Landing on a cometary surface is another critical issue that requires the effort of the Planetary Probe community. In this sense, Rosetta future landing will provide new clues on the surface properties of comets (Fig. 4). To land into a primitive body can be a technological challenge due to its presumably weak structure, and the presence of volatile phases that would be explosively released in case of heat exposure. Remote measurements, and laboratory evidence suggest that most of the studied comets are formed by poorly-consolidated pristine materials [18, 19] far more fragile than the most primitive meteorites arrived to Earth (Fig. 5). The missions deployed to comets have so far demonstrated the extreme fragility and tiny size of cometary materials. Below the surface most comets seem to have ice deposits that may have explosive behaviour when exposed to solar radiation, heat, or even compression. The extremely porous structure and low strength of most comets and their fragments is opposed to the properties observed in relatively pristine chondritic asteroids. Laboratory experiments and observational evidence [18] suggest that the originally porous aggregates were highly retentive of water and organic compounds present in their forming environment. After consolidation, many of them experienced a particular dynamic history. Bodies stored in the Kuiper Belt (KB) experienced a much lower degree of impact processing than these populating the Main Belt (MB). In such category would be comet 81P/Wild 2, whose materials have not experienced aqueous alteration because accreted water was never released to soak the body. Volatile-rich bodies transiting inner regions with higher impact rate experience significant compaction processing, together with subsequent aqueous alteration and loss of volatiles. The release of water from hydrated minerals or interior ices, participated in soaking the forming materials, and transforming their initial mineralogy and physical properties (Fig. 5).

In conclusion, planetary scientists and engineers need to work together to find solutions on these important issues. New techniques to return safely pristine samples from these primitive bodies are urgently required to get significant progress in our understanding of the physicochemical environment present in the outer regions of the protoplanetary disk. The prize of a sample-return mission for the scientific community would be having direct access to unprocessed solar system materials, particularly true if we use penetrators to obtain samples in depth [13]. We expect that such pristine materials provide new clues to better understand the processes that took place during the early stages of solar system formation.

References:

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Paper, Planetary Sci. Inst. [6] Trigo-Rodríguez J.M., & J. Llorca 2006. MNRAS 372, 655.[7] Atreya S.K. et al., 2009. In Titan from Cassini-Huygens, Brown

R.H., Lebreton J.P. & Hunter Waite J. (eds.), Springer, 177-199.[8] Bézard B. et al. 2007. Icarus 191, 397.[9] Niemann H.B. et al. 2005. Nature 438, 779.[10] Nixon C.A. et al. 2008. Ap.J. 681, L101[11] Gomes et al. 2005. Nature 435: 466-469. [12] Vinatier et al. 2007. Icarus 191, 712.[13] Trigo-Rodríguez J.M., & J. Blum 2009. Planet. Space Sci. 57,

243.[14] Oró J. 1961. Nature 190, 389.[15] Delsemme A. 2000. Icarus 146, 313.[16] Oró J. et al. 1990. Ann. Rev. Earth Planet. Sci. 18: 317-356. [17] Trigo-Rodríguez J.M. 2006. In Life as we know it. Seckbach, J.

(Ed.), Springer, NY, 383.[18] Blum J. et al. (2006) Ap J. 652, 1768. [19] Ball A.J. et al. (2004) In Mitigation of Hazardous Comets and

Asteroids, CUP, pp. 266-291.[20] Trigo-Rodríguez J.M., & Blum J. (2009) Publ. Astron. Soc. of

Australia 26(3), 289-296.

SAMPLE-RETURN MISSIONS: WHY ARE SO CRUCIAL?

SAMPLE-RETURN MISSIONS FROM COMETS AND PRIMITIVE BODIES: A FUTURE LANDMARK

IN SPACE EXPLORATION Josep M. Trigo-Rodríguez1,2

1 Institute of Space Sciences (CSIC), Campus UAB, Facultat de Ciències, Torre C5-2ª planta. 08193 Bellaterra, Barcelona, Spain; 2Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capità 2-4, Ed. Nexus. 08034 Barcelona, Spain.

e-mail: trigo@ieec.uab.es

DISCUSSION: SAMPLING OF COMETARY MATERIALS

Figure 5. A mm2 area of the CO chondrite ALH77307 can be a good example of the inner structure of primitive bodies. a) Backscatter electron image shows the texture of the matrix. b) Composite RGB ion microprobe image where red=Mg, green=Ca and blue=Al. This chondrite is one of the most primitive arrived to the Earth. Some carbonaceous chondrites may contain about a 10% of absorbed water in mass (mostly as phyllosilicates), but collisional processing would have depleted their primordial volatile abundances [20]

The successful Stardust mission to comet 81P/Wild 2 provided the second direct samples of a solar system body after the recovery of lunar rocks by Apollo, and Luna missions. Recovering cometary materials by using an aerogel collector, while the spacecraft was overflying this Jupiter Family Comet at a speed of 6.1 km/s, was a huge technological challenge [1]. This significant achievement was obtained in the framework of the multidisciplinary Stardust science team [2]. Recovered 81P/Wild 2 materials suffered significant heating during deceleration in the Stardust aerogel collectors that have partially altered some of the carbonaceous compounds during track excavation [3, 4]. To study low-melting temperature organic and mineral phases, future sample return missions will require new techniques to collect essentially unaltered materials from presumably extremely fragile bodies (Fig. 1). An interesting step ahead has been proposed with the Comet Coma Rendezvous Sample Return (CCRSR) mission concept [5] in which the spacecraft is designed to rendezvous with a comet, making extended observations within the cometary coma, and gently collect coma samples at much lower relative velocities than Stardust.

Minor bodies also played a major role in the enrichment in organics and volatile species of planetary bodies. Particularly the Earth was formed from materials condensed in a hot environment, strongly depleted in volatiles. Despite of this, the Earth have similitude with planetary bodies formed in the outer solar system. For example, Titan’s atmosphere is dominated by molecular nitrogen (N2) like the Earth. The volume percent of N2 in the atmosphere of Titan makes this world similar to Earth, and it seems to require a secondary processing of N2 from NH3 (photolysis, impacts and endogenic processes are good candidates) [7]. The measured isotopic ratios in Earth and Titan atmospheres (Fig. 2) seem to be inherited from their cometary (volatile-rich) building blocks. The D/H ratio is a central parameter to reconstruct the origin of water in the solar system. In Titan atmosphere this ratio has been recently measured as 1.3210-4 [8] that is close to the terrestrial value 1.510-4. The Titan ratio of 12C/13C82 fits well to the terrestrial value of 89 [9]. The 16O/18O ratio was recently estimated to be 346110 compared with 500 for Earth [10]. The O ratios could have been fractionated in the nebula or during aqueous alteration of bodies formed nearby the snow line, lately scattered due to the migration of the giant planets that produced the Late Heavy Bombardment [11]. Finally, the ratio of 14N/15N=568 corresponds to an enrichment in 15N of about five times the terrestrial ratio [12] as consequence of secondary processing.

THE IMPORTANCE OF ORGANICS CHARACTERIZATION

Figure 1. What is the tensile strength of poorly consolidated aggregates? Here is plotted the geocentric velocity (V) versus the (dynamic) tensile strength measured for meteoroids associated with different comets that are producing meteor showers [6]. The strength is inferred from the height, and the load atmospheric pressure at which the particles break apart. Sporadic (SPO) particles with high entry velocities (suggesting cometary origin) have typical disruption strengths of 105 dyne/cm2. Below this value are fluffy meteoroids as CAP particles (from 169P/NEAT), LEO (55P/Tempel-Tuttle) and ORI (1P/Halley). Particles from 21P/Giacobini-Zinner (GIA) are exhibiting extremely low strengths. Particles from old streams like e.g. TAU (3P/Encke), GEM (3200 Phaeton) and QUA (2003EH1) associated with evolved parent bodies have the highest strengths.

Figure 4. The Rosetta lander will touch down on the comet 67P/Churyumov-Gerasimenko’s surface in November 2014. The planned analyses will provide clues on the surface structure and composition of this comet (Image Rosetta/ESA)

From a cosmochemical and astrobiological perspective the study of H, C, N, and O isotopes can be the key to decipher the processes occurred in the early stages of solar system formation. These essential elements in organic chemistry were preferentially depleted from inner disk materials (Fig. 3). Volatile-rich particles were overheated when falling towards the Sun at its high radiative stage. Consequently, volatile species were vaporized and recycled when the solar wind swept such gases to the outer disk. Phase transitions produced most isotopic exchanges because, for kinetic reasons, heat of ices is needed to bring back the isotopes to the gaseous phase. 81P/Wild 2 materials have revealed that tiny minerals, plus ices and organics inherited these anomalies at the time of cometesimal formation [2]. Some of these objects escaped collisional processing, and subsequent aqueous alteration [13], playing an important role of comets in the volatile enrichment of terrestrial planets [14, 15, 16]. The main population of volatile-rich impactors was formed by easily disrupting ice-rich bodies that could enrich the volatile-depleted content of terrestrial planets [16, 17]. These bodies were scattered from the outer regions as consequence of the giant planets migration invoked in the Nice model that originated the Late Heavy Bombardment (LHB) [11].

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Figure 2. Titan isotope ratios of the light elements relative to Earth.

COMETS AND ATMOSPHERES

Figure 3. Sample return missions to primitive bodies (comets and carbonaceous asteroids) will be crucial to characterize the chemical and isotopic patterns inherited from the protoplanetary disk, and to better understand what was the primordial volatile composition of the bodies that enriched the terrestrial planets during the LHB.