Astronomers studying interstellar Comet 2I/Borisov as it zipped past Sun in December have found that while in many ways it resembled solar system comets, the differences were revealing.

During its trip through the solar system, the comet lost nearly 61 million gallons (230 million liters) of water — enough to fill over 92 Olympic-size swimming pools. As it moved away from the Sun, Borisov’s water loss dropped off — and did so more rapidly than any previously observed comet. Xing said this could have been caused by a variety of factors, including surface erosion, rotational change and even fragmentation. In fact, data from Hubble and other observatories show that chunks of the comet broke off in late March.

…Swift’s water production measurements also helped the team calculate that Borisov’s minimum size is just under half a mile (0.74 kilometer) across. The team estimates at least 55% of Borisov’s surface — an area roughly equivalent to half of Central Park — was actively shedding material when it was closest to the Sun. That’s at least 10 times the active area on most observed solar system comets. Borisov also differs from solar system comets in other aspects. For example, astronomers working with Hubble and the Atacama Large Millimeter/submillimeter Array, a radio telescope in Chile, discovered Borisov produced the highest levels of carbon monoxide ever seen from a comet at that distance from the Sun.

Because more of the comet’s entire surface had water ice than seen in solar system comets, it suggests that the comet has never been close to another star before. That the water release dropped off precipitously however also suggests that that surface layer of ice was not very deep.