# Application Of Precision Gravity To Uranus And Neptune's Interior Structures

The Juno Mission to Jupiter and the Cassini Grand Finale orbits around Saturn have revolutionised our understanding of these planets over the last decade since the application of precision gravity has grown by two orders of magnitude.

Application of precision gravity is maybe the finest window into Uranus and Neptune's internal structure and chemistry. Many orbiter mission ideas have been proposed with specific science cases, including enhancing our understanding of the inner structure of these planets. One such scientific point is improving our experience of the internal structure of these planets.

The advancements in big planet gravity field data have demanded advancements in modelling efforts, first in the form of new techniques of ultra-precise gravity field calculations and then in the form of recent ultra-precise gravity field calculations. A second effort has been made to re-examine the struct models' assumptions.

What may not be entirely a plethora of assumptions generally included in models may not be fully recognised call every conclusion regarding the structure of a giant planet being very model reliant.

The researchers, Naor Movshovitz and Jonathan Fortney, from the Department of Astronomy and Astrophysics at the University of California, USA, evaluated the mission's potential deliverables and limitations in terms of planet structure.

COPYRIGHT_SZ: Published on https://stationzilla.com/application-of-precision-gravity/ by Alexander McCaslin on 2022-03-31T13:09:09.731Z

Their study is purely academic, analysing the relative relevance of low-order gravity, high-order gravity, rotation rate, and moment of inertia (MOI) in limiting planetary structure in a broad sense. To look at all possible internal density structures of a planet that would be consistent with hypothetical gravity data, they use MCMC sampling of parameterised density profiles.

## Gravity's Function In Internal Structure Model

Physical interior models attempt to create a self-consistent description of the planet's surface's composition, density, pressure, and temperature. Gravity enters the situation due to the hydrostatic equilibrium, connecting pressure and density at all points.

Whenever the gravity field is unknown, as it is today, errors in the rotational velocity on the order of 10 minutes are insignificant since they are equivalent to uncertainties in the gravity coefficient.

Similarly, the rotation rate must be known with comparable accuracy when the gravitational field is established correctly. When gravity and rotation are understood, the moment of inertia is well-defined. This makes it hard to use the independent moment of inertia measurements unless they are exact.

Converting hard-won gravity data into assumptions about a planet's interior is likewise a complex undertaking. There is no simple inversion; instead, the typical process involves building models of the planet's interior and deducing features of interest, such as the existence and size of a heavy element core, based on how well the calculated gravity of model planets with such features matches the observed gravity of the actual planet.

The Juno Mission to Jupiter and the Cassini Grand Finale orbits around Saturn have revolutionised our understanding of these planets over the last decade since the precision of their gravitational fields has grown by two orders of magnitude. The bulk compositions of these planets are dominated by hydrogen and helium, and joint problems concern the mass and distribution of heavy elements inside the planets better to comprehend their structure, origin, and evolution.

Breakthroughs in big planet gravity field data have demanded advances in modelling efforts, initially in new ultra-precise gravity field computation methods. A second effort has been made to re-examine model assumptions. What may not be fully realised is the slew of assumptions that often go into models, making practically every conclusion regarding the structure of a giant planet heavily model-reliant.

## Equation Of State And Layered Composition Models

A thermodynamic equation of state also connects pressure and density. Iteration is used to find a self-consistent solution. The value of pressure meets hydrostatic equilibrium, which depends on gravity and matches the value given by the equation of state everywhere. An equation cannot compute the composition; it must be specified. Several excellent assumptions may be made.

In the case of the ice giants, determining the dominant species is more complicated. Recent models within the physical picture show that H/He, water and other volatiles, and rock and iron play essential roles. The inferred Z (s) for Jupiter and Saturn are not overly sensitive to whether heavy elements (i.e., volatiles or rocks) are used in the model, but this is not the case for Uranus and Neptune.

For an extended period, the three-layer model was the most prevalent type of model for both gas and ice giants. The planet is considered to have radial areas of uniform composition. For example, adding layers of composition gradients to layered-composition models has made them more complicated. Modelers are likely to keep doing this while still keeping the core paradigm.

## Power Of Gravity

Even a rudimentary estimate of a planet's gravitational potential, such as the J2 coefficient, narrows the range of possible density profiles compared to a baseline confined solely by mass, radius, and boundary constraints. This narrowing disappears within the first ten to twenty percent of the planet's interior. The length of the distribution of related moments of inertia values is a scalar quantity integrated from (and sensitive to!) (s) across the planet's equilibrium form.

Higher-order gravity coefficients are far more helpful than low-order gravity coefficients. Examine figures 5 and 7, focusing on the rightmost panel. However, higher-order coefficients are susceptible to dynamic phenomena not reflected by the basic rigid rotation rate, such as zonal winds and deep differential rotation.

These dynamic consequences must be taken into account. When high precision gravity is used to limit internal models, the planet's rotation time must also be known with the same level of precision.

One cannot expect the gravitational field and rotation state to accurately characterise the density of a planet's core. Figure 8, coloured yellow, Getting a factor of two or more In addition to the gravity field measurement, a measurement of a planet's moment of inertia may help restrict the internal mass distribution, but it must be very exact.

Uranus and Neptune have low-order J2 and J4 values known with approximate accuracy, so the distribution of the associated values is already limited to a substantial degree: around 0.6% for Uranus and 1% for Neptune. The moment of inertia is fixed and cannot be changed if a higher-order Jn is known.

## Gravity Field Of Uranus And Neptune

Uranus and Neptune are far too dense to be devoid of elements heavier than helium. Both planets enable considerable enrichment by constituents thicker than water in their central regions. At solar atmospheric abundances, a part of Uranus' envelope is compatible with an adiabatic area of H/He.

On the other hand, Neptune's envelope is not, and it should be highly metal-enriched, if not he-rich. An orbiter mission to better define either or both worlds' gravitational fields and rotation would be precious. Even if high-order gravity cannot be reliably split into hydrostatic and dynamic components, the interior barotrope may be limited.

This would allow us to directly compare Uranus and Neptune's formation and evolution models' compositions and depth profiles. This would open up a new era in our understanding of these planets.