On March 12, the Rosetta spacecraft was imaging Comet 67P/Churyumov-Gerasimenko from a distance of 75 kilometers (46 miles) and by pure chance it spotted an eruption of dusty material from the shaded nucleus.
Long-duration spacecraft are essential if we are to fully understand the evolution of a comet as it gradually heats up during its approach to the sun. And it just so happens that Rosetta is always in orbit around 67P's nucleus, ready to spot any transient event that could erupt at any time on the surface.
ANALYSIS: Rosetta Gets Confused by Rough Comet ‘Weather'
This latest event focuses on the comet's shaded underside. It is assumed that some sunlight slowly heated an outcrop, providing enough energy to sublimate subsurface ices, ejecting vapor and dust as a jet. The transient jet was imaged and measured by Rosetta's scientific imaging system OSIRIS.
With only 4 months before 67P makes closest approach with the sun (perihelion), the comet is producing many similar jets on the sun-facing side of the comet, but seeing such a pronounced eruption from the Imhotep region on the comet, while the region was in darkness, provides a valuable scientific opportunity.
"This was a chance discovery," said OSIRIS principal investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. "No one has ever witnessed the wake-up of a dust jet before. It is impossible to plan such an image."
NEWS: Rosetta's Comet May be Made of Pebbles
"In these images we see Imhotep on the brink of dawn," added OSIRIS scientist Jean-Baptiste Vincent, also from the MPS, in a Rosetta mission blog. "It is possible that the first rays of sunlight hit some cliffs or outcrops that remained hidden to Rosetta due to the orbital position at the time."
There is also the possibility that a wave of heating passed through the icy material, eventually producing a more explosive jet event.
Scientists have watched long-lived jets that continuously erupt on the sun-facing side of the comet. As the nucleus rotates, these jets continue outgassing until the sun rotates into view once more. But as this event proves, transient jets are also possible, and Rosetta is there to record and study them, furthering our knowledge of how these icy vagabonds increase in activity as the sun's heat bakes their complex surfaces.