# Moment of inertia factor

In planetary sciences, the moment of inertia factor or normalized polar moment of inertia is a dimensionless quantity that characterizes the radial distribution of mass inside a planet or satellite. Since a moment of inertia must have dimensions of mass times length squared, the moment of inertia factor is the dimensionless coefficient that multiplies these.[note 1]

## DefinitionEdit

For a planetary body with principal moments of inertia A<B<C, the moment of inertia factor is defined as

${\displaystyle {\frac {C}{MR^{2}}}}$ ,

where C is the polar moment of inertia of the body, M is the mass of the body, and R is the mean radius of the body.[1][2] For a sphere with uniform density, ${\displaystyle C/MR^{2}=0.4}$ . For a differentiated planet or satellite, where there is an increase of density with depth, ${\displaystyle C/MR^{2}<0.4}$ . The quantity is a useful indicator of the presence and extent of a planetary core, because a greater departure from the uniform-density value of 0.4 conveys a greater degree of concentration of dense materials towards the center.

## Solar System valuesEdit

The Sun has by far the lowest moment of inertia factor value among Solar System bodies; it has by far the highest central density (162 g/cm3,[3][note 2] compared to ~13 for Earth[4][5]) and a relatively low average density (1.41 g/cm3 versus 5.5 for Earth). Saturn has the lowest value among the gas giants in part because it has the lowest bulk density (0.687 g/cm3).[6] Ganymede has the lowest moment of inertia factor among solid bodies in the Solar System because of its fully differentiated interior,[7][8] a result in part of tidal heating due to the Laplace resonance,[9] as well as its substantial component of low density water ice. Callisto is similar in size and bulk composition to Ganymede, but is not part of the orbital resonance and is less differentiated.[7][8] The Moon is thought to have a small core, but its interior is otherwise relatively homogenous.[10][11]

Body Value Source Notes
Sun 0.070 [3] Not measured
Mercury 0.346 ± 0.014 [12]
Venus unknown[note 3]
Earth 0.3307 [14]
Moon 0.3929 ± 0.0009 [15]
Mars 0.3662 ± 0.0017 [16]
Ceres 0.37[note 4] [17] Not measured (Darwin-Radau relation)
Jupiter 0.254 [18] Not measured (approximate solution to Clairaut's equation)
Io 0.37824 ± 0.00022 [19] Not measured (Darwin-Radau relation)
Europa 0.346 ± 0.005 [19] Not measured (Darwin-Radau relation)
Ganymede 0.3115 ± 0.0028 [19] Not measured (Darwin-Radau relation)
Callisto 0.3549 ± 0.0042 [19] Not measured (Darwin-Radau relation)
Saturn 0.210 [18] Not measured (approximate solution to Clairaut's equation)
Rhea 0.3911 ± 0.0045 [21] Not measured (Darwin-Radau relation)
Titan 0.3414 ± 0.0005 [22] Not measured (Darwin-Radau relation)
Uranus 0.23 [18] Not measured (approximate solution to Clairaut's equation)
Neptune 0.23 [18] Not measured (approximate solution to Clairaut's equation)

## MeasurementEdit

The polar moment of inertia is traditionally determined by combining measurements of spin quantities (spin precession rate and/or obliquity) with gravity quantities (coefficients of a spherical harmonic representation of the gravity field). These geodetic data usually require an orbiting spacecraft to collect.

## ApproximationEdit

For bodies in hydrostatic equilibrium, the Darwin–Radau relation can provide estimates of the moment of inertia factor on the basis of shape, spin, and gravity quantities.[23]

## Role in interior modelsEdit

The moment of inertia factor provides an important constraint for models representing the interior structure of a planet or satellite. At a minimum, acceptable models of the density profile must match the volumetric mass density and moment of inertia factor of the body.

## NotesEdit

1. ^ For example, for a uniform long thin rod rotating about its end, the moment of inertia is ${\displaystyle (1/3)ML^{2}}$ , so its moment of inertia factor is 1/3.
2. ^ A star's central density tends to increase over the course of its lifetime.
3. ^ Based on a theoretical model of Venus's interior, its mean moment of inertia has been predicted to be 0.338.[13] Because this model makes assumptions about the interior that are not verified by observations, the prediction is of limited value.
4. ^ The value of 0.37 given for Ceres is the mean moment of inertia, which is thought to better represent its interior structure than the polar moment of inertia (0.39), due to its high polar flattening.[17]

## ReferencesEdit

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6. ^ Williams, David R. (7 September 2006). "Saturn Fact Sheet". NASA. Archived from the original on 9 April 2014. Retrieved 31 July 2007.
7. ^ a b Showman, Adam P.; Malhotra, Renu (1999-10-01). "The Galilean Satellites" (PDF). Science. 286 (5437): 77–84. doi:10.1126/science.286.5437.77. PMID 10506564.
8. ^ a b Sohl, F.; Spohn, T; Breuer, D.; Nagel, K. (2002). "Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites". Icarus. 157 (1): 104–119. Bibcode:2002Icar..157..104S. doi:10.1006/icar.2002.6828.
9. ^ Showman, Adam P.; Stevenson, David J.; Malhotra, Renu (1997). "Coupled Orbital and Thermal Evolution of Ganymede" (PDF). Icarus. 129 (2): 367–383. Bibcode:1997Icar..129..367S. doi:10.1006/icar.1997.5778.
10. ^ Brown, D.; Anderson, J. (6 January 2011). "NASA Research Team Reveals Moon Has Earth-Like Core". NASA. NASA.
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15. ^ Williams, James G.; Newhall, XX; Dickey, Jean O. (1996). "Lunar moments, tides, orientation, and coordinate frames". Planetary and Space Science. 44 (10): 1077–1080. Bibcode:1996P&SS...44.1077W. doi:10.1016/0032-0633(95)00154-9. ISSN 0032-0633.
16. ^ Folkner, W. M.; et al. (1997). "Interior Structure and Seasonal Mass Redistribution of Mars from Radio Tracking of Mars Pathfinder". Science. 278 (5344): 1749–1752. Bibcode:1997Sci...278.1749F. doi:10.1126/science.278.5344.1749. ISSN 0036-8075.
17. ^ a b Park, R. S.; Konopliv, A. S.; Bills, B. G.; Rambaux, N.; Castillo-Rogez, J. C.; Raymond, C. A.; Vaughan, A. T.; Ermakov, A. I.; Zuber, M. T.; Fu, R. R.; Toplis, M. J.; Russell, C. T.; Nathues, A.; Preusker, F. (2016-08-03). "A partially differentiated interior for (1) Ceres deduced from its gravity field and shape". Nature. 537: 515–517. Bibcode:2016Natur.537..515P. doi:10.1038/nature18955. PMID 27487219.
18. ^ a b c d Yoder, C. (1995). Ahrens, T., ed. Astrometric and Geodetic Properties of Earth and the Solar System. Washington, DC: AGU. ISBN 0-87590-851-9. OCLC 703657999.
19. ^ a b c d Schubert, G.; Anderson, J. D.; Spohn, T.; McKinnon, W. B. (2004). "Interior composition, structure and dynamics of the Galilean satellites". In Bagenal, F.; Dowling, T. E.; McKinnon, W. B. Jupiter : the planet, satellites, and magnetosphere. New York: Cambridge University Press. pp. 281–306. ISBN 978-0521035453. OCLC 54081598.
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