Free Astronomy Magazine January-February 2020

JANUARY-FEBRUARY 2020 ! N ew observations with ESO’s SPHERE instrument on the Very Large Telescope have revealed that the surface of Hygiea lacks the very large impact crater that scientists expected to see on its surface. Since it was formed from one of the largest impacts in the history of the asteroid belt, they were expecting to find at least one large, deep impact basin, simi- lar to the one on Vesta (bottom right in the central panel). The new study also found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System. The team used the SPHERE observations to constrain Hygiea’s size, putting its diameter at just over 430 km, while Ceres is close to 950 km in size. [ESO/P. Vernazza et al., L. Jorda et al./MISTRAL algorithm (ONERA/CNRS)] Pluto, the most famous of dwarf planets, has a diameter close to 2400 km, while Ceres is close to 950 km in size. Surprisingly, the observations also revealed that Hygiea lacks the very large impact crater that scien- tists expected to see on its surface, the team report in the study pub- lished today in Nature Astronomy . Hygiea is the main member of one of the largest asteroid families, with close to 7000 members that all orig- inated from the same parent body. Astronomers expected the event that led to the formation of this numerous family to have left a large, deep mark on Hygiea. “This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” says Vernazza. Although the astron- omers observed Hy- giea’s surface with a 95% coverage, they could only identify two unambiguous craters. “Neither of these two craters could have been caused by the impact that originated the Hy- giea family of asteroids whose vol- ume is comparable to that of a 100 km-sized object. They are too small,” explains study co-author Miroslav Brož of the Astronomical Institute of Charles University in Prague, Czech Republic. The team decided to investigate fur- ther. Using numerical simulations, they deduced that Hygiea’s spherical shape and large family of asteroids are likely the result of a major head- on collision with a large projectile of diameter between 75 and 150 km. Their simulations show this violent impact, thought to have occurred about 2 billion years ago, com- pletely shattered the parent body. Once the left-over pieces reassem- bled, they gave Hygiea its round shape and thousands of companion asteroids. “Such a collision between two large bodies in the asteroid belt is unique in the last 3–4 billion years,” says Pavel Ševe č ek, a PhD student at the Astronomical Insti- tute of Charles Univer- sity who also partic- ipated in the study. Studying asteroids in detail has been possible thanks not only to ad- vances in numerical computation, but also to more powerful tele- scopes. “Thanks to the VLT and the new generation adaptive-optics instru- ment SPHERE, we are now imaging main belt asteroids with unprece- dented resolution, clos- ing the gap between Earth-based and inter- planetary mission ob- servations,” Vernazza concludes. C omputational simulation of the fragmentation and reassembly that led to the formation of Hygiea and its family of asteroids, following an impact with a large object. While changes in the shape of Hygiea occur after the impact, the dwarf-planet candidate even- tually acquires a round shape. [P. Ševeček/Charles University]