The promise and limitations of improved-accuracy gravity field measurements for Uranus and Neptune

<p>Uranus and Neptune present unique challenges to planetary modelers. The<br>composition of the so-called ice giants is very uncertain, even more so than the<br>composition of the gas giants. For instance, it is far from clear that either<br>planet's composition is dominated by water. Instead, the composition of Uranus and<br>Neptune likely includes water and other refractory elements in large quantities as<br>well as a substantial H/He envelope. Furthermore, formation models also predict<br>that composition gradients are likely in the interiors of these planets, rather<br>than a neat differentiation into layers of homogeneous composition. (See Helled<br>and Fortney 2020 and references within.)</p><p>A key question that impacts the science case for a potential orbiting mission to<br>Uranus or Neptune is how will more precise measurements of the gravitational field<br>better constrain either planet's interior density profile and composition.<br>Surprisingly, there is yet no published answer to this question.  Here, we present<br>new work that explores this issue, using a Bayesian framework that allows<br>exploration of a wide range of interior density profiles.</p><p>Our approach, which builds off our previous work for Saturn (Movshovitz et al.,<br>2020) and that of others  (e.g. Marley et al., 1995, Helled et al., 2011) takes a<br>relatively unbiased view of the interior structure by employing so-called<br>empirical density profiles. A parameterization is applied to the density profiles<br>directly (via mathematical base functions) instead of to an assumed layered<br>composition (H/He, water, rocks). While some of these empirical density profiles<br>may imply unrealistic compositions, they can also probe solutions that would be<br>missed by the standard layered-composition approach.</p><p>Here we will present models of Uranus and Neptune constructed with this approach,<br>and ask two questions: 1) How large is the space of possible solutions today? 2)<br>How much will it be reduced should a future mission to Uranus and Neptune improve<br>the precision on their gravity field measurements by several orders of magnitude,<br>to the level now available for Jupiter and Saturn?</p>