HIRMES science - planetary deuterium

The Giant Planets in the outer solar system are composed primarily of molecular hydrogen, helium, and methane. Deuterium is an important indicator of the formation mechanism for these planets. We plan to measure HD and a thermometer, such as H2 or CH4, on all 4 Giant Planets using SOFIA/HIRMES. High spectral resolution and a broad bandpass are required. From formation models, Jupiter and Saturn are expected to exhibit the protosolar value of deuterium to hydrogen (D/H) in the Solar Nebula. Uranus and Neptune, in contrast, are expected to have D/H ratios close to those in the icy planetesimals where these planets formed. Comets are the closest analog that we can measure today to the icy planetesimals that formed Uranus and Neptune. HIRMES will measure D/H in comets over the next few years. From ground-based data, the D/H ratio has been measured in 11 comets, ranging from 8 to 32 times protosolar. However, from Herschel-PACS data, both Uranus and Neptune appear to have a D/H ratio of only 2 times the protosolar value. This is a major problem for formation models where volatiles are trapped within icy planetesimals that end up inside Uranus and Neptune. As a consequence, either the measured D/H in Uranus and Neptune has been underestimated or planet formation models are incomplete. HIRMES will help to resolve these issues by providing new HD observations of all of the Giant Planets at 100 times higher spectral resolution than previous data. This will improve our understanding of the formation of the outer planets as well as the formation of Neptune-sized exoplanets, which are common in the galaxy