[meteorite-list] Newfound Moon Craters Point to Asteroid Puzzle

[Ron Baalke]

http://www.space.com/31003-moon-asteroid-impacts-mystery.html 

Newfound Moon Craters Point to Asteroid Puzzle
by Charles Q. Choi
space.com
November 3, 2015

Newfound lunar craters suggest that asteroids that smashed into the moon 
long ago were very different from the ones that now occupy the asteroid 
belt, researchers say.

Solving this asteroid mystery could yield answers regarding the habitability 
of the early Earth, scientists added.

Scientists think swarms of asteroids and comets pummeled Earth, the moon 
and the other worlds of the inner solar system during an era known as 
the Late Heavy Bombardment about 4.1 billion to 3.8 billion years ago. 
The many giant, round craters known as lunar basins that pockmark the 
moon's surface now stand as mute testimony to this violent time. 

Although many lunar basins are readily apparent to the naked eye, their 
exact number, sizes and origins remain unclear. Lunar basins are difficult 
to study because their details are often concealed by the destructive 
effects of subsequent impacts and volcanic eruptions.

To learn more about impact basins on the moon, scientists relied on gravity 
datagathered by NASA's Gravity Recovery and Interior Laboratory (GRAIL) 
mission, which consisted of two spacecraft named Ebb and Flowthat were 
in the same orbit around the moon. Tiny changes in the distance between 
the GRAIL probes caused by the gravitational pull of clusters of rock 
have allowed researchers to probe the moon's structure and composition 
in unprecedented detail.

The researchers scanned the GRAIL data for craters 100 miles (160 kilometers) 
or more wide. They concentrated on basins with craters shaped like concentric 
rings, a kind of structure unique to impact basins.

The scientists not only confirmed 27 concentric-ring impact basins, they 
also identified 24 more structures that might be such craters, including 
three new to science. These newly identified craters - named Asperitatis, 
Bartels-Voskresenskiy, and Copernicus-H - are located on the lunar near 
side, and substantially increase the known number of large impacts on 
the moon.

The researchers also confirmed that larger impact basins are preferentially 
found on the moon's near side, which always faces Earth, whereas the far 
side features smaller basins. Previous research suggests this is because 
the lunar near side was warm during the early formation of the moon, creating 
an ideal environment for big craters to form, said study lead author Gregory 
Neumann, a planetary geophysicist at NASA Goddard Space Flight Center 
in Greenbelt, Maryland.
                             
The new findings support previous research suggesting that the asteroids 
that pummeled the moon long ago differed in surprising ways from the rocks 
now seen in the main asteroid belt between Mars and Jupiter, Neumann said.

If ancient asteroids occupied the same size range as modern asteroids, 
there are more intermediate-size lunar impact basins than one would expect, 
and half as many giant impacts "that would obliterate almost half the 
moon," Neumann told Space.com. "In other words, a deficit in the number 
of objects capable of producing megabasins that would destroy all forms 
of life."

More research is needed to understand the nature of asteroid impacts in 
the early solar system, which has implications for understanding "the 
habitability of the early Earth," Neumann said.

The scientists detailed their findings online Oct. 30 in the journal Science 
Advances.