Asteroids and comets are relics of the distant past – revealing to us the well-kept ancient secrets about how our sun and its family of planets, moons, and smaller objects came into being about 4.6 billion years ago. Our Solar System emerged from the collapse of a reliably weakened region embedded within one of the many beautiful, dark, cold molecular clouds that haunt our Milky Way Galaxy like billowing, undulating phantoms, and these clouds serve as the strange nurseries that cradle sparkling baby stars . There are millions of asteroids, and they are generally considered to be the shattered remnants of planetesimals – rocky and metallic objects that danced around within the primordial solar nebula, never managing to grow large enough to become planets. Most of the known asteroids circle our Sun in the Main Asteroid Belt located between the orbits of Mars and Jupiter – however, other orbital families of asteroids exist that host significant populations, including near-Earth objects (NEOs). In October 2016, a team of astronomers announced that they have discovered possible clues indicating that water exists on the surface of the largest metallic asteroid in our Sun's family.
Named 16 Psyche , the possibly wet metallic worldlet is one of the most massive denizens of the Main Asteroid Belt , measuring 186 miles across and composed of almost pure nickel-iron metal. It is often considered to be the naked core of a planetary embryo – a planetesimal –that was almost , but not completely, blasted into oblivion by catastrophic impacts billions of years ago when our Solar System was young and still in the process of forming .
The planets of our Solar System emerged from a protoplanetary accretion disk at about the same time our baby Star, the Sun was born. Protoplanetary accretion disks can whirl around a baby star for as long as 10 million years, and they feed the developing young star (protostar) a nourishing formula of dust and gas.
Very fine dust particles bounce around within the extremely strict environment of the protoplanetary accretion disk , and they possess a natural "stickiness." The tiny motes of dust easily "glue" themselves to one another when they collide, merging to create ever larger and larger objects – the planetesimals. The motes of "sticky" dust that cling to one another create objects up to several centimeters in size, and these continue to merge together to extremely form the planetesimals – the building-blocks of planets. Plantesimals can reach sizes of 1 kilometer across, or even larger. These primordial planetary building-blocks were an extremely abundantly populated through the entire protoplanetary accretion disk , and it is possible for some planetesimals to linger and survive long enough to remain as tattle-tale remnants of billions of years after a fully developed planetary system – such as our own Solar System – has formed. Asteroids that are members of our own Sun's family of objects are all that is left of the heavy population of planetesimals that were denizens of our primordial Solar System. The asteroids are the tattered remnants of the rocky and metallic planetesimals that merged together to form the quarter of inner rocky terrestrial planets in our Solar System: Mercury, Venus, our Earth, and Mars. Conversely, the dusty, muddy, icy comets are the left overs of the icy planetesimals that contributed to the construction of the four giant outer gaseous planets: Jupiter, Saturn, Uranus, and Neptune.
"Vermin Of The Skies"
The dwarf planet Ceres was the first asteroid to be discovered, and it was originally classified as a major planet. Detected for the first time by Giuseppe Piazzi in 1801, the discovery of Ceres – the largest body inhabiting the Main Asteroid Belt –was soon followed by the discovery of other kindred bodies. The instruments used by astronomers of that era could only resolve these small worldlets as glittering tiny points of light – like stars – and were unable to detect the existence of planetary disks. However, the astronomers were able to distinguish them from stars because of their similar motives. This was the reason why the English astronomer Sir William Herschel proposed that they be designated "asteroids" . The term asteroid was derived from the Greek word for "star-like" or "star-shaped" and "star, planet". During the early second half of the 19th century, the terms asteroid and planet were still used interchangeably.
During the latter half of the 18th century, Baron Franz Xaver von Zach organized a team of 24 astronomers to go on the hunt for a missing planet that should exist at about 2.8 astronomical units (AU) from our Sun. One AU is equivalent to the average distance between Earth and Sun, which is 93,000,000 miles. The prediction that there should be a planet at 2.8 AU from the Sun was derived from the Titius-Bode law, which was given additional credibility as a result of the discovery of the planet Uranus by William Herschel in 1781, who discovered this giant outer planet at the distance predicted by this law. The quest for the missing planet predicted that it should be located about 30 seconds of arc per hour – which could easily be spotted by observers.
Asteroids, in contrast to comets, are primarily composed of mineral and rock, while comets are mostly dust and ice. Furthermore, asteroids formed closer to the golden light and melting heat of our Star, thus preventing the formation of cometary ice. Asteroids are also distinguished from meteoroids primarily because of their different sizes. While meteoroids sport diameters of less than one meter, asteroids have a diameter greater than one meter. In addition, meteoroids can be composed of either asteroidal or cometary materials.
Only one asteroid, 4 Vesta, which has a relatively reflective (bright) surface, can be seen with the naked human eye – and this only when the night sky above our planet is extremely dark, and when Vesta itself is in a favorable location to be observed. On rare occasions, small asteroids zipping by close to Earth may be seen with the naked eye for one brief shining moment. As of March 2016, the Minor Planet Center had data on over 1.3 million objects dwelling in both the inner and outer regions of our Solar System, out of which they had about 750,000 of them to give them numbered designs. The instruments that astronomers now use to detect asteroids have improved significantly since the discovery of Ceres.
Ceres itself was discovered by accident by Giuseppe Piazzi, who was then director of the observatory of Palermo in Sicily. He detected a new star-like body located in the constellation Taurus, and he kept track of the displacement of this object for several nights. Late in 1801, Carl Friedrich Gauss used these observations to determine the orbit of this mysterious object, which was situated between the planets Mars and Jupiter. Piazzi named the brave new little world after Ceres, who was the Roman goddess of agriculture.
A trio of other asteroids were discovered over the next few years: 2 Pallas, 3 Juno, and 4 Vesta. Vesta was discovered in 1807. After an additional eight years of searches, that came up empty-handed, most astronomers began to think that there were no more asteroids and brave up the chase.
However, Karl Ludwig Hencke did not give up, and he began to hunt for more asteroids in 1830. Fifteen years later, he hit pay dirt with the discovery of the asteroid 5 Astraea – the first new asteroid to be discovered in 38 years. Hencke also discovered 6 Hebe less than two years after his discovery of 5 Astraea. Following close on the heels of Hencke's discoveries, other astronomers again joined in on the hunt and, as a result, at least one new asteroid was discovered every year after that – with the exception of 1945, as a result of World War II.
In 1891, Max Wolf demonstrated how the use of astrophotography could have been used to spot asteroids, which appeared as short streaks on long-exposure photographic plates. This new technique dramatically increased the rate of discovery compared with previously used visual methods. Wolf, himself, discovered 248 asteroids using this technique, beginning with 323 Brucia –only about 300 asteroids had been discovered up to that point. It was finally understood that asteroids are an abundant population, but most astronomers did not want to waste their time with the tiny objects, dismissing them as the "vermin of the skies". Even a century later, only a few thousand asteroids were discovered, numbered, and named.
Water May Exist On A Mysterious Metal Worldlet
The discovery of possible water on the surface of Psyche was announced by University of Arizona (Tucson) researchers to coincide with a presentation at the joint 48th annual meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) and the 11th annual European Planetary Science Congress (EPSC). The meeting was held in October 2016 in Pasadena, California.
Earlier observations of Psyche had revealed no evidence of water on its surface. However, in a paper published in The Astronomical Journal , Dr. Vishnu Reddy argues that new observations derived from NASA's Infrared Telescope Facility show evidence of the existence of volatiles such as water or hydroxyl on Psyche's surface. Dr. Reddy is an assistant professor at the University of Arizona's Lunar and Planetary Laboratory.
Hydroxyl is a free radical composed of one hydrogen atom bound to one oxygen atom. In our own planet's atmosphere, hydroxyl is extremely reactive and it helps eliminate many chemical compounds. Therefore, it is also referred to as the "detergent of the atmosphere."
"We did not expect a metallic asteroid like Psyche to be covered by water and / or hydroxyl," Dr. Reddy told the press at the October 2016 joint meeting of the DPS and EPSC, where he was presenting these results. Dr. Reddy is second author on the paper led by Dr. Driss Takir of the US Geological Survey (USGS) in Flagstaff, Arizona. "Metal-rich asteroids like Psyche are thought to have formed under dry conditions without the presence of water or hydroxyl, so we were puzzled by our observations at first," Dr. Reddy added.
The finds are intriguing in the context of a proposed $ 500 million mission to dispatch a spacecraft to Psyche , which is under review by NASA, as of this writing. Images obtained by a spacecraft in orbit around Psyche may help planetary scientists to distinguish between water and hydroxyl on the asteroid's surface.
Most asteroids fall into two main categories: those rich in silicates, and those rich in carbon and volatiles. Metallic asteroids such as Psyche are extremely rare, making it valuable natural laboratory where scientists can study how planets formed.
While the origin of this water on the surface of Psyche is still a lingering mystery, Dr. Reddy and his team propose two possible mechanisms for its formation. "We think the water we see on Psyche might have been delivered to its surface by carbonaceous asteroids that impected Psyche in the distant past," Dr. Reddy continued to explain to the press at the Planetary Science meeting in Pasadena.
"Our discovery of carbon and water on an asteroid that is not required to have those compounds supports the knowledge that these building blocks of life could have been delivered to our Earth Early in the history of our Solar System," Dr. Reddy noted. Dr. Reddy discovered similar dark, carbonaceous impactors rich in volatiles on the surface of asteroid Vesta by studying images obtained from NASA's Dawn Mission . As an alternative, the hydroxyl could have the product of solar wind interacting with silicate minerals on Psyche's surface.