A close-up of Arrokoth reveals how planetary building blocks were constructed

The farthest, most primitive object in the Solar System ever to be visited by a spacecraft – a bi-lobed Kuiper Belt Object known as Arrokoth – is described in detail in three new reports. The reports expand upon the first published results on this object, announced in a May 2019 issue of Science, and which were based on just a small amount of data downlinked from the New Horizons spacecraft after the flyby. The new reports are based on over ten times as much data from the flyby. Together, they provide a far more complete picture of the composition and origin of Arrokoth, and point to the resolution of a longstanding scientific controversy about how such primitive planetary building blocks called planetesimals were formed.

In the first study based on these results, William McKinnon and colleagues used simulations to better understand how Arrokoth formed. Their analysis indicates that the two lobes were previously independent bodies formed close together that assembled into the present-day object very gently. The finding points to formation in a local collapse cloud of the solar nebula, and not by the other longstanding theory of planetesimal formation, called hierarchical accretion, in which objects from disparate parts of the nebula collided to form the object.

In another study based on these new results, John Spencer and colleagues report that Arrokoth’s binary lobes are less flat than initially inferred and have larger volumes than previous estimates suggested. Spencer and colleagues further report that Arrokoth has a smooth, lightly cratered surface, different from that of previously visited solar system bodies, indicating its face has mostly remained well-preserved since the end of the planet formation era. From the crater density, they infer an ancient age of its surface of about 4 billion years, supporting the discovery that Arrokoth was formed in a local solar nebula collapse cloud.

Finally, Will Grundy and colleagues investigate the composition, color and temperature of Arrokoth’s surface and find it to be uniformly red, cold, and covered with methanol ice and unidentified complex organic molecules. The red color is likely due to the presence of organic molecules. Grundy et al. also offer several suggestions as to how methanol could have formed on this object, including formation by cosmic ray irradiation of mixed water and methane ices. Although water was not detected on Arrokoth, it could be present, the authors say, but somehow masked or hidden from view. The uniform color and composition of Arrokoth’s surface also support the discovery that Arrokoth was formed in a local solar nebula collapse cloud. The significance of the three studies is further discussed in a related Perspective.

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