Triton's eruptions analogous to comet Halley's?

Adv. Space Res. Vol.12, No.11, pp.(l1)133—(1 1)138, 1992 Printed in Great Britain. All rights reserved.

0273-1177/92 $15.00 Copyright © 1992 COSPAR

TRITON’S ERUPTIONS ANALOGOUS TO COMET HALLEY’S? Max K. Wallis and N. Chandra Wickrainasinghe School of Mathematics, University of Wales, Cardiff CF2 4AG, U.K.

ABSTRACT We hypothesise that Triton’s “geysers” are analogous to the remnant activity of comet Halley, whose outbursts producing a particulate coma are persisting out beyond 12 A.U. Halley’s outbursts are understood as resulting from inward freezing of a subsurface lake or sea that’s maintained at >10 m depth via metabolic energy release. Cracking of the ice due to thermal expansion forces generates sporadic emissions of H20 and other gases. Analogous activity on Triton is a manifestation of internal freezing of an interior sea under a km or so of ice. This sea at the present epoch is quite separate from the interior liquid ocean, expected to be maintained at > 100 km deep by radiogenic heating. The near-surface sea is maintained also by chemical or metabolic energy and sporadically emits gases and condensates through cracks induced during its freezing. Emissions through the ice cover on both comets and Triton would drive surface geology, thus enabling access to new nutrients and trace elements that may be vital for subterranean biology. Triton’s biology would be a relic from its tidal heating phase, when the interior was liquid below a 1-2 km frozen lithosphere.* 1.

INTRODUCTION

The “volcanos” on Triton appear to be either vents or sublimating areas just a few km across. This eruptive phenomenon was quite unexpected, particularly once it was recognized that Triton’s surface is completely frozen. The discovery that the surface is substantially colder than predicted, at 38 ~ 301, is consistent with the unexpectedly high albedos A~t~ = 0.8-0.9, in that most of the incoming solar radiation is reflected. On the one hand, Triton is likely to be a differentiated body with a silicate core containing some 2/3 of the total mass and radiogenic heat sources; it shows resurfacing probably due to water and ice flow in its past /1/. On the other hand, the geysers are too small and present in too a limited region to play a significant role in releasing internal heat, in the manner of the huge Olympus Ions volcano on lars. The geysers arise from dark patches, not associated with linear faults or ridges, and contain fine “dust” that drops to the surface up to 150 kin downwind. We can draw analogies with Halley’s comet, which is also unexpectedly venting gases and frozen particles far from the sun. On the icy-conglomerate model, it had been assumed that ices of elementary gases underlie such activity. During its perihelion passage, however, Halley showed little CL., NH3 and N2 (and these fractions 1% may have been disintegration products). CO2 seems to have surged on at least one occasion /2/ while the CO was partly if not entirely produced from degrading organics /3,4/. The one possible “ice”, C02, is relatively volatile enough to explain ilalley’s activity on its inbound orbit from about 9AU /5/, though perhaps not at 9-12 AU where brightness variations were first detected /6/. Joreover, at 12 AU on the outbound orbit, Halley was still showing irregular production of a particle coma /7/ by a process presumed similar to that causing the preperihelion variations. CO is the only elementary ice possibly present that could provide an explanation on the conventional model. Triton, in contrast, at 30 All and inside Halley’s 35 AU aphelion, has no CO but abundant N2, of similar sublimation properties. CH4 was detected at only 10~of the 16 pbar N2 atmosphere, implying little input from the venting process. Both Triton and Halley have abundant 1120. We can pose the question: are there similar mechanisms and energy sources that can act in both cases? We presume comets were formed in the Neptune-Uranus region, being remnants after accretion of these planets, and so would originally share the same composition. They were ejected by perturbations of Neptune and Uranus out to an inner Kuiper cloud of comets, biased towards low inclinations. Some of these objects are perturbed inwards stochastically, to give the short-period comets /8/. Satellite systems were formed with *Univ. of Wales, College of Cardiff, Atmospheric & Space Science preprint No. 6. (11)133

M. K. Walls and N. C. Wickramasinghe

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TABLE 1 Structural characteristics of Triton and Halley Triton

Halley

Heliocentric r

30 AU

lean radius

1350 km

6 km

lain composition

lIaO

(outer shell) mineral (interior) complex organics(?) N2 (surface ice) little CH4, CO

1120

Surface (Bond) albedo

0.8

0.04

Differentiation mechanisms

radiogenic heating via ~°Ketc. tidal (early capture phase)

radiogenic heating, possibly strong via 26At

Present energy sources

radiogenic, tidal? solar, chemical?

solar;

-

35 AU

C,11,O,N complex organics mineral grains C02, CO? little CH4, NH3, N2

0.95

chemical/metabolic

the giant planets as their gas/dust condensed. But outer and retrograde satellites are later captures /9/ probably via collisions with small proto-satellites. Triton is retrograde, but in a circular well-bound orbit; presumed to have evolved via tidal dissipation. A solid state greenhouse effect was originally proposed to explain cometary activity far from the sun /10/. It depends on the well known fact that 1120-ice transmits visible but blocks infrared radiation, and has since been detailed for ice or snow surfaced satellites /11/. However, 1120 freezes too deep around Halley’s perihelion passage for the greenhouse mechanism to maintain potency. And ilalley lacks the N2 or CO that might be sufficiently near the liquid state at comparable distances to Triton. 2.

ENERGY SOURCES AND INTERNAL STRUCTURE OF TRITON

The sublimation properties of the N2-C114 mixture had been postulated as a conventional energy source, drawing on sunlight. However, this would hardly explain the small scale geysers and would not work on comets, that lack the N2 and cannot retain an atmosphere. Tidal heating was thought to be significant /12,13/ because of the orbit’s 20°obliquity. Radiogenic sources are also likely in the 1000 km rocky core. We model these following Whipple & Stefanik /14/ and Wallis /15/. Adopting the view that Triton has a siliceous core of a 1000 km radius, as inferred /1/ from the mean density of 2.07 g/cm3 with a share of radioactive elements as found in chondritic meteorites, the heating rate in the present epoch is about S = 5.0 x 10’5 W/g (Table 2). The total heat content and current decay rate are E Sj/.lj

=

690 j/g, E S~/E.~jSi = 2.4

x

10~yr

indicating that latent heat available through freezing a significant fraction of Triton (e.g. 1120 at 340 J/g) on a ion yr time scale would supplement S, but not importantly on longer time scales. TABLE 2 Parameters* of assumed radioactive constituents /after 14/. SJ 238U

0.55 x 10~yr-I

1.011 x 10~ V/g

115.5 J/g

232Th

0.05 0.972

0.044 1.138

2260 459

0.154

2.844

6840

*Decay rates content

Qj

=

current heating rates S~ in the mineral silicate core and original heat

~

J~S~exp (-Ait) dt T

=

S~ exp (-A~T)/~~ for T

=

-

4.6

x 10~ yr.

Triton’s Eruptions

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Below a diurnally heated surface skin of a few cm /i6/, we can therefore seek quasi-stationary solutions to the spherical heat conduction equation pc~+

-

pSH(c-r)

jjr2a~=-

(1)

where the step function H describes sources restricted to the core r
Time

independence allows integration to satisfy the heat flux condition at r=c: pSr2dr = pSc3 c2a[dT/dr]r~c = -

-

~

and solution outside the core between radii a and b

jrTb

a(T)dT

=

j- pSc3(a-b)/ab

(2)

Ta

Because the core is compressed, we take an elevated p = 3.83 g/cm3 to satisfy the 757. silicate, c = 1000 km model of Smith et al. /1/: this gives the numerical values ~pSc2= 64 V/cm, ~.pSc3/a2 = 3.50

x

1O~V/cm2

(3)

The second figure shows that the radiogenic heat is about 57. of the mean absorbed solar radiation ~(1-A)F® , so induces a 0.501 higher surface temperature above cases without internal heat sources. If the mantle primarily comprises crystalline ice with high conductivity between the suface and melting radius b, with a = 5.67 T-1 V/cm /17/, rT b

,c(T)dT

=

5.67 ln (273/38.5)

=

11.1

V/cm

.

(4)

a

Formula (2) then gives its thickness a-b

=

(11.1/64)a/[c

+

(11.1/64)aJ

250 km.

=

The intermediate layer r=b to r=c would be liquid and convecting, as believed is the case in larger icy satellites like Europa /18/. The outer region could be powdered regolith or amorphous ice with lower conductivity /17/ but only at T < 1400K. There could be covering layers of low conducting N 2- ice (a = 1.2 x 10~V/cmoK) but only below its melting point T < 600K (note: heat transport via the vapour would raise the effective a at T 5501). At most, the thermal resistivity of the crystalline ice layer could be reduced to 5.67 in (273/140)

=

3.79 V/cm

with the other contributions from amorphous ice and frozen nitrogen 40(2.09T + 25.3)dT = 2.6 x 10~+ 0.188 V/cm edT = J60 38~5 1.2 x 10~dT+ 105f 60

J

(5) (6)

The corresponding thicknesses using formula (2) are 74m of N 2, 5.4 km amorphous ice and 105 km crystalline ice. Evidently only thin deposits of N2-ice are feasible; in practice, however, any deposits are mobile and uneven. Neglecting its small effect and noting that the surface is uneven to the 1 km scale, we would expect the average amorphous-ice contribution to be less than the 4 km. The presence of cliffs /1/ suggests refrozen crystalline ice and little regolith of one-time powdered ice over parts of the surface. On the other hand, a substantial fraction of organics would lower the conductivity compared with pure crystalline ice. Also km-thick re~olith arising from geyser particles and atmospheric precipitation (sect. 3) might provide a better insulating cover in some regions. It is thought that tidal forces were important at an early stage of Triton’s history, after its capture by Neptune and before its rotation became synchronous. Scaling the estimate of Chyba et al. /19/ to the smaller radius now known, the heating rate in “Cassini state 2” was 0.24 x 1013 V, compared with the present “Cassini state 1” rate of 108V. The larger value is 100 times larger than present radiogenic so implies an outer frozen shell of only 1-2 km. Early radiogenic heating was (Table 2j some 15 times larger than present, implying a 10 km frozen-ice shell. Rupture of the shell during the

(11)136

M. K. Wauis and N. C. Wickramasinghe

strong tide epoch, with outflows of water and substantial resurfacing some 2 Ga ago thus appears very plausible. On this viewpoint, we expect much of the ice to be crystalline and the ice shell to be still refeeezing inwards. Time-dependent corrections via equation (1) imply a 10-207. change, with an inner T = 2730K radius b 200 km.

The structure of Triton is thus very similar to that hypothesised and elaborated for Europa /18,20/ with a silicate core, liquid water layer and ice lithosphere. Unlike Europa, however, tidal flexure and heating is currently minor on Triton. Its 1-200 km ice lithosphere is expected to be unstable against solid convection via creep of at least the warmer inner parts /21/. Over and above this are the huge expansion forces generated as the water freezes inwards. Satellite-scale creep must extend right to the surface and cracks would develop if the cold outer layers cannot creep sufficiently. This driving force was hypothesised to operate in a comet /10/ but would not apply to a non-compact structure now conceived for those bodies. In Triton, it drives tectonic circulation and produces a lithosphere of variable thickness. While cracks may well have penetrated to the surface and water gushed out during the early tidal epoch, the morphology and crater counts imply this is no longer happening. Under the solid convection and variable lithosphere thickness, heat would be transported out faster and the mean thickness should be larger. The enhanced transport would speed up the freezing of the water down to the core /21/. On the other hand, no model nor time-scale for maximum rates of physical creep have been given. 3.

REMNANT BIOLOGY

In Triton’s earlier tidal phase, with the frozen lithosphere only a km thick (section 2) and episodic fracturing events allowing water to gush to the surface, there were presumably many opportunities for primitive biology to flourish. Circulation of both the atmosphere and the ocean created a variety of habitats with variable “weather”. A range of species and ecologies could develop analogous to those hypothesised for Europa /22/ and comets /23,24/, supposing initial seeding by genetic material, probably in spores. With light levels very low on Triton, life-forms using chemical energy (chemotrophs) are more likely than photosynthetic ones. A range of archaebacteria and diatoms are plausible /25/. Mineral nutrients may be available from fissures in the core, but little would reach 350 km higher in the surface layers. The initial organic material would be the main source of chemical energy. Such subterranean ecologies can be long-lived as evidenced by that discovered in the organic-rich deposits 6 km deep below the Siljan crater in Sweden /26/. In the present epoch as Triton freezes inwards, any deep ocean biology dependent on the core-ocean interface is largely disconnected from the near surface regions where biology is largely in deep-freeze. Transport of nutrients from the ocean and ultimately the core via solid-ice convection is extremely slow. As in comets, metabolic heat can be the major source for keeping subsurface seas from freezing. Potentially there’s about 0.6 eV per C-bond or 3 kJ/g available in organic material /10/ and, allowing for the 1120, some 750 J/g average, so twice the latent heat of fusion. The upward heat flux from a sea at depth d is d1 x (4-11) V/cm (7) according to whether there’s pure crystalline ice or an insulating regolith (3-5). The downward heat flux could be of similar magnitude. For example a km-deep sea having metabolic energy 7.5 x 10~J/cm2 would lose that in about 10~yr. The reason for survival of such a sea so close to the surface could be the availability of nutrients. Jeteoritic dust and hydrocarbons photolysed in the atmosphere /27,28/ are raining onto the surface and becoming buried, perhaps at the current rate of a few cm/yr sublimation flux /29/. The deficit of impact craters on parts of the surface indicates that such nutrients can be transported down to depths of order of the topography scale (1 km) perhaps over 10~or 106 yr. There are presumably certain elements (perhaps P) in very short supply, that once discovered by the remnant subsurface biology allows its rapid development with release of the metabolic heat. It appears however that the sea migrates laterally into new nutrient sources aided by the cracks and vents produced as exhausted regions freeze inwards. In the absence of oxygen, the biology is probably methanogenic as on the early Earth. The methane generated as gas needs to be vented and the geysers or plumes are manifestations of such releases. From the distribution of mostly inactive vents, one 2. The absence of linear would infer the area of subsurface sea of about x 10~km arrangements implies thatthetectonic (convective ice) 3motions are not significant in weakening the uppermost lithosphere. The venting can to some degree be self-sustaining, via the insulating effects of the emitted snow. With 1O~g/s /29/ from a vent falling back as methane snow over some 2000 km2, we would expect metres per (terrestrial) yr. Despite much sublimation, as methane is far undersaturated in the atmosphere, a few 7. of 1120- ice and organic material entrained in the plume and snow would accumulate and bury -

Triton’s Eruptions

(11)137

some methane over the 100 yr Triton winter. Its insulating effect reduces the heat loss locally and enables liquid water with its biologic heat source to penetrate locally and temporarily much closer to the surface than our postulated 1 km deep sea, thus tapping the store of nutrients perhaps as high as lOOm below the surface. For comet Halley, the structural dimensions are smaller and seasonal variations over its 76-yr orbital period much more ,ext,reme. The vents seen by Giotto /30/ were of 1 km scale diameter and depth but the base area of exposed ice is perhaps only lOOm diameter. It may be frosted but under cometary microgravity there would be little snow cover, so 2 through heat losses the upper figure inWe (10) are the expected, viz.depth 100 V/rn iOn deep ice,corresponding or around 2 to x 10152 per orbit. choose lOm scale as comparable to the several metre ice thickness on sublimation-dominated Antarctic lakes. The heat loss is equivalent to the latent heat of 7m depth of a 1 km diameter sea. So without a metabolic heat source, the sea would freeze inwards by about 3m per orbit (under microgravity, we assume sufficient stirring from rotational forces). During passage through the inner solar system, material sublimates from the sides of an erosion crater and of the ice at its base. Photosynthesis may occur under the ice with the weak sunlight that penetrates the lOm thickness. Larger fragments of the organic crust, cooked in the sun, accumulate as scree in the base of the crater. Methane generated by bacteria active in the sea may vent through the thinned and weakened ice. Diurnal insolation changes induce cracks due to thermal expansion strains. But on the passage through the outer solar system as the sea freezes, the expansion forces crack open the surrounding ice, principally in the vent region. Not only methane but also water would gush forth in the base of the vent but quickly freeze and bury the scree. It would give a burst of cometary ice grains, principly of 1120. The biological significance lies in the burial of the processed organic crust, which after a sequence of outgushings reaches the bottom of the ice plug and is available as nutrients to the sea biology. Metabolic heat, presumably, keeps the ice from thickening in this region. The existence of seas on comets seems very precarious. While liquid water might have existed in an early post-accretion phase, for larger comets under 26A1 radiogenic heating /15/, the interior sea and any biology within it would have long since frozen-in. Chance collision with an interplanetary boulder might, however initiate a new sea, bringing both heat energy and nutrients below the cornet surface. Metabolic energy would melt further ice and keep it liquid for a few thousand years on Halley. The persistence of water on Triton appears to draw on “Gaian” mechanisms /32/, whereby biology modifies the environment to better suit itself. The emission of methane and precipitation as snow provides insulating material around the vents to help biology tap the near-surface nutrients; it also provides insulating cover regionally over the sea, as indicated by the large number of currently quiescent vents. On the global level, the methane emissions replenish the atmosphere for photolytic production of hydrocarbons /27/ which precipitate and are buried to provide nutrients for further generations. REFERENCES 1.

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