Icarus 264 (2016) 264–273

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Rhea gravity field and interior modeling from Cassini data analysis
Paolo Tortora a,⇑, Marco Zannoni a, Doug Hemingway b, Francis Nimmo b, Robert A. Jacobson c,
Luciano Iess d, Marzia Parisi d,1a

Dipartimento di Ingegneria Industriale, Università di Bologna, Forlì, Italy
Department of Earth & Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USAc
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USAdDipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza Università di Roma, Rome, Italyb

a r t i c l e

i n f oArticle history:
Received 9 June 2015
Revised 14 September 2015
Accepted 16 September 2015
Available online 26 September 2015
Keywords:
Interiors
Orbit determination
Satellites, composition
Saturn, satellites

a b s t r a c t
During its tour of the Saturn system, Cassini performed two close flybys of Rhea dedicated to gravity
investigations, the first in November 2005 and the second in March 2013. This paper presents an estimation of Rhea’s fully unconstrained quadrupole gravity field obtained from a joint multi-arc analysis of the
two Cassini flybys.
Our best estimates of the main gravity quadrupole unnormalized coefficients are J2  106 = 94
6.0 ± 13.9, C22  106 = 242.1 ± 4.0 (uncertainties are 1-r). Their resulting ratio is J2/C22 = 3.91 ± 0.10, statistically not compatible (at a 5-r level) with the theoretical value of 10/3, predicted for a hydrostatic
satellite in slow, synchronous rotation around a planet. Therefore, it is not possible to infer the momentof inertia factor directly using the Radau–Darwin approximation.
The observed excess J2 (gravity oblateness) was investigated using a combined analysis of gravity andtopography, under different plausible geophysical assumptions. The observed gravity is consistent withthat generated by the observed shape for an undifferentiated (uniform density) body. However, because
the surface is more likely to be water ice, a two-layer model may be a better approximation. In this case,and assuming a mantle density of 920 kg/m3, some 1–3 km of excess core oblateness is consistent withthe observed gravity. A wide range of moments of inertia is allowed, but models with low moments ofinertia (i.e., more differentiation) require greater magnitudes of excess core topography to satisfy theobservations.
Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction
Discovered on December 23, 1672 by Giovanni Domenico Cassini, Rhea is the second largest moon of Saturn, with a mean radiusof about 764 km.
Before Cassini’s arrival in the Saturn system, only the gravitational parameter GM was known from the analysis of Pioneer andVoyager data (Campbell and Anderson, 1989). Using this and theestimated volume (from camera images), a bulk density of about1200 kg/m3 was derived, relatively small and compatible with amixture of about 75% by mass water ice (density 1000 kg/m3)
and 25% rock-metal (density 3000 kg/m3).
During its mission in the Saturn system, Cassini performed four
close encounters of Rhea, of which only two were devoted togravity investigations. The first gravity flyby, referred to as R1,⇑ Corresponding author.
1
Current address: Department of Earth and Planetary Sciences, Weizmann
Institute of Science, 234 Herzl Street, Rehovot 7610001, Israel.

http://dx.doi.org/10.1016/j.icarus.2015.09.022
0019-1035/Ó 2015 Elsevier Inc. All rights reserved.

according to the numbering scheme used by the Cassini project,
was performed on November 26, 2005, during the main mission,and the second and last gravity flyby, referred to as R4, was performed on March 9, 2013, during the Solstice mission. The main
orbital and geometrical characteristics of R1 and R4 are summarized in Table 1, while Fig. 1 displays the ground track of the flybys,
for a time interval of about ±2 h around the closest approach (black
circles).
Radiometric data acquired during the first encounter (R1) wereused to estimate the gravity field of Rhea. A first estimate(Anderson and Schubert, 2007) was obtained under the assumption of hydrostatic equilibrium, i.e. constraining the unnormalized
gravity coefficients J2 and C22 to a ratio of 10/3. From this estimation, by applying the Radau–Darwin relation the authors obtained
a normalized moment of inertia of about 0.3911 ± 0.0045 (a valueof 0.4 would imply a constant density interior). The authors concluded that the satellite’s interior is a homogeneous, undifferentiated mixture of ice and rock, with possibly some compression of
the ice and transition from ice I to ice II at depth.

P. Tortora et al. / Icarus 264 (2016) 264–273
Table 1Main geometrical and orbital characteristics of R1 and R4 gravity fybys.
Values at C/A

Unit

R1

R4

Epoch
AltitudeRelative velocity

(UTC)
(km)
(km/
s)
(°)
(°N)
(°E)
(°)
(°)

26-NOV-2005, 23:50
502
7.3

09-MAR-2013, 19:40
999
9.3

17
10.2
91.5
106
113

106
18.8
176.2
117
128

Inclination
LatitudeLongitude
Normal-to-Earth angle
Sun–Earth–Probe angle

In parallel, the radiometric data acquired during R1 were independently analyzed by the Cassini Navigation team (Mackenzie
et al., 2007) and by the Cassini Radio Science team (Iess et al.,2007). Both analyses estimated the moon’s GM and quadrupole
gravity coefficients J2 and C22, obtaining different solutions, but
consistent at the 2r level, as a result of different analysis
approaches. The two approaches were then combined to obtain ajoint ‘‘best” unconstrained estimation of the quadrupole field(Mackenzie et al., 2008). The solution obtained is not statisticallycompatible with hydrostatic equilibrium, hence no useful constraint on Rhea’s interior structure could be imposed. Hydrostatic
equilibrium was also ruled out by applying this constraint to theestimated quadrupole field coefficients, and this led to a significant
degradation of the orbital fit at closest approach. To explain the
non-hydrostatic ratio J2/C22, the authors theorized that a large collision occurred after the completion of the thermal evolution of thesatellite, causing a redistribution of mass and a reorientation of thetidal bulge.
More recently Anderson and Schubert (2010) stated that thedifferences in the previously published gravity fields are probably
caused by a mis-modeling of the non-gravitational acceleration
acting on Cassini caused by anisotropic thermal emission. To avoid
this issue, these authors restricted the analysis to a subset of dataaround the closest approach (±2000 s), where ‘‘the informationfrom Rhea’s quadrupole gravitational field is confined”. Theyobtained a new solution in agreement with Anderson andSchubert (2007), using the hypothesis of hydrostatic equilibrium.

265

Moreover, these authors concluded that non-hydrostaticity is notsupported by the data.
The different estimations of J2 and C22 published to date areshown in Fig. 2. To resolve these discrepancies, a second and final
gravity flyby was planned in Cassini’s Solstice. No other flybys of
Rhea are scheduled in the Cassini mission. R1 was characterized
by a very low inclination, about 17° at the closest approach (C/A),in order to de-correlate the estimation of J2 and C22, while R4
was designed to be nearly polar, with a high inclination at C/A,
about 106°. However, the C/A of R4 was about 999 km, twice ashigh as R1 (about 502 km), thus significantly reducing the information content about Rhea’s quadrupole gravity field in this second
flyby. The Sun–Earth–Probe (SEP) angle was larger than 110° during both encounters, thus range-rate measurements were only
slightly affected by the harmful effect of solar plasma.
This paper is organized as follows: Section 2 describes the data
analysis approach for the estimation of Rhea’s gravity field, alongwith the spacecraft dynamical model, and the data selection andcalibration procedure. Section 3 provides a geophysical interpretation of the results, by means of a combined analysis of Rhea’s estimated gravity and topography. Finally Section 4 summarizes our
findings and conclusions.

2. Gravity analysis
2.1. Introduction
The determination of the gravity field of a celestial body plays acrucial role in the investigation of its internal composition, structure and evolution, because it provides one of the very few direct
measurements of its internal mass distribution, even if the inversion process is not unique.
The gravity field of Rhea was precisely determined by reconstructing the trajectory of Cassini during the two close encountersof the satellite. The main observable quantity used in the gravity
estimation was the spacecraft range-rate, obtained from the frequency shift due to the relativistic Doppler effect, averaged overa count time of 60 s, of a highly stable microwave carrier transmitted from an Earth ground station to the spacecraft, that coherently
retransmits the signal to Earth by means of a precise transponder.

Fig. 1. Cassini ground track on Rhea during R1 and R4, considering a time interval of ±2 h around the closest approach.

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P. Tortora et al. / Icarus 264 (2016) 264–273

Fig. 2. Previously published values of Rhea’s quadrupole coefficients J2 and C22.

Range observables were not used in the analysis because they areof very limited use in the estimation of gravity fields.
During a close flyby, the spacecraft velocity variations caused by
the main terms of the gravity field harmonic expansion are approximately (Iess et al., 2014a):DVðGMÞ 

GM
rV

ð1Þ

DVðJ2 Þ 

 2
GM R
J2
rV r

ð2Þ

DVðJ3 Þ 

 3
GM R
J3
rV r

ð3Þ

where M is the satellite mass, r is the radial distance at closest
approach, R is the satellite radius, V is the flyby velocity and G is
the gravitational constant. Using the geometrical values at C/Areported in Table 1 and the gravity coefficients of Mackenzie et al.
(2008) the expected velocity variations were computed and areshown in Table 2. In this simplified analysis a conservative upperbound of J3  106 = 25 was used, following Mackenzie et al. (2008).
The Cassini radio science subsystem, along with the ground stations of NASA’s Deep Space Network (DSN), allows a tracking accuracy in the spacecraft’s velocity variations of about 0.02–0.09 mm/s
on a time scale of 60 s, depending mainly on the Sun–Earth–Probeangle (Iess et al., 2014b).
This accuracy is well below the expected spacecraft’s velocity
variations induced by Rhea’s GM and J2 during R1 and R4, while
it is comparable to the upper bound contribution due to J3.
2.2. Dynamical model
To correctly estimate the orbit of Cassini, the dynamical model
implemented in the orbit determination program must take into
account all non-negligible accelerations acting on the spacecraft.
The implemented dynamical model includes the point-mass
relativistic gravitational acceleration exerted by all the main Solar
System bodies and the main satellites of Saturn. The gravitational

Table 2
Expected DV due to Rhea’s gravity during R1 and R4. The reference values are:
GM = 153.9398 km3/s2, J2  106 = 931, J3  106 = 25.
Contribution

Unit

R1

R4

DV(GM)
DV(J2)
DV(J3)(m/s)(mm/s)(mm/s)

16.7
5.6
0.09

9.4
1.6
0.02

parameters and state vectors of the planets, the Sun, the Moon
and Pluto were obtained from JPL planetary ephemerides DE430(Folkner et al., 2014). The gravitational parameters and a prioristate vectors of Saturn and its main satellites were obtained from
Saturn’s satellites ephemerides, SAT355, provided by the Cassini
Navigation Team by fitting a large number of radio, astrometric,
and optical data (Jacobson et al., 2006) (available at ftp://ssd.jpl.nasa.gov/pub/eph/satellites/).
The dynamical model also included the gravitational acceleration due to the even zonal spherical harmonics of Saturn from J2
up to J8, whose updated values were provided by the satellite ephemerides, along with an updated model of Saturn’s rotation. Rhea’s
gravity field was modeled using a degree-2 spherical harmonics
expansion, which was the minimum degree field capable of fitting
the data at the noise level, without clear signatures. Given the verysmall orbital eccentricity (about 0.001), the periodic tidal effects ofSaturn on Rhea were neglected: in this condition the tidal bulge isalmost constant, and the quadrupole coefficients J2 and C22 are afunction only of the secular Love number kf.
Given the very small eccentricity, Rhea is considered to be insynchronous rotation around Saturn, with the spin axis normal tothe orbital plane and prime meridian always oriented toward
Saturn. The rotational model adopted was based on the latest coordinate orientations adopted by the IAU (Archinal et al., 2011), withsmall corrections applied to obtain a better fit of Cassini and otherastrometric data. The adopted Rhea rotational model is reported in
Eqs. (4)–(6).

a0 ð Þ ¼ 40:347  0:17089132T  2:958 sin h þ 0:076 sin 2hð4Þ

P. Tortora et al. / Icarus 264 (2016) 264–273

d0 ð Þ ¼ 83:550  0:01277063T  0:332 cos h þ 0:004 cos 2hð5ÞW ð Þ ¼ 235:16 þ 79:69005069d þ 2:941 sin h  0:077 sin 2hð6Þ

where a0 and d0 are the right ascension and declination of the northpole with respect to EME2000, respectively, W is the angle measured easterly along the body’s equator between the prime meridian and the ascending line of nodes, T is the time measured in
Julian centuries (36,525 days) past J2000, d is the time measured
in days past J2000, and h is a nutation–precession term given by
Eq. (7).

h ð Þ ¼ 9:151048 þ 1004:6342250T

ð7Þ

A dynamically defined, perfectly synchronous, rotational model
was also used, but the solution was not affected significantly.
The dynamical model also included the most important nongravitational forces acting on Cassini during the flybys: the solar
radiation pressure (SRP) and the thermal thrust. Both accelerations
were described by the same models adopted by the Cassini navigation team and whose parameters were estimated using cruise andtour data.
The SRP is caused by momentum transfer between the spacecraft and the photons hitting its surface. In general a proper modeling of this acceleration is very difficult but, during tracking
passes, Cassini keeps the large High Gain Antenna (HGA) pointed
to the Earth, shadowing the spacecraft bus. Hence, during gravity
flybys the total frontal area exposed to the Sun is nearly constant,resulting in a constant acceleration on the order of 5  1013 km/s2(Di Benedetto et al., 2009).
During tracking passes, SRP is essentially a function of thethermo-optical coefficients of the HGA, whose value has been computed from the readings of two temperature sensor mounted on
the HGA backside, using a simple thermal model of the element(Di Benedetto et al., 2009). In order to assess the effect of an imperfect knowledge of the HGA thermo-optical coefficients their uncertainty was taken into account in the computation of the formal
covariance of the solution. In particular, using a conservative
uncertainty equal to 100% of the value, the solution covarianceincreases by a negligible amount.
The thermal thrust is caused by an anisotropic thermal emission of Cassini, and in particular caused by the thermal heat generated by the three onboard Radio-isotope Thermal Generators(RTG). Due to the spacecraft geometry, the main effect is an acceleration along the spacecraft Z body-fixed axis (which is the HGA
boresight axis) of about 5  1012 km/s2 (Di Benedetto et al., 2009).
The RTG-induced acceleration is modeled using a simple exponential model:AðtÞ ¼ A0 ebðtt0 Þ

ð8Þ

where A(t) is the thermal acceleration vector at time t, in body-fixed
coordinates, A0 is the spacecraft thermal acceleration at the reference epoch t0, and b is the time scale of the exponential law, which
derives from the 87.7-year half-life of 238Pu used as nuclear fuel in
the RTGs.
The magnitude of the RTG reference acceleration A0 has been
estimated during the cruise phase to a 3% uncertainty for the radial
component (Di Benedetto et al., 2009).
The force due to Saturn’s thermal radiation incident on Cassini
was neglected, because the resulting acceleration is on the orderof 1016 km/s2, increasing up to 5  1015 km/s2 near Saturn pericenter (Di Benedetto et al., 2009), i.e., about three orders of magnitude smaller than the RTGs acceleration.

267

2.3. Data selection and calibration
During the encounters, Doppler data at X (8.4 GHz) and Ka band
(32.5 GHz) were acquired by the antennas of NASA’s DSN at thethree complexes of Goldstone, Madrid and Canberra. In addition
to data obtained around the closest approach, the analysis also
used data obtained up to two days before and after the closest
approach, during standard navigation tracking passes. The additional data allow an improvement of the orbit determination, inparticular the estimation of Rhea’s ephemerides and Rhea’s GM,
because of the stronger constraints that are imposed on the relative trajectories between Cassini, Rhea and Saturn.
It is important to note that the closest approach of R1 wastracked only in three-way mode, meaning that the receiving andtransmitting stations were different. This represents a suboptimal condition, due to the possible delay between the reference
oscillators of the two different uplink and downlink stations. During R4, the closest approach was tracked in two-way mode.
Dual frequency X-band uplink and Ka-band downlink (X/Ka)
Doppler data were preferred to the standard X/X data when available, to reduce the effects of the dispersive noise sources, mainly
the solar corona and the Earth ionosphere. Two-way Doppler data
were preferred to three-way data in the same band when bothwere available, to remove the errors due to the clock synchronization between the uplink and downlink ground stations.
When only two-way X/X data and three-way X/Ka data wereavailable, the selection was made on a case by case basis.
When available, the wet path delay due to the Earth’s troposphere was calibrated using measurements from advanced water
vapor radiometers (Bar-Sever et al., 2007). When not available,
the Earth’s troposphere was calibrated using a combination of
weather data and dual frequency GPS measurements.
2.4. Estimation
Data analysis was carried out using two different approaches,
producing a multi-arc solution (SOL1) and a global solution (SOL2).
SOL1 was obtained using JPL’s orbit determination program
MONTE (Mission Analysis, Operations, and Navigation Toolkit
Environment) and a multi-arc approach, in which radiometric dataobtained during non-contiguous orbital segments, called ‘‘arcs”,
are jointly analyzed to produce a single solution of a set of ‘‘global”
parameters, which do not vary with time.
MONTE is the new orbit determination software developed by
JPL to replace the Orbit Determination Program (ODP) and it is
now used for the operations of all NASA’s space missions managed
by JPL. MONTE and ODP share the same mathematical formulation(described in detail in Moyer (1971, 2000)) which, apart from navigation, proved successful also for radio science data analysis.
The a priori values of GM, J2, C22 and S22 were retrieved fromMackenzie et al. (2008). The a priori values of C21 and S21 were
set to zero. In order to avoid constraining the estimation, the a priori uncertainties of the GM and the estimated quadrupole coefficients were set to at least one order of magnitude larger thantheir formal uncertainty. No hydrostatic equilibrium constraint
between J2 and C22 was imposed.
To properly fit the radiometric data, an updated state vector of
Rhea at a reference epoch was estimated, and the ephemerides ofall main Saturn satellites were updated by numerically integrating
the equations of motion, in order to keep constant the mass andthe trajectory of the system’s center of mass. The a priori values
for the Rhea state were retrieved from the Saturn satellite ephemerides SAT355.
For each arc, a separate initial condition for the Cassini statevector was estimated. The a priori values were obtained from thereconstructed trajectory provided by the Cassini Navigation Team.

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P. Tortora et al. / Icarus 264 (2016) 264–273

Fig. 3. Range-rate residuals around the closest approach of R1 (dashed vertical line), in mm/s. The RMS is about 76 lm/s.

(The updated Cassini trajectory Spice kernels are available at
ftp://naif.jpl.nasa.gov/.)
The residuals were weighted on a pass-by-pass basis using theirown RMS value.
Solution SOL2 was made using JPL’s original orbit determination
program, the ODP. Moreover, it was obtained as part of the ongoing
global analysis that seeks to determine the satellite ephemerides,
Saturn’s rotational model, and the gravitational parameters andgravity fields of Saturn and it major satellites. The global analysis,
described in detail in Jacobson et al. (2006), utilizes an extensive
data set that includes: Earth-based and Hubble Space Telescope
astrometry, satellite mutual events, Saturn ring occultations, imaging from the Voyager and Cassini spacecraft, and radiometric tracking of Pioneer 11, Voyager, and Cassini.
As in the SOL1 solution, the Doppler data that are sensitive tothe gravity parameters are weighted pass-by-pass at the RMS of
their residuals. However, the data are pre-processed to remove
the correlation between data points caused by scintillation in thesolar plasma, assumed to be the dominant noise source and having
a spectrum that follows the Kolmogorov power law.
2.5. Results
Figs. 3 and 4 plot the post-fit residuals of the tracking passes
around the closest approaches of R1 and R4. The residuals do notshow any evident signature around the closest approach, the mean
is approximately zero and the RMS are about 76 lm/s for R1 andabout 17 lm/s for R4.
Fig. 5 shows the different estimations of Rhea’s unnormalized J2
and C22 in the J2–C22 plane, along with their 1-r error ellipses,compared to the results published in Mackenzie et al. (2008) andAnderson and Schubert (2010). As a reference, Table 3 collects all
the estimated gravity coefficients for all the different solutions displayed in Fig. 5.
Although obtained with two independent analyses and following different approaches, SOL1 and SOL2 are statistically compati-

ble: the difference between the estimated values of J2 and S22 isless than 1-r, while the difference in the estimates of C22 is within
3-r. Moreover, both solutions are statistically compatible with
Mackenzie et al. (2008). This provides strong evidence of the reliability and robustness of the solution. However, the discrepancy
with Anderson and Schubert (2010) is statistically significant,being larger than 3-r and 5-r for J2 and C22, respectively. Recall,however, that the (Anderson and Schubert, 2010) solution was
obtained by constraining the hydrostatic ratio J2/C22 to 10/3.
Applying the same condition, our solution becomes fully compatible with Anderson and Schubert (2010), but the residuals show alarge signature at the closest approach of R1, in a similar way as
Mackenzie et al. (2008). Moreover, considering only a time intervalof ±15 min around C/A, where almost the entire quadrupole fieldsignal on the range-rate is concentrated, the RMS of the residuals
increases by about 56%, from 64 lm/s to 100 lm/s, a strong indication of a wrong dynamical model. Given the larger altitude at theclosest approach of R4, applying the hydrostatic constraint theresiduals of that pass do not show any evident signature, while
their RMS increases only slightly, from 17 lm/s to 18 lm/s.
From MacCullagh’s theorem the quadrupole gravity coefficientsare related to the body’s inertia tensor. In particular, J2 and C22 are afunction of the differences between the diagonal terms, while C21,
S21, and S22 are a function of the off-diagonal terms, which are nullin a frame of principal axes of inertia.
Under the assumption of small rotations, C21 and S21 are related
to a misalignment between the assumed spin axis and the real
maximum inertia axis, while S22 is related to a misalignment
between the adopted reference frame and the principal axes frame
around the spin axis.
The estimated values of C21 and S21 are null within 2-r, as
expected, while S22 is slightly larger, being about 3-r away fromzero. However, this proved to be due to the particular choice of
the adopted Rhea-fixed reference frame, as shown in the following.
Indeed, Rhea’s prime meridian used in the model and the Saturnpointing direction were found to be misaligned by about 3°, prob-

P. Tortora et al. / Icarus 264 (2016) 264–273

269

Fig. 4. Range-rate residuals around the closest approach of R4 (dashed vertical line). The RMS is about 17 lm/s.

Fig. 5. Estimated J2 versus C22 and formal 1-r uncertainty ellipses for the different approaches described in this paper. The most recent published values are also shown as
reference. Anderson and Schubert (2010) solution was obtained by constraining the J2/C22 ratio to the hydrostatic value 10/3.

ably because the former was retrieved using different Satellite
ephemerides and Rhea’s orbit was updated as part of the fitting
process. Instead, using a perfectly synchronous frame, dynamicallydefined using Rhea’s updated orbit, S22 decreases becoming nullwithin 2-r, while C22 increases in a statistically non-significant
way. Therefore, there is a good alignment within uncertainty
between the principal inertia axis of Rhea and a synchronous rotational state. Rhea’s quadrupole is dominated by J2 and C22, as

expected by a satellite in synchronous rotation around its planet.
However, the ratio J2/C22 is 3.91 ± 0.10 for SOL1 and 4.22 ± 0.19
for SOL2, which are equivalent to each other at the 2-r level andmore than 5-r away from the hydrostatic value of 10/3. Therefore
Rhea’s gravity field is significantly non-hydrostatic, meaning thatthe moment of inertia cannot be inferred directly from either J2
or C22 using the Radau–Darwin approximation. Estimating the
moment of inertia directly from the J2 or C22 coefficients of SOL1

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P. Tortora et al. / Icarus 264 (2016) 264–273

Table 3
Estimated values and 1-r formal uncertainties of Rhea’s quadrupole gravity unnormalized coefficients for the different approaches described in this paper compared to theresults published in Mackenzie et al. (2008) and Anderson and Schubert (2010). Mackenzie et al. (2008) did not provide the correlation between J2 and C22, a zero value wasassumed. Anderson and Schubert (2010) solution was obtained by constraining the J2/C22 ratio to the hydrostatic value 10/3. A priori values and uncertainties used for SOL1 arealso shown in the first column.

J2
C21
S21
C22
S22
J2/C22
Corr J2  C22

Unit

SOL1 (a priori)

SOL1

SOL2

Mackenzie et al. (2008)

Anderson and Schubert (2010)(106)(106)(106)(106)(106)

930 ± 600
0 ± 300
0 ± 300
240 ± 250
0 ± 250
3.9 ± 4.8
0.0

946.0 ± 13.9
19.9 ± 11.0
23.5 ± 21.3
242.1 ± 4.0
15.3 ± 5.0
3.91 ± 0.10
0.34

957.0 ± 20.3
0 ± 0.0
0 ± 0.0
227.0 ± 6.4
14.9 ± 5.0
4.22 ± 0.19
0.58

931.0 ± 12.0
0 ± 0.0
0 ± 0.0
237.2 ± 4.5
3.8 ± 3.8
3.92 ± 0.09
0.0 (N/A)

892.0 ± 1.6
0 ± 0.0
0 ± 0.0
267.6 ± 4.9
0 ± 0.0
3.33 ± 0.0
1.00

yields incompatible and unrealistic values of about 0.40 and 0.38,
respectively.
As pointed out by Anderson and Schubert (2010), errors in thethermal acceleration model may produce biases in the estimationof the gravity coefficients. In particular, a relatively high correlation (about 0.71) between Rhea’s J2 and the radial component of
the thermal acceleration was found. To take into account this possible error source, an updated value of the reference thermal acceleration vector was estimated, using a 5% a priori uncertainty for theradial component, and a 10% a priori uncertainty for the non-radial
components.
The estimated values of the thermal acceleration are compatible
within 1-r with the a priori values, which were adopted by the
Cassini navigation team. The formal uncertainty in the radial component is 3%, while the uncertainties in the other components areequal to the a priori value of 10%. Therefore, using data collectedduring Rhea’s flybys, only the knowledge of the radial componentof the thermal acceleration could be improved, because Doppler
measurements are directly sensitive only to velocity variations
along the line of sight.
To test the stability of the solution the a priori uncertainties of
the thermal accelerations were increased to 100% of their a priori

value, which can be considered a conservative upper bound. Rhea’s
J2 and C22 change by less than 2-r, their formal uncertainty
increase by about 25% and 7%, respectively, and the ratio J2/C22
becomes 3.85 ± 0.11, still 4.8-r away from the hydrostatic value.
Therefore, we conclude that even a modelization error in the thermal acceleration of 100% is not sufficient to absorb the excess of
the J2/C22 ratio with respect to the hydrostatic value.
3. Interpretation
There are a number of ways to explain the excess oblateness(greater than hydrostatic J2/C22 ratio) in Rhea’s gravity field thatare consistent with the observed shape, which has been determined via analysis of limb profiles (e.g., Thomas et al., 2007;
Thomas, 2010; Nimmo et al., 2011). Incorporating the latest available data (Thomas, pers. comm.), we adopt for this analysis a bestfitting triaxial ellipsoid with semi-axes:

a ¼ 765:68  0:25 km
b ¼ 763:60  0:25 km

ð9Þ

c ¼ 762:86  0:15 km

Fig. 6. Comparison of observed surface topography (blue square and large crosshairs, representing 1r uncertainties in the shape model) against the topography required tomatch the measured gravity for the models discussed in Section 3 (each shown with crosshairs illustrating the 1r uncertainties propagated from the SOL1 gravity model). For
the homogeneous interior case, the shape was determined via Eq. (10). For the spherical core case, the shape was determined via Eq. (11), with Hclm set to zero. For thehydrostatic core (hyd-core) cases, the core shape, Hclm , was determined using a theory-of-figures approach for a two-layer fluid body (e.g., Tricarico, 2014), assuming various
mantle thicknesses (d). Corresponding normalized moments of inertia (C=MR2 ) are also given in parentheses. Finally, the geoid was obtained via Eq. (12). For reference, thesurface topography expected for a hydrostatic two-layer body is indicated for various normalized moments of inertia along the dashed black line. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)P. Tortora et al. / Icarus 264 (2016) 264–273

from which we obtain the unnormalized degree-2 shape coefficients: J2 = 1190 ± 260 m and C22 = 350 ± 120 m (uncertainties are
all one-sigma) (blue square and crosshairs in Fig. 6). The large
uncertainties result from Rhea’s relatively small tidal/rotational distortion and rough topography. The shape is not as well determinedas the gravity; for instance, the topography coefficients are
consistent (within error) with a hydrostatic body, while the gravity
coefficients are not.
In the analysis that follows we will assume that the degree-2
coefficients provide a record of the time the shape and gravity were‘‘frozen in”. We do not consider the possibility that these coefficients were modified by later processes, such as the formation of
impact basins. Most such basins are sufficiently small that they
will not have an appreciable degree-2 signature. Tirawa represents
a possible exception, but we do not consider this issue further here.
We first take the simplest case of a homogeneous, uniform density Rhea. For a body of uniform density, it can be shown (Jeffreys,
1976; Hemingway et al., 2013; Iess et al., 2014a) that the shaperequired to explain the observed gravity is given byHlm ¼

ð2l þ 1ÞR
Glm
3

ð10Þ

where R is Rhea’s mean radius, and Glm and Hlm are spherical harmonic coefficients of degree l and order m, representing the dimensionless gravitational potential and the topography, respectively.
The green circle and crosshairs in Fig. 6 illustrate the topographycoefficients obtained from Eq. (10), representing the shape required
for a homogeneous Rhea to match the observed gravity field (forclarity, we use only SOL1 for the remainder of this analysis; results
are not significantly different when we use SOL2 instead). The large
uncertainties in the shape allow for the possibility of a homogeneous Rhea (we will discuss this issue further below). However,
because the surface is likely primarily water ice (Clark andOwensby, 1981; Stephan et al., 2012) we next consider a twolayer model consisting of an H2O mantle overlying a denser core.
Of course, as soon as the assumption of homogeneity is abandoned,
the problem becomes non-unique. Nonetheless, below we focus on
the two-layer model because it is relatively generic, and also consistent with the inferred structures of differentiated icy satellites suchas Enceladus (Iess et al., 2014a).
In general, and setting aside the point mass term, the dimensionless gravitational potential coefficients for a two-layer body,referenced to radius R, assuming constant density in each layer,
is given by a generalization of Eq. (10) (e.g., Lefevre et al., 2014):" lþ2 #3Rsc
Glm ðRÞ ¼
q H þ DqHlm c
 m lmð2l þ 1ÞRqR

ð11Þ

where the superscripts on the Hlm terms refer to the surface (s) and is Rhea’s bulk density (1236 kg/m3), qm iscore (c) topographies, q
the mantle density, which we take to be 920 kg/m3, Dqð¼ qc  qm Þis the density contrast at the core–mantle-boundary, and Rc is themean core radius. Hence, the observed gravity (Glm ) can be accommodated by a combination of core and surface topographies.
If we assume zero core topography (i.e., a spherical core), thensignificant surface topography (mass anomalies at the top of the
H2O mantle) would be required to account for the J2 and C22 terms
in the observed gravity field. Fig. 6 illustrates that this scenario (redtriangle and crosshairs) is not compatible with the observed shapeat the one-sigma level, although it remains compatible at the twosigma level. The case of a spherical core is not meant to be realistic
but rather to serve as a point of reference for subsequentdiscussion.
The addition of topography at the core mantle boundary would
contribute additional mass anomalies that, if placed in-phase,
could reduce the need for large surface topography to account271

for the observed gravity field. We start by assuming a weak (fluid)core and relax this assumption below. Taking a theory-of-figuresapproach (Murray and Dermott, 1999; Tricarico, 2014), we compute the expected hydrostatic core topography (Hclm ) and use Eq.
(11) to compute the surface topography (Hslm ) required to accountfor the remainder of the observed gravity (Glm ). We carry out thiscalculation for various mantle thicknesses ranging from 5 km (representing the maximum likely moment of inertia) to 300 km (corresponding to significant differentiation, with a normalized
moment of inertia of 0.335). The resulting surface topographies(Fig. 6) are consistent with the observed topography at the onesigma level, provided the mantle thickness is less than about
200 km.
The presence of deep impact basins such as Tirawa (5 km in
depth) suggests that the present day icy surface is rigid enoughto support considerable topography on spatial scales of up to afew hundred km. Support for degree-2 topography, however,may be more limited, especially if the degree-2 shape and gravity
field were established early, when Rhea’s heat flux was still high(White et al., 2013). In that case, it may be more appropriate toassume that the J2 (=C20) and C22 surface topography is relaxed
and therefore conforms closely to the geoid (equipotential surface
at reference radius R), given by
5
Hgeoid¼ R G20  q
20
6


1
¼ R G22 þ q
Hgeoid224

ð12Þ

where q ¼ ðR3 x2 Þ=ðGMÞ is the ratio of centrifugal to gravitational
acceleration, with x being Rhea’s rate of rotation (currently once
every 4.5 days).
In this scenario of the surface conforming to the geoid, an irregular (non-hydrostatic) core shape is required to account for theobserved gravity. We can use Eq. (11) to compute the required coreshape (Hclm ), setting the degree-2 surface topography (Hslm ) tomatch the present day geoid obtained from Eq. (12). For degree 2,
the efficacy of adding core topography in balancing Eq. (11)depends linearly on core radius (because Dq is proportional toR3c ). That is, the gravity observations can be most efficiently
accommodated by core topography when the H2O mantle is thin.
With larger mantle thicknesses, the core–mantle-boundary is deeper and so larger amplitudes of core topography are required togive rise to the same gravity signal at the surface. Fig. 7 shows,for various mean mantle thicknesses, the core topography required
to accommodate the observed gravity, assuming the surface topography conforms to the geoid. The magnitude of the required core
topography is not much greater than what is expected for a hydrostatic core. Even in the case of a thick (300 km) mantle, therequired excess core topography (i.e., beyond the hydrostatic
expectation) is less than 2 km (on a core of 460 km radius). For
comparison, Vesta’s longest and shortest dimensions are roughly
573 km and 446 km, suggesting that much larger core topography
is likely to be easily supportable. Thomas et al. (2007) and
McKinnon (2013) have likewise argued that a rigid, distorted coreis plausible for Enceladus and Tajeddine et al. (2014) have discussed a similar scenario regarding Mimas.
4. Conclusions
The main message of this paper is that the addition of a second
gravity flyby has allowed independent estimates of Rhea’s J2 andC22 without having to make a priori assumptions. The ratio of thedegree-2 gravity coefficients (J2/C22 = 3.91 ± 0.10) indicates a statistically significant departure from the expected hydrostatic ratio

272

P. Tortora et al. / Icarus 264 (2016) 264–273

600

C22 Core Topography (meters)

500

400
0.4

300

0.38

0.36

200
0.34

d=5km (C/MR2=0.399)
d=100km (C/MR2=0.375)
d=200km (C/MR2=0.353)
d=300km (C/MR2=0.335)
hydrostatic

100

0

0

500

1000

1500

2000

2500

J2 Core Topography (meters)
Fig. 7. Core topography required to give rise to the observed gravity under the assumption that the degree-2 surface topography conforms to the geoid. The required core
topography is computed via Eq. (11) and depends on the assumed mantle thickness (d); four examples are shown. In each case, the crosshairs illustrate the 1r uncertainties
propagated from the SOL1 gravity model. For reference, the core topography expected for a hydrostatic two-layer body is indicated for various normalized moments of inertia
along the dashed black line.

of 10/3. As a consequence, the moment of inertia cannot be
inferred directly using the Radau–Darwin relation.
As a point of comparison, the J2/C22 ratio for Enceladus wasrecently determined by Iess et al. (2014a) to be 3.51 ± 0.02, alsosignificantly in excess of the hydrostatic ratio (note that whereas
10/3 is a good approximation for Rhea, the appropriate hydrostatic
ratio to assume for Enceladus is closer to 3.24, due to its rapid rateof rotation (McKinnon, 2015; Tricarico, 2014)). The presence of significant non-hydrostatic topography at Enceladus suggested a differentiated interior with an isostatically compensated ice shell(Iess et al., 2014a). In contrast, Rhea’s large non-hydrostatic gravity
and small topography are more suggestive of internal mass anomalies that have little or no surface expression, as might be expected
for a body with an irregular core surrounded by a weak, relaxedmantle (e.g., McKinnon, 2013).
We investigated a number of internal structure models, none ofwhich can be strictly excluded due to the large uncertainties in theobserved shape and the inherent non-uniqueness of the problem.
The observed gravity and topography are consistent with Rhea
being of uniform density (undifferentiated). However, spectroscopic observations suggest that the surface is more likely to beuncontaminated water ice down to at least a few kilometers, leading us to prefer a two-layer model. Two-layer models, with water
ice mantles overlying denser cores, can accommodate the gravity
observations with modest amounts of topography at the surface
and/or at the core mantle boundary. However, if the current
degree-2 shape was established early, during an epoch of high heat
flux, then the degree-2 water ice surface topography may conform
closely with the geoid, in which case the majority of the excess J2
gravity must be accommodated by an irregular core (McKinnon,
2013). Low moment of inertia models (corresponding to highly differentiated interiors) are permitted and require only a modestly
oblate core (less than 2 km of excess core topography) and/or hundreds of meters of excess surface topography.
It is evident that the absence of hydrostatic equilibrium renders
any conclusions non-unique, and this problem is compounded by
the relatively poor shape determination. How might this situation

be improved? Theoretical arguments of whether Rhea should be
differentiated are not useful; even much larger bodies like Titan
could theoretically remain undifferentiated if they accreted sufficiently gently (Barr et al., 2010). Potentially more promising are
observational ways of determining the normalized moment ofinertia (C=MR2 ) of Rhea.
One possibility is to measure Rhea’s obliquity. As argued by Bills
and Nimmo (2008), given J2 and C22, a measurement of the obliquity is then sufficient to determine the normalized moment ofinertia of a satellite, assuming that it is in a Cassini state. Chenet al. (2014) calculated a predicted obliquity for Rhea of 0.03°(assuming hydrostatic equilibrium). This translates into a surface
distance of 0.4 km, which would be hard to detect with opticalimages, but perhaps not entirely out of the question.
Since C22 is known, one could also measure the longitudinal
libration amplitude to determine the moment of inertia, as wasdone at Mimas by Tajeddine et al. (2014). Unfortunately, Rhea’s
comparative lack of tidal distortion and its extremely low eccentricity (e = 0.001) conspire to produce a very small libration amplitude. For a normalized moment of inertia of 0.35, the amplitude is
12.6 m (cf. Comstock and Bills, 2003). Optical images would not be
sufficient to resolve such a small displacement, although radarmeasurements might.
In spite of the improved determination of Rhea’s gravity field,
the significant uncertainties in shape prevent us from drawing
strong conclusions about the interior structure. In particular, neither undifferentiated nor fully-differentiated structures can be
excluded. Without additional observational constraints, which
may be difficult to obtain, further insights into Rhea’s internal
structure may remain elusive.
Acknowledgments
The authors are grateful to John W. Armstrong and Sami W.
Asmar of the Cassini Radio Science Team, for the useful discussions
and suggestions regarding the procedures for Cassini data analysis.
Thanks to Peter Thomas for his helpful feedback and for providing

P. Tortora et al. / Icarus 264 (2016) 264–273

the updated solution for Rhea topography used in this work. P.T.,
M.Z., L.I., and M.P. acknowledge support from the Italian Space
Agency. D.H., and F.N. are grateful to NASA for support throughthe Cassini Project. The work of R.A.J. was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under acontract with NASA. The Doppler data and ancillary information
used in this analysis are archived in NASA’s Planetary Data System.
We thank the Cassini Project, A. Anabtawi and the JPL Radio
Science Systems Group, the NASA/JPL Deep Space Network andtheir operations personnel involved in the acquisition of Doppler
data analyzed here.
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