The moon may have been the victim of an ancient asteroid impact that radically altered its orientation relative to the Earth, astronomers now believe. Unusual features on the poles of our lunar companion, and the far side of the body, suggest the moon was not always aligned like it is today.
Extinct volcanoes and traces of past magnetism in rocks provides evidence our planetary companion was once geologically active, unlike the modern day. These magnetic traces provide part of the evidence suggesting a large meteor struck the moon long ago, changing its orientation.
Most astronomers believe the moon was created during the formation of our solar system, when the Earth was struck by an object the size of Mars. Such a traumatic collision would have created a molten core within the moon that would have taken hundreds of millions of years to cool. During that time, our planetary companion would have generated a magnetic field. Rocks would preserve a record of that magnetism within their structure.
During the late heavy bombardment period, planets were subject to frequent collisions with meteors and comets of all sizes. This era began 4.1 billion years ago, and ended 300 million years later.
Researchers from Japan closely studied data collected by the Lunar Prospector, launched in 1998, as well as the Kaguya craft, managed by the Japan Aerospace Exploration Agency (JAXA).
Looking at 57 regions of the moon without complex magnetic patterns, the researchers mapped the orientation of the magnetic field. Many of the rocks displayed magnetic patterns aligned with the current rotational poles of the moon. However, others were centered on another region, between 45 and 60 degrees away from the first.
"Since the Apollo era, many magnetic anomalies have been observed on the Moon. The magnetization of the lunar crust in some of these regions could preserve the signature of an early dipolar magnetic field generated by a core dynamo," researchers wrote in an article announcing their study.
A 2009 study suggested impact craters on the lunar surface are more common on the Earth-facing side than on the dark side. Astronomers believe this situation would normally be reversed. This could suggest the moon rotated 180 degrees long in the past.
" We show that there is less than a 2% probability that the oldest lunar impact basins are randomly distributed across the lunar surface. We postulate that the current "near side" of the Moon was in fact its "far side" when the oldest basins formed," researchers wrote in the article detailing their study.
Alignment of magnetic poles on Earth shifts from time to time, although geologists are uncertain what causes the phenomenon.
