enceladus - brown university · cassini finds molecular hydrogen in the enceladus plume: evidence...

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This image, acquired by the Cassini spacecraft, captures Saturn, its rings (edge on), and the moon Enceladus. It was discovered that this moon emits jets of ice from possible underground seas. It appears white because its surface is covered with relatively clean water-ice.

Enceladus

Orbital trajectory of Cassini spacecraft (2010 - 2017).

Image: solarsystem.nasa.gov

1. Major Surface Features & Structure

2. Geysers: composition

3. Geysers: origin

Overview

Enceladus as seen by Voyager 2 (August 1981)

Enceladus 5

Voyager 2 image of Enceladus Cassini Image

Enceladus 6

•6th largest moon of Saturn

•Discovered in August 1789 by William Herschel.

•Only ~500 km in diameter.

•Geysers observed in south polar region....what is heat source??

Enceladus 7

Enceladus contributes ice to Saturn’s E ring (see image to right).

High velocity material leads the moon, lower velocity material trails.

Enceladus 8

Enceladus preserves a surprising record of geologic activity. The smooth, uncratered terrain is geologically young, suggesting that Enceladus has recently experienced internal melting and resurfacing by an exotic form of volcanic activity in which water and icy slush were extruded. Linear sets of grooves tens of kilometers long are probably faults resulting from crustal deformation.

Enceladus 9

Enceladus 10

Enceladus: Internal Structure 11

NASA/JPL/Space Science Institute

Voyager data indicated body was mostly ice, but Cassini showed higher density that implies more silicate.

No evidence for Fe-rich (metal) core, but there is a silicate “core”/mantle that likely still contains some water.

Differentiation between silicate and water ice caused by fast formation and decay of 26Al and 60Fe.

Aqueous alteration of carbonaceous-chondrite like material?

Enceladus: Geysers 12

These spectacular plumes, both large and small, spray water ice out from many locations along the "tiger stripes" near the south pole of Enceladus.

The tiger stripes are fissures that spray icy particles, water vapor and organic compounds, showing that Enceladus is geologically active today.


The tiger stripes are also associated with thermal anomalies. They appear warmer than surrounding regions, which helps to explain why they are so geologically active.

The visible images can be used to locate the positions of the jets, which can in turn be combined with the thermal images to see if there are any clear associations.

101 jets have been identified and they seem to vary in strength over time.


This thermal map of the south polar region reveals never-before-seen details of warm fractures that branch off like split ends from the ends of the main trenches of two "tiger stripes." The features nicknamed "tiger stripes" are long fissures that spray water vapor and icy particles. These two fissures, Cairo Sulcus (left) and Alexandria Sulcus (right), extend to the lower right, off the bottom of the image. The map also shows an intriguing isolated warm spot, shown in purple-red in the upper left of the image, that is separated from other active fissures.

The pale blue color indicates regions that were mapped but that were too cold to emit significant radiation. The map shows a region approximately 130 km (80 miles) across. Away from the warm tiger stripes, which reach temperatures up to 190 K (-120°F), Cassini measured surface temperatures near the south pole as low as 52 K (-365°F), and colder temperatures are achieved during winter.


This region, called Damascus Sulcus, is one of several “tiger stripe” regions within the geologically active south polar region of Enceladus. It consists of two large parallel ridges separated by a deep V-shaped trough. The ridges are each 100 -150 m high, while the entire width of Damascus Sulcus is 5 km. The medial trough between the ridges is 200 to 250 m deep and may have formed by daily shear (sliding) faulting triggered by tidal forces. These medial troughs may be the primary source of numerous jets making up the large active water vapor plume over the south pole of Enceladus.

Small ridges along the floor could be blocks of crust that have slid down the walls of the trough or fractured blocks pushed up from below. Flanking Damascus Sulcus are repeating sets of broken and disrupted parallel ridges a few tens of meters high. These are typical of the plains that lie between the tiger stripe structures and resemble crumpled or folded rock patterns seen on Earth.

Relief has been exaggerated by a factor of ~10 to enhance clarity.

17Enceladus: GeysersGeophysical data indicates Enceladus contains an ‘ocean’ above a rocky interior.

This may provide water-rock interaction, leading to enrichment of CHNOPS in fluids and potential energy gradients.


Important source of energy for this activity is tidal forcing.

This produces internal friction which produces heat, and it also produces compression and extension in the crust.

Gravity data suggest an ocean is present in at least the southern polar region, but a global ocean is not required to fit the data.


Large volcano-like explosions blast icy particles high above the surface of Enceladus. The right image is color enhanced to show the abundance of the icy ejecta. Some particles go into orbit around Saturn and form the E ring (stable since 1966...ongoing process!!).


Tiger Stripe ‘Baghdad Sulcus’ on the South Polar

Terrain of Enceladus

Cassini thermal mapping at 1 km/pixel.

Jet

Jet ✷

✷


These figures show mass spectra that reveal the chemical constituents sampled in Enceladus' plume by Cassini's Ion and Neutral Mass Spectrometer during its fly-through of the plume on Mar. 12, 2008. Shown are the amounts, in atomic mass per elementary charge (unit of Daltons [Da]), of water vapor, methane, carbon monoxide, carbon dioxide, simple organics and complex organics identified in the plume.

In addition, measurements and models have estimated that there are ~3 g of salts per kg H2O.

Data like these are critical for determining the chemical composition of the plumes, which in turn provides information about the composition of the water and interior of Enceladus.


In addition to ice being ejected from the plumes, the infrared spectrometer on Cassini has also detected faint signatures of organic material, as shown in the image on the bottom and the reflectance spectra on the right.

Organic material absorbs light at wavelengths near ~3.4 µm, and we can map the strength of this absorption to see if organics are present on a planetary surface.

132 14 APRIL 2017 • VOL 356 ISSUE 6334 sciencemag.org SCIENCE

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By Jeffrey S. Seewald

Planetary bodies with global oceans

are prime targets in the search for life

beyond Earth owing to the essential

role of liquid water in biochemical re-

actions that sustain living organisms.

In addition to water, life requires en-

ergy and a source of essential chemical ele-

ments (C, H, N, O, P, and S). Although there

is compelling evidence for liquid water and

many of the essential elements on several

ice-covered planetary bodies in our solar

system and beyond, direct observation of

energy sources capable of fueling life has, to

this point, remained elusive. On page 155 of

this issue, Waite et al. (1) report that recent

flybys of the ice-covered saturnian moon

Enceladus by the Cassini spacecraft reveal

the presence of molecular hydrogen (H2)

in jets of vapor and particles ejected from

a liquid water ocean through cracks in the

ice shell. The abundance of H2 along with

previously observed carbonate species sug-

gests a state of chemical disequilibria in the

Enceladus ocean that represents a chemical

energy source capable of supporting life.

Enceladus is a midsized (504-km diam-

eter) satellite of Saturn that has an inferred

rocky silicate core covered by an estimated

2- to 60-km layer of water ice ( 2–4). Evi-

dence points to the existence of a global

ocean (3, 5) that is likely maintained in a liq-

uid state by heat generated during tidal de-

formation. The viability of life on planetary

bodies such as Enceladus can be assessed

through examination of biogeochemical

processes on Earth. Sunlight-fueled photo-

synthesis is the primary source of energy at

Earth’s surface, but is unlikely in the outer

solar system where energy from the Sun is

limited, especially at depth in ice-covered

oceans. In Earth’s oceans, however, there

are vast ecosystems where primary produc-

tion is sustained in the absence of sunlight

by chemical energy available from aqueous

fluids venting at the seafloor. Some of the

most primitive metabolic pathways utilized

by microbes in these environments involve

the reduction of carbon dioxide (CO2) with

H2 to form methane (CH

4) by a process

known as methanogenesis (6). Here lies

the connection with Enceladus. By operat-

ing the Cassini onboard mass spectrometer

in open-source mode during a 2015 flyby of

Enceladus that intersected the vapor and

particle plume, Waite et al. were able to

minimize analytical artifacts that had com-

promised H2 measurements during previ-

ous flybys. The new approach allowed them

to determine that the plume gas contained

0.4 to 1.4 volume % H2 along with 0.3 to

0.8 volume % CO2, critical ingredients for

methanogenesis.

Reconstructing the composition of the

Enceladus ocean from the abundance of

material in the plume is a difficult task be-

cause of unknown chemical fractionation

associated with the freezing of saline ocean

water in a vacuum as it is ejected through

cracks in the icy shell. By making the sim-

plifying assumption that molal abundance

ratios in the Enceladus ocean are preserved

in the plume, Waite et al. developed a geo-

chemical model that predicts a highly alka-

line (pH = 9 to 11) sub-ice ocean containing

dissolved H2 and carbonate species in a

PLANETARY SCIENCE

Detecting molecular hydrogen on Enceladus

Cassini spacecraft detects molecular hydrogen on one of Saturn’s moons

Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA. Email: [email protected]

INSIGHTS

PERSPECTIVES

DA_0414Perspectives.indd 132 4/12/17 10:49 AM

Published by AAAS

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Enceladus: Geysers

Recent findings published in Science:0.4 - 1.4% (by volume) of H2. 0.3 - 0.8% (by volume) of CO2.

methanogenesis: CO2 + 4H2 = CH4 + 2H2ORESEARCH ARTICLE◥

PLANETARY GEOLOGY

Cassini finds molecular hydrogen inthe Enceladus plume: Evidence forhydrothermal processesJ. Hunter Waite,1* Christopher R. Glein,1* Rebecca S. Perryman,1 Ben D. Teolis,1

Brian A. Magee,1 Greg Miller,1 Jacob Grimes,1 Mark E. Perry,2 Kelly E. Miller,1

Alexis Bouquet,1 Jonathan I. Lunine,3 Tim Brockwell,1 Scott J. Bolton1

Saturn’s moon Enceladus has an ice-covered ocean; a plume of material erupts fromcracks in the ice. The plume contains chemical signatures of water-rock interactionbetween the ocean and a rocky core.We used the Ion Neutral Mass Spectrometer onboardthe Cassini spacecraft to detect molecular hydrogen in the plume. By using the instrument’sopen-source mode, background processes of hydrogen production in the instrument wereminimized and quantified, enabling the identification of a statistically significant signal ofhydrogen native to Enceladus. We find that the most plausible source of this hydrogen isongoing hydrothermal reactions of rock containing reduced minerals and organic materials.The relatively high hydrogen abundance in the plume signals thermodynamic disequilibriumthat favors the formation of methane from CO2 in Enceladus’ ocean.

Inhydrothermal systemsonEarth,water reactswith rocks containing reduced iron-bearingminerals to producemolecular hydrogen (1, 2).Reduced ironacts as anoxygen sink, providingsufficient reduction potential to drive the con-

version of some H2O to H2. Because of rapid con-vective transport of fluids in thesedynamic systems,hydrothermally derived H2 is far from chemicalequilibrium when it mixes with oxidants in thecooler surrounding environment [e.g., seawater(3, 4)]. This state of disequilibrium is exploited bysome forms of life (chemolithotrophs) as a sourceof chemical energy.Oneexample ismicroorganismsthat obtainenergybyusingH2 toproduceCH4 fromCO2 in a process calledmethanogenesis. SuchH2-based metabolisms are used by some of the mostphylogenetically ancient forms of life on Earth (5).On themodernEarth, geochemically derived fuelssuch asH2 support thriving ecosystems (6–8) evenin the absence of sunlight.Previous flybys of the saturnian satellite En-

celadusby theCassini spacecraft providedevidencefor a global subsurface ocean residing above a coreof rocky material (9–12). The inference of warmwater cycling through silicates at the base of thisocean (13) raises the issue of whether this geolog-ically active moon of Saturn—which ejects gasesand ice grains through a system of fractures to

form a plume (14)—might have active hydrother-mal systems.Molecular hydrogenwould be prod-uced during hydrothermal alteration of reducedchondritic rock and could be observable in theplume gas (15, 16). Hence, H2may serve as amark-erofhydrothermalprocesses, althoughother sourcesof H2 (e.g., ice radiolysis) must be considered be-fore a hydrothermal origin can be deduced. Thepresence of H2 in the plume of Enceladus couldtherefore suggest the occurrence of temperaturesand chemical energy sources necessary for habit-able conditions in the moon’s interior (17).For the final Cassini in situ flyby of Enceladus

(designated E21), theOpen SourceNeutral Beam-ing (OSNB) mode of the Ion Neutral Mass Spec-trometer [INMS (18)]wasused to search for nativeH2 in the plume. The open source is a direct inletinto the mass spectrometer that minimizes gasinteraction with the walls of the instrument be-fore analysis in the quadrupolemass spectrometer.OSNB mode ameliorates the issue of hydrogenproduction inside the instrument from water-titanium interactions,whichoccurswhen the alter-native Closed Source Neutral (CSN)mode is used(19). The use of OSNBmode during E21 permits amore straightforward interpretation of the dataregarding the presence of H2 at Enceladus.

Final observations and analysis ofplume gas

All close Cassini Enceladus flybys are designatedaccording to their order of occurrence (E1, E2, etc.).INMS performed measurements of the Enceladusplume during eight flybys: the south-to-north dis-covery flybyE2 [year 2005–day 195 (14)]; thenorth-to-south E3 (2008-072) and E5 (2008-283) flybys(19), which flew close to the plume axis outbound

fromEnceladus; a series of low-altitude plume tra-versals, E7 (2009-306), E14 (2011-274), E17 (2012-087), and E18 (2012-105); and most recently E21(2015-301),whichwas thedeepest observationwith-in the plume at a closest approach of 49 km fromthe surface.During the E21 flyby, Cassini flew almost per-

pendicular to the Enceladus tiger stripes at a rela-tive speedof 8.5 kms−1. The INMSsensor alternatedbetween two different modes of operation. CSNmode increases the total signal by collecting andthermally equilibrating (“thermalizing”) gas in a ti-taniumantechamberprior to ionization,massselec-tion, and detection. OSNBmode directly samplesambient gas, ionizing the neutral beam as it trav-els through the instrument without striking thewalls. The use of OSNBmode during the INMS ob-servations of E21 on 28 October 2015 enabled thedetection and quantification of H2 in the plume.OSNBmode has a set of deflector elements that

prevent ion entry, as well as a velocity filter thataccepts incoming neutral molecules over a nar-row but adjustable range of angles and energiesafter they are ionized in the ion source. Althoughit has only 0.25% of the sensitivity of CSN mode,OSNBmodeminimizes themeasurement ofmol-ecules that are generated by surface interactionson the walls of the CSN antechamber. In OSNBmode, neutralmolecules are ionized and analyzedwithout contacting instrumental surfaces (18).The velocity vector of the molecules on arrival

at the OSNB aperture determines their apparentenergy in relation to the spacecraft. During E21,the electrostatic velocity filter was continuously ad-justed via sawtooth scans of ±2 km s−1 in ampli-tude to characterize the velocity distribution of theplume gas and consequently to accept moleculesarriving from the direction of Enceladus’ surface(fig. S9). In addition to OSNBmeasurements, CSNdata were acquired to determine whether therewere any major changes in plume compositionrelative to the earlier E14, E17, and E18 flybys.The E21 INMS data are shown in Fig. 1. A large

number of mass 2 counts were detected in OSNBmode, potentially indicating the presence ofH2 inthe plume. However, background (instrumental)sources of mass 2 counts must be considered to de-termine whether native H2 is present.We investigated sources of background [(20),

section 1] and found that the main source of back-ground is leakage of thermalized background H2Ogas from the rest of the instrument into the opensource [fig. S4; see also (21)]. Other backgroundsources include thermal leakage of H2 from therest of the instrument into the open source, disso-ciative ionization of the incomingH2Omolecularbeam in the open source, radiation noise in thedetector, and leakage of ions from the closed sourcethrough the quadrupole switching lens.We quan-tified the background sources by applying calibra-tiondata from literatureand laboratory experimentswith the INMSengineering model to the observedmass 18 OSNB counts and to themass 2 andmass18 CSN counts [(20), section 1].The estimatedbackground is plottedwith the raw

mass 2 data in Fig. 2. The >1s difference betweenthe observed counts and the total background

RESEARCH

Waite et al., Science 356, 155–159 (2017) 14 April 2017 1 of 5

1Space Science and Engineering Division, SouthwestResearch Institute, 6220 Culebra Road, San Antonio, TX78238, USA. 2Applied Physics Laboratory, Johns HopkinsUniversity, Laurel, MD 20723, USA. 3Department ofAstronomy and Carl Sagan Institute, Cornell University,Ithaca, NY 14853, USA.*Corresponding author. Email: [email protected] (J.H.W.);[email protected] (C.R.G.)

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Potential pathway for microbial metabolism (i.e., methanogens), but reaction may proceed in absence of biology.


Orientation of ~98 geysers based on triangulation.‘Hot spots’ are observed in these locations, but too small to be from tidal heating and they are on the walls of the fractures....the geysers cause the heat signature observed at the surface, not the other way around! (but tidal forcing still responsible for interior heating and extension)

25Enceladus: Geysers

However, it is now believed that the geysers are not simple point sources.

Instead, they appear to be more like curtains that have supersonic jets at certain locations.

Models need to be able to explain how the eruptions can occur along the entire length of a fissure and not just at local points.


A possible model suggests underground reservoirs of pressurized liquid water above 273 degrees Kelvin could fuel geysers that send jets of icy material into the air above the moon's south pole. A vent to the surface pierces one of the "tiger stripe" fractures seen in the southern polar terrain.

Some combination of internal radioactive decay, salts, and flexing -- perhaps concentrated within the tiger stripe fractures and brought about by the particular characteristics of Enceladus' orbit--is implicated as the source of the heat creating the liquid reservoirs.

However, it is not yet clear how the deep interior of Enceladus functions, nor whether the moon is fully differentiated.

Why Don’t the Cracks Freeze Over? 28

depth Z = 35 km, stress-free half-widthW0, and spacing S = 35 km.Slots are connected to vacuum at the top, and open to an ocean atthe bottom (Fig. 1 and SI Appendix). Subject to extensional slot-normal tidal stress σn = (5 ± 2) × 104 sinð2πt=pÞ Pa modified byelastic interactions between slots, the water table initially falls,water is drawn into the slots from the ocean (which is modeled asa constant-pressure bath), and the slots widen (Fig. 2). Wider slotsallow stronger eruptions because the flow is supersonic andchoked (17). Later in the tidal cycle, the water table rises, water isflushed from the slots to the ocean, the slots narrow, and eruptionsdiminish (but never cease) (Fig. 2). Solving the coupled equationsfor elastic deformation of the icy shell with turbulent flow of waterwithin the tiger stripes allow us to computeW ðtÞ (SI Appendix).W0 >2.5-m slots oscillate in phase with σn, W0 < 0.5-m slots lag σn by∼ π=2 rad, and resonant slots (W0 ∼ 1 m, tidal quality factor ∼1)lag σn by ∼1 rad (SI Appendix, Fig. S4). The net liquid flowfeeding the eruptions (< 10 μm/s) is much smaller than the peaktidally pumped vertical flow (approximately ±1 m/s for W0 ∼ 1 m,Re > 105). Although the amplitude of the cycle in water tableheight is reduced when the slot is hydrologically connected to theocean relative to a hypothetical situation where the slot is iso-lated from the ocean, the flow velocity, driven by the deviation ofthe water table from its equilibrium elevation, is very much largerthan in the hydrologically isolated case.Turbulent liquid water flow into and out of the slots generates

heat. Water temperature is homogenized by turbulent mixing,allowing turbulent dissipation to balance water table losses andprevent icing over. Ice forming at the water table is disrupted byaperture variations and vertical pumping; water cooled by evap-oration, if sufficiently saline, will sink and be replaced by warmerwater from below. A long-lived slot must satisfy the heat demandsof evaporitic cooling at the water table (about 1.1× the observedIR emission; SI Appendix) plus heating and melt-back of ice driveninto the slot (18) by the pressure gradient between the ice and thewater in the slot (19) (SI Appendix). Turbulent dissipation canbalance this demand forW0 = (1± 0.5) m, corresponding to phaselags of 0.5–1 rad, consistent with observations. Eruptions arethen strongly tidally variable but sustained over the tidal cycle,also matching observations. W0 < 0.5-m slots freeze shut, andW0 > 2.5-m slots would narrow. Power output is sensitive to

the amplitude k of conduit roughness, which is poorly constrainedfor within-ice conduits. For the calculations in this paper, we usek = 0.01 m; for discussion, see SI Appendix. Near-surfaceapertures ∼10 m wide are suggested by modeling of high-temperature emission (19), consistent with near-surface ventflaring (20). Rectification by choke points (18) [which are re-quired to explain the absence of sodium in the gas plume (21)],together with condensation on slot walls, and ballistic fallback(6) could plausibly amplify the less than twofold slot-widthvariations in our model to the fivefold variations in the flux ofice escaping Enceladus. Water’s low viscosity slows the feedbackthat causes the fissure-to-pipe transition for silicate eruptions onEarth (22, 23), which is suppressed for Enceladus by along-slotmixing (SI Appendix).The mass and heat fluxes associated with long-lived slots (24)

would drive regional tectonics (SI Appendix). Slow inflow of iceinto the slot (25) occurs predominantly near the base of the shell,where ice is warm and soft. Inflowing ice causes necking of theslot, which locally intensifies dissipation until inflow is balancedby melt-back. Melt-back losses near the base of the shell causecolder ice from higher in the ice shell to subside. Because sub-sidence is fast relative to conductive warming timescales, sub-sidence of cold more-viscous ice is a negative feedback on theinflow rate. This negative feedback adjusts the flux of ice con-sumed by melt-back near the base of the shell to balance the fluxof subsiding ice (SI Appendix, Fig. S6), which in turn is equal tothe mass added by condensation of ice from the vapor phaseabove the water table (SI Appendix, Fig. S6). The steady-stateflux of ice removed from the upper ice shell via subsidence andremelting at depth depends on Z, S, moon gravity, and the ma-terial properties of ice. Using an approximate model of ice shellthermal structure, this steady-state flux is approximately pro-portional to Z in the range 20 km <Z < 60 km (SI Appendix, Fig.S8) and is ∼3 ton/s (7 mm/y subsidence, Pe ≈ 6) for Z = 35 km.This long-term value for ice removal is comparable to the inferredpost-2005 rate of ice addition to the upper ice shell, 2 ton/s[assuming the observed 4.4 ± 0.2 GW cooling of the surface isbalanced by recondensation of water vapor on the walls of thetiger stripes above the water table (5, 11, 23, 26)]. If near-surfacecondensates are distributed evenly across the surface of the tiger

Fig. 1. The erupted flux from Enceladus (blue arrows) varies on diurnaltimescales, which we attribute to daily flexing (dashed lines) of the source fis-sures by Saturn tidal stresses (horizontal arrows). Such flexing would also drivevertical flow in slots underneath the source fissures (vertical black arrow), whichthrough viscous dissipation generates heat. This heat helps to maintain the slotsagainst freezeout despite strong evaporitic cooling by vapor escaping from thewater table (downward-pointing triangle). The vapor ultimately provides heat(via condensation) for the envelope of warm surface material bracketing thetiger stripes (orange arrows; “IR” corresponds to infrared cooling from thiswarm material).

Fig. 2. Tidal flexing cycle for interacting slots assuming two inboard (ib) andtwo outboard (ob) slots, E = 6 GPa, and L = 100 km. Slot half-widthW0 = 1 m,wall roughness k = 0.01 m. ΔWm is maximum width change, ΔV=V is frac-tional change in slot water volume, and σn is extensional stress to 90% of itsown peak amplitude.

2 of 4 | www.pnas.org/cgi/doi/10.1073/pnas.1520507113 Kite and Rubin

[Kite & Rubin, 2016]

Tiger stripes could represent slots that connect the surface to the underlying ocean.

Turbulent flow in the water dissipates and releases heat, preventing freeze-over.

Condensation of vapor onto slot walls releases heat (to explain observed thermal signature). Is this model more consistent with ‘curtain’ plumes than crack models?

Ocean Worlds 29

MoonName,Planet

Geophysically & Geochemically

Plausible?

Significant tidal energy to helpmaintain ocean?

Induced Magnetic

Field?

Activity Observed?

Ocean in contact with

rock?

Europa,Jupiter Yes Yes Yes No Yes

Ganymede, Jupiter Yes ~Yes No No No

Callisto,Jupiter Yes No Yes No No

Enceladus, Saturn ??? ??? ??? Yes! Yes?

Titan,Saturn Yes No ??? ??? No

Triton,Neptune Yes? No ??? Yes No

enceladus - brown university · cassini finds molecular hydrogen in the enceladus plume: evidence...

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