TITAN'S INTERNAL STRUCTURE
Dominic
1,2
Fortes
and Peter M.
1,2
Grindrod
1
Department of Earth Sciences, UCL, Gower Street, London, WC1E 6BT, UK. (andrew.fortes@ucl.ac.uk)
2Centre for Planetary Sciences at UCL/Birkbeck, University College London, Gower Street, London, WC1E 6BT
0.400
We have constructed a range of internal structure models for Saturn's moon Titan in order to interpret the results
of the Cassini space-craft's first four flybys devoted to measurements of the gravity field.
The presence of a subsurface ocean (Lorenz et al. 2008) indicates at least partial differentiation, with a rock-free
outer shell to depths of order few hundred kilometres.
50 %
0.390
0.380
0.360
45 %
40 %
35 %
~
30 %
25 %
~
Differentiated two-layer models
water ice shell with dense ocean - (NH4)2SO4-H2O
50
0.370
21
water ice shell with no ocean
water ice shell with a light ocean - CH3OH-H2O
0.350
methane clathrate shell with no ocean
0.360
190
0
0.340
0.330
1700
0.320
1700
Orgueil
0.310
1700
2) The low density may also be indicative of a partially differentiated core, i.e., mixed rock
and ice to considerable depth. For rock with a grain density similar to CM chondrites,
the observed C/MR2 is reproduced when 80 % of the core by volume is a uniform
mixture of rock and ice.
Murchison
0
0.300
CI
CO, CV
CM
180
0.290
2200
2400
2600
2800
3000
3200
3400
3600
uncompressed core density (kg m-3)
Calculated moments of inertia for a suite of partially differentiated three-layer structures, consisting of
an ice shell, a zone of mixed rock and ice, and an ice-free innermost core. The panel below left depicts
the contours of inner core radius as a function of rock density and MoI, whereas the panel below right
depicts the same data as inner-core volume as a fraction of the whole-core volume. In order to yield
the observed moment of inertia with a nominally anhydrous 'rock' component (density greater than ~
3000 kg m-3) requires that a very large volume fraction (> 80 %) of the core is undifferentiated.
In both cases the temperature in the core must be much lower than has
been previously supposed. The hydrous core cannot be above ~900 K,
and the mixed rock + ice core cannot be above ~450 K. Earlier models
had suggested core temperatures in excess of the the Fe-FeS eutectic
at ~ 1400 K (e.g., Tobie et al., 2005, 2006).
0.360
0.360
00
22
This result calls into question the interpretation of extensive
cryovolcanic resurfacing.
21
0.350
00
0.350
00
20
Moment of inertia, C/MR2
00
19
0.340
Moment of inertia, C/MR2
1) This may be indicative of a low-density silicate mineralogy, i.e., a pervasively hydrated core composed of
serpentine, clay-group minerals and organics, perhaps similar to CI chondritic rock.
Undifferentiated models
Moment of inertia, C/MR2
Our model results show that Titan's moment of inertia, C/MR2, requires a low-density core (2570 - 2460 kg m-3)
with a radius of 1980 - 2120 km, as predicted by Fortes et al. (2007).
Variation of the moment of inertia for
undifferentiated (uppermost dashed line) and
differentiated two-layer structures, as a function of
the uncompressed core density.
Solid lines are for models composed of water ice
shells around a rock core, covering the full range
of plausible subsurface ocean densities. The
lightest and densest oceans consistent with
gravitational stability correspond to ~950 kg m-3
and 1250 kg m-3 (approximately the eutectics in
the methanol-water and the ammonium sulfatewater systems, respectively).
The dashed green line corresponds to an oceanfree model with an outer shell composed only of
methane clathrate hydrate.
The bars near the bottom of the diagram show
grain densities for certain groups of chondritic
meteorites, and two well-known examples are
marked by arrows. Titan's moment of inertia is
consistent with a CI chondrite-like core.
18
00
17
00
0.330
120
0
130
0
140
150
160 0
0
0.320
0
3-layer models with a partially
differentiated outer core
(50:50 rock:ice)
40%
60%
0.330
0.320
80%
100
%
0.300
3-layer models with a partially
differentiated outer core
(50:50 rock:ice)
0.290
0.290
2200
20%
0.310
0.310
0.300
0.340
2400
2600
2800
3000
3200
3400
2200
3600
2400
2600
2800
3000
3200
3400
3600
uncompressed core density (kg m-3)
uncompressed silicate core density (kg m-3)
Calculated moments of inertia for a suite of differentiated three-layer structures, consisting of an ice
shell, a rocky outer core, and either an iron-sulfide inner core (below left), or a pure iron inner core
(below right). For a rocky core of density comparable to CI chondrite, then the admissible inner core
radii are ~ 360 km (Fe75S25 case) and ~290 km (for the Fe100 case). It is worth observing that a
metallic inner core is not consistent with the inferred low interior temperatures. The lack on an intrinsic
magnetic field is not diagnostic, but its absence supports the inference of low-T, low-density core.
organic rich atmosphere
and surface
0.360
0.360
de-coupled outer shell
3-layer models with an iron inner core
(in situ density ≈ 8000 kg m-3)
3-layer models with an iron sulfide (Fe75S25) inner core
(in situ density ≈ 5500 kg m-3)
(water- ice / clathrate)
0.350
0.340
0
00
2
00
0.330
19
00
18
0.320
600
0.310
global subsurface ocean
Moment of inertia, C/MR2
Moment of inertia, C/MR2
21
21
00
00
0.350
0
170
400
0.340
00
20
00
0.330
19
00
18
0.320
0
0.310
800
600
500
170
3
400 00
700
1000
800
0.300
0.300
900
0.290
0.290
2200
2400
2600
2800
3000
3200
3400
3600
uncompressed silicate outer core density (kg m-3)
high-pressure ice VI shell
References
hydrous silicate core
~ 2000 km radius
rock-ice interface = 0.9 GPa
core pressure = 4.9 GPa
Tobie, G. et al. Icarus 175(2), 496 (2005).
Tobie, G. et al. Nature 440, 61 (2006).
Fortes, A. D. et al. Icarus 188(1), 139 (2007).
Lorenz, R. D. et al. Science 319, 1649 (2008).
2200
2400
2600
2800
3000
3200
3400
uncompressed silicate outer core density (kg m-3)
3600