Comets


Meteor

Comets, Not Asteroids, to Blame for Moon's Scarred Face

Moon
© NASAA new study suggests comets gouged out the vast majority of craters on the moon.
Icy comets - not rocky asteroids - launched a dramatic assault on the Earth and moon around 3.85 billion years ago, a new study of ancient rocks in Greenland suggests. The work suggests much of Earth's water could have been brought to the planet by comets.

"We can see craters on the moon's surface with the naked eye, but nobody actually knew what caused them - was it rocks, was it iron, was it ice?" says Uffe Gråe Jørgensen, an astronomer at the Niels Bohr Institute in Copenhagen, Denmark. "It's exciting to find signs that it was actually ice."

Evidence suggests that the Earth and moon had both formed around 4.5 billion years ago. But almost all the craters on the moon date to a later period, the "Late Heavy Bombardment" 3.8 to 3.9 billion years ago, when around 100 million billion tonnes of rock or ice crashed onto the lunar surface. The Earth would have been pummelled by debris at the same time, although plate tectonics on our restless planet have since erased the scars.

Meteor

Jupiter increases risk of comet strike on Earth

Image
© Julian BaumEarth experienced an especially heavy bombardment of asteroids and comets early in the solar system's history.
Contrary to prevailing wisdom, Jupiter does not protect Earth from comet strikes. In fact, Earth would suffer fewer impacts without the influence of Jupiter's gravity, a new study says. It could have implications for determining which solar systems are most hospitable to life.

A 1994 study showed that replacing Jupiter with a much smaller planet like Uranus or Neptune would lead to 1000 times as many long-period comets hitting Earth. This led to speculation that complex life would have a hard time developing in solar systems without a Jupiter-like planet because of more intense bombardment by comets.

But a new study by Jonathan Horner and Barrie Jones of Open University in Milton Keynes, UK, shows that if there were no planet at all in Jupiter's orbit, Earth would actually be safer from impacts.

The contradictory results arise because Jupiter affects comets in two different, competing ways. Its gravity helps pull comets into the inner solar system, where they have a chance of hitting Earth, but can also clear away Earth-threatening comets by ejecting them from the solar system altogether, via a gravitational slingshot effect.

Meteor

California's Channel Islands Hold Evidence of Clovis-Age Comets

California
© NOAA and UC Santa BarbaraThis map shows California's Channel Islands with the islands of the previously combined islands of Santarosae encircled at the top.
A 17-member team has found what may be the smoking gun of a much-debated proposal that a cosmic impact about 12,900 years ago ripped through North America and drove multiple species into extinction.

In a paper appearing online ahead of regular publication in the Proceedings of the National Academy of Sciences, University of Oregon archaeologist Douglas J. Kennett and colleagues from nine institutions and three private research companies report the presence of shock-synthesized hexagonal diamonds in 12,900-year-old sediments on the Northern Channel Islands off the southern California coast.

These tiny diamonds and diamond clusters were buried deeply below four meters of sediment. They date to the end of Clovis -- a Paleoindian culture long thought to be North America's first human inhabitants. The nano-sized diamonds were pulled from Arlington Canyon on the island of Santa Rosa that had once been joined with three other Northern Channel Islands in a landmass known as Santarosae.

Telescope

Subaru Telescope observes Comet-shaped Knots inside the Helix Nebula

near-infrared image of the Helix Nebula
© Subaru Telescope, NAOJNew near-infrared image of the Helix Nebula, showing comet-shaped knots within. These features look like a fireworks display in space.
The Helix Nebula, NGC 7293, is not only one of the most interesting and beautiful planetary nebulae; it is also one of the closest nebulae to Earth, at a distance of only 710 light years away. A new image, taken with an infrared camera on the Subaru Telescope in Hawaii, shows tens of thousands of previously unseen comet-shaped knots inside the nebula. The sheer number of knots -- more than have ever been seen before -- looks like a massive fireworks display in space.

Telescope

Cosmic surprise: Many asteroids are comets

meteor streaking across the sky
© AFP/NASA-HO/FileThis November 2000 NASA file image shows a meteor streaking across the sky during the Leonid meteor shower. Many of the primitive bodies wandering the asteroid belt between Mars and Jupiter are former comets, tossed out of orbit by a brutal ballet between the giant outer planets, say a team of astrophysicists.
Many of the primitive bodies wandering the asteroid belt between Mars and Jupiter are former comets, tossed out of orbit by a brutal ballet between the giant outer planets, say a team of astrophysicists.

A commonly accepted theory is that the asteroid belt is the rubble left over from a "proto-planetary disk," the dense ring of gas that surrounds a new-born star.

But the orbiting rocks have long been a source of deep curiosity. They are remarkably varied, ranging from mixtures of ice and rock to igneous rocks, which implies they have jumbled origins.

The answer to the mystery, according to a study published by the British journal Nature on Wednesday, is that a "significant fraction" of the asteroid population in fact comprises ex-comets.

Famously described as "dirty snowballs" of ice and dust, comets are lonely, long-distance wanderers of the Solar System whose elliptical swing around the Sun can take decades.

Researchers in France and the United States ran a mathematical model of the development of the early Solar System, when the planets were accreting from clustering masses of dust and gas.

Telescope

Meteor shower, comet highlights in July

Summer is now upon us and this will be a good month to enjoy the sky in spite of the short nights. The bright planets are evenly split this month, with the gas giants Saturn and Jupiter visible in the evening sky and our neighboring terrestrial planets, Venus and Mars, visible in the morning sky.

There will be two more very exciting celestial events taking place this month, but only one of them will be visible for us in New England. The annual Delta Aquarid Meteor Shower will peak during the morning hours of July 28. This shower actually begins around the middle of July and blends right into the famous Perseid Meteor Shower, which starts at the end of July and peaks on August 12.

Caused by Comet Machholz, you can expect around 15 to 20 Delta Aquarids per hour that morning. The moon will be first quarter and will set around midnight. Meteor showers are usually better after midnight, anyway, since that is when the earth is spinning directly into the meteors, like snowflakes on the front windshield of your car during a snowstorm. The whole earth can be seen as a little spaceship continually orbiting the sun at 18.6 miles per second, or 67,000 miles per hour.

Meteor

Comets Seeded Earth's Early Atmosphere

The ratio of nitrogen isotopes in several comets almost exactly matches the ratio on Earth, implying that our early atmosphere probably came from a cometary bombardment.

Astrobiologists have long puzzled over the origin of Earth's oceans. But they've dwelt a little less long over a related question: where does the nitrogen in our atmosphere come from? Now a new analysis by Damien Hutsemekers and pals at the Universite de Liege, in Belgium, suggests an answer to both questions.

One of the most attractive theories of the origin of our water is that Earth was once bombarded by icy comets that left a watery residue. The trouble is that the ratio of deuterium to hydrogen in water on Earth is much lower than it is in the few comets we've been able to measure it in (i.e., Halley, Hyakutake, Hale-Bopp, and C/2002 T7 LINEAR). So if these types of comets, which we know came from the Oort Cloud, did supply Earth's water, it must have mixed with water already on Earth that had a very low deuterium content.

Hourglass

Comets, Meteors & Myth: New Evidence for Toppled Civilizations and Biblical Tales

"...and the seven judges of hell ... raised their torches, lighting the land with their livid flame. A stupor of despair went up to heaven when the god of the storm turned daylight into darkness, when he smashed the land like a cup."

-- An account of the Deluge from the Epic of Gilgamesh, circa 2200 B.C.
If you are fortunate enough to see the storm of shooting stars predicted for the Nov. 18 peak of the Leonid meteor shower, you'll be watching a similar but considerably less powerful version of events which some scientists say brought down the world's first civilizations.

The root of both: debris from a disintegrating comet.

Biblical stories, apocalyptic visions, ancient art and scientific data all seem to intersect at around 2350 B.C., when one or more catastrophic events wiped out several advanced societies in Europe, Asia and Africa.

Increasingly, some scientists suspect comets and their associated meteor storms were the cause. History and culture provide clues: Icons and myths surrounding the alleged cataclysms persist in cults and religions today and even fuel terrorism.

Magnify

U2 comet dust predates solar system

An innovative plan to retrieve comet particles from earth's stratosphere has hit pay dirt, with the discovery that some predate the formation of the solar system.

"It was the largest number ever found," says Dr Henner Busemann of the University of Manchester's School of Earth, Atmospheric and Environmental Sciences.

Bizarro Earth

Nova program focuses on rain of comets 12,900 years ago

Mammoth
© Royal BC Museum, Victoria, British ColumbiaImpact victim? Nanodiamonds suggest to some scientists that a huge impact did in the mammoths.

Last night at Southern Illinois University in Carbondale, a dozen faculty members and students gathered for a "mammoth barbecue" before the U.S. Public Broadcasting System's NOVA program laid out the story of the provocative--and highly controversial--proposal that a huge impact drove the mammoths and dozens of other large North American animals to extinction 12,900 years ago. The verdict?

"It was NOVA theater," says geologist and host Nicholas Pinter. "It was enjoyable, there were nice animals, but there was skepticism [expressed at the gathering] about the impact story." That, despite the first revelation of evidence from Greenland, added further support to an extraterrestrial killer.