The Building Blocks of Life
It was a real-life Alien Fireball and one of the best-documented meteorite falls in history. A stunning sphere of fire hurtled toward Earth and was captured by a vast network of specialized technology. Now, the race is on to recover minute fragments that may help explain the origins of life on Earth.
Hoba is a meteorite that lies on the farm "Hoba West", not far from Grootfontein, in the Otjozondjupa Region of Namibia. It has been uncovered but, because of its large mass, has never been moved from where it fell.
The main mass is estimated at over 60 tons, and it is the largest known meteorite (as a single piece) and the most massive naturally-occurring piece of iron known at the Earth's surface.
The Hoba meteorite is thought to have landed less than 80,000 years ago. It is inferred that the Earth's atmosphere slowed the object down to the point that it fell to the surface at terminal velocity, thereby remaining intact and causing little excavation.
The meteorite is unusual in that it is flat on both major surfaces, possibly causing it to have skipped across the top of the atmosphere in the way a flat stone skips on water.
Meanwhile, at NASA's Johnson Space Center, scientists question if a controversial space rock from Mars will provide evidence of life elsewhere in the universe.
Although meteors have been known since ancient times, they were not known to be an astronomical phenomenon until early in the 19th century.
Prior to that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky.
Thomas Jefferson wrote "I would more easily believe that (a) Yankee professor would lie than that stones would fall from heaven."
He was referring to Yale chemistry professor Benjamin Silliman's investigation of an 1807 meteorite that fell in Weston, Connecticut.
Silliman believed the meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833.
People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.
The astronomer Denison Olmsted made an extensive study of this storm, and concluded it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted its return in 1867, which drew the attention of other astronomers.
Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct. With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle, the cosmic origin of meteors was now firmly established.
Still, they remain an atmospheric phenomenon, and retain their name "meteor" from the Greek word for "atmospheric". Earth moves at about 30 km/s in its orbit.
Meteoroids also have significant orbital speeds. The differences in velocities where these orbits intersect combined with the gravitational attraction of Earth result in the velocities observed for meteors.
Meteoroids may range in size from large pieces of rock to tiny dust particles floating in space that did not form planets. Once the meteoroids enter Earth's atmosphere and begin to heat up and break apart, they are known as meteors.
Meteors are distinct from comets or asteroids, but some, especially those associated with showers, are dust particles that were ejected from comets.
Most meteorite falls are recovered on the basis of eye-witness accounts of the fireball or the actual impact of the object on the ground, or both.
Therefore, despite the fact that meteorites actually fall with virtually equal probability everywhere on Earth, verified meteorite falls tend to be concentrated in areas with high human population densities such as Europe, Japan, and northern India.
A small number of meteorite falls have been observed with automated cameras and recovered following calculation of the impact point. There are several reported instances of falling meteorites having killed both people and livestock, but a few of these appear more credible than others.
The most infamous reported fatality from a meteorite impact is that of an Egyptian dog that was killed in 1911, although this report is highly disputed. This particular meteorite fall was identified in the 1980s as Martian in origin.
However, there is substantial evidence that the meteorite known as Valera hit and killed a cow upon impact, nearly dividing the animal in two, and similar unsubstantiated reports of a horse being struck and killed by a stone of the New Concord fall also abound.
Throughout history, many first and second-hand reports of
meteorites falling on and killing both humans and other animals abound,
but none have been well documented.
A 13 year old boy was hit in the head by a 3-gram fragment but was not injured, as the meteorite's fall was broken by banana tree leaves.
The first known modern case of a human hit by a space rock occurred on
30 November 1954 in Sylacauga, Alabama. There a 4 kg stone chondrite
crashed through a roof and hit Ann Hodges in her living room after it
bounced off her radio. She was badly bruised.
Other than the Sylacauga event, the most plausible of these claims was put forth by a young boy who stated that he had been hit by a small (~3 gram) stone of the Mbale meteorite fall from Uganda, and who stood to gain nothing from this assertion.
The stone reportedly fell through a number of banana leaves before striking the boy on the head, causing little to no pain, as it was small enough to have been slowed by both friction with the atmosphere as well as that with banana leaves, before striking the boy.
Although it is impossible to prove this claim either way, it seems as though he had little reason to lie about such an event occurring.
In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space.
Several Martian meteorites have been found to contain what some think is evidence for fossilized Martian life forms. The
most significant of these is a meteorite found in the Allan Hills of
Antarctica (ALH 84001). Ejection from Mars seems to have taken place
about 16 million years ago.
Arrival on Earth was about 13 000 years ago. Cracks in the rock appear
to have filled with carbonate materials (implying groundwater was
present) between 4 and 3.6 billion-years-ago.
Tales From Other Worlds
Evidence of polycyclic aromatic hydrocarbons (PAHs) have been identified with the levels increasing away from the surface.
Other Antarctic meteorites do not contain PAHs. Earthly contamination should presumably be highest at the surface.
Several minerals in the crack fill are deposited in phases, specifically, iron deposited as magnetite, that are claimed to be typical of biodepositation on Earth.
There are also small ovoid and tubular structures that might be nanobacteria fossils in carbonate material in crack fills (investigators McKay, Gibson, Thomas-Keprta, Zare).
Micropaleontologist Schopf, who described several important terrestrial bacterial assemblages, examined ALH 84001 and opined that the structures are too small to be Earthly bacteria and don't look especially like lifeforms to him. The size of the objects is consistent with Earthly "nanobacteria", but the existence of nanobacteria itself is controversial.
Many studies disputed the validity of the fossils. For example, it was found that most of the organic matter in the meteorite was of terrestrial origin. But, a recent study suggests that magnetite in the meteorite could have been produced by Martian microbes. The study, published in the journal of the Geochemical and Meteoritic Society, used more advanced high resolution electron microscopy than was possible in 1996.