Earth bears evidence of cosmic impacts, both ancient and more recent

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Asteroids and meteors have been much in the news recently, with an exploding meteor injuring hundreds in Russia and an asteroid making a close fly-by of the Earth, both on Friday.

Earlier in the week, researchers announced they had improved the accuracy of the dating of an asteroid collision with Earth 65 million years ago that left a giant crater in the Caribbean and was considered the final event leading to the extinction of the dinosaurs.

Evidence of violent cosmic collisions can be found throughout the solar system, and in fact one need look no farther than the moon for signs.

Even to the naked eye the scars and craters of a cosmic bombardment are clearly visible.

The moon is old, of course, but the Earth is even older and must have undergone eons of the same kind of bombardment.

So where, we can ask, are the giant craters, the "smoking gun" evidence of the pounding Earth must have experienced in the violent, crowded early solar system?

They're there, it turns out, if you know where to look -- and scientists are learning where to look. More to the point, they're learning from where to look: from on high, using the satellites that gaze down on our world and see it in large-scale difficult to appreciate from our vantage point firmly on the Earth's surface.

Part of the problem is that Earth's long history of climate, of wind and rain and erosion, has left our "smoking guns" not looking what we see in our mind's eye as a classic impact crater.

We do have one of those, of course, clearly etched in the desert near Flagstaff, Ariz.

Meteor Crater is considered the best preserved impact carter on earth: Crisp, intact and with sharp edges, it owes its look to its youth, with scientists estimating the age of the 4,000-foot-diameter crater at 50,000 years -- just yesterday in terms of Earth's geological age.

But there are giant scars aplenty on the Earth; it's just that their age has given the powers of erosion and earth movement time to blur and disguise them.

Most famous is, of course, the Chicxulub crater -- the dinosaur killer -- at the edge of the Yucatan peninsula in Mexico.

Its existence was first suggested by gravity anomalies and a huge underwater arc of geologic features.

Subsequent studies would provide evidence from sediment of intense heat and pressure, suggesting an Earth impact involving an object at least 6 miles wide that left a crater 110 miles wide.

But with the help of satellite observations, scientists have identified even larger impact craters.

A geologic structure in Ontario, Canada, known as the Sudbury Basin, has been confirmed as an impact crater -- what scientists also call an astrobleme -- considered the second largest on Earth and one of its oldest.

The impact 1.8 billions years ago of an object perhaps 9 miles across created a 160-mile crater -- now almost completely eroded -- and likely scattered debris across the globe.

But for the largest confirmed impact crater on Earth, eyes must turn to Africa, and a 190-mile-diameter surface feature known as the Vredefort crater.

Named after a town in the Free State province of South Africa, it was created 2 billion years ago from an impact of such high energy, some scientists suspect it may have induced volcanic activity in the region that left signs still visible today.

And the search for Earth impact sites has gone on, recently resulting in a possible new candidate for the largest and oldest, this time in Greenland.

Scientists say they found evidence in 2012 for a 3-billion-year-old impact crater that currently measures 62 miles across but before erosion began its work was probably more than 300 miles in diameter, which would make it the largest discovered on Earth -- so far.

The meteor or asteroid that created it would have likely been nearly 20 miles across, and scientists say such an impact today would almost certainly kill off a lot more life than the "dinosaur" event of 65 million years ago.

Whether it's confirmed as Earth's largest cosmic blemish or not, the fact remains that our world has not escaped the bombardment that characterized much of the early history of our solar system's formation.

And though the solar system is a tidier, slightly more orderly place today, it still abounds with asteroids, comets and meteors that could find themselves sharing the same close cosmic neighborhood with Earth.

Just ask people in Russia.