The Moon is home to one of the largest known impact craters in the solar system known as the Lunar South Pole-Aitken Basin, but because it is located on the far side of the moon, it cannot be viewed directly from Earth. At 2,500 kilometers (1,550 miles) across, the South Pole-Aitken Basin covers nearly a quarter of the lunar surface - and there's something massive buried beneath it. Detailed readings made using lunar orbiters indicate there is something huge enough under that crater to be causing a significant gravitational anomaly. Researchers say it could be a huge lump of metal from the asteroid that formed the South Pole-Aitken Basin. Instead of sinking down into the moon's interior, it remained buried in the moon's mantle. Computer simulations suggest that this is a plausible explanation, but other explanations have also been proposed.
Researchers led by Baylor University have discovered something massive lurking underneath the far side of the moon: a mysterious blob with the mass akin to a pile of metal five times the size of the Big Island of Hawaii. The structure, described in a recent study published in Geophysical Research Letters, sits at least 180 miles beneath the South Pole-Aitken basin—a colossal crater punched into the lunar landscape billions of years ago, when the moon's initially molten surface had cooled just enough for impacts to leave a lasting mark. In the past, the South Pole-Aitken crater has garnered plenty of interest both for its surface composition and its size which makes it the biggest preserved crater that we know of in the solar system. The discovery of the odd mass only adds to the intrigue, especially since the crater and the nearby lunar south pole are potential targets for multiple future missions to the moon.
The anomaly was discovered in two sets of data. The first was from NASA's GRAIL mission, a pair of orbiting spacecraft that mapped the Moon's gravitational field in 2011 and 2012 to try to shed some light on its interior structure. This data had already indicated a gravitational anomaly, and that the basin had higher-than-average density compared to the rest of the lunar surface; the team attributed this to its iron-rich surface composition. But when the team compared these findings with the lunar topography data collected by NASA's Lunar Reconnaissance Orbiter, the results showed something else: a mass of about 2.18 quintillion kilograms extending more than 300 kilometers (184 miles) below the surface.
This mass, the researchers believe, is weighing the floor of the basin downward by more than 800 meters, around 10 percent of its total depth, explaining a depression in the bottom of the basin previously attributed to contraction. According to computer simulations, if conditions are just right, the iron-nickel core of an impacting asteroid can be dispersed into the upper mantle, between the Moon's crust and core. This is what could have happened 4 billion years ago, when the object that created the basin slammed into the Moon.
Another possible explanation has to do with volcanism, of which the Moon was once a hotbed. There is a high concentration of titanium oxides in the lunar mantle, thought to have been produced by the cooling and solidification of oceans of lunar magma. These oxides have a great deal of mass, which somehow could have been concentrated beneath the South Pole-Aitken Basin when the magma ocean that existed on the moon solidified.
In addition to spotting the mysterious blob, the new study retraced the boundary of the basin's inner rim, revealing that scientists previously underestimated the crater's size, a potentially important find as NASA and others prepare to send missions to the basin and the nearby lunar south pole. The last researchers to map out these limits used data from the Clementine mission, which had a gap near the basin's southern extent. The latest work, however, used more complete data from LRO and GRAIL, which revealed that the crater is roughly 40 miles larger than once thought.
Meteorologist Paul Dorian