Last year, the European Space Agency’s Rosetta spacecraft reached comet 67P/Churyumov-Gerasimenko. For months, Rosetta studied the sun-facing portions of the comet. But what about the dark side of the comet? That was a bit trickier.

Just like Earth, comet 67P has seasons. But, they’re not like ours. The comet’s orbit lasts for 6.5 years. For 5.5 years, the northern hemisphere experiences a long summer, while the southern hemisphere sits in a cold, dark winter. But a few months ago that changed. As the comet reaches its closest point to the sun (also called perihelion), the southern hemisphere entered a very short and very hot summer.

Rosetta didn’t wait for the comet to reach perihelion. It had an instrument that could observe the dark side of the comet. It’s called the Microwave Instrument for Rosetta Orbiter (MIRO).

Rosetta MIRO

In a new paper, a team of scientists explain what they found from data collected by MIRO between August and October 2014.

“We observed the ‘dark side’ of the comet with MIRO on many occasions after Rosetta’s arrival at 67P/C-G, and these unique data are telling us something very intriguing about the material just below its surface,” said Mathieu Choukroun from NASA’s Jet Propulsion Laboratory (JPL) and lead author of the study.

Using MIRO, the scientists found differences in the millimeter and sub-millimeter wavelength channels while observing the comet’s southern polar regions. The differences hint at large amounts of ice at the surface or just below.

“Surprisingly, the thermal and electrical properties around the comet’s south pole are quite different than what is found elsewhere on the nucleus,” said Choukroun. “It appears that either the surface material or the material that’s a few tens of centimeters below it is extremely transparent, and could consist mostly of water ice or carbon-dioxide ice.”

Why the differences? Scientists think it could be because of the comet’s unique seasons. When the southern hemisphere enters the very short, hot summer – water and other gases are released. But then the water condenses again and falls back to the surface as the season changes back. This explanation matches up with what other scientists learned about the comet’s water-ice cycle.

The results discussed in this paper are preliminary. “We plan to revisit the MIRO data using an updated version of the shape model, to verify these early results and refine the interpretation of the measurements,” said Choukroun.

Plus, Rosetta was able to observe the southern polar regions with its other instruments as the region became brighter at perihelion.

“We hope that, by combining data from all these instruments, we will be able to confirm whether or not the south pole had a different composition and whether or not it is changing seasonally,” said MIRO Principal Investigator Mark Hofstadter.