The Earth is long overdue for a visit from space, not by aliens but by great chunks of rock. New research related to what caused the 1.2 km wide hole in the ground at Meteor Crater in the Arizona desert is a reminder of the mess that these troublesome visitors can leave behind.
The study, published in the journal Science, provided evidence that the impactor that caused Meteor Crater near Winslow melted rather than vaporised on impact. Such fine distinctions would seem moot if you had the misfortune to be standing nearby, but not to the scientists who strive to understand what takes place during a major impact by an asteroid or comet.
What caused the 180m deep crater is not in dispute. It was an iron-rich rocky asteroid about 30m in diameter, alarmingly small to have caused such a gouge in the Earth's surface. Yet this is considered "modest by geological standards", the researchers said.
The scientists carried out complex measurements of radioactive Nickel 59 and developed computer models to determine what happened to the impactor. They compared the isotope, formed when Nickel 58 changes to Nickel 59 after exposure to cosmic rays, in fragments of the impactor found nearby and in "spheroids", sand-grain-sized particles formed from the melted material.
The team, which included researchers from Rutgers University, the University of Arizona, Tucson, Australian National University, the University of Rhode Island and University of California-Berkeley, decided the meteorites were from surface material and that the spheroids probably came from material one or two metres deep. About 85 per cent of the object melted, leaving only about 15 per cent of the back, outer part of the asteroid remaining solid after impact.
Their calculations suggest the impactor was travelling at about 20 kilometres per second (45,000 m.p.h.) when it hit, delivering energy equivalent to a 20 to 40 megaton bomb. Such comparatively puny events are relatively common however, they are happy to remind us.
Meteor Crater was formed about 50,000 years ago, but Earth-crossing asteroids of this size are predicted to reach us every 1,600 years or so. Most would strike the oceans, but such a meteor might slam into a land mass every 6,000 years on average.
No mathematician is needed to realise that we seem long overdue for our next visit. The recent disaster movies about cataclysmic impacts with asteroids or comets all assume an object of one to two kilometres across. The world wouldn't end if another 30 metre object hit us, but it certainly would make things difficult for anyone living nearby.