A primordial collision of two moons that once orbited Earth explains why the present-day moon is a bit lopsided and its far side is much rockier than the lunar surface facing our planet, scientists said on Wednesday.

Research published in the journal Nature suggests the early moon was shadowed by a smaller companion satellite, about one-thirtieth of its own mass. The two bodies circled the infant Earth in tandem more than 4 billion years ago.

But the moons moved farther from the Earth, coming under greater influence from the Sun's gravitational force. The stability of their co-orbit was upset, putting them on a collision course, said Erik Asphaug, a planetary scientist at the University of California at Santa Cruz.

After about 100 million years of cohabitation, the smaller moon finally crashed into the larger moon in an impact that unfolded over several hours and resulted in a merger of the two celestial bodies, Asphaug said.

The violent but slow-motion celestial union explains much about the striking difference between the mountainous far side of the moon and the smoother lunar surface of vast plains visible to Earth in the night sky, the scientists concluded.

The study suggests that the moon-forming giant impact also created a smaller, companion body that eventually coalesced with the larger satellite to form today's single moon.

Questions:

1. Why is the moon lopsided, with one side rockier than the other?

2. Why did the little moon collide with the existing moon?

3. How long had the two moons existed before the collision?

Answers:

1. A collision was created from two moons that used to exist near the current moon. That collision was lopsided, causing the lopsided look of the moon.

2. Their co-orbit was upset as they moved farther away from the Earth’s gravitational pull.

3. They existed together for 100 million years.

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About the broadcaster:

Renee Haines is an editor and broadcaster at China Daily. Renee has more than 15 years of experience as a newspaper editor, radio station anchor and news director, news-wire service reporter and bureau chief, magazine writer, book editor and website consultant. She came to China from the United States.