All of these changes resulted from the comet losing mass. Comparing Doppler measurements early and late in the mission yielded an estimated mass loss of 10.5 million tons, about 0.1 percent of the comet’s total. This is equivalent to a 70-centimeter-thick (2-foot-thick) layer of material removed from the comet nucleus during each orbit. Yes, this means that comets have a finite lifetime.

Comets: Not Earth's main water source

In planetary science, elements that have a low sublimation temperature are often referred to as “volatiles.” A comet outgasses more volatiles when it’s closer to the Sun and less when it’s farther away. Like planets, comets can also have seasons. Comet 67P has pronounced seasons due to the tilt of its rotation axis with respect to the orbital plane. The northern hemisphere summer is much longer and weaker compared to the short and intense summer around perihelion in the south, and the ROSINA, MIRO, OSIRIS, and VIRTIS instruments monitored these seasonal variations.

The comet’s releases of gas and dust must originate at or near the nucleus’ surface. Yet despite all of the changes we witnessed, we had difficulty locating active areas or exposed ices on the surface of the nucleus. On rare occasions, the VIRTIS instrument detected patches of water and carbon dioxide frost on the dawn side of the nucleus.

One of the surprising results was the detection of abundant amounts of molecular oxygen (O2) by ROSINA and Alice. We saw that the abundance of oxygen correlated well with water but less well with molecular nitrogen. The relationship between oxygen and water in the comet likely dates back to their origin in the interstellar medium before the solar system formed. We also studied the isotopes of hydrogen in the comet’s water and came to a similar conclusion: the comet’s water originated in the interstellar medium.

We were surprised by the relatively large amount of heavy hydrogen (deuterium) in the comet’s water. That discovery ruled out a popular hypothesis for the origin of Earth’s water. We now know that comets of 67P’s type were not a major source of the water that became Earth’s oceans. However, noble gas measurements suggested that maybe 22 percent of Earth’s atmosphere has a cometary origin. If this is true, then organic chemicals detected by the ROSINA, COSAC, and Ptolemy mass spectrometers on both the lander and the orbiter would have also found their way to the early Earth. Organic materials that Earth potentially received from comets include a host of molecules relevant to early life, such as the simplest amino acid: glycine.