or comet smacked into the Moon to form the crater itself. Copernicus has a series of mountains in its center, the tallest over a kilometer high. These weren’t created in tectonic events like on Earth, though - giant impacts that cause big craters have weird physics. The pressure upon impact can be so high that the rock in the surface flows like a liquid. It splashes outward, then flows back in, surging upwards in the middle of the impact point. If you checked video above you will get an idea. The crater itself is more than 90 km wide.




Second crater below is called Giordano Bruno. Obviously this one is different (apart being smaller - only 21 km wide). The floor is flat, and noticeably lacking in a central peak indicating the impactor wasn’t big enough to create the same conditions as in Copernicus so the crater shape and structure is different. In fact, you can see places where the crater wall has collapsed inward a bit, like in the upper right. The slumped material forms a flat bench halfway down the wall. That sort of feature is common on Earth too, where sometimes mountains or canyon walls subside a bit and form those benches.





And here is the video of Giordano Bruno crater made by NASA folks.





The Orientale basin, the Moon's most recently formed sizeable crater, stands out from the rest. The crater, which lies along the southwestern boundary between the near and far sides of the moon, appears as a dark spot ringed by concentric circles of ejecta that reach more than 900 kilometers from the impact location (see below). Though other craters have similar rings, the lunar surface surrounding the Orientale basin is unusually rough with reduced concavity. The fact that other craters - even those of similar size and age - lack similar features suggests that mechanisms such as weathering or gravitational settling cannot explain the anomaly. Instead, the Orientale basin, which formed about 3.8 billion years ago, stands out simply because it is the youngest large crater. Researchers proposed that whenever a large body slams into the Moon, seismic waves produced during the impact travel through the solid lunar material, inducing seismic shaking that causes landslides and surface settling. They estimate that the impactor would need to be at least 100 km across to cause sizeable seismic shaking.




The lunar soil was originally white but is known to have is been darkened over time by exposure to the charged particles of the "solar wind". Lunar swirls are enigmatic features found across the Moon’s surface, which are characterized by having a high albedo, appearing optically immature and (often) having a sinuous shape. Their curvilinear shape is often accentuated by low albedo regions that wind between the bright swirls. They appear to overlay the lunar surface, superposed on top of craters and ejecta deposits, but impart no observable topography. Swirls have been identified on the lunar maria and highlands - they are not associated with a specific lithologic composition. Swirls on the maria are characterized by strong albedo contrasts and complex, sinuous morphology, whereas those on highland terrain appear less prominent and exhibit simpler shapes, such as single loops or diffuse bright spots.



It has long been thought that the swirls were a result of magnetic shielding of the lunar surface from the solar wind, but nobody understood how the relatively weak magnetic fields associated with lunar swirls could sufficiently protect the moon's surface over hundreds of millions of years to prevent surface darkening and produce such finely detailed patterns. Close to the moon's surface, the strength of a magnetic anomaly is likely to be very irregular, featuring overlapping "cavities" and "gradients".  We cannot know the precise arrangement without going there to see for ourselves. Over an estimated 3.8 billion years these anomalies would have been deflecting the solar wind particles streaming in from space, slowly creating these amazing patterns, which can be clearly seen on the lunar surface today.


The idea was confirmed by experiments done in the laboratory with the University of York in the U.K. using their "Plasma Wind Tunnel". The particles generated were indeed "corralled" by a narrow electrostatic field, so protecting areas of the exposed surface. The interaction of the solar wind with the magnetic field anomalies have been shown, to be effective enough to create protected voids above the surface of the moon, sufficient to stave off against weathering caused by the bombardment of solar particles.




Credits: Wikipedia, Phil Plait, University of Maryland, Cardiff University, Carnegie Institution, NASA, University of Wisconsin-Madison, Ludwig-Maximilians-Universität München, Jay Kimball, Purdue University, University of Copenhagen, University of Alberta, Carnegie Institution, National University of Singapore, Space Telescope Science Institute, Southwest Research Institute, University of Western Australia, European Synchrotron Radiation Facility, University of Chicago, arXiv, Technology Review, American Geophysical Union