50 for Uranus and Neptune. Overall, these measurement do, therefore, give decisive support to the nucleation model for the giant planets. In addition, it seems to indicate that the giant planets formed from planetoids of solar composition.

There is, however, one significant, unanswered question. The calculation just made assumes trapping of the elements in the form of ice, whatever the temperature considered. Laboratory experiments, however, have shown that certain elements, in particular nitrogen and argon, cannot be trapped at temperatures above approximately 30 K. So did the planetoids that later formed Jupiter agglomerate at a temperature less than 30 K? Models for the evolution of the protosolar nebula favour temperatures that are significantly higher, at least equal to 100 K. The question is currently unanswered.

Valuable information is provided by isotopic ratios, especially the D:H ratio. Laboratory measurements have shown that ices are enriched in deuterated molecules, because of the ion-molecule and molecule-molecule reactions that come into play at low temperatures. The enrichment is greater, the lower the temperature. So a measurement of the D:H ratio in various bodies in the Solar System provides information on the temperature at which the planetoids formed, and from which the bodies we are investigating later accreted.

In the giant planets, the deuterium came from two sources: the protosolar nebula (the most important reservoir for Jupiter and Saturn), and the ices in the initial core (the most important reservoir for Uranus and Neptune). Jupiter's D:H ratio should, therefore, reflect the protosolar value; Saturn's value should be slightly higher, while the D:H ratio for Uranus and Neptune should be significantly enriched.

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