Titan Seismology with Dragonfly : Probing the Internal Structure of the Most Accessible Ocean World R
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50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2173.pdf Titan Seismology with Dragonfly : Probing the Internal Structure of the Most Accessible Ocean World R. D. Lorenz1, M. Panning2, S. C. Stähler3, H. Shiraishi4, R. Yamada5, E. P. Turtle1. 1Johns Hopkins Applied Physics Laboratory, Laurel, MD. 2JPL/Caltech, Pasadena, CA. 3ETH, Zurich, Switzerland. 4Japan Aerospace Exploration Agency. 5Aizu University, Japan. ([email protected]) Introduction: The Ocean Worlds (OW) of the e.g. [8]. This modeling, scaled from Apollo observa- outer solar system, provide an alternative scenario of tions of tidal quakes on the Earth's moon, suggests planetary habitability. Among them, Titan is of particu- (with large uncertainty) that ~one Magnitude-4 event lar interest, being both large and organic-rich. Titan is can be expected per Tsol (1 Titan day = 16 Earth days) also the easiest OW surface to which in situ instrumen- and many smaller events. tation can be delivered, permitting the nature of the ice Developments in terrestrial seismology, and work crust, and thus possible processes allowing exchange of stimulated by the InSight mission (e.g. [9]) show how material between the ocean and the surface, to be diag- measurements at a single station can be used to deter- nosed directly by seismic measurements. NASA is cur- mine source direction and distance, as well as diagnose rently evaluating two candidates for the New Frontiers interior structure – see Figure 1. Indeed, elements of 4 mission, to launch circa 2025: one of which is a Titan the waveforms can indicate not only the thickness of lander, Dragonfly [1,2]. the overlying ice crust, but can even probe the presence In situ exploration of Titan: Dragonfly would ex- or thickness of a high pressure ice layer at the ocean ploit Titan's dense atmosphere not only for affordable floor [5]. delivery (using a heat shield and parachute like Huy- gens, rather than complex rocket propulsion as needed for landing on other OWs) but also for mobility. Using a set of rotors, this "dual-quadcopter" could traverse tens to hundreds of kilometers to seek areas of poten- tial prebiotic synthesis (e.g. cryovolcanic flows or im- pact melt sheets, where liquid water may have interact- ed with the abundant surface organics). Important con- text for these astrobiology studies is how thick Titan's ice crust may be and what composition the ocean might have: Cassini/Huygens data suggest a 50-150 km ice crust, and models have considered ammonia-water oceans, or water with abundant sulfate. Accordingly, Dragonfly will attempt to constrain these properties, via seismic means (as well as by observations of the Schumann Resonance, and by measurements of Titan's Figure 1: Simulated measured waveforms (see also [5,8]) rotation state). Composition data (noble gas abundanc- from a Magnitude-4 event on Titan at a distance of 18° es and isotopes in the atmosphere, and the presence of (~800 km). The train of P-wave arrivals (200–350 s) is a e.g. sulfur, chlorine, sodium etc. in the ice) will help straightforward diagnostic of the ice crust, the interval inform an overall picture of Titan's formation and evo- between then being simply proportional to the thickness of lution. the ice. Rayleigh and Love wave amplitudes, ~20 and >100 Titan Seismology: Seismology was identified as an µm/s, respectively, are easily detectable even with important element in post-Cassini Titan science by the Dragonfly’s skid-mounted geophones. The value of 2007 Titan Explorer Flagship study [3], and more re- measurement in all three axes is evident. cent studies have underscored the importance of this technique in resolving icy satellite interiors, e.g. [4]. Other Signals: Present modeling [10] suggests that Indeed, consideration of the rich range of propagation Dragonfly's initial low-latitude landing site is probably modes through ice crusts, internal oceans, possible out of range of possible microseisms generated by high-pressure ice phases etc. demands a new taxonomy gravity waves in Titan's polar seas. On the other hand, of seismic waves [5]. Observations over even just a the seismic detection of booming dunes (e.g. [11]) is an fraction of a Tsol with a simple geophone would dis- interesting possibility. Other speculative sources in- criminate activity levels comparable with active and clude bolides or atmospheric excitation. quiet regions on Earth [6]. Instrumentation: Dragonfly's skids are (after ini- Following similar work for Europa [7], we have tial absorption of landing energy) rigidly coupled to the developed models of Titan's tidally induced seismicity, 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 2173.pdf lander body, and thus a body-mounted instrument is Beyond passive seismic monitoring, Dragonfly has able to measure ground motion relatively effectively actuators which can be used to probe the near-surface (as recently demonstrated even on a rover [12]). Two mechanical properties. Specifically, ground stimulation sets of geophones, one mounted on each skid, record by the rotary-percussive sampling drills (one on each motion in three axes. Although skid mounting is supe- skid) can be sensed by the geophones and/or seismo- rior to body-mounting, such an installation is still sen- meter. DraGMet also features a microphone (principal- sitive to noise caused by lander systems and motion ly as a drill diagnostic) that could also be used to char- due to wind. To mitigate these effects, a more sensi- acterize booming dunes. tive seismometer can be lowered to the ground by a winch. This seismometer is equipped with a wind shield, which is held securely against the body of the lander when the seismometer is retracted for flight. The lowered seismometer is vertical-axis only, and thus is tolerant of modest (~10o) off-pointing and needs no levelling mechanism. Testing during Phase A has shown that the geo- phones operate largely unaffected by Titan temperature (in fact, sensitivity increases slightly, due to the drop in resistance of the coil windings). Similarly, testing of the JAXA seismometer (a shock-tolerant heritage unit qualified for the Lunar-A mission [13], with a sensitivi- ty hundreds of times better than the geophones at ~1 Hz frequency) shows very satisfactory electrical opera- tion at 94 K, and the Titan atmosphere provides im- Figure 2: The Dragonfly lander sitting on an interdune proved damping; a minor spring adjustment brings the plain: the MMRTG is the cylinder at left, and the high gain coil into the sensitive range for Titan gravity and tem- antenna is deployed for direct-to-Earth communication. A perature. seismometer (not visible) can be lowered between the skids. Some lander wind loads may be detected by elastic Each skid features a sampling drill and geophones. deformation of the ground, and indeed lander noise is even noticeable on instruments deployed tens to hun- Conclusions: Dragonfly offers the prospect of de- dreds of meters away, as observed by Apollo [14]. The tailed seismological investigation of an Ocean World. records of the skid geophones will allow this disturb- If Dragonfly is selected by NASA in summer 2019, ance to be subtracted from the seimometer signal. Ad- Titan will be set to become one of only few extraterres- ditionally, as on InSight, continuous wind and pressure trial bodies to receive sustained seismic studies. measurements can be used to identify and decorrelate meteorological effects. References: [1] Turtle E.P. et al. (2018) LPSC 49. Measurement Approach: Dragonfly only flies a #1641 [2] Lorenz R.D. et al. (2018) APL Tech Digest, small fraction of the time (~1%) and spends two Titan 34(3), 374–387 [3] Leary, J. et al. (2007) days or more at each landing site. The seismic sensors https://www.lpi.usra.edu/opag/Titan_Explorer_Public_ are part of the APL-led DraGMet (Dragonfly Geophys- Report.pdf [4] Vance , S.,et al. (2017) Astrobiology, ics and Meteorology) package [2], facilitating simulta- 18, 37-53 [5] Stähler, S. et al. (2018), Journal of Geo- neous wind/pressure and seismic observation. physical Research – Planets, 123(1),206-232. [6] Lo- The DraGMet instrument (much like InSight) has renz R. and M. Panning (2018) Icarus, 303, 273-276 provisions to perform seismic and meteorological mon- [7] Panning, M. et al. (2018) Journal of Geophysical itoring when the lander is in recharge mode, e.g. during Research: Planets, 123(1), 163-179 [8] Panning, M. et the Titan night when Earth is not in view. al. (2018) LPSC 49. #1662 [9] Panning, M. et al. In addition to dedicated observing sessions to make (2015) Icarus, 248, 230-242 [10] Stähler, S. et al., 'open loop' recording of seismic activity and back- submitted [11] Lorenz, R. and J. Zimbelman (2014) Dune Worlds: How Windblown Sand Shapes Planetary ground noise at different tidal phases (local solar times) Landscapes, Springer [12] Panning, M. and S. Kedar and locations, DraGMet can perform continuous moni- (2019) Icarus 317, 373-378 [13] Yamada, R. et al. toring, with triggered events flagged for data downlink. (2009) Planetary and Space Science, 57,751–763 [14] Ample onboard storage permits later retrieval of events Lorenz, R. D. (2012) Planetary and Space Science, 62, determined to be of interest. 86–96 .