Photosynthesis Evolution and Oxygen Revolution on Earth

Photosynthesis Evolution

Early photosynthetic systems, such as those in green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, and used various other molecules as electron donors rather than water. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as electron donors. Green nonsulfur bacteria used various amino and other organic acids as an electron donor. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that Earth's early atmosphere was highly reducing at that time.

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.

The main source of oxygen in the Earth's atmosphere derives from oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modern photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor, which is oxidized to molecular oxygen (O2) in the photosynthetic reaction center.

Oxygen Revolution

The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Revolution, or Great Oxidation, was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Although geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.3 billion years ago (2.3 Ga), the actual causes and the exact date of the event are very contested amongst the scientific community. It has been argued that current geochemical and biomarker evidence for the development of oxygenic photosynthesis before the Great Oxidation Event has been mostly inconclusive.

Oceanic cyanobacteria, having developed into multicellular forms more than 2.3 billion years ago (approximately 200 million years before the GOE), became the first microbes to produce oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE, the excess free oxygen started to accumulate in the atmosphere.

The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Besides marine cyanobacteria, there is also evidence of cyanobacteria on land.

A spike in chromium contained in ancient rock deposits shows that these rocks, formed underwater, had accumulated chromium washed off from continental shelves by rivers. The researchers chose to focus on chromium because it is not easily dissolved and its release would have required the presence of a powerful acid. One such acid is sulphuric acid, that would have been created through bacterial reactions with pyrite. Though cyanobacteria are responsible for the GOE, they are not the only organisms capable of releasing oxygen. Research has shown that microbial mats of oxygen-producing microbes will produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere, organisms living on these mats would already be adapted to being exposed to oxygen. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest snowball Earth episode in Earth's history.

Eventually, aerobic organisms evolved, consuming oxygen and bringing about an equilibrium in its availability. Free oxygen has been an important constituent of the atmosphere ever since.

Explanation from: https://en.wikipedia.org/wiki/Photosynthesis#Evolution and https://en.wikipedia.org/wiki/History_of_Earth#Oxygen_revolution

Flying Through an Aurora

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

The NGC 3125 Galaxy

This NASA/ESA Hubble Space Telescope image reveals the vibrant core of the galaxy NGC 3125. Discovered by John Herschel in 1835, NGC 3125 is a great example of a starburst galaxy — a galaxy in which unusually high numbers of new stars are forming, springing to life within intensely hot clouds of gas.

Located approximately 50 million light-years away in the constellation of Antlia (The Air Pump), NGC 3125 is similar to, but unfathomably brighter and more energetic than, one of the Magellanic Clouds. Spanning 15 000 light-years, the galaxy displays massive and violent bursts of star formation, as shown by the hot, young, and blue stars scattered throughout the galaxy’s rose-tinted core. Some of these clumps of stars are notable — one of the most extreme Wolf–Rayet star clusters in the local Universe, NGC 3125-A1, resides within NGC 3125.

Despite their appearance, the fuzzy white blobs dotted around the edge of this galaxy are not stars, but globular clusters. Found within a galaxy’s halo, globular clusters are ancient collections of hundreds of thousands of stars. They orbit around galactic centres like satellites — the Milky Way, for example, hosts over 150 of them.

Image Credit: ESA/Hubble & NASA, Judy Schmidt
Explanation from: https://www.spacetelescope.org/images/potw1629a/

Evolution, Evolutionary history of Life on Earth and Geologic Time Scale

Evolution

Evolution is change in the heritable characteristics of biological populations over successive generations. Evolutionary processes give rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.

All life on Earth shares a common ancestor known as the last universal common ancestor (LUCA), which lived approximately 3.5–3.8 billion years ago, although a study in 2015 found "remains of biotic life" from 4.1 billion years ago in ancient rocks in Western Australia. In July 2016, scientists reported identifying a set of 355 genes from the LUCA of all organisms living on Earth.

Repeated formation of new species (speciation), change within species (anagenesis), and loss of species (extinction) throughout the evolutionary history of life on Earth are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences. These shared traits are more similar among species that share a more recent common ancestor, and can be used to reconstruct a biological "tree of life" based on evolutionary relationships (phylogenetics), using both existing species and fossils. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilized multicellular organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction. More than 99 percent of all species that ever lived on Earth are estimated to be extinct. Estimates of Earth's current species range from 10 to 14 million, of which about 1.2 million have been documented. More recently, in May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.

In the mid-19th century, Charles Darwin formulated the scientific theory of evolution by natural selection, published in his book On the Origin of Species (1859). Evolution by natural selection is a process demonstrated by the observation that more offspring are produced than can possibly survive, along with three facts about populations: 1) traits vary among individuals with respect to morphology, physiology, and behaviour (phenotypic variation), 2) different traits confer different rates of survival and reproduction (differential fitness), and 3) traits can be passed from generation to generation (heritability of fitness). Thus, in successive generations members of a population are replaced by progeny of parents better adapted to survive and reproduce in the biophysical environment in which natural selection takes place. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Natural selection, including sexual selection, is the only known cause of adaptation but not the only known cause of evolution. Other, nonadaptive evolutionary processes include mutation, genetic drift and gene migration.

In the early 20th century the modern evolutionary synthesis integrated classical genetics with Darwin's theory of evolution by natural selection through the discipline of population genetics. The importance of natural selection as a cause of evolution was accepted into other branches of biology. Moreover, previously held notions about evolution, such as orthogenesis, evolutionism, and other beliefs about innate "progress" within the largest-scale trends in evolution, became obsolete scientific theories. Scientists continue to study various aspects of evolutionary biology by forming and testing hypotheses, constructing mathematical models of theoretical biology and biological theories, using observational data, and performing experiments in both the field and the laboratory.

In terms of practical application, an understanding of evolution has been instrumental to developments in numerous scientific and industrial fields, including agriculture, human and veterinary medicine, and the life sciences in general. Discoveries in evolutionary biology have made a significant impact not just in the traditional branches of biology but also in other academic disciplines, including biological anthropology, and evolutionary psychology. Evolutionary computation, a sub-field of artificial intelligence, involves the application of Darwinian principles to problems in computer science.

Evolutionary history of Life on Earth

The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life appeared on the planet, until the present day. Earth formed about 4.5 Ga (billion years) ago and there is evidence that life appeared within 0.5 billion years. The similarities between all present-day organisms indicate the presence of a common ancestor from which all known species have diverged through the process of evolution. More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described.

Geologic Time Scale

The geological time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time, and is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth’s history. The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy.

Evidence from radiometric dating indicates that Earth is about 4.54 billion years old. The geology or deep time of Earth’s past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually marked by changes in the composition of strata which correspond to those, and indicate major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life. Older time spans, which predate the reliable fossil record (before the Proterozoic Eon), are defined by their absolute age.

Some other planets and moons within the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus, Mars and the Earth's Moon. Dominantly fluid planets, such as the gas giants, do not preserve their history in a comparable manner. Apart from the Late Heavy Bombardment, events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets. Construction of a time scale that links the planets is, therefore, of only limited relevance to the Earth's time scale, except in the whole-solar-system context. The existence, timing, and terrestrial effects of the Late Heavy Bombardment is still debated.

Image Credit: United States Geological Survey
Explanation from: https://en.wikipedia.org/wiki/Evolution and https://en.wikipedia.org/wiki/Evolutionary_history_of_life and https://en.wikipedia.org/wiki/Geologic_time_scale

Moonrise seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

Spiral Galaxy NGC 24

This shining disc of a spiral galaxy sits approximately 25 million light-years away from Earth in the constellation of Sculptor. Named NGC 24, the galaxy was discovered by British astronomer William Herschel in 1785, and measures some 40 000 light-years across.

This picture was taken using the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys, known as ACS for short. It shows NGC 24 in detail, highlighting the blue bursts (young stars), dark lanes (cosmic dust), and red bubbles (hydrogen gas) of material peppered throughout the galaxy’s spiral arms. Numerous distant galaxies can also been seen hovering around NGC 24’s perimeter.

However, there may be more to this picture than first meets the eye. Astronomers suspect that spiral galaxies like NGC 24 and the Milky Way are surrounded by, and contained within, extended haloes of dark matter. Dark matter is a mysterious substance that cannot be seen; instead, it reveals itself via its gravitational interactions with surrounding material. Its existence was originally proposed to explain why the outer parts of galaxies, including our own, rotate unexpectedly fast, but it is thought to also play an essential role in a galaxy’s formation and evolution. Most of NGC 24’s mass — a whopping 80% — is thought to be held within such a dark halo.

Image Credit: ESA/Hubble & NASA
Explanation from: https://www.spacetelescope.org/images/potw1639a/

Abiogenesis - Origins of Life on Earth

Abiogenesis or biopoiesis or OoL (Origins of Life), is the natural process of life arising from non-living matter, such as simple organic compounds. It is thought to have occurred on Earth between 3.8 and 4.1 billion years ago. Abiogenesis is studied through a combination of laboratory experiments and extrapolation from the characteristics of modern organisms, and aims to determine how pre-life chemical reactions gave rise to life on Earth.

The study of abiogenesis involves geophysical, chemical, and biological considerations, with more recent approaches attempting a synthesis of all three. Many approaches investigate how self-replicating molecules, or their components, came into existence. It is generally thought that current life on Earth is descended from an RNA world, although RNA-based life may not have been the first life to have existed. The classic Miller–Urey experiment and similar research demonstrated that most amino acids, the basic chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Various external sources of energy that may have triggered these reactions have been proposed, including lightning and radiation. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication. Complex organic molecules have been found in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.

The panspermia hypothesis alternatively suggests that microscopic life was distributed to the early Earth by meteoroids, asteroids and other small Solar System bodies and that life may exist throughout the Universe. It is speculated that the biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years. The panspermia hypothesis therefore answers questions of where, not how, life came to be; it only postulates that life may have originated in a locale outside the Earth.

Nonetheless, Earth remains the only place in the Universe known to harbour life, and fossil evidence from the Earth supplies most studies of abiogenesis. The age of the Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago, and possibly as early as the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia. Other early physical evidence of biogenic substances includes graphite and possibly stromatolites discovered in 3.7 billion-year-old metasedimentary rocks in southwestern Greenland, as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. According to a scientist who commented on the study, "If life arose relatively quickly on Earth … then it could be common in the universe."

Explanation from: https://en.wikipedia.org/wiki/Abiogenesis

Sunset seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

Reflection Nebula BFS 29

NASA's Wide-field Infrared Survey Explorer, or WISE, captured this colorful image of the nebula BFS 29 surrounding the star CE-Camelopardalis, found hovering in the band of the night sky comprising the Milky Way. Most of the gas and dust in this image cannot be seen directly in visible light, but WISE's detectors revealed exquisite new details, and even some hidden stars.

The nebulous interstellar gas and dust in this image is known as BFS 29. "BFS" stands for Blitz, Fich, and Stark -- the three astronomers who identified and catalogued 65 new star-forming regions in 1982 (the "29" simply means that it's the 29th object in their catalog). In visible light, BFS 29 can be seen, but only very slightly. This is because the dust scatters and reflects some of the light from nearby stars, hence its classification as a reflection nebula. The gas in BFS 29 also contains large amounts of ionized hydrogen -- referred to by astronomers as "H II." Hence, the nebula is also classified as an HII region. Reflection nebulae and HII regions are often associated with star formation.

Most of the illumination and energy in BFS 29 is likely provided by the star CE-Camelopardalis. The "CE" in its name comes from a complex naming system for variable stars. Camelopardalis is the name of the constellation in which it is found, and means giraffe in Latin (from a camel wearing a leopard's coat). Of the three brightest stars in this image, it is the bright pink-colored star nearest to the center of the image. The other two bright stars cannot be seen in visible light; they are hidden behind the clouds of gas and dust. In infrared light, however, they shine through brilliantly. CE-Camelopardalis is a variable supergiant star, which means it will eventually end its life in a supernova, likely leaving behind a black hole. It is near the giraffe's hind foot, making a sort of ankle bracelet, as compared to the emerald necklace featured in the Nov. 9, 2010 image.

All four of WISE's infrared detectors were used to make this image. The colors used represent specific wavelengths of infrared radiation. Blue and blue-green (cyan) represent 3.4- and 4.6-micron light, respectively. These wavelengths are mainly emitted by stars within the Milky Way. Green represents 12-micron light, which is emitted by the warm gas of the nebulae. Red represents the longest wavelength, 22-micron light emitted by cooler dust within the nebulae.

Image Credit: NASA/JPL-Caltech/UCLA
Explanation from: http://photojournal.jpl.nasa.gov/catalog/PIA13459

Origin of water on Earth

The origin of water on Earth, or the reason that there is clearly more liquid water on Earth than on the other rocky planets of the Solar System, is not completely understood. There exist numerous more or less mutually compatible hypotheses as to how water may have accumulated on Earth's surface over the past 4.5 billion years in sufficient quantity to form oceans.

Comets, trans-Neptunian objects or water-rich meteoroids (protoplanets) from the outer reaches of the asteroid belt colliding with Earth may have brought water to the world's oceans. Measurements of the ratio of the hydrogen isotopes deuterium and protium point to asteroids, since similar percentage impurities in carbon-rich chondrites were found in oceanic water, whereas previous measurement of the isotopes' concentrations in comets and trans-Neptunian objects correspond only slightly to water on Earth.

Planetesimals heated by the decay of aluminium. This could cause water to rise to the surface. Recent studies suggest that water with similar deuterium-to-hydrogen ratio was already available at the time of Earth's formation, as evidenced in ancient "eucrites" meteorites originating from the asteroid Vesta.

That Earth's water originated purely from comets is implausible, since a result of measurements of the isotope ratios of deuterium to protium (D/H ratio) in the four comets Halley, Hyakutake, Hale-Bopp, and 67P/Churyumov–Gerasimenko, by researchers such as David Jewitt, is approximately double that of oceanic water. What is however unclear is whether these comets are representative of those from the Kuiper Belt. According to Alessandro Morbidelli, the largest part of today's water comes from protoplanets formed in the outer asteroid belt that plunged towards Earth, as indicated by the D/H proportions in carbon-rich chondrites. The water in carbon-rich chondrites point to a similar D/H ratio as oceanic water. Nevertheless, mechanisms have been proposed to suggest that the D/H-ratio of oceanic water may have increased significantly throughout Earth's history. Such a proposal is consistent with the possibility that a significant amount of the water on Earth was already present during the planet's early evolution.

Recent measurements of the chemical composition of Moon rocks suggest that Earth was born with its water already present. Investigating lunar samples carried to Earth by the Apollo 15 and 17 missions found a deuterium-to-hydrogen ratio that matched the isotopic ratio in carbonaceous chondrites. The ratio is also similar to that found in water on Earth. The findings suggest a common source of water for both objects. This supports a theory that Jupiter temporarily migrated into the inner Solar System, destabilizing the orbits of water-rich carbonaceous chondrites. As a result, some of the bodies could have fallen inwards and become part of the raw material for making Earth and its neighbors. The discovery of water vapor out-gassing from Ceres provides related information on water-ice content of the asteroid belt.

Explanation from: https://en.wikipedia.org/wiki/Origin_of_water_on_Earth

Clouds over Pacific Ocean seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

Mercury

Mercury is the smallest and innermost planet in the Solar System. Its orbital period (about 88 Earth days) is less than any other planet in the Solar System. Seen from Earth, it appears to move around its orbit in about 116 days. It has no known natural satellites. It is named after the Roman deity Mercury, the messenger to the gods.

Partly because it has almost no atmosphere to retain heat, Mercury's surface temperature varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day in some equatorial regions. The poles are constantly below 180 K (−93 °C; −136 °F). Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 degree), and its orbital eccentricity is the largest of all known planets in the Solar System. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.

Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance, and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

Because Mercury orbits the Sun within Earth's orbit (as does Venus), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth. Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years, before exhausting its fuel and crashing into the planet's surface on April 30, 2015.

Image Credit: NASA
Explanation from: https://en.wikipedia.org/wiki/Mercury_(planet)

Late Heavy Bombardment

The Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm) is an event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, corresponding to the Neohadean and Eoarchean eras on Earth. During this interval, a disproportionately large number of asteroids are theorized to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars. The LHB happened after the Earth and other rocky planets had formed and accreted most of their mass, but still quite early in Earth's history.

Evidence for the LHB derives from lunar samples brought back by the Apollo astronauts. Isotopic dating of Moon rocks implies that most impact melts occurred in a rather narrow interval of time. Several hypotheses are now offered to explain the apparent spike in the flux of impactors (i.e. asteroids and comets) in the inner Solar System, but no consensus yet exists. The Nice model is popular among planetary scientists; it postulates that the giant planets underwent orbital migration and scattered objects in the asteroid and/or Kuiper belts into eccentric orbits, and thereby into the path of the terrestrial planets. Other researchers argue that the lunar sample data do not require a cataclysmic cratering event near 3.9 Ga, and that the apparent clustering of impact melt ages near this time is an artifact of sampling materials retrieved from a single large impact basin. They also note that the rate of impact cratering could be significantly different between the outer and inner zones of the Solar System.

The main piece of evidence for a lunar cataclysm comes from the radiometric ages of impact melt rocks that were collected during the Apollo missions. The majority of these impact melts are believed to have formed during the collision of asteroids or comets tens of kilometers across, forming impact craters hundreds of kilometers in diameter. The Apollo 15, 16, and 17 landing sites were chosen as a result of their proximity to the Imbrium, Nectaris, and Serenitatis basins respectively.

Under study on Earth, the ages of impact melts collected at these sites clustered between about 3.8 and 4.1 Ga. The apparent clustering of ages of these was first noticed in the mid-1970s led to postulation that the ages record an intense bombardment of the Moon. They called it the "lunar cataclysm" and proposed that it represented a dramatic increase in the rate of bombardment of the Moon around 3.9 Ga. If these impact melts were derived from these three basins, then not only did these three prominent impact basins form within a short interval of time, but so did many others based on stratigraphic grounds. At the time, the conclusion was considered controversial.

As more data has become available, particularly from lunar meteorites, this theory, while still controversial, has gained in popularity. The lunar meteorites are believed to randomly sample the lunar surface, and at least some of these should have originated from regions far from the Apollo landing sites. Many of the feldspathic lunar meteorites probably originated from the lunar far side, and impact melts within these have recently been dated. Consistent with the cataclysm hypothesis, none of their ages was found to be older than about 3.9 Ga. Nevertheless, the ages do not "cluster" at this date, but span between 2.5 and 3.9 Ga.

Dating of howardite, eucrite and diogenite (HED) meteorites and H chondrite meteorites originating from the asteroid belt reveal numerous ages from 3.4–4.1 Ga and an earlier peak at 4.5 Ga. The 3.4–4.1 Ga ages has been interpreted as representing an increase in impact velocities as computer simulations using hydrocode reveal that the volume of impact melt increases 100–1000 times as the impact velocity increases from the current asteroid belt average of 5 km/s to 10 km/s. Impact velocities above 10 km/s require very high inclinations or the large eccentricities of asteroids on planet crossing orbits. Such objects are rare in the current asteroid belt but the population would be significantly increased by the sweeping of resonances due to giant planet migration.

Studies of the highland crater size distributions suggest that the same family of projectiles struck Mercury and the Moon during the Late Heavy Bombardment. If the history of decay of late heavy bombardment on Mercury also followed the history of late heavy bombardment on the Moon, the youngest large basin discovered, Caloris, is comparable in age to the youngest large lunar basins, Orientale and Imbrium, and all of the plains units are older than 3 billion years.

Explanation from: https://en.wikipedia.org/wiki/Late_Heavy_Bombardment

Earth seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

Herbig Haro 32

HH 32 is an excellent example of a "Herbig-Haro object," which is formed when young stars eject jets of material back into interstellar space. This object, about 1,000 light-years from Earth, is somewhat older than Hubble's variable nebula, and the wind from the bright central star has already cleared much of the dust out of the central region, thus exposing the star to direct view. Many young stars, like the central object in HH 32, are surrounded by disks of gas and dust that form as additional material is attracted gravitationally from the surrounding nebula. Material in the disk gradually spirals in toward the star and eventually some of it accretes onto the star, increasing its mass. A fraction of the gas, however, is ejected perpendicularly to the disk at speeds near 200 miles per second, and forms two oppositely directed jets. These jets plow into the surrounding nebula, producing strong shock waves that heat the gas and cause it to glow in the light of hydrogen atoms (green) and sulfur ions (blue), several other atoms and ions, and sometimes radiation from the exciting star that is reflected by the surrounding gas (red). This glow is called a Herbig-Haro object, in honor of astronomers George Herbig and Guillermo Haro, who did much of the early work in this area in the 1950's. The jet on the top side, whose furthest extent is about 0.2 light-year from the star, is pointed more nearly in our direction, while the opposite jet on the bottom lies on the far side of the star and is fainter either because it is partially obscured by dust surrounding the star or because there is much less material in front of the star.

Image Credit: NASA and The Hubble Heritage Team (AURA/STScI).
Explanation from: http://hubblesite.org/newscenter/archive/releases/1999/35/image/a/

Formation of the Moon

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago, some 30–50 million years after the origin of the Solar System. Recent research presented by Rick Carlson indicates a slightly lower age of between 4.40 and 4.45 billion years. These mechanisms included the fission of the Moon from Earth's crust through centrifugal force (which would require too great an initial spin of Earth), the gravitational capture of a pre-formed Moon (which would require an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon), and the co-formation of Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metals in the Moon). These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.

The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact, where a Mars-sized body (named Theia) collided with the newly formed proto-Earth, blasting material into orbit around it that accreted to form the Moon.

This hypothesis perhaps best explains the evidence, although not perfectly. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular.

Before the conference, there were partisans of the three "traditional" theories, plus a few people who were starting to take the giant impact seriously, and there was a huge apathetic middle who didn’t think the debate would ever be resolved. Afterward there were essentially only two groups: the giant impact camp and the agnostics.

Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system and the small size of the lunar core. These simulations also show that most of the Moon came from the impactor, not from the proto-Earth. However, more-recent tests suggest more of the Moon coalesced from Earth and not the impactor. Meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to Earth, whereas Earth and the Moon have nearly identical isotopic compositions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions, although this is debated.

The large amount of energy released in the giant impact event and the subsequent re-accretion of material in Earth orbit would have melted the outer shell of Earth, forming a magma ocean. The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km (300 miles) to the entire radius of the Moon (1,737 km (1,079 miles)).

Despite its accuracy in explaining many lines of evidence, there are still some difficulties that are not fully explained by the giant impact hypothesis, most of them involving the Moon's composition.

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. To their surprise, the team found that the rocks from the Apollo program carried an isotopic signature that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System. Because most of the material that went into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from the California Institute of Technology announced that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as Earth, which conflicts with what is expected if the Moon formed far from Earth's orbit or from Theia. Variations on the giant impact hypothesis may explain this data.

Image Credit: Dana Berry, Robin Canup, SWRI
Explanation from: https://en.wikipedia.org/wiki/Moon

Sun's reflection on Atlantic Ocean seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: NASA/ESA

Planetary Nebula NGC 6826

NGC 6826's eye-like appearance is marred by two sets of blood-red 'fliers' that lie horizontally across the image. The surrounding faint green 'white' of the eye is believed to be gas that made up almost half of the star's mass for most of its life. The hot remnant star (in the centre of the green oval) drives a fast wind into older material, forming a hot interior bubble which pushes the older gas ahead of it to form a bright rim. (The star is one of the brightest stars in any planetary.) NGC 6826 is 2, 200 light- years away in the constellation Cygnus. The Hubble telescope observation was taken Jan. 27, 1996 with the Wide Field and Planetary Camera 2.

Image Credit: NASA/ESA
Explanation from: https://www.spacetelescope.org/images/opo9738d/

Early Earth - Theia collision

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact suggests that the Moon formed out of the debris left over from a collision between Earth and an astronomical body the size of Mars, approximately 4.5 billion years ago, in the Hadean eon; about 20 to 100 million years after the solar system coalesced. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in 2016, suggests that the impact may have been a direct hit, causing a thorough mixing of both parent bodies.

As of 2001 the giant-impact hypothesis is the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes:

• Earth's spin and the Moon's orbit have similar orientations.

• Moon samples indicate that the Moon once had a molten surface.

• The Moon has a relatively small iron core.

• The Moon has a lower density than Earth.

• Evidence exists of similar collisions in other star systems (that result in debris disks).

• Giant collisions are consistent with the leading theories of the formation of the solar system.

• The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.

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 have heated Earth to produce a global "ocean" of magma, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle has been documented. However, as of 2015 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 the Moon lost its share of volatile elements and why Venus—which experienced giant impacts during its formation—does not host a similar moon.

Indirect evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygen isotope ratios nearly identical to those of Earth. The highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich samples, suggest that a large portion of the Moon once was molten; and a giant impact scenario could easily have supplied the energy needed to form such a magma ocean. Several lines of evidence show that if the Moon has an iron-rich core, it must be a small one. In particular, the mean density, moment of inertia, rotational signature, and magnetic induction response of the Moon all suggest that the radius of its core is less than about 25% the radius of the Moon, in contrast to about 50% for most of the other terrestrial bodies. Appropriate impact conditions satisfying the angular momentum constraints of the Earth–Moon system yield a Moon formed mostly from the mantles of the Earth and the impactor, while the core of the impactor accretes to the Earth.

Comparison of the zinc isotopic composition of Lunar samples with that of Earth and Mars rocks provides further evidence for the impact hypothesis. Zinc is strongly fractionated when volatilized in planetary rocks, but not during normal igneous processes, so zinc abundance and isotopic composition can distinguish the two geological processes. Moon rocks contain more heavy isotopes of zinc, and overall less zinc, than corresponding igneous Earth or Mars rocks, which is consistent with zinc being depleted from the Moon through evaporation, as expected for the giant impact origin.

Collisions between ejecta escaping Earth's gravity and asteroids would have left impact heating signatures in stony meteorites; analysis based on assuming the existence of this effect has been used to date the impact event to 4.47 billion years ago, in agreement with the date obtained by other means.

Warm silica-rich dust and abundant SiO gas, products of high velocity (> 10 km/s) impacts between rocky bodies, have been detected by the Spitzer Space Telescope around the nearby (29 pc distant) young (~12 My old) star HD172555 in the Beta Pictoris moving group. A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades cluster appears similar to the predicted results of Theia's collision with the embryonic Earth, and has been interpreted as the result of planet-sized objects colliding with each other. A similar belt of warm dust was detected around the star BD +20°307 (HIP 8920, SAO 75016).

Image Credit: NASA/JPL-Caltech
Explanation from: https://en.wikipedia.org/wiki/Giant-impact_hypothesis

Aurora seen from the International Space Station

ISS, Orbit of the Earth

August 2016

Image Credit: ESA/NASA

The Crab Pulsar

The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant.

The Crab Pulsar is one of very few pulsars to be identified optically. The optical pulsar is roughly 20 km in diameter and the pulsar "beams" rotate once every 33 milliseconds, or 30 times each second. The outflowing relativistic wind from the neutron star generates synchrotron emission, which produces the bulk of the emission from the nebula, seen from radio waves through to gamma rays. The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the surrounding nebula, forming a termination shock. The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar into the main body of the nebula. The period of the pulsar's rotation is slowing by 38 nanoseconds per day due to the large amounts of energy carried away in the pulsar wind.

The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar itself. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'crab' and 'millicrab' are sometimes used as units of flux density. A millicrab corresponds to a flux density of about 2.4x10−11 erg s−1 cm−2 (2.4x10−14 W m−2) in the 2–10 keV X-ray band, for a "crab-like" X-ray spectrum, which is roughly a powerlaw in photon energy, I(E)=9.5 E−1.1. Very few X-ray sources ever exceed one crab in brightness.

Image Credit: NASA/CXC/ASU/J. Hester et al., HST/ASU/J. Hester et al.
Explanation from: https://en.wikipedia.org/wiki/Crab_Pulsar