27 June 2008


Two meteors have been in the news lately. The big one, of course, is the one that may have hit Mars a few billion years ago. ZUI this NASA press release:
New analysis of Mars' terrain using NASA spacecraft observations reveals what appears to be by far the largest impact crater ever found in the solar system.

NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor have provided detailed information about the elevations and gravity of the Red Planet's northern and southern hemispheres. A new study using this information may solve one of the biggest remaining mysteries in the solar system: why does Mars have two strikingly different kinds of terrain in its northern and southern hemispheres? The huge crater is creating intense scientific interest.

The mystery of the two-faced nature of Mars has perplexed scientists since the first comprehensive images of the surface were beamed home by NASA spacecraft in the 1970s. The main hypotheses have been an ancient impact or some internal process related to the planet's molten subsurface layers. The impact idea, proposed in 1984, fell into disfavor because the basin's shape didn't seem to fit the expected round shape for a crater.


A giant northern basin that covers about 40 percent of Mars' surface, sometimes called the Borealis basin, is the remains of a colossal impact early in the solar system's formation, the new analysis suggests. At 5,300 miles across, it is about four times wider than the next-biggest impact basin known, the Hellas basin on southern Mars. An accompanying report calculates that the impacting object that produced the Borealis basin must have been about 1,200 miles across. That's larger than Pluto.

"This is an impressive result that has implications not only for the evolution of early Mars, but also for early Earth's formation," said Michael Meyer, the Mars chief scientist at NASA Headquarters in Washington.

This northern-hemisphere basin on Mars is one of the smoothest surfaces found in the solar system. The southern hemisphere is high, rough, heavily cratered terrain, which ranges from 2.5 to 5 miles higher in elevation than the basin floor.


One complicating factor in revealing the elliptical shape of the basin was that after the time of the impact, which must have been at least 3.9 billion years ago, giant volcanoes formed along one part of the basin rim and created a huge region of high, rough terrain that obscures the basin's outlines. It took a combination of gravity data, which tend to reveal underlying structure, with data on current surface elevations to reconstruct a map of Mars elevations as they existed before the volcanoes erupted.

ZUI also this article from the New York Times:
The lopsided shape of Mars may well be a result of a cataclysmic impact of a Pluto-size meteor billions of years ago, three teams of scientists are reporting. That would suggest that the lowlands of Mars’s northern hemisphere are a single gigantic impact crater, the largest crater in the solar system.

“The early solar system was a pretty exciting place,” said Francis Nimmo, an associate professor of earth and planetary sciences at the University of California, Santa Cruz, and the lead author of one of three scientific papers to appear in Thursday’s issue of Nature. “There were big collisions happening fairly frequently, and those collisions affected what the planets ultimately ended up looking like.”

About the same time, more than four billion years ago, Earth is believed to have been hit by a Mars-size object, which created the Moon, and signs of a giant impact have also been detected on Mercury.

The San Francisco Chronicle also has a good article.

Another meteor hit what is now Chesapeake Bay some 35 million years ago. ZUI this article from Fox News:
The true impact of an asteroid or comet crashing near the Chesapeake Bay 35 million years ago has been examined in detail for the first time. The analysis reveals the resilience of life in the aftermath of disaster.

The impact crater, which is buried under 400 to 1,200 feet (120 to 365 meters) of sand, silt and clay, spans twice the length of Manhattan.

The sprawling depression helped create what would eventually become Chesapeake Bay. About 10,000 years ago, ice sheets began to melt and once-dry river valleys filled with water.

The rivers of the Chesapeake region converged directly over the buried crater, according to the U.S. Geological Survey (USGS).

ZUI also this article from the Baltimore Sun:
The effort to study the Chesapeake Bay impact crater is a work in progress. An international team of 40 scientists will continue testing the shattered rocks, sediment and microbes in core samples extracted on a privately owned farm five miles north of Cape Charles, Va., between July and December 2005.

But so far, scientists say evidence in the shattered and superheated rocks and sediments shows that when the mile-wide meteorite splashed into the underwater coastal plain, it created a tsunami that threw vapors into space, incinerated everything in its path and sent shattered rocky material flying for thousands of miles.

"It would've been a huge splash," said David A. Kring, a Houston-based crater expert at the Lunar and Planetary Institute, who reviewed the report but was not a part of the study.

The bay crater is the largest in the United States and by some estimates the sixth-largest in the world.

The meteorite that formed it helped shape the bay, continues to affect water supplies in surrounding Virginia communities and is used by teachers in Maryland and elsewhere to spark interest in geology.

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