For decades, planetary scientists have been trying to understand the origins of two colossal geological anomalies inside our planet. New insights suggest they could be leftovers from a cosmic collision
OUR planet is like a bad cake in a cosmic baking contest. On inspection of the first slice, the judges might say its layering is quite neat. The crunchy crust sits on a solid-but-squidgy mantle, which wraps around a gooey outer core. But cut another slice and they will soon see that something has gone awry. Looming inside the neat layers are two giant, messy lumps.
Since the late 19th century, geologists have used vibrations called seismic waves, normally generated by earthquakes, to map the interior of our planet. These waves move slowly in less dense and rigid rock, but faster through more tightly packed matter.
Working separately, Lau and Koelemeijer have both been trying to figure how dense these blobs are. In 2017, using GPS sensors to measure tidal changes to the shape of the crust caused by the blobs, Lau and her colleagues estimated the blobs to be fairly dense. But that same year, Koelemeijer and her colleagues used a type of seismic wave sensitive to deep mantle structures, to study where the blobs sit in relation to the core.
Around 4.5 billion years ago, when Earth was just an infant, an object the size of Mars, known as Theia, is thought to have slammed into the planet. This giant impact sent molten matter screaming into orbit around our magma-covered world, material that. This idea of how the moon formed has been around since the 1970s, and remains the leading hypothesis. In recent years, however, some have taken it further, wondering if Theia may also be the origin of the blobs.
Most scientists hope to do more with the tools they already have – seismology, chief among them. Most seismometers, the devices that detect seismic waves, are on land, which makes up less than a third of Earth’s surface area. The oceans, on the other hand, are “one massive blind spot that global seismology is yet to really improve upon”, says Lau., or vast arrays of sea-floor seismometers that can peer into the planet in considerable detail, are starting to fix that.
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