THE CONTRIBUTIONS OF COMETS TO PLANETS,

ATMOSPHERES, AND LIFE: Insights from Cassini-

Huygens, Galileo, Giotto, and Inner Planet Missions

Tobias Owen1

1 University of Hawaii Institute for Astronomy Honolulu, HI 96822 USA(owen@ifa.hawaii.edu)

Received ; accepted

Abstract.

Comets belong to a group of small bodies generally known as icy planetesimals. Todaythe most primitive icy planetesimals are the Kuiper Belt objects (KBOs) occupying aroughly planar domain beyond Neptune. KBOs may be scattered inward, allowing themto collide with planets. Others may move outward, some all the way into the Oort cloud.This is a spherical distribution of comet nuclei at a mean distance of ∼50,000 AU. Thesenuclei are occasionally perturbed into orbits that intersect the paths of the planets, againallowing collisions. The composition of the atmosphere of Jupiter—and thus possibly allouter planets—shows the effects of massive early contributions from extremely primitiveicy bodies that must have been close relatives of the KBOs. Titan may itself have acomposition similar to that of Oort cloud comets. The origin and early evolution of itsatmosphere invites comparison with that of the early Earth. Impacts of comets must havebrought water and other volatile compounds to the Earth and the other inner planets,contributing to the reservoir of key ingredients for the origin of life. The magnitude of thesecontributions remains unknown but should be accessible to measurements by instrumentson spacecraft.

Keywords: comets, collisions, origins: solar system, icy planetesimals

1. Introduction

The possibility that comets may have played an important role in the devel-opment of planets was independently proposed by Edmond Halley and IsaacNewton in the late 17th century (Genuth 1997). Halley thought a collisionwith a comet would have caused a chaos on Earth that could have producedthe Biblical flood. Newton suggested that the vapors from comet tails wouldbe collected by planetary atmospheres, thereby delivering water that wouldsustain terrestrial life. Considering that no one knew the composition orstructure of comets at that time, these were bold conjectures indeed!

The steady increase in knowledge about comets made these hypotheseseven more attractive, but not because they might explain the need for Noah’sark! The discovery of the radicals CN, C2, and C3 in the spectra of comets im-plied the presence of parent molecules that could be simple or even complexorganics. This chain of reasoning in turn led to the hypothesis that impacts

c© 2008 Springer Science + Business Media. Printed in the USA.

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2 Owen

WHAT WE WISH FOR:

ISM

|

SOLAR NEBULA

|

COMETS

|

EARTH

|

US!!

Figure 1. The simple track we would prefer.

by comets could have aided or even initiated the pre-biological chemistrythat ultimately resulted in the origin of life on Earth. In fact, similar impactsmust have occurred on Venus and Mars, thereby providing them with thesame chemicals. This idea represents a modern form of Newton’s hypothesis.More recently, we have further broadened the possible influences of cometsby suggesting that the accretion of these objects may have been responsiblefor the formation of giant planet cores, thereby enabling the formation ofthe planets themselves. The accretional impacts thus represent a modernversion of Halley’s idea.

2. Comets As Icy Planetesimals

An idealized pathway from the interstellar medium to the origin of life isgiven in Figure 1. The current conception of the actual situation for cometarydeliveries appears in Figure 2. This figure is still idealized, but closer toreality.

It is evident from this figure that there are multiple opportunities for alltypes of comets to collide with planets and satellites. Some of these collisionscan contribute to the composition of these objects and their atmospheres.Others will simply make craters as they explode upon impact.

The common feature in all these suggestions is the use of comets to aid inthe explanation of some otherwise intractable yet fascinating problem. Butdoes any of this make sense?

Figure 2 shows that the spectacular comets with which people are mostfamiliar because of their bright comae and immensely long tails are actuallya subset of a class of icy planetesimals that have originated in different places

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Contributions of comets 3

ISMSolar Nebula

KBOsPlanetesimalsOort Cloud

Planets Subnebulae

Satellites Planetesimals

JFC

LPC

MBC

Figure 2. A still incomplete depiction of the actual situation!

in the early solar nebula. Ever since Whipple’s epochal study of the problem(Whipple 1950), it has become clear that a traditional comet is actually anice-rich small body called the nucleus that has wandered close enough to theSun to produce a coma and tail by sublimation. Millions if not billions ofthese icy nuclei form a spherical shell around the plane of the solar system ata mean distance of 50,000 AU. This shell is known as the Oort cloud, after itsdiscoverer. These nuclei are commonly thought to have been scattered fromtheir hypothesized place of origin among the giant planets. Another reservoirof icy objects extends outward in a disk beyond the orbit of Neptune. Thisis the Kuiper Belt, home of the Kuiper Belt objects (KBOs). The largestKBOs are far larger than any Oort cloud comets seen so far. They includePluto and often have satellites. Some KBOs could have been scattered intothe Oort cloud, from which they could occasionally visit the inner solarsystem.

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4 Owen

The comet most likely to fit this description is Comet Sarabat (1729).This is the brightest comet ever observed, reaching naked-eye visibility atits perihelion of 4.0 AU (Hurst 1997). Could this comet have been a KBO?Unfortunately, we will never know.

Icy planetesimals also originated in the subnebulae surrounding the giantplanets at the time they formed. Some of these accreted to form the icysatellites; others crashed into the forming planets, and still others couldhave been scattered into the Oort cloud.

Finally, several objects in the outer region of the main asteroid belthave recently been discovered to exhibit cometary characteristics (Hsiehand Jewitt 2006).

These different locations for the origins of icy planetesimals imply dif-ferent compositions as well, largely determined by the temperature of theenvironments in which they formed. Thus we can expect bodies rangingfrom nearly pure ice—formed by collisions among differentiated objects—toextremely primitive objects containing all the elements except He and Ne,formed at temperatures below 30 K. But ground-based observations have notyet found such variations with the possible exception of some differences inabundances of organic compounds (Mumma et al. 2007).

To understand these differences we need to start at the beginning—theinterstellar cloud fragment that collapsed to form the solar nebula. Here wefind that C/N ∼ 4, the solar value, but the disposition of these two cosmicallyabundant elements is very different. We can therefore use them to track theprocesses of comet formation. C is mostly (>50%) present in solid form—asamorphous carbon, macromolecular carbon, and frozen simple compoundssuch as CO2. In contrast, N is primarily (∼90%) in the form of a highlyvolatile gas—N2 or even atomic N. This means that it will be much easierfor a forming planetesimal to capture carbon than nitrogen, so we mayexpect the ratio C/N > 4 in these objects.

Given the possibilities of fractionation by atmospheric escape, formationof compounds locked in planetary interiors including sequestration in highdensity cores, C/N by itself is a rather blunt instrument. Nevertheless, it isat least a beginning, and indeed we find a significant depletion of nitrogen inOort cloud comets, exactly as predicted (Figure 3). Because N2 is commonlyassumed to have been the dominant form of nitrogen in the solar nebula,the inability of ice to trap this molecule at T > 30 K (Owen and Bar-Nun1995) has been identified as the probable cause of the nitrogen deficiency incomets. This conclusion obviously requires that Oort cloud comets scatteredfrom the region of the giant planets formed at T > 30 K. A discussion of thecompounds that carry the nitrogen in comets is given in the next section. Aswe shall see, the isotopes of nitrogen provide another way of distinguishingOort cloud comets from other icy planetesimals.

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Contributions of comets 5

Figure 3. Abundances in Halley’s Comet (Geiss 1988).

3. The Giant Planets

Having established what to expect for the composition of comets, we can seewhether the same value of C/N shows up in the atmospheres of the planets,testifying to cometary impact. In the case of the giant planets, it was widelyassumed that it would, since comets were considered to be the major sourceof the planets’ heavy elements (Pollack and Bodenheimer 1989).

The mass spectrometer on the Galileo probe revealed that this was def-initely not the case for Jupiter (Niemann et al. 1998). Instead both carbonand nitrogen were found to be enriched by the same amount compared withsolar values relative to hydrogen. Thus C/N was solar, and both C/H andN/H were 4 ± 2 × solar (Owen et al. 1999; Owen and Encrenaz 2006; seeFigure 4). Assuming these heavy elements were all delivered by solid bodies,these carriers must have been formed at very low temperatures ( T ≤ 30 K)

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6 Owen

7.0

6.0

5.0

4.0

3.0

2.0

1.0

Enrichment of Heavy Elements on Jupiter

Grevesse et al. 2007

Grevesse et al. 2007, Lanz et al. 2008, Lodders 2008

Xe Kr Ar S N C

Solar Abundance

Figure 4. The enrichment of heavy elements/hydrogen on Jupiter compared to solarabundances. Red circles, Grevesse et al. (2007); green square, from average of Lanz etal. (2008) and Lodders (2008). Error bars deleted for clarity.

to trap the N2. This holds true whether the gas was trapped as clathrates,by adsorption on amorphous ice grains, or simply condensed on grains.

Such low-temperature objects are very different from the comets we havestudied in the inner solar system so far, as these have all been deficient innitrogen (Figure 3; Cochran 2002). Owen and Encrenaz (2003) coined theterm “SCIPs” (solar composition icy planetesimals) to describe these low-temperature planetesimals. These objects are essentially identical to the typeIII comets postulated by Owen and Bar-Nun (1995). If the outer Kuiper Beltobjects were formed where we find them today, they may represent remnantSCIPs. We may yet observe one of these objects in the inner solar system ifindeed some of them were scattered out to the Oort cloud. They could thenbe perturbed into orbits that bring them in, just like the bright long-periodcomets that give such spectacular displays when they appear in our skies.This is the intriguing possibility offered by Comet Sarabat.

We can make one further test of this model if we can prove that thenitrogen on Jupiter arrived as N2. This would show that the planetesimals

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Contributions of comets 7

PRIMITIVE SECONDARY

NITROGEN IN THE SOLAR SYSTEM

(ATM)

TITAN

5.46

EARTH MARS (ALH84001)

VENUS

Escape (14N)

SUN [?]

(N in Solar Nebula)

JUPITER (N in Solar Nebula)

{GPMS}

1

2

3

4

5

6

7

15N/ 14N (X10–3)

Meibom et al(2007)

CN(Comets)

Figure 5. The ratio 15N/14N in the solar system. Note low value for Jupiter, formerlyconsidered to be the “solar” value and the high value for CN in comets (Owen et al. 2001;Arpigny et al. 2003).

that carried in the nitrogen must have formed at the low temperaturesrequired to trap N2. In this case, they would be SCIPs. To make this test,we have evaluated the nitrogen isotopes in Jupiter’s NH3, using the GalileoProbe Mass Spectrometer. NH3 is the major product of nitrogen chemistryin the planet’s accessible atmosphere, starting from N2.

The results of that determination are shown in Figure 5 (Owen et al.2001). Here we see the isotopic ratio 15N/14N in the atmospheres of Jupiter,

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8 Owen

Titan, and the inner planets. The Jovian value is clearly lower than theothers, exactly what we would expect if Jovian nitrogen were contributedby N2 (Owen and Bar-Nun 1995; Terzieva and Herbst 2000), while nitrogenon the inner planets came from condensed compounds such as NH3. Theatmospheres of Titan and Mars show the fractionation of isotopes that canoccur when bodies have sufficiently small masses that the lighter isotope canescape. Rocks from Mars also contain nitrogen that has not participated inthe escape process and reveals the same isotope ratio we find on Earth.There is further support that the Jovian value of 15N/14N represents thenitrogen that dominated the solar nebula from a measurement of these sameisotopes in the nitrogen in TiN found in a high-temperature CaAl inclusionof a meteorite (Meibom et al. 2007). The inclusion number agrees with theJovian value.

The nitrogen we find in comets—like that on Earth and other solidbodies—is predominantly derived from condensable nitrogen compounds,most probably primarily NH3, but including minor compounds such as HCN.The Oort cloud comets studied so far are distinctly different from SCIPs,which must include N2 as well as NH3.

Comets contain another type of nitrogen that is even more different fromthe solar nitrogen trapped in SCIPs. Arpigny et al. (2003) have discoveredand Jehin et al. (2004, 2006) and Cochran et al. (2007 and references therein)have confirmed in 15 comets, embracing all types, that 15N/14N in the CNradical in comets is consistently close to 7×10−3 (Figure 5). This and highervalues are seen in the organic components of cometary interplanetary dustparticles (Messenger 2000). Higher values of D/H are also found in thesematerials. Both of these isotopic anomalies point toward the interstellarmedium as their source. In comets, the CN radical must be derived from aminor component relative to NH3, while in the interplanetary dust particles,the NH3 is missing owing to its high volatility. Thus we can assume withsome confidence that any nitrogen delivered to solid bodies by comets—ortrapped in any ices forming at T > 35 K—will predominantly exhibit theratio of 15N/14N we find in the atmospheres of Earth and Venus and in rocksprotected from the atmosphere on Mars (Figure 5).

Adopting their hypothesis that SCIPs delivered heavy elements to allthe giant planets, Owen and Encrenaz (2003) predicted that the enrichmentof heavy elements on Saturn would follow the pattern on Jupiter. Unfor-tunately, only remotely detected carbon (in methane) could be evaluatedon Saturn because no atmospheric probe has yet been sent there to mea-sure additional species. The value of C/H for Saturn determined by the IRspectrometer on the Cassini orbiter indeed shows the value predicted bythe model we have just described (Flasar et al. 2005). It appears that themass of heavy elements delivered to Saturn is the same as that delivered toJupiter. The smaller mass of the planet then requires the enrichment to be

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Contributions of comets 9

correspondingly greater. This pattern is repeated for C/H on Uranus andNeptune, where the observed values of D/H also match the values predictedby the SCIP hypothesis (Owen and Encrenaz 2006).

The Jupiter results already indicate that SCIPs represent a distinct typeof comet nucleus, one formed at sufficiently low temperatures to capture allthe heavy elements. Are SCIPs required to explain abundances and isotoperatios on Saturn, Uranus, and Neptune? If so, this discovery would demon-strate that these low-temperature comet nuclei were the most abundant formof solid matter in the early solar system.

4. Titan

We turn now to a consideration of Titan, Saturn’s biggest satellite. It isslightly larger than the planet Mercury, has a surface temperature of 94 K,and a nitrogen-dominated atmosphere with a surface pressure of 1.5 bars.The density of Titan indicates that it is approximately half ice and half rockby mass, which is close to the solar ratio for these materials. Thus in bulkcomposition, this satellite is similar to a comet.

As determined by the mass spectrometer on the Huygens Probe, theatmosphere of Titan is 98% N2 and 1.6% methane, with a trace of 40Ar(Niemann et al. 2005). Here is an object that appears to violate our earlyparadigm of C/N ∼ 15 on small bodies. Yet Titan must have been builtfrom planetesimals containing more carbon than nitrogen, even if they wereSCIPs. So where is Titan’s carbon?

Earth also has an atmosphere in which nitrogen dominates carbon; N/C= 80/3.5 × 10−4. On Earth we know exactly where the missing carbon is.Most of it is buried as carbonate rocks such as huge deposits of limestone. Asmaller amount is buried organic material, the most well-known being coaland petroleum.

To produce the carbonates, it is necessary to have liquid water; the buriedorganics are contributed by long-dead organisms. Since neither of theseagents are available on Titan, we must look elsewhere. All the methanepresently in the atmosphere of Titan will be destroyed by photochemistryin 10–20 million years, so there must be an internal source that replenishesit.

At present, there are two competing hypotheses: delivery of carbon asmethane in clathrate hydrates that get trapped in the planet’s interior, anddelivery in the form of CO2, organics, and other forms of solid carbon thatare converted to methane in a two-step process (Niemann et al. 2005). Hy-drogen is first produced by a water-rock reaction known as serpentinizationand then reacts with the various forms of carbon in a kind of Fischer-Tropschreaction to produce methane.

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10 Owen

To see what Titan can tell us about cometary contributions, we can againconsult the atmosphere. Here we find from the fractionation of the nitrogenisotopes in N2 (Figure 5) that Titan must have produced ∼5× the amountof nitrogen we now find in the atmosphere. This tells us that the ice onTitan could not have been contributed by SCIPs, because N/Ar on Titanis ∼108 instead of 30, as it would be if the ice had trapped gases whilemaintaining solar abundances. In fact, to reach the observed ratio from aninitial mixture of NH3 and 36Ar, the temperature at which the ice formedmust have been close to 100 K, based on the laboratory work of Bar-Nunet al. (1988). This may be compared to the ∼50 K temperature at whichthe Oort cloud comets appear to have formed (Owen and Bar-Nun 1995).Evidently, the icy planetesimals that accreted to form Titan originated inSaturn’s “warm” subnebula rather than in the solar nebula as is postulatedfor the Oort cloud comets.

This conclusion can be tested eventually by examining D/H in Titan’sH2O in some future mission that will heat the surface of the satellite tovaporize some of the ice. Oort cloud comets have D/H = 3.1×10−4 (Balsigeret al. 1995; Eberhardt et al. 1995), whereas we would expect Titan’s ice tohave a lower value, representing isotope exchange with H in the ∼100 Ksubnebula.

5. Venus, Earth, and Mars

We now return to the question first posed by Newton and Halley: What roledid comets play in bringing water and other volatiles to Earth?

We can begin with an investigation of C/N in inner planet atmospheresto establish a context for the source(s) of water. We choose Venus as ourstandard, as this planet appears to have all its volatile carbon and nitro-gen in its atmosphere. The massive atmosphere consists of 96.5% CO2 and3.5% N2, with small amounts of Ar and other gases. The surface pressureis 90 bars. The ratio C/N in this atmosphere is then ∼15 (Donahue andPollack 1983), consistent with the value found in comets (cp. Section 2).However, this same ratio is also found in meteorites, so this is not a proofthat cometary bombardment produced the atmosphere of Venus, merely apositive consistency check. Any original water on Venus has been dissociatedwith the subsequent escape of hydrogen. We know this because of the hugefractionation of D/H, which is now 150 times the value in seawater on Earth(Donahue et al. 1982).

Thus there is no hope of using the residual water we find as vapor in theatmosphere to search for evidence of cometary bombardment.

However, a possible fingerprint left by one or more comets can be foundin the abundances of the noble gases (Figure 6). The ratio Ar/Kr = 103

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Contributions of comets 11

Figure 6. Noble gases in the atmospheres of Venus, Earth, and Mars. Kr and Xe on Venusstill poorly determined. Note similarity between Mars and Earth, and unusually high valueof 36Ar on Venus, giving Ar/Kr a nearly solar value.

is much closer to the solar value of 2 × 103 than to the value of 30 foundin Earth’s atmosphere. Recall that SCIPs are postulated to contain solarabundances. Furthermore, the abundance of Ar per gram of planetary rockis the highest found anywhere in the solar system. Both of these results couldbe explained by the impact of one or more SCIPs. Assuming that to be thecase, one can easily show that the amounts of C and N delivered with thenoble gases would be far too small to account for the CO2 and N2 presentlyfound in the atmosphere. Furthermore, if these elements were also deliveredby SCIPs, C/N would be solar instead of 15× solar, as observed.

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12 Owen

Evidently, there must be a noncometary source for the bulk of the planet’svolatiles. It could be either meteorite bombardment or outgassing of therocks making up most of the planet’s mass. In either case, some “seasoning”must be added to produce the noble gases, and SCIPs provide an attractivesource.

Turning to Earth, we may anticipate a similar story because Venus andEarth are such similar planets, located in the same part of the solar system.At first glance, the two atmospheres appear totally different. Earth’s atmo-sphere has a surface pressure of only 1 bar and is 78% N2, 21% O2, and1% of radiogenic Ar. As we saw in Section 4, however, the missing carbonis largely bound up in carbonate rocks and deposits of coal and petroleum.If this carbon were returned to the atmosphere as CO2, it would producesurface pressure of about 70 bars that would be 98% carbon dioxide. Thepresent 23% O2 would disappear in the absence of life. We would find a ratioof C/N = 20±10, again consistent with our scenario for the capture of thesetwo elements by planetesimals, but not conclusive evidence for cometarydelivery.

On the other hand, the noble gases in the atmosphere argue against themeteorites as a source. The ratio Kr/Xe is distinctly different from thatfound in meteorites. We don’t yet know whether this pattern also exists incomets as no noble gases have yet been detected in any icy planetesimals. Asearch for these gases is one of the prime objectives of the Rosetta missionto Comet 67P/Churyumov-Gerasimenko.

The abundances of the noble gases are important because they provide astandard that can be used to calculate the contribution of comets to all thevolatiles on Earth. The possibility that these volatiles may have includedcompounds that were important to the origin of life on Earth has beenproposed by Oro (1961) and developed in detail by Delsemme (1998, 2000).Unfortunately, it is not possible to search for complex organic compounds incomets by remote observations. The Rosetta Mission will help, but returnedsamples will probably be required to provide a definitive answer. However,in the specific case of water, we can search for evidence of cometary bom-bardment directly by measuring D/H in seawater and comparing it to D/Hin the water that forms cometary ice. Pristine comets are ∼50% water ice;the question is how much of the water on Earth was contributed by thiscelestial delivery system.

D/H in seawater is well determined as 1.6 × 10−4. Measuring this ratioin comets has proved to be much more difficult. The most precisely de-termined value thus far is D/H = 3.1 ± 0.2 determined for Halley’s cometby Balsiger et al. (1995) and Eberhardt et al. (1995). These measurementswere made by mass spectrometers that were carried through the coma ofHalley’s comet. Ground-based observations using telescopes at submillimeter

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Contributions of comets 13

~

~

Comets

HCN

Hale-Bopp

Hyakutake

H2O

ATM

ESC

Minerals(S, C)

H2O (SMOW)

Jupiter

H 4.5BY

PRIMITIVE REPROCESSED

Hale-BoppHalley

Mars

H20

~

~

10–5

10–4

10–3

10–2

H2

LISM

D/H

TitanCH4Uranus

CH4

NeptuneCH4

Earth

Mars

Earth

Deuterium in the Solar System

Figure 7. D/H in the planets and comets of the solar system. Major differences existbetween Jupiter, presumed to represent the primordial value in the solar nebula, thefractionated values among the inner planets, and the mixture of primordial D/H withD/H from the icy cores of Uranus and Neptune. The origin of the value on Titan remainsuncertain.

wavelengths led to the same result with greater uncertainty in two othercomets (Figure 7).

All three of these icy planetesimals came from the Oort cloud, leavingopen the possibility that comets from other reservoirs might be made of icewith different values of D/H. In particular, the comets found in the outer partof the main belt of asteroids have been suggested as a source of Earth’s water(Hsieh and Jewitt 2006). This hypothesis may run into trouble with the

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14 Owen

noble gases in these comets if they have the same proportions as noble gasesfound in carbonaceous chrondrite meteorites. These volatile rich meteoritesare commonly thought to originate from the same region of the asteroid beltas the newly discovered comets. Their noble gases have a distinctly differentabundance pattern than noble gases found in Earth’s atmosphere.

But perhaps the noble gases in these comets are different from those inthe meteorites. The noble gases in meteorites are concentrated in the carbonfound in a curious macromolecular compound called “Q” for quintessence,that comprises a tiny fraction (<0.04%) of the meteorite (Swindle 1988).The noble gases in the much greater mass of ice originally in these cometscould be both more abundant and different in their relative abundances.

On the basis of existing evidence, we must conclude that comets bythemselves could not have supplied Earth’s water. Once again we look to therocks that made the planet. In this case, we must postulate that the grainsthat made these rocks adsorbed water vapor in the inner solar nebula, whereisotope exchange between H2O and HDO could produce low values of D/Hin H2O. Mixing this water with cometary water would then produce theobserved D/H in the ocean water. This isotope exchange rate has been mea-sured by Lecluse and Robert (1994) and appears to be sufficiently efficientto produce the desired effect.

What about other volatiles? We have suggested that Earth’s nitrogenwas delivered as NH3. This compound appears to be the major carrier ofnitrogen in comets. Unfortunately, the value of 15N/14N in cometary NH3 isunknown—another project for Rosetta. If it turns out that 15N/14N in thisNH3 has the same value as the ratio found in CN, instead of the value foundin the nitrogen we breathe, we can rule out comets as the source of Earth’snitrogen (Figure 5).

There is one more approach we can use to assess cometary contributionsto Earth and the other inner planets. This involves the planet Mars.

On Mars we find small amounts of water vapor in the atmosphere, plusa ratio of C/N = 15. Here we have to admit that this value is probably acoincidence as a huge amount of CO2 has evidently been blown off the planetby impact erosion (Melosh and Vickery 1989), while approximately 10 × thepresent amount of post-impact nitrogen has left the planet by nonthermalescape (McElroy et al. 1977).

We therefore turn once again to the noble gases for enlightenment. Herewe find a remarkable correspondence between the abundance patterns ofAr/Kr/Xe found in the atmospheres of Earth and Mars (Figure 6). Fur-thermore, the relative abundances of xenon isotopes are also the same forthe two planets, again different from any other source (Owen and Bar-Nun2000; Figure 8). We emphasize once more that the Rosetta mission will tellus if the ices in comets have trapped noble gases with the same pattern

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Contributions of comets 15

of abundances and isotope ratios, thereby furnishing a crucial test of thecometary bombardment hypothesis.

Could it be that we are seeing the result of a post-impact cometarycontribution on Mars? If so, near-surface water on Mars that has not beenexposed to the atmosphere might exhibit the cometary value of D/H. Theexisting measurements of D/H in H2O from Martian meteorites indicate thatthe lowest values of D/H in water not subject to escape may have cometaryrather than terrestrial value (Leshin et al. 1996; Figure 9). It is worthy ofnote that a single comet with a diameter of just 10 km could supply allthe water that exchanges seasonally between the two Martian polar caps.It is also important to realize that there is apparently no exchange betweensurficial water and water in silicates (Karlsson et al. 1992). Evidently, thelack of plate tectonics on Mars keeps the hydrosphere separated from thelithosphere, unlike the situation on Earth.

Thus we have the fascinating possibility that the surface of Mars is lightlyfrosted by a thin layer of cometary water. The Mars Science LaboratoryMission planned for 2012 will be able to determine if that is the case bystudying D/H in the surface water.

6. Conclusions

We can summarize the evidence for cometary contributions to planets asfollows:

Outer Planets: Low-temperature, icy planetesimals with solar relativeabundances (SCIPs) have apparently delivered the heavy elements to Jupiterand possibly all of the giant planets. If this is correct, SCIPs would have beenthe most abundant form of solid matter in the early solar system. While wedon’t find any evidence of SCIPs among the comets we have studied thusfar, they may be identical to the most primitive Kuiper Belt objects wefind in orbits beyond that of Neptune. We may characterize both of thesefamilies as comets of Type III formed at T < 30 K and thus containing solarabundances of heavy elements.

Titan: The ice making up ∼50% of Titan’s mass was apparently formedfrom planetesimals that originated at T ≈ 100 K in Saturn’s subnebula. Thisconclusion follows from the high value of N/Ar in the satellite’s atmosphere.This high ratio also excludes N2 as the source of Titan’s nitrogen, indicatingthat both N2 and 36Ar must be strongly depleted on the planet. However,carbon will be trapped in these “warm” planetesimals in the form of solidCO2, amorphous grains, and macromolecular compounds, all of which canserve as sources for the atmospheric CH4.

Inner Planets: The abundances of noble gases on Venus offer a hintthat collisions with one or more SCIPs could have occurred. However, the

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16 Owen

Chondrites

Earth atmosphere

1.3

1.2

1.1

1.0

0.9

0.8

0.7136134132130128126124

[i Xe

/13

0 X

e]

Ma

rs /

[i X

e/1

30 X

e]

So

lar

Xe Isotopic Mass

Xenon Isotopes – I

Solar

(a)

(b)

Mars atmosphere

Chondrites

Earth atmosphere

1.3

1.2

1.1

1.0

0.9

0.8

0.7136134132130128126124

[i Xe

/13

0 X

e]

Ma

rs /

[i X

e/1

30 X

e]

So

lar

Xe Isotopic Mass

Xenon Isotopes – Mars= Earth!

II

Chassigny

(solar)

EETA 79001 Glass Inclusions

Figure 8. A comparison of xenon isotopes I (a) Earth, Sun, meteorites, and II (b) Earth,Mars, meteorites (Bogard et al. 2001). Mars and Earth are remarkably similar anddistinctly different from the Sun and meteorites.

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Contributions of comets 17

D/H in Shergottites and Chassigny

Chassigny kaersutites

Shergotty kaersutite

Zagami kaersutites

Chassigny biotite

Zagami N apatite

QUE 94201

MarsAtmosphere

4000 50003000200010000

4

3

2

1

0

δD (per mil)

Nu

mb

er

of

an

aly

se

s

Halley'sComet

TerrestrialHydrogen

Figure 9. D/H determined from minerals in Martian meteorites. The lowest value observedin these rocks from Mars is clearly closer to the cometary value than to D/H in Earth’soceans (Figure 5). (Leshin et al. 1996; Greenwood et al. 2007).

dominant sources of N and C appear to be the rocks making up the planet.The water initially present on Venus has almost completely disappeared,removed by a combination of photodissociation in the upper atmosphereand escape of the resulting hydrogen.

On Earth the noble gas abundance pattern is distinctly different fromthat of Venus, the Sun, or the meteorites. Water in the oceans on Earthexhibits a value of D/H that is clearly lower than that in comets, indicatingthat cometary water, if it exists on Earth at all, must be mixed with waterhaving a lower value of D/H. That water presumably came from the rocksmaking up the planet, as did the bulk of the C and N, just as for Venus. OnMars, we find the same abundance pattern for noble gases and Xe isotopesas on Earth, suggesting that both planets received their noble gases fromthe same source.

Water in the atmosphere of Mars has a fractionation of D/H caused byatmospheric escape, which has also affected the nitrogen isotopes. Waterin the rocks of Mars has a minimum value of D/H that is identical withthe cometary value, about 2× the value in Earth’s oceans. Unlike Venus

comets_rev3.tex; 7/01/2008; 12:04; p.17

18 Owen

and Earth, Mars exhibits no signs of communication between crustal andinterior volatiles, offering the possibility that cometary water may still bepresent as an extremely thin veneer on the Martian surface.

7. Future Work: Some Key Experiments

− Detailed analysis of the composition of nuclei and comae of severalcomets. Once the critical Rosetta results are in hand, plan for missionsto comets with different source regions and especially dynamically newcomets coming into the inner solar system as close as possible to the firsttime and ultimately comet sample return. These analyses must includeabundances and isotope ratios of noble gases, H, C, O, N, and othermajor elements. These elements should be studied in all compounds inwhich they are found.

− Atmospheric probes for all of the major planets to investigate isotopesand abundances of noble gases and other major elements.

− Surface investigations on Titan to enable comparison of D/H in Titan’ssubcrustal water ice with D/H in atmospheric methane and hydrogen.

− Detection and abundance measurements of Kr, Xe, and their isotopeson Titan, Venus, and comets for comparison with solar and terres-trial/Martian values.

− Further investigation of D/H in Martian water isolated from the atmo-sphere.

− All of the existing measurements mentioned in the text would benefitfrom increased precision with the brilliant exception of the N isotopesmeasured in a high-temperature inclusion in a meteorite (Meibom etal. 2007).

Acknowledgements

Thanks must go to and NASA and ESA for developing and flying the hugelysuccessful Cassini-Huygens spacecraft, as well as the other spacecraft thatsupplied the data used here. I am grateful to George Gloeckler, NicolasGrevesse, and the referees for their helpful comments and to Louise Goodfor her extensive and timely help with the manuscript.

This paper was written to honor the eightieth birthday of Johannes Geiss,whose contributions to our knowledge of the physics and chemistry of the

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Contributions of comets 19

solar system defy enumeration. The author expresses his personal thanks formany instructive and enjoyable conversations with our honoree.

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