Particles from Ephemeral Plume Activities ofEnceladus Deposit on Saturnian Satellites.

Naoyuki Hirata
The University of Tokyo
The University of Museum

Background 1• Enceladus is one of saturnian icy satellites• Enceladus has the ocean in its sub‐surface.
• Icy particles released from satellites form Ering
Enceladus

Icy particles released from Enceladus

PIA00347

PIA07762Interior structure

PIA13420

Background 2
The lifetime of the ocean is long or short?
•Thermal source is not sufficient to maintain thesubsurface ocean–•the subsurface ocean must freeze over 30 My (Robert & Nimmo 2008)

No geological evidence to support the large massdecreasing of Enceladus.
–The current eruption rate is estimate to ~200kg/s (Hansen et al.
2006)

–

This rate should cause significant geological tectonics(Spencer et al. 2009)

PIA07727

Short?
However, observational facts
are not obtained yet
PIA12564

Point of this study• Addressing this issues,– we point out the coefficients between rings and satellites
PIA03550• Materials erupted from Enceladus form the E ring
• The E ring materials should affect the surrounding satellites.
• Based on this view, we investigate 5 mid‐sized and 7 small satellites.

Finding
We found Helene has the obviously deposits originated from E ring.

(右からN1646319549,N1687121104,N1646315085,N1563643679)

This satellite is located at L4 point of Dione
Its mean diameter is ~30 km
We findHelene has the unusual smooth surface and streaky depression.
These features is formed by the depositions of E ring materials

Discussion• These features can be explained by the depositions of E ringmaterials– Concentration to the leading side.
• E ring particles tend to collide to the leading side of the satellite• In fact, Dione’s leading side is more strongly contaminated than its trailingside. (Hamilton & Burn 1994)•

Helene’s Bimodal appearance may be explained– This phenomena makes the slope unstabilize
and small craters erase– Spectral features of leading side is similar to that of E ring
Leading side

trailing side
E ring deposits

Helene’s original surface

Other small satellites of E ring region
Pallene’s mean diameter is 4 km
Methone’s mean diameter is 3 kmLocated within the orbit of Mimas
Spherical shape andQuite smooth surface
パレネ
PallenePallene Telesto
Methone Calypso

PIA03550
HeleneUpper N1665947247
Lower N00189072

メトネ
Methone••••Located at the L4 and L5 points of theorbit of Tethys
There are a lot of large craters while
there are few small craters
Surface is somehow smoothStreaky depressions can be identified on
CalyspoTelestoCalypsoUpper N1630076968, N1514163666, lower N1644754662, N1506184171

Irregular features of these Small satellites
can be explained by the depositions of Eringdense– Telesto and Calypso↑densityof E ring↓• E ring of their orbits is denser than that ofHelene’s orbits
thinwhich means the accumulations isstronger• Their shapes may bury their ordinary
surface

Distance from Saturn– Pallene and Methone• E ring of their orbits is much denser thanthat of Telesto’s or Calypso’s orbits
which means the accumulations is muchstronger• The large amount of E ring materials may
completely bury their ordinary surface andtheir shape may be near‐spherical ones

HelenePalleneMethoneTelestoCalypso

E ring accumulations concentrate to
the leading side in the case of Dione and Helene
the trailing side in the case of Mimas
the global in the case of Tethys, Telesto, and Calypso
•••••
•

Sa

The angular velocity decrease in apoapsis
The angular velocity increase in periapsis

M

Inner region of E ring, ring particles exceed satellites and collidewith trailing side of the satellites
Surrounding region of Enceladus, particles collide anywhere ofsatellites
Outer orbit of E ring, ring particles are exceeded by satellites andcollide with leading side of the satellites
In fact, this nature was occurred on Mimas, Tethys, Dione–•TIcy particles emitted from Enceladus has the same orbital velocity
with that of Enceladus
After that, the eccentricity of these particles increase
Then, eccentric orbits cause––•D

Based on the albedo, spectral features, and radar reflectanceThis view has the good agreement with the appearances of small
satellites of E ring region.

EMimastrailing side

Calypso leading side

trailing sideDione

E ring particle
E ring particle Globalaccumulation

Trailing side accumulation

Telesto leading sideEnc.
TethysE ring partic

leading side accumulatio

HeleneTelesto, Calypso(occupiesthesame orbit of Dione)(occupy the same orbit
Leading sideof Tethys) GlobalImplications 1•

Streaky depressions indicate– The friction angle of E ring particles is low, lower than 10degree– the friction coefficient between E ring particles is quitesmall– E ring particles may have a spherical shape with asmooth surface– A particle attains a spherical shape due to the surface
tensional force of a droplet emitted from Enceladus
before frozen as a particle– If so, almost all E ring particles are originated from thesubsurface ocean (not surface regolith layer)•Sample return mission– A future sample return mission might consider utilizing
small landers to obtain a sediment core to help reveal
the activity of Enceladus

12 satellites of E ring region

EnceladusImplication 2•

Cratering age of the deposit of E ring on Helene is––––•Less than 50 My, if the cratering rate is the same as the case of Dione
Less than 400 My, if the trailing hemisphere was formed at 4 Ga
From 3 My to 500 My if I use the cratering estimate of Zahnle et al. 2003
The deposits of other small satellites also show the same result

Based on the eruption rate
– the mass of solid material per second escaping from Enceladus is most likely ~5 kg/s, currently– Assuming that 10 to 100% of particles ejected from Enceladus would be lost by collisions tosatellites– the total volume of the E ring deposits on satellites would accumulate in only 3.8 My to 270 My••

there is the possibility that the accumulations of the E ring material on satellites
began several My ago
the initiation of cryovolcanism on the surface of Enceladus is also the same age?
– This result is consistent with the theoretical study of Robert and Nimmo (2008)•

the subsurface ocean must freeze over 30 My– This result is the first one indicating the geologic age of thecryovolcanism on Enceladus
1Ga

100Ma

10Ma

1Ma

0.1 MaConclusion• The surfaces of small satellites is strongly
affected by the cryovolcanism in Enceladus• The accumulations due to E ring may havebegan at a few million years ago• The lifetime of subsurface ocean of Enceladus
may be also limited to a few million years