A tektite

Tektites (from Greek tektos, molten) are glass objects, up to a few centimeters in size, which may be formed by the impact of large meteorites on Earth's surface. However, some tektite researchers disagree with the popular terrestrial-impact theory; they suggest that tektites are more likely volcanic ejecta from the Moon.

The terrestrial-impact theory states that an impact melts material from the Earth's surface and catapults it up to several hundred kilometers away from the impact site. The molten material cools and solidifies to glass. Although a meteorite impact causes the formation the precursor material of tektites is primarily of terrestrial origin. The color of tektites are black or olive-green and their shape varies from rounded to irregular.

Because of the formation process, locations where tektites can be found are sometimes associated with impact craters. Curiously, the largest, geologically youngest tektite deposit in Southeast Asia, called the Australasian strewnfield, has no known impact crater associated with it.

In the 1960s and 1970s, NASA aerodynamicist Dean Chapman and others advanced the "lunar origin" theory of tektites. Chapman used complex orbital computer models and extensive wind tunnel tests to demonstrate that the so-called Australasian tektites originated from the Rosse ejecta ray of the large crater Tycho on the Moon's nearside. Chapman's scientific detective work has apparently stood the test of time, although Tycho ray material has yet to be retrieved and returned to Earth for study. At the same time, U.S. lunar geologist Jack Green observed "recent" volcanic features on the floor of Tycho.

During the 1980s and 1990s, researchers such as planetary scientist John A. O’Keefe of NASA, astronomer and long-time tektite researcher Hal Povenmire , and petrologist Darryl Futrell observed that the slow way in which tektite glass formed (called "fining"), and the volcanic features observed within some layered tektites, couldn’t be explained by the terrestrial-impact theory. Unlike all terrestrial impactite glasses, tektites are nearly free of internal water similar to lunar rocks. O'Keefe suggested explosive, hydrogen-driven lunar volcanoes as the original source of tektites.

Based on still more circumstantial evidence, such as the fact that Apollos 12 and 14 astronauts found several lunar highland and subcrustal rocks with tektite-like chemistry, the space-science community may need to rethink what mechanisms caused these ancient stones to fall to Earth.

Discussion: Some scientists have proposed that tektites are material from deep inside the Moon. Others claim geochemical evidence from the Moon and from tektites themselves clearly shows that this is unlikely. Furthermore, they claim, the clear association of tektites with at least three "young" craters on Earth provides strong evidence that tektites are a product of terrestrial impact. Looking at the problem from another viewpoint, Hal Povenmire said, "If impact events produce tektites, why are tektites not found associated with nearly all of the 250 known impact craters on Earth? We have massive amounts of tektite glass spread over more than 20 percent of the Earth's surface from the Australasian tektite event and yet we cannot find the crater. When Apollo sample 14425 was analyzed under the electron microprobe , it was essentially identical to some Australasian tektites. This led John A. O'Keefe to state, 'If this specimen had been found on the Antarctic ice shelf instead of Fra Mauro on the Moon, it would be declared a tektite.'" [1]

External links

• http://www-pat.llnl.gov/tektite/
• http://webplaza.pt.lu/guyhein/
• http://www.physicstoday.org/pt/vol-54/iss-6/p76.html
• http://www.spacedaily.com/news/deepimpact-02k.html
• http://www.meteoritetimes.com/Back_Links/2002/october/upcoming_in_meteorite.htm