| Introduction: Enceladus is a medium sized icy satellite (MIS) of Saturn. MIS
are built of mixtures of rocks and ices. Enceladus with its radius of 250 km is one
of the smallest of MIS, however, contrary to the rest of them, it is geologically
active.
According to [1]: “For life to have emerged [/¦] on the early Earth, a sustained source of
chemically transducible energy was essential. The serpentinization process is emerging as
an increasingly likely source of that energy. Serpentinization of ultramafic crust
would have continuously supplied hydrogen, methane, [/¦] to off-ridge alkaline
hydrothermal springs that interfaced with the metal-rich carbonic Hadean Ocean” (see also
[2]).
We consider here conditions for origin of life in the early Enceladus and possible
proliferation of the life.
Mass of serpentinite: The serpentinization on the Earth is often considered with
hydrothermal activity in neovolcanic zones along mid-oceanic spreading centers. The total
length of present spreading centers is ~80Â000 km. However, only in small part of them the
hydrothermal activity really occurs. Even if in Hadean oceans the hydrothermal
activity was more widespread, still only small part of terrestrial rocks could be
serpentinized.
After [3] we consider the following reaction of serpentinization:
Mg2SiO4 (forsterite) + MgSiO3 (enstatite) + 2H2O –> Mg3Si2O5(OH)4 (antigorite).
This reaction releases 241Â000 J per kg of serpentine produced. A simple calculations
(e.g. [4]) indicate that mass fraction of silicatesfmas in Enceladus is ~0.646, hence the total
mass of its silicate is ~6.97 1019 kg.
[4] considered the process of differentiation and core forming in Enceladus. He found
that the result of differentiation is a relatively cold core of loosely packed grains
with water between them. At that time, there is not mechanism of removing the
water.
Since terrestrial rocks are permeable up to the pressure of ~300 MPa then the entire core
of Enceladus was probably permeable for liquids and gases. This could lead to formation of
extensive hydrothermal convective systems. Note that in Enceladus most of silicate could be
serpentinized (contrary to the Earth). It indicates that total mass of serpentinized silicate in
Enceladus could be larger than on the Earth.
T-p conditions in Enceladus: The pressure in the center of Enceladus is ~2.3 107 Pa that
correspond to pressure on the depth 2300 m in the terrestrial ocean.
The evolution of temperature in the Enceladus interior for the first a few hundreds Myr is
considered by [4] (no tidal heating is included). If Enceladus accreted later than 2.4 Myr
after formation of CAI then the temperature allows for existing the life even in the
center of the satellite. It is possible that for hundreds of Myr the conditions in the
interior of Enceladus were more favorable for origin of life than on the Earth [5,
6].
Proliferation of life: We do not know the probability of life origin. The life could
be a common phenomenon originating in relatively short time if conditions are
favorable. However, it is possible also that the life had originated only one time
in the Universe. If this option is true then the transport of primitive organism is
critical.
From the core to the surface. The volcanic activity offers occasion to transport organisms
from the core to the surface of early Enceladus. The form of this activity could be essentially
the same as present .
From the surface to E-ring. The existence of E-ring is an evidence that cryo-volcanic jets
could eject gas and solid particles (possibly with primitive organism) into orbit around
Saturn.
From E-ring to an orbit around Sun. The mechanism of gravity assist could be
responsible for acceleration of some particles from the orbit around the Saturn into
orbit around the Sun. The existence of several satellites of Saturn increases the
probability of this mechanism. The sequence of close encounters with these satellites
could eventually transfer enough energy to the grains to leave the orbit around
Saturn.
From orbit of Saturn to terrestrial planets. To reach the terrestrial planets from the
orbit close to Saturn the grain must be substantially decelerated. There are a few
possible mechanisms of loosing energy: Poynting-Robertson mechanism (for grains
larger than a few μm), Yarkovsky diurnal effect (if the grain is a retrograde rotator)
and Yarkovsky seasonal effect (for grains of diameter of a few meters); e.g. [7].
Deceleration leads the particle to move closer to the Earth and other terrestrial
planets.
After [6] we consider here the Poynting-Robertson effect which is effective
for the grains size of E-ring particles. Assume the grain on the circular orbit of
with radius R and the photons radially emitted by the Sun. In a grain’s frame of
reference the photons have some tangent component of the velocity. It gives rise to
tangential force opposite to the velocity of the grain. This force is given by formula (e.g.
[7]):
FPR = vW/c2, (1)
where v is orbital velocity of the grain, W is the power of Sun’s radiation and c is speed
of light. The power of drag is:
Pdrag= -FPRv = v2D/(R2c2). (2)
Note that D = CSunESgr (RE)2, where CSunE= 1350 W m-2 is the Solar constant at
the Earth orbit,Sgr= Ï rgr2 is the cross section of the grain, rgr is the grain radius andRE is
the radius of the Earth’s orbit. Note also that the orbital energy of the grain is given
by:
Eorb = -½ G Mm/R . (3)
Comparison of dEorb/dt and Pdrag and integration indicate that time of falling from the
orbit with radius Ri to the orbit with radius Re is given by:
t =[D/(4 mc2)](Ri2 - Re2) , (4)
where m is the mass of the grain. For the grain’s radius of 10 μm the time of reaching
Earth’s orbit from the Saturnian one is ~650Â000 yr. Note that for large grains (e.g. ~1 m)
other processes, like Yarkovsky effect, could be more effective than the Poynting-Robertson
effect.
Decelaration in the upper atmosphere. Small ratio of mass of the considered particles to
their cross section makes possible to decelerate them in upper atmospheres of terrestrial
planets without substantial increase of temperature – e.g. [7]. During deceleration of larger
bodies the dissipation of heat could be high, but cooling effect of ablation would reduce the
temperature.
Acknowledgments: This work was partially supported by the National Science Centre
(grant 2011/01/B/ST10/06653).
References: [1] Russell, M. J., Hall, A. J., And Martin W. (2010). Geobiology (2010), 8,
355–371. [2] Izawa M.R.M. et al. (2010). Planet. Space Sci. 58, 583–591. [3] Abramov, O.,
Mojzsis, S.J., (2011) Icarus 213, 273–279. [4] Czechowski, L. (2014) Planet. Sp. Sc. 104,
185-199. [5] Czechowski, L. (2014). Enceladus, a cradle of life of the Solar System.
Presented in EGU 2014, Vienna. [6] Czechowski, L. (2014) Enceladus as a place of
origin of the life in the Solar System. - submitted. [7] Pater (de), I, and Lissauer
J.J., (2001). Planetary Sciences, Cambridge University Press, Cambridge, UK,
pp. 528. [8] Wainwright, M., Wickramasinghe N. Ch., Rose, Ch. E., Baker, A. J.,
(2014). Astrobiology & Outreach, http://dx.doi.org/ 10.4172/2332-2519.1000110. |