Real Science: Rosetta Measures Nitrogen

OK, I’ve been gushing over the Rosetta probe, concentrating on the cool hardware and navigation.

It’s time for me to eat my vegetables, and try to grok some of the actual science being done.  (Apologies in advance to the science team:  this is pure amateur stuff, from a fan.  No real substantive science in this post.)

This week one of the Rosetta science teams reported measurements that detected molecular Nitrogen (N2) on 67P/CG. (See [1}, below) While Nitrogen compounds have been detected by remote spectroscopy, this is the first in situ measurement of N2.

First detection of molecular nitrogen at a comet 19/03/2015 8:00 pm Spacecraft: ESA/ATG medialab; comet: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; Data: Rubin et al (2015)

Not being a planetary scientist, I had to read the article carefully to understand not only the observation, but the implications.  I’m sure I missed quite a bit.

First, the measurements were done last fall, using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA), which measures gas near the comet (i.e., the atmosphere as the comet heats up). Over 100 of the measurement were done from an orbit about 10km above the center of 67P.

Direct measurements, pretty close up.

The report indicates that the amount of N2, and the ratio of N2 to CO is consistent with hypotheses that the ice was formed at a very cold temperature, about 30 degrees K. This isn’t surprising, but does support the notion that 67P/CG was formed in the Kuiper belt, and that its orbit was later perturbed into its current Jupiter/Sun orbit.

The authors also argue that the measurements of N2 / CO do not match the Nitrogen in the Earth’s atmosphere, strongly suggesting that our N2 probably did not come primarily from comets. I note that earlier findings about the water ice on 67P/CG also did not match Earth’s water.

We are starting to get a picture of the origins of 67P/CG itself, and strong suggestions that the Earth has not been substantially “seeded” by comets, as some have hypothesized in the past. This leaves the question of where our terrestrial water and Nitrogen did come from.

This just goes to show that there is no substitute for close up measurement, “ground truth” for the remote sensing, dunnit?

  1. Rubin, M., K. Altwegg, H. Balsiger, A. Bar-Nun, J. J. Berthelier, A. Bieler, P. Bochsler, C. Briois, U. Calmonte, M. Combi, J. De Keyser, F. Dhooghe, P. Eberhardt, B. Fiethe, S. A. Fuselier, S. Gasc, T. I. Gombosi, K. C. Hansen, M. Hässig, A. Jäckel, E. Kopp, A. Korth, L. Le Roy, U. Mall, B. Marty, O. Mousis, T. Owen, H. Rème, T. Sémon, C. Y. Tzou, J. H. Waite, and P. Wurz, Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature. Science, 2015.


Space Saturday

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