Hartley 2

Accepted Manuscript Geologic Control of Jet Formation on Comet 103P/Hartley 2 M. Bruck Syal, P.H. Schultz, J.M. Sunshine, M.F. A’Hearn, T.L. Farnham, D.S.P. Dearborn PII: DOI: Reference:

S0019-1035(12)00494-0 http://dx.doi.org/10.1016/j.icarus.2012.11.040 YICAR 10481

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Icarus

Received Date: Revised Date: Accepted Date:

1 May 2012 16 October 2012 29 November 2012

Please cite this article as: Syal, M.B., Schultz, P.H., Sunshine, J.M., A’Hearn, M.F., Farnham, T.L., Dearborn, D.S.P., Geologic Control of Jet Formation on Comet 103P/Hartley 2, Icarus (2012), doi: http://dx.doi.org/10.1016/ j.icarus.2012.11.040

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Geologic Control of Jet Formation on Comet 103P/Hartley 2 M. Bruck Syala,∗, P. H. Schultza , J. M. Sunshineb , M. F. A’Hearnb , T. L. Farnhamb , D. S. P. Dearbornc a

Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA b Department of Astronomy, University of Maryland, College Park, MD 20742, USA c Lawrence Livermore National Laboratory, P.O. Box 808 L-16, Livermore, CA 94551, USA

Abstract The EPOXI mission flyby of comet 103P/Hartley 2 revealed numerous discrete dust jets extending from the nucleus, thereby providing an unprecedented opportunity to visually connect these features to the nuclear surface. The observed distribution of jets provides fresh insight into the conditions under which these cometary features may form. This study examines the geomorphology associated with areas of jet activity and then applies observed topographic correlations in the construction of a 2-D hydrodynamic model of a single dust jet. Visible light images of Hartley 2 show correlations between specific surface structures with both narrow-angle and fan-shaped dust jets; associations include pits, arcuate depressions, scarps, and rimless depressions. Notably, many source regions for jets appear finer than the practical mapping resolution of the imaging instruments (∼12 m). This observation indicates that the processes controlling jet formation operate at significantly ∗

Corresponding author, 1-401-863-3594, 1-401-863-3978 (fax) Email address: [email protected] (M. Bruck Syal)

Preprint submitted to Icarus

December 8, 2012

finer scales than the resolution of most cometary activity models and motivates a complementary numerical investigation of dust jet formation and evolution. In order to assess controlling variables, our parametric numerical study incorporates different geometries and volatile abundances for the observed source regions. Results indicate that the expression of jet activity not only depends on local topography but also contributes to the evolution and development of surface features. Heterogeneous distributions of volatiles within the nucleus also may contribute to differences in local styles of jet activity. Keywords: Comets, nucleus, Comets, dynamics, Comets, Ices

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1. Introduction Images obtained with the Deep Impact Flyby spacecraft’s Medium-Resolution

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Instrument (MRI-VIS) and High-Resolution Instrument (HRI-VIS) during

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the EPOXI mission’s closest approach to comet 103P/Hartley 2 reveal the

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presence of numerous highly collimated, filamentary dust jets emanating from

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the nucleus. Remarkably, several of these narrow jet features also exhibit

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strong activity well beyond the evening terminator, persisting without direct

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solar heating. Following cometary science conventions, here we invoke the

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term “jet” to describe linear brightness enhancements in broad-band visi-

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ble light images. As this paper focuses on jet activity recorded in visible

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light images, the observed well-defined enhancements in brightness are dom-

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inated by dust and ice particles. Hence, in this paper, the word “jet” refers

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generally to dust/ice jets emanating form the nucleus of Hartley 2. Similar

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fine-scale filamentary type features have been observed on all four comet nu-

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clei previously visited by spacecraft: 1P/Halley, 19P/Borelly, 81P/Wild 2,

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and 9P/Tempel 1 (Keller et al., 1987; Soderblom et al., 2004; Sekanina et al.,

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2004; Farnham et al., 2007; Farnham et al., 2012). However, the extraor-

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dinary levels of activity at comet Hartley 2 (evident from its classification

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as a “hyperactive” comet A’Hearn et al. (2011)) provide unparalleled views

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of these tightly collimated, narrow features and their source regions. While

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Farnham et al. (2007) connected a few small jets on Tempel 1 to surface

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features, the Hartley 2 observations are the first for which a wide range of

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jet structures are traceable to specific surface features (rather than regions)

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on the nucleus.

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Past observations of cometary nuclei from spacecraft reveal diverse styles

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of activity, including: diffuse sublimation over larger surface areas, wide-

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angle (fan-shaped) outflows, and narrow-angle (highly collimated) features.

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The latter two styles have both been termed types of “cometary jets,” e.g.,

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Soderblom et al. (2004); Yelle et al. (2004), although the term “jet” is some-

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times reserved for the more collimated structures, e.g., Belton (2010); Farn-

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ham et al. (2007). Gas release in a near vacuum results in rapid gas expan-

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sion. Particles above a certain size range entrained in such a gas (within a

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factor of 5-8) effectively decouple from radial gas expansion as the inertial

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forces outweigh gas dynamic forces (e.g., Schultz and Gault (1979)). Con-

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sequently, the observed well-defined streams of dust must represent released

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particles on ballistic trajectories. In reality, the spherically expanding gas

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from a surface vent also contains much smaller (and larger) size fractions

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that are not easily detected optically (but see Hermalyn et al. (2012)).

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While broad-scale cometary jets far from the nucleus were initially iden-

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tified as early as the year 1835 with Earth-based telescopes (Festou et al.,

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1993), the advent of comet flyby missions allowed such features to be resolved

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into groups of many discrete, highly collimated outflows (Keller et al., 1986).

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Since then, comet flyby missions have accumulated a wealth of data on the

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various types of cometary activity (Keller et al., 1987; Soderblom et al., 2004;

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Sekanina et al., 2004; A’Hearn et al., 2005; A’Hearn et al., 2011). Observa-

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tions from previous comet-flyby missions have recently been synthesized by

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Belton (2010) into a detailed taxonomic organization of activity types.

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Over the past two decades, researchers have developed increasingly so-

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phisticated models to simulate the activity of comets. Some of the earliest

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three-dimensional hydrodynamical models of the collisional inner coma in-

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cluded Kitamura (1990); Crifo et al. (1995); Crifo and Rodionov (1997).

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Later numerical work successfully coupled the evolving thermophysical state

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of the nucleus to the hydrodynamic state of the inner coma (Rodionov et al.,

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2002; Crifo et al., 2002; Szego et al., 2002). Other studies focused on the

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interior of the nucleus, constructing detailed simulations of its evolving ther-

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mophysical conditions in order to explain cometary activity patterns (Rosen-

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berg and Prialnik, 2007; Prialnik et al., 2008; Rosenberg and Prialnik, 2010).

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Many of the most recent advances in numerical methods for calculating

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cometary comae have applied the Direct Simulation Monte Carlo Method

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to the problem (Crifo et al., 2005; Zakharov et al., 2009; Tenishev et al.,

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2008; Combi et al., 2012). Since most observations of cometary activity rely

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on Earth-based telescopes, models generally focus on broad-scale outgassing

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processes and incorporate relatively coarse meshing schemes in order to cal-

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culate total production rates. By necessity, the connection to surface geology

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had to be theoretically deduced or imagined, rather than observed.

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Recent flyby missions now provide high-resolution views of the surface

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that reveal the source regions of specific jets for the first time, thereby mo-

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tivating the use of fine-resolution numerical models. In the case of Hartley

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2, many source regions for jets appear smaller than the practical mapping

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resolution of the imaging instruments (∼12 m). Consequently, the processes

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controlling the release of volatiles and dust require model resolutions on

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the order of meters, rather than tens or hundreds of meters. It is impor-

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tant to note that past studies employing three-dimensional cometary activ-

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ity models could produce spatial variations in dust density, even without

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the introduction of discrete source areas for jet activity (Crifo and Rodi-

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onov, 1997; Rodionov et al., 2002; Crifo et al., 2002; Szego et al., 2002).

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For example, pioneering work by Crifo and Rodionov (1997) demonstrated

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that homogeneously-distributed activity over an aspherically-shaped nucleus

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is capable of producing broad, jet-like features in the inner coma. Such a

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mechanism may contribute to the broader-scale variations in observed dust

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jet brightness at Hartley 2. We argue, however, that the structure observed

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in the finest-scale dust jets imaged during the EPOXI mission flyby supports

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a role for heterogeneous activity levels across the nuclear surface.

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Previous models of localized cometary jets employed one-dimensional

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steady-state solutions (Belton, 2010; Yelle et al., 2004). Collimated jets may

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persist long enough to be considered steady-state features for a portion of

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their lifetimes. But our current constraints are only the duration of the flyby

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(on the order of minutes) or possibly the rotational period (though this is an

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assumption). Regardless, the extreme variability in observed dust jet activ-

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ity in the near-nucleus region of Hartley 2 motivates this investigation of the

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initial triggering of an active area (within an otherwise inactive region of the

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nucleus) and how it evolves. The time variability of individual jets must re-

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main uncertain until a spacecraft imager acquires continuous, high-resolution

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data on jet initiation and evolution. The Rosetta Mission (Glassmeier et al.,

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2007; Colangeli et al., 2007; Kissel et al., 2007), for example, should provide

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critical insights into the timescales over which jets evolve.

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The following discussion first reviews the location of jet source regions

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and their relationships to a variety of geologic structures. Numerical values

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for the physical properties of these jets, however, are beyond the scope of

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this contribution. Second, we develop a two-dimensional numerical model

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of jet evolution, from initiation to near steady state. Finally, we interpret

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these results and consider possible implications for other cometary nuclei.

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Our overall objective is to understand the formation of different styles of

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jets observed from the EPOXI flyby and to apply new constraints from their

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associations with specific features on the surface.

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2. Geology of Jet Source Locations

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2.1. Background

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The relation, if any, between nuclear geology and the locations of jet

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outflows has been vigorously debated in recent literature (Yelle et al., 2004;

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Crifo et al., 2002; Szego et al., 2002; Belton, 2010). Of central concern is the

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mechanism for collimating the jets. Some studies propose that an initial vent

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or hole in the crust may provide essential geometric conditions for collimation

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(Sekanina, 1991; Keller et al., 1994; Yelle et al., 2004). Other studies conclude 6

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that such conditions are unnecessary and suggest that jets emerge from an

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effusive pathway through the porous regolith (Belton, 2010; Gortsas et al.,

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2011; Fanale and Salvail, 1984; Gombosi et al., 1985). In particular, the

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Belton (2010) study relies on the comet’s ambient H2 O near-nucleus coma as

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the primary collimation agent. Still other models find that jet-like dust coma

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morphologies will arise from homogeneous sublimation across an irregularly

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shaped comet nucleus (Crifo and Rodionov, 1997; Rodionov et al., 2002;

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Crifo et al., 2002; Szego et al., 2002). This effect may very well contribute

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to the broad-scale variations in dust coma density at the very irregularly

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shaped Hartley 2 nucleus. However, observations of the very fine-scale dust

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jets and the topography associated with these features (outlined in Sections

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2.2 and 2.3) provides evidence for local heterogeneities in activity levels.

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These two effects, one purely shape-controlled, and one controlled by spatial

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variations in activity over the nuclear surface, may both operate concurrently

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to produced observed variations in dust coma brightness.

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One specific mechanism for producing a collimated jet assumes entrain-

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ment of dust particles in gases released from the interior of the comet (Yelle

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et al., 2004; Combi et al., 2012). Gas expands radially after being released in

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the near-vacuum coma environment just above the surface; however, the en-

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trained dust particles remain on ballistic trajectories that reflect conditions

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of their release. Although the Yelle et al. (2004) and Combi et al. (2012)

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models were applied to somewhat larger scale jets on comets 19P/Borrelly

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(200-400 m jet diameter) and 67P/Churyumov-Gerasimenko (∼1600 m jet

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diameter), respectively, the entrainment mechanism should remain valid at

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smaller scales as well.

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2.2. Jet Source Locations at Hartley 2

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Images acquired during the flyby of the Hartley 2 nucleus reveal distinct

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correlations between styles of outgassing activity and terrain type. Figure 1

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provides a general terrain and context map of key features discussed below.

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A separate contribution reviews the general shape and geology of Hartley 2

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(Thomas et al., 2012). Here we review the connections between near-nucleus

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dust jets and specific geologic features or terrains.

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The elongate nucleus divides into three distinct regions: the small, hy-

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peractive lobe with a distinctly rugged (“spiked”) texture; a large lobe with

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contrasting smooth and rugged terrains; and a waist with a much smoother

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texture connecting the two lobes. A’Hearn et al. (2011) described how the

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numerous narrow dust jets emerging from the rough terrain on the smaller

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lobe correlate with locally enhanced abundances of H2 O ice and CO2 in

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the inner coma. An enhancement in near-nucleus H2 O ice abundance also

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correlates with jet activity on parts of the larger lobe. The diffuse coma sur-

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rounding the smooth waist region, on the other hand, is associated with an

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enhancement in near-nucleus H2 O vapor (A’Hearn et al., 2011). IR-derived

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abundance maps of H2 O ice on the nuclear surface reveal that it is confined

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to the morning terminator region of the nucleus (Sunshine et al., 2012) in

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patches at the meter or sub-meter scale (Groussin et al., 2012). The surface

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water ice distribution does not (directly) spatially correlate with the distri-

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bution of H2 O vapor in the near-nucleus coma, e.g., above the waist region.

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This observed heterogeneity in the distribution of volatile species near the

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nucleus, taken together with the terrain type correlations, provides insight

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into possible differences between the mechanisms that could power various

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styles of cometary outgassing.

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Figures 2 and 3 provide stereo views of two perspectives of Hartley 2

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captured during the flyby and reveal the discrete source regions and different

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jet morphologies. Specific jets for discussion have been labeled (Figs. 2B

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and 3B). Two different image stretches allow linking jets to surface features.

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Because these jets are best expressed on the limb, certain examples disap-

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pear once the sunlit surface moves into the background during the flyby. Our

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criterion for a jet is that it can be identified visually in the available imaging

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after making image contrast stretches. It is not yet possible to assign a quan-

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titative limit on the detection limit due to the complexities of background

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contrasts (nucleus shadow, background coma, etc.).

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Collimated (narrower) jets appear associated with small features such as

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scarps and arcuate depressions (jet nos. 1 and 3 in Fig. 2B and c in Fig.

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3B) or from the spiked terrains on the smaller lobe (collectively labeled no.

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16). Narrow jets at the end of the larger lobe (nos. 1, 2, 3, a, b, and c)

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continue beyond the sunlit terminator (Figs. 2B, 3B, 4A). A jet emerging

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from exposed ice, such as a scarp, might be expected to emerge at an angle.

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Instead, jets typically extend normal to the surface from the inferred exposure

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or opening (e.g., nos. 1 and 3; Figs. 2B and 4A). An exception is the diffuse

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jet no. 6 (Fig. 2B), which is directed more sideways from a feature that

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appears to be a large scarp extending beyond the sunset terminator.

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Fan or funnel-shaped jets (subtending wide angles) appear to originate

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from low-rimmed or rimless depressions (e.g., jet nos. 11, 12, and 15 in Fig.

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2B; nos. 6, 12-14 and h in Fig. 3B). The jet boundaries are relatively sharp,

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thereby indicating ballistic boundaries of light-scattered dust and ice, rather

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than expressions of an expanding gas. A closer stereo view of jet no. 12

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situated on the boundary between the large lobe and waist (Fig. 4B, 4C,

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4D) illustrates this connection. During the flyby, this jet could be viewed

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from different perspectives (Fig. 5). The dashed line in the first three frames

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of Fig. 5 traces the center axis of the jet that extends from the center of

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a low-rimmed depression with a central pit. As the source region is viewed

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from above, the jet seems to disappear, as the nucleus provides a brighter

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background. This depression resembles some of the shallow, low-rimmed

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crater-like structures on 9P/Tempel 1 (Thomas et al., 2007). The paucity of

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impact structures on Hartley, however, suggests an endogenic origin.

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The smaller lobe contains numerous, closely spaced jets that cannot be

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directly associated with identifiable geologic structures. This lobe is in near-

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direct sunlight and jets appear to emerge from between spires, some 60m

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high (discussed below). In this location, the jets radiate perpendicular to the

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surface, rather than in the direction of illumination. Consequently, gravity

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and/or surface structure appears to control both their directionality and

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degree of collimation.

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A diffuse plume occurs above the contact between the smooth waist and

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the large lobe. Under small phase angles, this region (arrows, Fig. 3B)

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contains clusters of large clumps ∼100 m above the surface of the nucleus

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(Fig. 4B). Stereo imaging establishes that most of these clumps do not

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represent background stars (e.g., Fig. 4C and 4D); rather, their parallax

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offsets indicate that they represent bodies above the surface. Such objects

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are consistent with low-speed aggregates lofted off the surface and tracked

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farther from the nucleus (Hermalyn et al., 2012; Kelley et al., 2012).

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In the fully sunlit portion of the large lobe, jets appear to be less active

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(or lacking light scattering dust), even with a dark background off the nucleus

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surface (Fig. 2A). This contrasts with the hyperactive southern lobe, which is

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near the sub-solar point. Moreover, jets are very active in the large lobe near

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the sunset terminator (Fig. 6), which prompts three interpretations. First,

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activity away from the sub-solar point might indicate delayed release by

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some unknown mechanism. Second, reduced gas emission (lower expansion

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speeds) near the sunset terminator increases the number density of dust

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and ice particles within the jet near the surface. Conversely, widespread

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gas emission from the fully sunlit large lobe prevents narrow and funnel-

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shaped jets from forming. Third, the observed active areas along the sunset

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terminator simply represent the most active areas on the large lobe. This

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pattern is enigmatic if only direct solar radiation drives the process.

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2.3. Significance of Large Blocks

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Large blocks and clumps are scattered across the nucleus but are most

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evident in the highly active smaller lobe (short, solid lines, Fig. 3B). De-

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convolved high-resolution images reveal much greater surface detail (Figs. 6

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- 8). Correlated features in a corresponding MRI image demonstrate that

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smaller features of interest are not products of the de-convolution processing.

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While most rugged terrains coincide with hyperactive small lobe (Fig. 8,

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region D), other regions that appear hummocky in medium-resolution images

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are also rugged at higher resolutions (Figs. 7, 8). The largest angular blocks

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generally occur in two settings: (a) along the boundary between the waist

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and the two lobes (Figs. 3B, 6, 7); and (b) in exceptionally textured areas as

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in the rubbly and spiked/pitted surface of the small lobe or near the sunrise 11

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terminator (Figs. 6-8).

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The concentration of large blocks in the spiked/pitted terrains and nar-

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row jets in the smaller lobe indicates a connection in their formation. The

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blocks could arise either as surface lags and/or from constructional jet-related

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processes (accretive lags). As lag features, active jets remove volatiles and

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fines, leaving behind relict inhomogeneities from the interior (e.g., cold-

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welded refractory materials). High-albedo blocks (BB, Fig. 7B) may rep-

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resent materials with greater amounts of ice (or silicates, higher in albedo

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than organics), whereas low-albedo blocks (B2, Fig. 7B and Fig. 4C and

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4D, lower right) may represent aggregates with less ice. Large blocks along

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the edge of the waist (Figs. 3B, 6, 8) are not all associated with active jets.

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The bands around the smaller lobe (Fig. 3B) may represent exposures of

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more resistant material that emerge after sufficient loss of volatiles. The

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large block (arrow, Fig. 6) could have migrated (or toppled over) from one

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of these bands toward the waist after being exhumed.

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As constructional jet-related features, blocks could represent large accre-

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tions of ices resulting from jet activity. The large block near the waist (arrow,

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Fig. 6) disrupts the sharp boundary between the waist and larger lobe. This

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is in a similar setting as the active jet highlighted in Figs. 4C, 4D, and 5 and

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could indicate an end product of an active jet source region.

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Small, bright patches (speckles) cover a broad area near the sunrise ter-

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minator (Fig. 7, upper right). Profile views of these regions reveal that the

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bright specks may represent spires and blocks (as revealed on the limb in

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Fig. 9, inset) that retain patches of ice after sunrise (Sunshine et al., 2012).

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Careful examination of different raw and restored images of these near-limb

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features indicates that these spires are not artifacts of the deconvolution

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(Lindler and A’Hearn, 2012) and should be interpreted as real. Numerous

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large angular blocks (Fig. 9, B1, B2, arrows) rest on the surface and cast

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long shadows. Block B1 (Fig. 7B) is one of the largest. Although just com-

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ing into sunlight, one side of this block is in full illumination, yet remains

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relatively dark.

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The large lobe (away from the spiked/pitted terrain) contains fewer large

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blocks and displays more rounded features. This observation suggests that

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blocks degrade with time: from angular blocks to clumps to mounds. The

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inferred evolution of block morphology implies that they are comprised of

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weakly bonded (or ice-welded) material. Such areas also have accumulations

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of smooth materials (Fig. 1). In fact, blocks must disappear with time;

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otherwise, the surface would be littered to saturation. Instead, large angular

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blocks appear localized in certain regions associated with rugged terrains.

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In summary, the observed distribution of jets on Hartley 2 indicates geo-

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logic control of jet morphology, location, and block formation. Deep Impact

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and Stardust-NExT images of comet Tempel 1 (Farnham et al., 2007; Farn-

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ham et al., 2012) also indicate a connection between jet source locations and

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geologic features, such as steep slopes and terraces. Such associations on

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two very different comet nuclei demonstrate that surface processes may af-

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fect (if not control) jet formation and evolution. An additional role for the

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exothermic interior processes emphasized in Belton (2010) is supported by

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the persistence of bright, collimated jets into the night side of the Hartley 2

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nucleus. Nightside jet activity was also reported at Tempel 1 by Farnham

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et al. (2007) and found in Feaga et al. (2007) to correlate with CO2 out-

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gassing. The different surface morphologies associated with the jet source

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regions, however, motivate an investigation into the effects of time-dependent

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surface conditions on jet activity. Numerical models allow such conditions

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to be simulated, so that the role of surface features in jet formation and

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evolution may be quantified.

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3. Modeling Jet Initiation: Numerical Approach

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A model for initiation of jet formation follows a qualitative description of

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the evolution of a vent previously proposed by Sekanina (1991) and Keller

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et al. (1994). Specifically, structural weakness within the comet’s dusty man-

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tle leads to a slumping of warm material into underlying frozen volatiles. As

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a result, warmed volatiles rapidly sublimate through a confined source region

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to form the jet. A parametric study of this jet initiation model is constructed

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by varying the initial conditions that could affect jet development, including

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depth and width of the vent and variations in the ratio of H2 O to CO2 ices.

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While slumping has not been directly observed at a comet, the evolving

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surface conditions of the nucleus (through observed, volatile-driven mass loss)

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can be reasonably expected to result in the eventual release of support near

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the toe of geologic structures. Such a process will inevitably cause slumping,

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which is simply defined as the sliding of loose materials down a slope. The

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geomorphic associations between jets and geologic structures, along with the

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inferred disappearance of large blocks over time (outlined in Sections 2.2 and

310

2.3), support the idea that such a process occurs.

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Calculations used the CALE hydrodynamics code, a two-dimensional Ar-

312

bitrary Lagrangian Eulerian code written in C. CALE was chosen for this 14

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particular study because of its known accuracy in handling solid to vapor

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phase transitions, which are central to the problem of cometary activity.

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The code’s flexibility in tracking particle motions via both Lagrangian and

316

Eulerian methods makes it well-suited to the problem. The physics algo-

317

rithms and approximations incorporate a wide variety of ideas and techniques

318

developed by many different workers at Lawrence Livermore National Labo-

319

ratory over the last 30 years (Barton, 1985; Wilkins, 1964; Cloutman et al.,

320

1982; Winslow, 1963; Tipton, 1987). Previous work to model hydrodynamic

321

jets with CALE (Stone et al., 2000; Foster et al., 2002) demonstrates its

322

applicability to the problem investigated in this study. We chose to use a

323

two-dimensional (2-D) code, rather than a fully three-dimensional simula-

324

tion, in order to conserve computational time and to allow a greater number

325

of jet simulations to be completed for this first-results contribution.

326

The initial conditions for the 2-D axisymmetric model are illustrated in

327

Fig. 10. Initial conditions in each simulation consisted of various mixes of

328

cold (50 K) H2 O and CO2 ices (see Table 1 for a comprehensive list of the

329

varied parameters), overlain by a layer comprised of granular (0.1 mm-sized)

330

SiO2 , which served to represent a porous particulate dust mantle. Though

331

the actual dust composition at a comet will be a diverse mix of silicates and

332

organic materials (Meech et al., 2011; Sugita et al., 2005), here we are using

333

SiO2 as a proxy material.

334

For the present study, a mantle thickness (Hm ) of 2.5 m was used in

335

the majority of the models. This mantle thickness was based upon results

336

from the Deep Impact cratering experiment (Schultz et al., 2012; Schultz

337

et al., 2007; Sugita et al., 2005), which currently provide the only direct

15

338

measurement of cometary subsurface structure. The ejecta sequence observed

339

at Deep Impact suggested a layer at 2-5 m depth (Schultz et al., 2007).

340

Later analysis of Startdust-NExT observations of the crater yielded a mantle

341

thickness estimate of 1-2 m (Schultz et al., 2012). A second line of evidence

342

for a meter-scale mantle thickness is provided by the paucity of spectrally-

343

identified water ice at Hartley 2 (Sunshine et al., 2012), given the observed

344

scale of topographic variations (e.g., scarps and depressions) on the nucleus.

345

While mantle thickness may be thinner, as indicated by numerical results

346

from cometary activity models (Rosenberg and Prialnik, 2009; Thomas et al.,

347

2008), the precise thickness of the mantle should not affect the results of

348

this work, so long as the vent depth exceeds the thickness. A comparative

349

calculation using Hm = 0.5 m (Run No. 2 in Table 1) found that the model

350

results did not change substantially for a thinner Hm value.

351

As we observe jet source regions at Hartley 2 near the ∼10 m scale (just

352

below mapping resolution of the imaging instruments), hole diameters (D)

353

slightly below that scale (3 m) were chosen as a working model. While

354

the precise depths of holes at Hartley 2 cannot be directly measured, the

355

topography of the rough terrain provides a guide. The presence of local

356

topographic variations on the order of tens of meters suggests that a 5 m

357

hole depth is a reasonable working assumption. The depth of the vents (z)

358

varied from 5.0 meters to 0.0 meters, with the 0.0 meter depth case equivalent

359

to a flat, icy patch at the surface (see Table 1). As a first step, our study

360

focused on a single jet and did not consider interactions with adjacent activity

361

sources or other topographic variations beyond the immediate vicinity of the

362

jet. Near-vacuum conditions (ρambient = 10−10 g/cm3 ) were implemented

16

363

above the surface of the nucleus, which is consistent with the very tenuous

364

atmosphere of even a hyperactive comet such as Hartley 2.

365

Each model incorporated the assumption that slumping of warmed mate-

366

rials into the vent initiated a transition from amorphous to crystalline water

367

ice, which then provided the energy necessary to trigger outgassing activ-

368

ity. Though the presence of amorphous water ice has yet to be detected

369

at a comet (Huebner, 2009), the exothermic amorphous to crystalline water

370

ice transition has been suggested as a candidate energy source for driving

371

cometary activity (Prialnik and Bar-Nun, 1992; Bar-Nun and Laufer, 2003;

372

Meech et al., 2009).

373

Sourcing internal energy into the base of the vents over one second simu-

374

lates the amorphous to crystalline transition. The energy-release mechanism

375

depends on the presence of near-surface amorphous H2 O ice; this approach is

376

supported by recent thermal modeling of comet nuclei interiors (Rosenberg

377

and Prialnik, 2007), which indicates that cometary amorphous ice may per-

378

sist even at relatively shallow depths (on the order of a few meters). This

379

exothermic phase change releases 100 Joules of internal energy per gram of

380

converted water ice (Ghormley, 1968; Schmitt et al., 1989). The timescale

381

on which this transition takes place decays exponentially with temperature: tc = A · exp(5370/T )

(1)

382

where tc is the characteristic timescale of the amorphous to crystalline phase

383

change, A = 1.59×10−15 seconds and T is temperature in Kelvin (Schmitt

384

et al., 1989).

385

In order for the energy to be released over the course of one second, the 17

386

model uses the assumption that ices within the vent are heated to at least 180

387

K by the slumped warm materials. The infrared-derived surface temperatures

388

at Hartley 2, which range between 250 and 360 K (Groussin et al., 2012),

389

justify such an assumption. The details of heat transfer are not considered

390

here; rather, an energy-balance approach is taken, in which internal energy

391

from the warm materials is transferred to the underlying ice. The total energy

392

sourced into the models amounted to 2.0×106 J, which is equivalent to the

393

energy released by 20 kg of amorphous H2 O ice as it crystallizes. Spatially,

394

this amount of energy corresponds to the crystallization of water ice over

395

approximately one square meter of surface area, with a layer thickness of 2

396

cm.

397

An alternative way to formulate the problem is to consider thermal diffu-

398

sion from the slumping, warmed surface materials as the only heat transfer

399

mechanism affecting the ices in the source region (no amorphous to crys-

400

talline transition). While such a process may be sufficient to trigger the

401

initiation of a jet outflow, the initial rate of heat transfer would be signifi-

402

cantly lower than the rate of energy release used in this study. Hence, while

403

the amorphous to crystalline transition is not explicitly necessary for trigger-

404

ing jet initiation in the general style described here, the model inputs used

405

in this study are more specific to the timescales on which the amorphous

406

to crystalline transition operates. This study focuses on the response and

407

growth of a jet source region to some initial trigger, with full recognition

408

that the exothermic response (leading to vaporization) may have a variety

409

mechanisms.

410

Equations of state from the Livermore Equation Of State (LEOS) tabular

18

411

library were used to describe the various materials in the model. The volatile-

412

depleted mantle was assigned a low compressive yield strength (1.46 kPa), no

413

tensile strength, and a shear modulus of 3.5 kPa. The underlying volatile ices

414

were assigned zero strength. These modest strength values are appropriate

415

for loosely consolidated bodies such as comets and asteroids. The volatile

416

densities were left at their nonporous quantities: ρH2 O = 0.917 g/cm3 , ρCO2

417

= 1.56 g/cm3 . The equation of state used for the mantle is a mix of porous

418

(φ = 0.18) SiO2 and H2 O, which fills the void space; its bulk density is ρSiO2

419

= 1.91 g/cm3 . This assumed density for the surface grains is significantly

420

greater than the likely low-density, high-porosity material comprising the

421

Hartley 2 nucleus; calculations described in A’Hearn et al. (2011) estimate a

422

bulk density between 180 and 880 kg/m3 for Hartley 2. Implementing addi-

423

tional porosity, either from an empirically determined crush curve or using

424

an analytic model, could be a useful extension of this study in the future.

425

The average gravitational acceleration of Hartley 2 (∼ 2.3 × 10−3 cm/s2 ) was

426

also included within all models (assumed to act downward, normal to the

427

surface).

428

The spatial resolution of the models was 0.5 meters, which is sufficiently

429

small to account for the finer jets emanating from the Hartley 2 nucleus.

430

Models with different resolutions allowed testing the effects of zone size,

431

from the nominal 0.5-meter resolution to resolutions of 0.25 meters and 0.167

432

meters. These finer-scale studies offered no clear quantitative or qualitative

433

differences in their calculations, other than being computationally much more

434

expensive.

19

435

4. Jet Model Results and Discussion

436

4.1. General Characteristics of the Outflows

437

CALE enables the user to track mixing ratios of the various materials

438

present in the simulation (here, H2 O, CO2 , and SiO2 ) within each zone

439

throughout the calculations. For each of the collimated flows established

440

within the various models, dust (SiO2 ) comprised the collimated portion of

441

the outflow, while the volatiles expanded in a more or less hemispherically

442

symmetric pattern (Fig. 11). Such a result supports the hypothesis that

443

jet-like morphology results from ballistic launch of dust particles initially en-

444

trained by the flow, rather than a collimation of the gases themselves. The

445

1-D steady-state model put forth by Yelle et al. (2004) relied on this same

446

principle to achieve collimation. More recently, a similar dust-volatile decou-

447

pling mechanism has been used by Combi et al. (2012) to describe the dust

448

jet collimation produced in models of comet 67P/Churyumov-Gerasimenko.

449

Because the icy substrate beneath the dusty mantle was not explicitly seeded

450

with dust grains, the dust comprising the collimated flow in the model origi-

451

nated from the dusty edges of the vent, where outgassing volatiles entrained

452

sufficiently small dust and directed it upward within a narrow angle above

453

the vent. While computational time constraints prevented running the models beyond several hundred seconds after initiation (typical time steps, controlled by initial conditions and mesh resolution, were 1 - 100 μs), the models approached a steady-state condition (using the criteria of constant maximum particle velocities and jet widths), described by jet features with constant outflow diameters of ∼40 m near the base of the jet and particle accelera20

tions near 0.5 cm/s2 . Such accelerations are more than sufficient to propel the micron-size ice grains that were calculated to dominate the icy particle size distribution in the inner coma of Hartley 2 (A’Hearn et al., 2011). Much like the dust-dominated jets calculated here, solid icy grains also will be entrained by the expanding gases and exhibit greater collimation than the gas-phase volatiles initially driving the activity. Equating the drag force (Eqn. 2) with the gravitational forces acting on a spherical particle within the flow provides an estimate of the maximum particle size entrained by the jet. 1 FD = ρv 2 Cd A 2

(2)

454

Here FD is the drag force, ρ is the fluid density, v is the velocity of the flow,

455

Cd is the drag coefficient, and A is the reference area.

456

Using the characteristic densities and velocities from the numerical results

457

with the drag equation (Eqn. 2), we find that the calculated flow can entrain

458

icy particles (ρice = 0.5 g/cm3 ) up to a few meters in diameter. This process

459

could account for the observations of low-velocity, larger aggregates of icy

460

grains observed at Hartley 2 (Hermalyn et al., 2012; Kelley et al., 2012;

461

A’Hearn et al., 2011).

462

4.2. Surface Constraints on Initial Jet Formation

463

For convenience, the source region here is termed a “vent,” even though

464

it could be rootless, i.e., without extending to the deep interior. For a 3.0

465

meter-wide, cylindrical hole or vent penetrating through a 2.5 meter-thick

466

SiO2 mantle into the icy substrate, two different vent depths (2.5 and 5.0

467

meters) were considered. A 4:1 volume ratio of H2 O to CO2 was used for

21

468

the underlying ices in each case (Run Nos. 3 and 4 in Table 1). After the

469

energy was sourced into the 5.0 meter-deep vent, the resulting outflow of

470

entrained dust required only a few seconds to achieve a jet-like morphology

471

(Fig. 12). However, in the case of the 2.5 meter-deep vent, the model failed to

472

establish a jet-like outflow; instead, the process merely widened and deepened

473

the initial hole (Fig. 13). Consequently, jet formation may be significantly

474

hindered by the presence of a thick, dusty mantle. In order for a vent to be

475

successfully activated by surface mechanisms, it may not be sufficient to just

476

pierce through the mantle. The depth of the vent may need to significantly

477

exceed the thickness (2.5 meters, in this case) of the topmost layer of dust-

478

rich material.

479

Similarly, a 5.0-meter-deep but wider vent (10 m) did not establish the

480

same type of strong, collimated flow exhibited by the models with higher

481

depth-to-width ratios (Run No. 9 in Table 1). Such a result is consistent

482

with observations of Hartley 2, which indicate that the morphology of local

483

surface features may play a role in the formation of jets. As an extreme test

484

of this idea, one model placed the energy source onto a 3.0-meter-wide patch

485

of ice at the comet’s surface (Run No. 8 in Table 1). This initial condition

486

did not produce a collimated flow; instead, it exhibited the type of diffuse

487

activity seen in the smooth region at the waist of Hartley 2 (Fig. 14).

488

4.3. Effects of Different H2 O:CO2 Ratios

489

General trends with increasing H2 O:CO2 ratios include: enhanced lateral

490

growth of the vents with time, less-collimated outflows (partly as a result of

491

vent widening), and greater outflow velocities. These effects, illustrated in

492

Fig. 15, accord well with the observed volatile distributions near the Hartley 22

493

2 nucleus, including the association between enhanced CO2 abundance and

494

the most tightly collimated jets at the smaller lobe (low H2 O:CO2 ) and the

495

most diffuse outgassing activity around the comet’s waist (high H2 O:CO2 ).

496

Such an association also suggests that these distinct outgassing styles may

497

be related to the pre-existing variations in volatile distributions within the

498

interior of the nucleus.

499

4.4. Possible Mechanisms for the Development of Spiked and Pitted Terrains

500

The variety of expression of surface roughness and general terrains across

501

the nucleus could be related to the state of evolution of jet source regions. The

502

smaller lobe contains numerous spikes and pits; the jets arising from this part

503

of the nucleus are also characterized by a higher CO2 :H2 O ratio. In contrast,

504

the larger lobe has regions containing numerous isolated blocks along with an

505

elongated, low-lying region containing smooth material. Nevertheless, there

506

is also a broad scarp-bounded region on this lobe that contains numerous

507

spires and blocks similar to the smaller lobe, most easily seen along the limb

508

(Fig. 4). This region is just coming into sunlight and contains a bright

509

surface deposit.

510

The high-relief knobs, blocks, spires, and smooth surfaces are likely prod-

511

ucts of evolving jets. Two models of formation can be envisioned: relict

512

constructs and lag features; both processes likely occur. Active vents widen

513

as they evolve. During less vigorous gas release, surface materials around

514

the perimeter of the vent slump into the pit or cracks, thereby bringing more

515

warm material in contact with the ices below. This process should also apply

516

to surface materials slumping from scarps or ridges; the connection between

517

ridges and jet activity is visible in both EPOXI images of Hartley 2 (Fig. 2B) 23

518

and Stardust-NExT observations of Tempel 1 (Farnham et al., 2012). As a

519

cylindrically shaped vent or pit grows into a shallow bowl, activity becomes

520

strongest near its edges, where mass-wasting resupplies warmed materials to

521

the floor. The boundary condition along the wall of the vent drives a sig-

522

nificant portion of the flow in the horizontal direction, towards the center

523

of the flow. Over time, entrained particles with smaller velocities should be

524

deposited in a central mound within the vent.

525

A model using a wider vent (Fig. 16) reveals that material accumulates

526

near the center. Initially, this should result in a rimless, bowl-shaped source

527

region having a central mound composed of water ice and dust surrounded

528

by a ring vent along the base of the walls (e.g., the edge of the larger lobe in

529

Fig. 6). After many orbits, this process should leave behind a central mound

530

as the surrounding surface disappears. Consequently, the knobs and spires

531

could represent constructional features related to the evolution of the jet and

532

gradual shrinking of the nucleus due to mass loss. The lower abundance of

533

the supervolatile CO2 within certain regions may prevent the construction of

534

spires found elsewhere on the nucleus.

535

Alternatively, the knobs and spires represent lag features. In this case,

536

loss of volatile-rich surface materials over many orbits gradually expose in-

537

trinsic (less-volatile) inhomogeneities, which are left behind as resistant ma-

538

terials expressed as blocks, knobs, or spires. The different shape of the lag

539

features appears to depend on the activity rate: while high gas release rates

540

result in rugged terrains with spires, lower activity rates result in knobs or

541

mounds. The narrow gaps created between such features would enhance

542

the collimation mechanism for entraining dust and icy grains, promoting the

24

543

formation of the many filamentary-type jets. If all blocks, knobs, and spires

544

represent surface lags, then they would eventually cover the surface. Because

545

this does not occur, the lag deposits also may be water ice or ice-welded dust.

546

With enough time (orbits), these relicts disaggregate, only to be replaced by

547

others.

548

In either model (construction or lag), large features eventually becomes

549

unstable at the base and either collapse or fall over (Fig. 5; bottom dashed

550

line). In the most active regions, the closely packed spires create a distinctive

551

rough terrain, best illustrated on the smaller lobe (Fig. 8D). An active region

552

on the sunrise limb of the larger lobe also could develop into similar activity

553

(Fig. 9). Some of the isolated block-like features scattered over the larger lobe

554

(Fig. 9) are interpreted as remnants of structures constructed by jet activity,

555

as their dimensions (tens of meters) are similar to those of the knobs at the

556

smaller lobe. Their isolation could be the result of infilling due to ballistic

557

fines with the jets that return to the surface and are trapped.

558

The boundary between the waist of both lobes is characterized by parallel

559

ridges containing individual spires, knobs, and angular blocks creating the

560

appearance of a crown of thorns. A circular scarp-bounded depression also

561

occurs at the end of the larger lobe (identified in Figs. 2B, 3B, 4, 7B, 8). Jets

562

(nos. 1, 3, b, c) do not emanate from the scarp wall but from the base of this

563

scarp at an angle from the scarp-surface normal, which indicates a source

564

within a fracture. A possible example of a jet emanating from a scarp is jet

565

no. 6 (Figs. 2B and 6). Collectively, these strings of blocks around the long

566

axis create a banded appearance (Fig. 3). Blocks occur along low-relief scarps

567

(S2, S3) in the large lobe and elsewhere (e.g., Fig. 9 inset, upper left). The

25

568

formation mechanism for these strings and rings of blocks remains unknown.

569

Possible processes include layering (contrasts in strength or composition)

570

related to accretion or relict layers of indurated materials. Alternatively,

571

localized regions of enhanced volatile degassing result in concentrations of

572

jets that widen with time until depleted, leaving behind the blocky ring or

573

string. Regardless of origin, jets are observed to emanate from scarps along

574

these rings, adjacent to the blocks, which indicates a connection with the

575

associated blocks.

576

4.5. Comparing Comets Hartley-2 and 9P/Tempel-1

577

The nucleus of Comet 9P/Tempel-1 exhibits both diffuse and fan-shaped

578

dust jets but not in the abundance or in the well-collimated morphology of

579

the jets of Hartley 2 (Farnham et al., 2012). Nevertheless, there are small

580

narrow jets that can be identified during the closest approach to the Tempel

581

1 (Fig. 17). Stereo images (and movies) reveal this feature more clearly.

582

Without the sequential views during approach, this jet would appear to be

583

one of numerous bright patches on the surface. This jet occurs along a narrow

584

fracture identified in an exhumed terrain (Thomas et al., 2007; Schultz et al.,

585

2007).

586

Source regions for jets on Hartley 2 appear to evolve to rimless shallow

587

depressions, then to rings of blocks and knobs. Those on 9P/Tempel 1 appear

588

to leave high-rimmed circular or semi-circular depressions resembling volcanic

589

vents. The numerous pitted and breached-rim mounds on 9P/Tempel likely

590

represent depleted or dormant jet source regions. In some cases, the interior

591

of these structures have been filled, resulting in a feature that resembles a

592

flat-floored crater. 26

593

The large number of dormant or relict vent structures on 9P/Tempel-

594

1 contrasts with the large number of discrete collimated jets on Hartley 2.

595

The lower gravity and higher abundance of CO2 at hyperactive Hartley 2

596

cause entrained fines to be well dispersed, whereas the higher gravity and

597

lower CO2 abundance at Tempel 1 result in near-source deposits. Occasional

598

outbursts on Tempel 1 (Farnham et al., 2007), however, result in source pit

599

morphologies similar to those observed on Hartley 2.

600

5. Conclusions

601

The clear association between collimated dust jets and structures on the

602

nucleus demonstrates that their formation mechanism must be related to

603

processes below the surface, in addition to the thermal wave from solar ra-

604

diation. The combination of these observations and numerical models yield

605

the following conclusions:

606 607

• Jets originate from geologic structures including: pits/vents/holes, arcuate depressions, scarps, and rimless depressions.

608

• While exoergic, internal processes are likely to play a central role in

609

cometary jet activity, the connection between surface features and jets

610

indicate geologic control on the release of dust, if not origin.

611 612

• Collimated jets represent dust or icy grains entrained in H2 O and CO2 that emerge from narrow geologic features (e.g., pit or fracture)

613

• Large blocks are tied to the evolution of jet activity. The absence

614

of large blocks over portions of the nucleus indicates disaggregation

615

through time. 27

616 617

• Jet activity provides a mechanism for ejecting large clumps of particles, which contribute to the water vapor observed in the inner coma

618

• Our numerical simulations suggest that the collimation mechanism re-

619

quires a relatively narrow vent or hole. Emission from a flat patch of

620

ice on the nucleus surface results in only diffuse activity.

621

• Models reveal that CO2 -dominated-gas release result in greater colli-

622

mation of dust jets, whereas release depleted in CO2 (relative to H2 O)

623

result in a diffuse dispersal.

624

• As jet source regions widen, low-velocity entrained dust and ice are

625

driven inward where they accumulate. This process could result in

626

relict knobs or spire-like structures at the smaller lobe of Hartley-2.

627

Alternatively, the spires are relict features as the jets erode the volatile-

628

rich surface.

629

The results of this work rely on a time-dependent, dynamic view of

630

jet activity, in which discrete active areas may evolve on relatively short

631

time scales, rather than remaining unchanged for several apparitions. Time-

632

resolved data of activity on comet 67P/Churyumov-Gerasimenko from the

633

Rosetta Mission (Glassmeier et al., 2007; Colangeli et al., 2007; Kissel et al.,

634

2007) should provide new clues to the role of time-dependent versus steady-

635

state solutions for the formation of narrow dust jets.

636

Acknowledgements

637

This material is based upon work supported by NASA’s Discovery Pro-

638

gram, which supported the EPOXI mission via contract NM071102 to the 28

639

University of Maryland and task order NAS7-03001 between NASA and Cal-

640

Tech, and the National Aeronautics and Space Administration through the

641

NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A

642

issued through the Office of Space Science.

643

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644

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645

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38

Table 1: Jet Model Parameters

No.

z

1

5.0

2

D

Hm

c

H2 O/CO2 d

3.0

2.5

1

5.0

3.0

0.5

1

3

2.5

3.0

2.5

4:1

4

5.0

3.0

2.5

4:1

5

5.0

3.0

2.5

9:1

6

5.0

3.0

2.5

all H2 O

7

5.0

3.0

2.5

all CO2

8

0.0

3.0

2.5

1

9

5.0

10.0

2.5

1

a

b

a

Hole depth (m) Hole diameter (m) c Mantle thickness (m) d Ratio by volume b

39

Figure 1: Illustration of the associations between surface features, including scarps and ridges, and dust jet source locations. Here we have divided the nucleus into three basic terrain types: knobby (smaller lobe), smooth (waist and portions of the larger lobe), and blocky (most of the larger lobe). Dashed lines represent selected individual jets emerging from the nucleus, which will be detailed in other views. In this and following figures, jets were identified by visual inspection with locations based on the perceived axis of each jet, aided by stereo imaging. MRI-VIS frame 6000002. In this view, solar illumination is from below to the lower right (and out of the page).

40

Figure 2: (A) Stereo views (direct viewing) of the nucleus of Hartley 2 during approach optimized for surface features. MRI-VIS frames 5004054 (left) and 6000003 (right). (B) Two different contrasts of the left frame (MRI-VIS frame 6000003) in Fig. 2A. At left, image is optimized for accentuating surface features; at right, for identifying dusty jets extending from surface. Dusty jets generally extend normal to the local surface from small sources or along linear features. Specific jets are labeled for comparison with other views to be shown. At right, triangles along the solid or dashed line delineate a low-relief scarp (labeled from S1to S3). Several jets occur at or beyond the sunset terminator (nos.1, 2, 3, 4, 5, 7). Solar illumination is from below41 (and out of the page).

Figure 3: (A) Stereo medium-resolution view optimized for dusty jets. Left: MRI-VIS frame 5004031; Right: MRI-VIS frame 6000001. (B) Two different contrasts of right frame of stereo pair (MRI-VIS frame 6000001) in Fig. 2A. Terrain maps of regions shown in Fig. 2A optimized for surface features (left) and dusty jets (right). Triangles along the solid line indicate a low-relief transition between the rugged terrain and smooth terrain (appears to be a scarp). Dashed lines correspond to linear concentrations of blocks (idealized). Short solid lines delineate large blocks occurring near the boundary between the smooth waist and smaller lobe (right image). The dashed circles in Figure 2B (left) indicate regions above the surface containing clumps above the waist region identifiable in successive frames (and shown in more detail in Fig. 4b). Solar illumination is from below (and out of the page).

42

Figure 4: Close views of nucleus (MRI-VIS frame 6000001). (A) The end of large lobe contains jets associated with scarp S1 (jets nos. 1, b, and c) and S-2 (no. 3) noted in Fig. 4B. (B) At right, a close view of the waist region near jet no. 12 shows more clearly the diffuse clumps disconnected from the nucleus (arrows). Stereo views (Fig. 3A) demonstrate that these clumps are not background stars that would have shifted out of view. Similar clumps occur above the waist on the other side of the sunlit nucleus. (C, D) Stereo view (direct view) of surface associated with jet nos. 11 and 12 indicated by dashed line centered (see Fig. 2B). Stereo imaging reveals that cone-shaped jet no. 12 converges on a shallow circular feature near the edge of the waist. In addition, two large clumps (circles) occur above the surface of the nucleus, presumably ejected at low speeds from the nearby jets. Solar illumination is from below and to the lower left (slightly out of the page). MRI-VIS frames 5004045 (left) and 5004044 (right).

43

Figure 5: Series of images isolating a fan-shaped jet (no. 12, Fig. 2B, 4C, and 4D), which originates from a rimless depression. Arrows outline the depression. In the top row, the dashed line centers on the perceived axis of the jet; in the bottom row, the line extends from the approximate center of the depression since the jet no longer can be identified due to the bright background of the nucleus. These views also can be viewed in stereo (reversed: left eye on right image; right eye on left) and reveal the orientation of these jets emanating from the surface. As the spacecraft viewpoint shifts (from the side to above), the fan-shaped jet becomes invisible as the nucleus becomes the background. MRIVIS frames, from left to right: 5004044, 5004045, 5004052 (top row); 5004054, 5004057, 5004061 (bottom row). Solar illumination varies due to the changing perspective during the spacecraft flyby: in the first image at upper left, from below (and out of the page); in the last image at lower right, from the right.

44

Figure 6: Comparison between de-convolved high-resolution view (left) and mediumresolution (inset). Dashed lines identify common features in both the HRI and MRI images. Circle co-locates a darker block that is in profile in the HRI. This view provides a closer view of the jets near (and beyond) the sunset terminator on the nucleus of Hartley 2. Jet no. 12 (tracked in Fig. 5) becomes invisible due to the sunlit nucleus in the background. Curiously, well-defined jets do not occur above the smooth bulged region, even with favorable lighting. Arrow identifies a large angular block discussed in the text. Left: HRI-VIS frame 5006015; Right: MRI-VIS frame 5004057 (inset). Solar illumination is from the right (as indicated by shadows).

45

Figure 7: (A) Stereo view of large lobe using de-convolved high-resolution images (straight viewing). Dashed lines indicate visible jets near the end of the elongated nucleus (no. 3) and closer to the equator. The latter jet is most likely associated with no. 12, beyond the limb. The stereo pair clearly reveals large blocks and pits, also identifiable by shadows. The largest blocks occur in the stippled terrain (above the waist, at right). HRI-VIS frame 5004012 (left); 5004015 (right). (B) Annotated view of the nucleus from de-convolved highresolution view (HRI-VIS frame 5004012). Large blocks (B1-B8-9) are scattered across spiked and pitted terrain. They also occur in silhouette or peeking into sunlight near the end of the large lobe (circled area). Some bright clumps (BB) may be more ice rich. Block B-2 is also visible in Fig. 3B under a small local emission angle. The sunrise terminator (to the right) appears stippled, in part due to trapped ice deposits in the rough terrain. Well-defined scarp (S1, see Fig. 2B) is associated with jet no. 1, while the low-relief scarp (S2) is associated with jet no. 3. Scarp S1 appears to be an arcuate linear depression in this view, but with one side higher. This high-resolution view also reveals arcuate depressions that encircle the smaller lobe (bottom). Solar illumination is from below and to the lower left (slightly out of the page). 46

Figure 8: Top: High-resolution view of sunset terminator side of Hartley with specific areas highlighted shown in Fig. 9. HRI-VIS frame 5006015. Solar illumination is from the right (as indicated by shadows). Bottom: Isolated areas shown at top of Fig. 8. Profile of a well-defined scarp (S1) encircles at least part of the end of the long axis (region A) from which jets emerge (also visible in Figs. 2B, 4C, 4D and 5). Block B2 in area B (also shown in Figs. 3B and 7B) is rounded and darker than other areas. Area D shows the hyperactive southern lobe with numerous large angular littering the surface (compare with Fig. 2). The bright feature surrounded by a dark ring in C (left) is likely an artifact of the deconvolution. Solar illumination is from the right. Shadows provide the local sun angle that varies in different regions due to the shape of the nucleus and local topography. HRI-VIS frame 5006015.

47

Figure 9: Limb view reveals needle-shaped spires extending from surface on larger lobe (inset upper left). These features occur in a currently less-active area (lower left image). When viewed from above during flyby, this region appears hilly but not fully resolved (right). Ellipse corresponds to limb region enlarged in the inset (upper left). Upper right inset provides a corresponding medium-resolution view for reference. Arrows identify blocks. The arcuate scarp at the end of the large lobe is identified by label “S.” labels B1 and B2 refer to large blocks viewed in multiple images at different resolutions and perspectives. Left: HRI-VIS frame 5006000; Right: HRI-VIS frame 5004012. Solar illumination is from below. Shadows provide the local sun angle that varies in different regions due to the shape of the nucleus and local topography.

48

Figure 10: Initial conditions of a jet model. Colors denote various densities, with red indicating the mantle of SiO2 grains, yellow-green indicating a mix of H2 O and CO2 ices, and blue indicating ambient conditions (near vacuum). Dotted black line represents the axis of symmetry for this 2-D model. The hole is 5 meters deep and 3 meters wide.

49

Figure 11: Plot of dominant materials (by volume fraction) within various zones of the model. Purple denotes SiO2 grains, which dominate the collimated portion of the flow. The H2 O (dark blue) and CO2 (green), on the other hand, expand above the surface in a more hemispherically-symmetric manner. Plot depicts the model at 510 seconds into the simulation.

Figure 12: A well-collimated flow of dust is established in the case of a 3 meter-wide and 5 meter-deep hole (Run No. 1), shown here 510 seconds into the simulation. Colors denote densities within the left-hand plot and dust velocity magnitudes within the right-hand plot.

50

Figure 13: In the case of a vent that is the same depth as the mantle’s thickness (2.5 meters), a strong flow of dust is not established. The vent simply widens and deepens (note the lack of vectors to denote velocities above the vent). Plot depicts Run No. 3 at 40 seconds.

51

Figure 14: Sourcing energy into a patch of ice on the surface of the comet results in diffuse lofting of dust, rather than a jet-like flow. This result implies a key role for surface features in the formation and development of jets. Plots depict Run No. 8 (in Table 1) at 400 seconds; colors denote densities within the left-hand plot and dust velocity magnitudes within the right-hand plot.

Figure 15: With increasing H2 O:CO2 ratios, the lateral growth of the jets’ vents is enhanced, the flows become slightly less collimated, and maximum release velocities increase. Vectors denote dust velocity fields; the V-like shape of the plotted vectors is a result of the plotting routine and not representative of the cross-sectional velocity variations (velocity at a given height within the jet is relatively constant, as shown in Fig. 12). Plots depict Run Nos. 1, 4, and 5 (in Table 1) at 480 seconds.

52

Figure 16: A knob-like feature approximately 20 meters in height was constructed by radially inward-flowing material in an initially wide (10 meter diameter) vent. As material slumps from the sides of an ever-widening jet source, central knobs can be built by the deposition of dust and ices. This may explain the abundance of knobby or spire-like structures seen at the highly-active smaller lobe (see Fig. 8, area D).

53

Figure 17: Stereo view of small narrow jet on 9P/Tempel 1 from the Deep Impact mission. (A) Diffuse light-colored feature appears to shift with respect to the background. In stereo, this shift can be explained as a small jet extending from an exhumed terrain. Other bright patches do not exhibit similar offsets. (B) Lines trace the feature in sequential views. In stereo, this feature is more easily recognized as a possible dust/ice.

54

We correlate source locations of jets with surface features, such as pits and scarps We build a numerical model of jet initiation from material slumping into a vent Initial geometric conditions affect the expression of jet activity Observations and model results indicate a role for surface geology in jet formation Ongoing jet activity can produce knobby/spired terrain at Hartley 2