Impact craters on the surface of the moon tell the tale of a troubled, violent childhood that may have continued into the moon's adolescence -- a history shared, but obliterated, on Earth.
The findings have implications for understanding how, when and how often life got a toehold on Earth.
"Large impacts in the early solar system could be a factor in keeping life from getting started or evolving rapidly in the early period," Brown University planetary geologist James Head told Discovery News. "Turning it around the other way... so much stuff and larger projectiles moving around the solar system might even increase the possibility of anything that was harboring life to have impacted the Earth as well."
"It makes you think about those early years in the context of what would maybe enhance or deliver life, and what would maybe keep it from gaining a foothold because of the heavy bombardment," Head said.
Working from high-resolution maps of the moon's surface, Head and colleagues homed in on 5,185 craters that are 20 kilometers (12.4 miles) in diameter or larger, an area roughly the size of the city of Atlanta, from among the billions pitting the lunar surface.
The team identified the most heavily cratered and therefore the oldest parts of the moon (fewer craters means an area was more recently resurfaced by volcanic lava flows or other face-changing events). They found that the projectiles smashing in the lunar surface were bigger in the moon's early days than those that impacted later.
The transition appears to have taken place roughly about 3.8 billion years ago.
"This is confirming that there were different populations (of projectiles) coming in early on," Head said. "It's likely to have affected all the inner planets as well, including Earth."
"What was happening on the moon was happening on Earth too," added Noah Petro, a planetary geologist with NASA's Goddard Space Flight Center in Maryland. "I think of the moon as another continent of Earth. In studying the moon, you learn about the history of Earth."
The maps, derived from data collected by NASA's Lunar Reconnaissance Orbiter, identify the moon's southern near side and its north-central far side as its oldest regions.
The research also confirms that the 2,500-kilometer (1,553-mile) diameter Aitken Basin, which lies between the moon's south pole and Aitken Crater south of the moon's equator, is the moon's oldest impact structure. A robotic mission called MoonRise to retrieve soil samples from Aitken Basin is among three projects vying for NASA funding.
The new analysis of the moon's craters is among a trio of papers appearing in this week's Science with results from the ongoing Lunar Reconnaissance Orbiter mission. The probe has been circling about 31 miles above the moon's surface for the past year.
Initially intended to hunt landing spots for future human missions to the moon, the spacecraft on Thursday transitioned to NASA's science department for another two years of work.
