Hidden crater on asteroid Lutetia could explain its unusual surface grooves

(Lutetia in 3D – Photo Credit: ESA)

Grooves found on Lutetia, an asteroid encountered by the European Space Agency’s Rosetta spacecraft on its journey to Comet 67P, indicate that a large impact crater is hiding on its dark side: a side of the asteroid not yet photographed or analysed. 

Rosetta passed Lutetia at a distance of 3168km in July 2010 while en route to its 2014 rendezvous with its target comet.

The spacecraft took a number of images of the 100km-wide asteroid during its two hour flyby, revealing numerous impact craters and hundreds of grooves scattered across its surface.

Impact craters are seen commonly on solid surfaces throughout our Solar System, revealing an intense history of collisions between cosmic bodies. However, grooves are much less prevalent. To date, they have only been discovered by spacecraft visiting the martian moon Phobos and the asteroids Eros and Vesta.

The process that forms these grooves is still widely debated, but it is thought to likely involve impacts. Shock waves from impacts can travel through the interior of a small, porous body and fracture the surface to form the grooves.

“For Lutetia, by assuming that the grooves were formed in concentric patterns around their source impact crater, we identified 200 such features falling into distinct ‘families’, correlated with three different impact craters,” describes Sebastien Besse, a research fellow at ESA’s Technical Centre, ESTEC, and lead author of a paper on the grooves.

One of the groove systems on Lutetia is associated with the Massilia crater and another with the North Pole Crater Cluster, which comprises a number of superimposed craters. Both are on the asteroid’s northern hemisphere.

But another group of grooves points to a crater not seen during Rosetta’s brief flyby, in the asteroid’s southern hemisphere.

Its implied presence has earned it the nickname ‘Suspicio’.

(An overlay tracing the asteroid’s grooves – Photo Credit: ESA)

The grooves related to Suspicio cover a large area on the asteroid, suggesting it may span several tens of kilometres. By comparison, Massilia, the largest known crater on Lutetia, is about 55 km wide, and the largest of the polar cluster is about 34 km across.

“These three major impacts seriously deformed Lutetia’s surface,” adds Sebastien.

“As with grooves seen on other asteroids that may also be associated with impact events, this study provides new insights into the catastrophic history of these small bodies.”

By observing how subsequent small craters lie over the grooves on Lutetia, the scientists determined the relative ages of the three larger cratering events. Massilia is thought to be the oldest of the three craters and the polar cluster the youngest, with Suspicio in between.

The authors also looked at other, independent measurements of Lutetia, including ground-based observations with the Infrared Telescope Facility and space-based observations with ESA’s Herschel and NASA’s Spitzer.

“Our study ties together several independent analyses of Lutetia into one coherent story that is consistent with the presence of a large impact crater on the far side of the asteroid,” says co-author Michael Küppers, from ESA’s Space Astronomy Centre.

“Four years on and we are delighted still to be learning from just two hours’ worth of data collected during the Lutetia flyby,” says Matt Taylor, ESA’s Rosetta project scientist.

“Rosetta is now in its main mission phase at its comet, where we are on the cusp of fantastic results. Rosetta is a true small bodies mission, two asteroids and one comet in a single trip.”