Enceladus: Cassini
Observations and Implications
for the Search for Life
(In press, Astronomy & Astrophysics)
Christopher Parkinson, Mao-Chang Liang, Hyman
Hartman, Candice Hansen, Giovanna Tinetti, Victoria
Meadows, Joseph Kirschvink, and Yuk Yung
Cassini UVIS Science Team Update, January 2007
Context
• Previous efforts to detect evidence for
life elsewhere in the solar system have
been focused on Mars and Europa
• On July 14, 2005, the Cassini
spacescraft had a close encounter with
the Saturnian moon, Enceladus,
resulting in one of the most unexpected
discoveries from the Cassini mission
Evidence
• Multiple instruments on Cassini spacecraft
show a large water vapour plume (Dougherty
et al. 2006) eminating from Enceladus’ south
polar region
Radius (km)
Gravity (cm s-2)
Geometric albedo
250
11.5
0.99 (visual wavelengths)
Escape velocity (m s-1)
Thermal velocity (m s-1)
Scale height of jet (km)
240
460 (H2O at 180 K)
80
UVIS
INMS
VIMS
CIRS
• The weak gravitational field prevents
retention of an atmosphere, indicating the gas
is likely resulting from some currently active
geothermal venting process
• A hydrological cycle governing the weathering
of rocks by liquid water + any other
concomitant radioactive emissions + other
geothermal energy sources are possible
incipient conditions for life
Cassini Data and Analysis
• The Cassini UltraViolet Imaging Spectrograph
(UVIS) observed the occultation of Bellatrix
(γ-Orionis) as it passed behind Enceladus as
seen from the spacecraft (Hansen et al.
2006)
• Analysis of UVIS spectra for the presence of
other molecules done by method of leastsquares linear regression method (Bevington
& Robinson 1992)
• The residual is obtained by subtracting the
H2O signal (Hansen et al. 2006) from the
UVIS spectra
• We then apply the linear regression to
estimate upper limits for several molecules of
astrobiological interest using normalized
cross sections
• Our calculated values for the upper limit of
the column density for several species are
consistent with detections from other
instruments aboard Cassini spacecraft
Species
Abundance
Reference
CO2
0.032
a
N2
0.040
a,b
CH4
0.016
a
C2H2
~0.01
a
C3H8
~0.01
a
NH3
<0.005
a
CO
<0.01
b, c
SO2
<0.15
c
dust
0.1 m-3
d
electron
~100 cm-3
e
N+
3% of total ion
f
Organics
Detected in ice
g
CO2
Detected in ice
g
a. Waite et al. 2006 (INMS); b. Hansen et al. 2006 (UVIS);
c. Parkinson et al. 2006; d. Spahn et al. 2006; e. Tokar et al. 2006;
f. Bouhram et al. 2006; g. Brown et al. 2006 (VIMS)
Organics
The main sources of organics are believed to
be either from
1. extraterrestrial influx (i.e. comets - nucleus
and interplanetary dust, macro- and micrometerorites)
2. synthesis of organic matter from inorganic
molecules in the vicinity of submarine
hydrothermal vents, as well as in shallow
basins (Brack 1998; Raulin 2005)
• Organic products can be obtained from
submitting gas mixtures of various starting
composition to electron or photon irradiation
• Only reducing mixtures such as CH4-NH3-H2H2O or CO-N2-O2 allow the production of
organic molecules (Raulin 2005)
• The primitive atmosphere of the Earth was
probably not the starting point of the prebiotic
organic processes, since its main compostion
(CO2-N2-H2O) was not favorable to the
formation of organics
From Becker and Epstein (1982) organic
matter in carbonaceous chondrites
can be separated into three fractions
1. fraction insoluble in both methanol and
chloroform
2. fraction soluble in chloroform and
3. fraction soluble in methanol
•
Hartman et al. (1993) interpreted
1. the first insoluble component to be of interstellar
origin and
2. the other two soluble fractions to have been
synthesized on a hydrothermally altered
planetoid body
•
The simultaneous synthesis of iron-rich
clays with the polar organics may be
indicative of events related to the origin of
life on Earth
Hydrological Cycle
• Icy particles ~1 μm icy are emanating from
Enceladus’ plume (Porco et al.,2006).
• We use this value guiding our modeling effort
with CARMA (Toon et al. 1988) to simulate
the growth of water ice in the plume of
Enceladus.
• Start with 0.1 μm sized condensation nuclei
with an abundance of 10-3 particles cm-3
• The reaction temperature is fixed at 170 K,
190 K, 210 K, and 225 K during the course of
simulation, with a water vapor abundance of
xx ×101x molecules cm-3
• In order to grow 1 μm size particles a reaction
time of ~10 seconds is required (coagulation
unimportant).
• Our models infer that the formation and
growth of ice particles takes place in the
regions where the temperature has to be at
least ~200 K.
• Once the 1 μm sized particles particles from
Enceladus’ plume land back on the surface of
the moon, they must rapidly grow to 30 μm.
• A microphysical description of sintering
(Colbeck,1998) can only enhance the particle
radius by ~1.1, implying sintering is not the
contolling process for grain growth.
• Thermally-activated normal grain growth
could yield particle sizes ~30 μm in ~2 x 107
years where T ~ 145K, comparable to the
estimated temperature in the center of the
tiger stripes (Spencer et al., 2006).
•
However, these large grains are observed in
the flanks of the tiger stripes , where
temperatures may be as low as 70K
(Spencer et al., 2006).
•
Therefore, we conclude the ice grains either
1. formed in the center of the tiger stripes and were
moved outwards due to a mid-ocean-ridge-type
spreading mechanism (Stempel et al., 2005),
2. or that a non-thermally controlled process
(perhaps vapor diffusion) causes the ice grains to
grow rapidly.
Silicates: Exogenic or
Endogenic?
• Silicate material required for nucleation of the
icy particles in the plume could come from the
interior of the moon or from micrometeorites.
• Cassini’s Cosmic Dust Analyzer (CDA)
estimates dust influx ~0.2 kg s-1.
• Both Porco et al (2006) and Hansen et al
(2006) estimate material from the plume to be
escaping at a rate of ~150 kg s-1.
• ~1% dust fraction in plume (Kempf, private
communication) obtains ~0.15 kg s-1
escaping dust.
• Assumed micrometeroid flux of ~10-16
g/cm2s (Moses et al.,2000) gives an influx of
dust of ~0.2 g/s over the south polar region.
• Comparison with the Cassini’s CDA
observation shows that the total influx of
dust from exogenic sources is insufficient.
• Clearly the vast majority of silicate material
in plume must be endogenic, viz., coming
from under the ice mantle.
Origin of Nitrogen: where is
the ammonia?
• Impacting energetic ions will destroy any
surface ammonia present, as the resurfacing
rate isn't enough to shield them from
destruction. There are radiogenic sources as
well. (Loeffler et al., 2006).
• Possible existence of NH3 in early moon
history, but recycling of the ice mantle would
"scrub" the ammonia out leaving mainly N2
• Over 1 billion years, at a recycling rate of 1 200 million years, obtains 5 -1000 cycles.
• Cycling also results in an estimated ice
mantle mass loss of ~20% (Kargel 2006).
• The N2 (along with H2O2) could be subducted
over time down to the clay/rock/ice/water
interface.
• Current estimates (Hansen et al., 2006; Waite
et al., 2006; Parkinson et al., 2006) put N2 in
the plume at about 4%.
• 4% NH3 by mass can be delivered to the
proto-saturnian disk by NH3 clathrated
planetesimals from the cold outer regions of
the disk (Rs >80).
• Assuming the ice mantle had a similar value
at formation, this suggests an ≥50% N2
depletion over 4 billion years due to the
hydrological cycle of Enceladus.
• What about the northern region? Perhaps
there are ammonia reservoirs deep in the ice.
• However, if the ice crust "moves around" at
all, this may provide additional sources of
ammonia, and hence, N2.
• Additionally, acetylene, propane from
methane by high temperature processes
have been observed by the Cassini
spacecraft.
• Such high temperatures have been proposed
by Matson et al (2006) but it is not at all clear
at this point that this is indeed the case.
• The processes governing the hydrological
cycle apply to all species, with methane being
a lot more stable than ammonia.
• Therefore some methane may survive, where
ammonia seems not to have done so.
• Consistent with the Cassini observations, viz.,
H2O ~ 91%, N2 ~ 4%, CO2 ~ 3%, CH4 ~1.6%,
ammonia < 0.5% (cf. Hansen et al., 2006,
Waite et al., 2006 and Parkinson et al., 2006).
Oxidants
• The most abundant oxidant on icy satellites is
most likely H2O2, which has been detected on
Europa
• we assume that the ratio of the H2O2
production rate and the H2O sputtering rate is
the same for Europa and Enceladus
• This implies that the production rate of H2O2 at
Enceladus would be of the same magnitude as
that on Europa ~ 1011 molecules cm−2 s−1
• NOTE: Baragiola (private communication)
suggests this value may be one or two orders
of magnitude too high
• Following Chyba & Phillips (2001) for Europa,
it is possible to estimate the total number
cells that could exist in an ecosystem
underneath Enceladus’ ice crust in the vicinity
of the plume vent
• Using estimated value of ∼150 kg/s for the ice
resurfacing rate at the south pole of
Enceladus suggests a crust turn-over time of
∼106 yr for an ice thickness of 1 km
• Assume the conditions (such as the
concentrations of H2O2 and HCHO, and a
recycling ice thickness of 1.3 m) of Enceladus
are similar to those of Europa
• We estimate the microbial ecology in the
region of Enceladus’ plume to be ∼1020 -1021
cells (for a 103 yr biological turn-over time)
• Our estimated value is an order of magnitude
greater per unit area than that for Europa.
Conclusions
• Presently Enceladus is the most exciting
object in the solar system for the search of
extant life
• We have compelling evidence supporting the
view that Enceladus has active hydrological,
chemical and geochemical cycles, which are
essential ingredients for originating and
sustaining life
• To our knowledge, these conditions are not
duplicated anywhere else in our solar system
except our planet
• Compared to Mars, Titan and Europa,
Enceledus is the only other object in our solar
system that appears to satisfy the conditions
for originating life at present
What’s next?
Future ground based and space observations and
future space missions are urgently needed to
advance our understanding of Enceladus.
1. long-term observations to establish whether the
plumes are transient or in steady-state
2. searching for molecules of astrobiological
significance, such as NH3
3. searching for the presence of photopigments
(e.g. chlorophyll)
4. identifying the presence of oxidants on the
surface, such as H2O2 and O3
5. mapping the North-South gradient in surface
properties to quantify the rate of impact erosion
and resurfacing
Laboratory studies are needed,
1. to identify the chemical species responsible
for 3.4 μm absorption feature in the ice in
the “tiger stripes” (Berstein et al. 2005),
2. to study the chemical evolution of organics
in ice in the presence of energetic photons
and particles, and
3. to quantify the rate of production of oxidants
such as H2O2 and O3 for conditions
appropriate for Enceladus.
• Modeling studies are needed to link the
observations and laboratory experiments to
the evolution of the hydrological, chemical
redox and geochemical cycles on Encaladus
• Another Cassini flyby in the works (extended
mission)!