Chimera: a Mission of Discovery to the First Centaur

EPSC Abstracts Vol. 13, EPSC-DPS2019-1094-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license.

Chimera: A Mission of Discovery to the First Centaur

Walter Harris (1), Laura Woodney (2), Geronimo Villanueva (3) and the Chimera Science team. (1) University of Arizona, USA, (2) California State University San Bernardino, USA, (3) Goddard Space Flight Center, USA ([email protected])

Abstract planet migration sent them into cold storage. Centaurs are chimeric objects, with properties in common with Chimera is a mission concept to visit the highly active both TNOs and JFCs. Understanding their structure, Centaur, 29P/Schwassmann-Wachmann 1 (SW1), chemistry, and processes that drive their activity are which has been submitted to the NASA 2019 call for key to understanding the evolution of icy Discovery mission proposals. Chimera will study planetesimals and how the planetesimals we observe evolution, chemistry, and mechanisms driving activity today, especially the most accessible JFCs, inform us of an icy planetesimal beyond Jupiter. New Horizons about the origin of the solar system. taught us about the most primitive type of icy planetesimal and many missions from Giotto to Rosetta have visited highly evolved comets: Chimera will explore the evolutionary middle ground between Trans Neptunian Objects (TNOs) and Jupiter Family Comets (JFCs) (Figure 1).

Stardust Deep Space-1 Giotto/Vega New Horizons Rosetta Figure 2: The modern family of icy planetesimals.

Chimera Deep Impact EPOXI Stardust-NExT Lucy 2. Why SW1? While ~20% of Centaurs are known to be at least Figure 1: The mission legacy to which Chimera will sporadically active [1], SW1 stands out in multiple contribute. ways. Discovered during an outburst in 1927, SW1 was the first object other than the giant planets found 1. Why Centaurs? to occupy a heliocentric orbit entirely beyond Jupiter. Its persistent coma and photometric variability while Comets, Centaurs and TNOs are believed to be in a near-circular orbit at 6 AU was considered cryogenically preserved samples of the gas and grains enigmatic, making it a favorite target of study. From of the proto-solar nebula out of which the solar system the resulting 90-year observational baseline, we now formed. Having been dynamically perturbed to large know that SW1 maintains a near-constant background heliocentric distances early in the history of the solar level of activity in all phases of its orbit. This system, their composition and structure retain quiescent activity is frequently interrupted by large information that can be used to quantify the conditions outbursts that brighten by 1-5 magnitudes [2]. The and mixing in the disk from which they formed. modern outburst cadence is ~7 events/year with little evidence of seasonal variability (Figure 3). The Centaurs themselves exist in a relatively short dynamical phase (Figure 2): they have been disturbed The persistence and magnitude of SW1’s activity is out of the scattered disk of the Kuiper Belt back into greater than that observed from any other Centaur and the region between Jupiter and Neptune where they comparable to that observed from similar-sized long likely originally formed. From here, interactions with period comets (LPCs) at the same heliocentric the giant planets can either scatter them out of the solar distance (Figure 4). Combined with the known long- system or inwards to become the JFCs. It is in the term consistency of its activity, this makes SW1 Centaur region that these objects develop nascent uniquely positioned to address the most fundamental activity, modifying them for the first time since giant A Gateway to thequestions Centaurs and of the planetesimal Secrets of Small-Body formation, Formation volatile References composition, and evolution. nation of activity, size, evolutionary state, and ac- [1] Jewitt, D.: The Active Centaurs, The Astronomical Journal, Volume 137, Issue 5, pp. 4296-4312, 2009. cessibility. From orbit, the sub-surface character- 11 Nearly Constant Activity istics of this object can be probed by studying the Observations 12 [2] Miles, R.: Discrete sources of cryovolcanism on the structures exposed by mass wasting and during e 13 d nucleus of Comet 29P/Schwassmann-Wachmann and their tu outbursts, from mapping geologic processes (e.g. i

n 14 origin, Icarus, Volume 272, pp. 387-413, 2016. g slope failures) indicative of cohesion, friction, a M

15 s or near-surface degradation, and through radar u

e [3] Wierzchos, K., Womack, M. and Sarid, G.: Carbon l

c 16 sampling of outer surface layering, porosity, and u Monoxide in the Distantly Active Centaur (60558) N dielectric properties. For a sufficiently massive 17 174P/Echeclus at 6 au, The Astronomical Journal, Volume object, radio science can further determine the 18 155, Number 5, pp. 230-243, 2017. mass to high precision (<0.1%), measure the rate 19 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 of mass loss, and obtain detailed measurements Year . of gravity field perturbations resulting from the CM093 Figure D-7: Historical rate of outbursts on SW1. interior mass distribution. Figure 3: A decade of outburst detections from SW1. The Astronomical Journal, 153:230 (8pp), 2017 May Wierzchos, Womack, & Sarid D.1.1.3 Determine the Afect, Timing, and Period = 57.708 d Source Source Source Source Source Source Implications of Evolutionary Processes ‘b’ ‘c’ ‘d’ ‘e’ ‘f’ ‘a’ 2010-2014 Te Centaurs are chimeras: evolutionary bridg- season es between the KBOs and JFCs. 2006-2010 If we are to understand what end state plan- season 2002-2006 etesimals such as JFCs and Trojan Asteroids tell season us about the origin of the solar system, we must understand the evolutionary processes they have 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 undergone since they first formed. As orbits of the Rotational Phase of Nucleus CM094 icy planetesimals dynamically evolve, variation in Figure D-8: Identifcation of outburst sites as a function of sur- solar heating drives the onset, type, and rate of gas face phase from rotational debias. production. Orbital Evolution of 29P/SW1 For most of the history of the solar system 8.0 7.5 2019 these objects, whether in the cold storage of the 7.0 1975

(au) Q Oort Cloud or the Kuiper belt, have experienced 6.52 Figure 3. Specific production rates, Q(CO)/D , for distant comets and Centaurs. The solid line is a fit to data for Hale–Bopp, assuming Q CO=´3.5 1029 r- 2, and q, 6.0 () diameter DHale–Bopp=60 km; values for Chiron and 29P are plotted assuming production rates from Womack et al. (2017) and DChiron=218 km and D29P = 60 km. very little change. Galactic-cosmic-ray irradiation a, 5.5 2038 – – This plot shows that, when adjustedFigure for surface area4: andThe heliocentric area distance,-averaged Echeclus (D Echeclusproduction=65 km) and Chironrate producefrom∼10 50 times less CO than Hale causes chemical alteration to a depth ofBopp, only a proxy for4-6 a relatively unprocessed5.0 comet. Upper limits to specific production rates for other Centaurs were calculated from Q(CO) upper limits and diameters provided in Drahus et al. (2016), and showSW1 that1900 29P is is larger the only CO-rich1950 than Centaur that found of2000 to date.other All Centaurs Centaurs 2050 with values below[ 3 ] .2100 the Hale–Bopp line are considered to m (Stazzula and Johnson, 1991). Whenshow athey reduced COare activity level. Date (years) dynamically disturbed into the region between 14 Last Deep Jupiter Next Deep Jupiter 12 fi layers with little to no accessible10 volatileClose species Encounter surviving selection effect,Close Encounter because only signi cant limits have been made

Jupiter and Neptune, some new Centaurs(e.g., Meech and & Svoren 3.2004 ;Summary Prialnik(au) et al. 2004). Whenand we Conclusionsto date only for Centaurs with diameters greater than 65 km. Q look at gas activity measurements, depletion8 of volatile species, We point out that 29P, the most CO prolific Centaur, is on the LPCs begin to develop persistent comae from a q, 6 in terms of bulk compositionThe 2013 ratios a, 4 may decadal not be functionally survey identifiedsmall size forthe Centaurs Centaurs with a diameteras an of ∼60 km. It will be type of continuous activity (often referreddistinct from to thoseas mechanisms of2 mantling or desiccation. very useful to obtain more constraining Q(CO) measurements However, we suggest thatimportant CO may be present 1500target at higher for 2000 levels,future exploration. on 2500 smaller Centaurs. 3000 Chimera It may also 3500 will be helpful to extend this quiescent) that is distinct from water sublima- fi at deeper sub-surface layers, but suppressed in detectable Dateanalysis (years) to other comets; however,CM082 we were unable to nd any tion (Section D.1.2.2). Tey also experienceemissions. spo- deliver a capable suite of remoteother distant sensing comet Q instruments(CO) measurements beyond 4 au which Small bodies in theto outer SW1 solar that system will present characteriz a real hade independently its activity, determined searchQ(CO for) and diameters. radic outbursts: sudden, explosive releaseschallenge in of determining a Figure compositional D-9: Forward trends. Unlike and thebackwardAlthough projection Centaurs of are the thought SW1 to shareor- a common origin in large amounts of gas and entrained dust.growing chemicalTese databasebitsignatures forthe comets shows( Cochran of stable evolutionary et al.behavior2015), over the processing, Kuipera 1200 Belt, year the measure well-known span (below). different the outgassing behavior the more distant and less active Centaurs lack sufficient in Chiron and 29P has been attributed to the differences in typically last for tens of minutes to a fewinformation, days and either fromEvolutioncomposition surface reintoflectance and volatiles or out from of gas andSW1's icescomposition low on eccentricity the and surface evolution orbit historyand during ( inDe Sanctis et al. 2001; emission, to determine mixingmostthe coma, ratiosrecent between and 200 determine volatileyears is species. shown the above. physicalCoradini et al. characteristics2008). The apparent of CO deficiency in some generally occur in localized areas (A’HearnThus, we et make al. the distinction that depletion in these cases Centaurs, consistent with Figure 3, may be explained by the 2005, Agarwal et al. 2017). Outburstsrefers have to the been reduced activitythe levelnucleus. detected for a given species devolatilization of small KBOs, in which only large KBOs are (such as CO) relative toTable other objects D-3: orThe a certain SW1 reference.orbital andable physical to retain characteristics. volatiles since the rate of volatile loss is observed in all classes of active planetesimals,That reference but in our case here is Hale–Bopp. This is analogous controlled by the gravity and surface temperature (Schaller & to the taxonomy framed for highlyProperty active objects (Mumma & Brown 2007Value; Brown 2012). the large-scale events detected from remoteCharnley 2011sens).- We noteAcknowledgements that in the absence of complete We bring up another potentially important characteristic: ing are infrequent and unpredictable,knowledge with of the mixing ratios (at least relativea to water), it is activity rate.5.989 Chiron and Echeclus (both very low CO complicated to distinguish between different ratios of carbon outgassers) outburst only rarely, often with years between singular exception of SW1 (Section D.2.1species). in theT nucleuse andThe production authors ratese ofacknowledge molecules in the theoutbursts; support whereas,0.0451 and 29P assistance(CO-rich) has multiple outbursts per coma overall (Sarid et al. 2005). i year. Since9.393° Echeclus and Chiron are only occasionally active, mechanism(s) behind outbursts are unknown,Interestingly, with theof notable the exception Chimera of 29P, science no Centaurs team onein possiblethe development cause is that emitted of gas the(possibly CO) originates and, while the crystallization of amorphousproduce the same ice amountresearch of CO per investigation surfaceq area as Hale and– measurementfrom sub-surface5.719 ice requirements patches, or possibly as trapped volatiles Bopp. Thus, there may be a signiQficant difference in that are freed6.260 by the crystallization of amorphous water ice. and the associated release of latent heatcomposition (Prialnik and/or bulkof propertiesthe mission between concept. most Centaurs Echeclus and Chiron may have had more CO in the past and et al. 2004 Comets II) is most frequentlyand invoked, Hale–Bopp. We note that most ofPeriod the Centaurs below the substantially14.66 devolatilized yr after moving into their Centaur line in Figure 3 have the largest nuclei. As discussed in orbits. In contrast, we note that 29P undergoes an outburst the release of subsurface gas pockets (GronkowsWomack et al. (2017- ), CO devolatilizationAlbedo may be expected in every ∼0.03350 days ± on0.15 average. 29P looks like an outlier Centaur, large Centaurs, partly due to increasedDiameter radiogenic heating. It is by producing48 ± so14 much km CO. Perhaps it is a more recent entrant ki and Wesolowski 2015), cliff collapseimportant events to keep in mind, however, that this is partly a to its current orbit than frequently thought, or perhaps it has a (Steckloff et al., 2016), and exothermic processes Density 800 ± 300 kg/m3 6 D-5 Draft - 5/6/2019 1:45 PM PROPRIETARY INFORMATION Use or disclosure of data contained on this sheet is subject to the restriction on the title page of this proposal.