TITAN's TROPICAL HYDROLOGICAL CYCLE : CONSTRAINTS from HUYGENS, CASSINI and FUTURE MISSIONS Ralph D
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Comparative Climatology III 2018 (LPI Contrib. No. 2065) 2046.pdf TITAN'S TROPICAL HYDROLOGICAL CYCLE : CONSTRAINTS FROM HUYGENS, CASSINI AND FUTURE MISSIONS Ralph D. Lorenz1 1Space Exploration Sector, JHU Applied Physics Laboratory, Laurel, MD 20723, USA. ([email protected]) Introduction: Only two worlds in the solar sys- The question naturally arose of 'when was this area tem, Earth and Titan, feature rain falling onto a solid last rained on?' (although in principle it could have surface in the present epoch. By presenting familiar rained elsewhere in a catchment area and the moisture cloud convection, precipitation and hydrological pro- conveyed by the ephemeral river generated as a 'flash cesses [1] in an exotic environment with a different flood'.) This is difficult to constrain: one could apply working fluid, Titan serves as a planet-scale laboratory models of vapor transport in a regolith to see how deep in which to understand these important phenomena at a the surface should dry out (like many models applied to more fundamental level. Additionally, these processes Mars water vapor exchange) but many parameters are significantly modify Titan's landscape, transporting poorly constrained. More importantly, the moisture in organic material via fluvial sediment transport and via the pore space evidently contained less volatile com- solution erosion and evaporite formation. Thus to de- pounds than methane, such as ethane and perhaps ben- code Titan's geological record and to understand the zene. The equilibrium vapor pressure of methane provenance of surface organics, the rates and character above such an organic mixture could easily be as low of meteorological and hydrological processes need to as 50% of the saturation value for the pure liquid, in be assessed. which case this moisture would never dry out, being in Huygens Measurements: The Huygens probe equilibrium with the humidity in the air. made a single sounding of Titan's atmosphere at 10oS Cassini Geomorphology and Circulation Mod- o around 9am local solar time, in 2005 (Ls~300 , late els: The large-scale circulation on Titan is of course southern summer). This revealed multiple features in different due to the different atmospheric pressure, the potential temperature profile, indicating a nascent planetary rotation rate and annual timescale, such that planetary boundary layer at ~300m, and inflections at the mean meridional ('Hadley') circulation is usually 1,2 and 3km which may be vestigial and/or seasonal interhemispheric, with only a transient symmetry phase boundary layers [2]; the 3km layer likely is the control around equinox. The effect – indicated in Global Cir- on the spacing of dunes that circle Titan's low latitudes. culation Model (GCM) results for over a decade – is to The near-surface methane humidity was about 50%, dessicate the low latitudes. Indeed, Cassini mapping an amount too small to provide Convective Available shows that Titan's equatorial regions are dominated by Potential Energy (CAPE) for strong cumulus convec- large sand seas, whose dunes indicate dry conditions tion. However, parts of the tropospheric profile were for much of the time. Similarly, the polar regions (and saturated, and could permit stratiform rain or drizzle; the north in particular) have lakes and seas of methane, some turbulence characteristics in the atmosphere are which models suggest accumulate during the summer consistent with those measured in clouds on Earth. rains (the configuration of Titan's solar eccentricity and However, no direct evidence for such hydrometeors pole orientation giving a longer cooler summer in the was seen, beyond a thin layer of cloud opacity at 21km. North). Further geomorphological indicators being In a comparative climatology sense, the methane used to compare with GCMs are detailed dune mor- profile is an interesting allegory for the likely water phologies and orientation, and the presence at interme- vapor profile in the early Venus atmosphere (and that diate latitudes of alluvial fans [3] which are associated of the Earth in the future when the solar luminosity with particularly intense rainfall and fluvial transport. evolves to high levels), in that the tropopause 'cold Cassini Cloud Observations: Compared to the trap' is a rather leaky one on Titan – from a value of Earth, where average cloud cover is of the order of 50- ~5% at the surface, the methane mixing ratio falls to 65%, Titan is relatively cloud-free, with pre-Cassini only ~1.5% in the stratosphere, in contrast to the much observations indicating cover of ~0.2-1% cloud cover. smaller abundance of water in the Earth's stratosphere. A recent paper [4] summarizes Cassini observa- An important observation was made at the surface, tions of cloud activity throughout the 13 year mission: which was the apparent release of methane and other Clouds were generally more prevalent in the summer organics from the heated inlet of the mass spectrometer hemisphere, but there were surprises in locations and instrument, the surface science lamp, and perhaps the timing of activity: southern clouds were common at body of the probe itself. Images of the probe environs mid-latitudes, northern clouds initially appeared much suggest it landed in a stream bed, littered with cobbles sooner than model predictions, and north-polar summer on a sandy substrate. convective systems did not appear before the mission Comparative Climatology III 2018 (LPI Contrib. No. 2065) 2046.pdf ended. Differences from expectations constrain atmos- D., P. Claudin, J. Radebaugh, T. Tokano and B. pheric circulation models, revealing factors that best Andreotti, A 3km boundary layer on Titan indicated by match observations, including the roles of surface and Dune Spacing and Huygens Data, Icarus , 205, 719– subsurface reservoirs. The preference for clouds at 721, 2010 [3] Faulk, S.P., Mitchell, J.L., Moon, S. and mid-northern latitudes rather than near the pole is con- Lora, J.M., 2017. Regional patterns of extreme precipi- sistent with models that include widespread polar near- tation on Titan consistent with observed alluvial fan surface methane reservoirs in addition to the lakes and distribution. Nature Geoscience, 10(11), p.827. [4] seas, suggesting a broader subsurface methane table is Turtle, E. P. et al., 2018. Titan's meteorology over the accessible to the atmosphere. Cassini mission: Evidence for extensive subsurface Rain Observations: Cassini has observed two methane reservoirs, Geophysical Research Letters, in events of surface darkening associated with cloud ac- press doi:10.1029/2018GL078170 [5] Turtle, E.P., et tivity; these are best interpreted as rain events. In 2004 al., 2011. Rapid and extensive surface changes near Arrakis Planitia (34,000km2, 80oS) and in 2010 Con- Titan’s equator: Evidence of April showers. science, cordia Regio (510,000km2, 20oS [5]). Together, these 331(6023), pp.1414-1417. [6] Lorenz R.D. et al. represent ~0.7% of Titan’s surface, in 6 years. Crudely, (2018) APL Tech Digest, in press.see also 100% of the surface would then be rained on in http://dragonfly.jhuapl.edu 6*100/0.7~860 years. In reality of course, the Cassini record is unlikely to be complete (‘missing’ events might be estimated by assuming that rain cells, as on Earth, follow some distribution like a power law) and thus the recurrence interval will be somewhat shorter. However, the order of magnitude is remarkably con- sistent with the other considerations herein. Future Missions : While there is an important con- tribution to be made from groundbased observations in tracking the seasonal distribution of large-scale cloud systems, significant progress will require both orbital and landed/aerial measurements. A Titan orbiter using cameras, spectrometers, and ideally a cloud-profiling radar could observe the evolution of individual storms and constrain the precipitation process. Extended in- situ measurements of winds, temperatures and humidity would be important to understand variability in these properties and to constrain GCMs. Additionally, meas- urements of soil moisture and hydrological parameters of the surface are important to understand The Drag- onfly relocatable lander [ ] proposed to NASA's New Frontiers program could contribute in these areas via long-term landed weather and surface property meas- urements, imaging and repeated flights that profile the meteorological parameters of the lower atmosphere. Conclusions and perspective : Considered as a 'relaxation oscillator' the current (heat flux) going through the Titan climate system is small, but the ca- pacitor (moisture content of the atmosphere) is much higher, leading to violent but rare precipitation events. Thus Titan may serve as an example of the evolution of the terrestrial climate (with a warming atmosphere, able to hold moisture) to a destructive extreme. It therefore begs further study by observation and model- ing. References: [1] Haynes, A., R. D. Lorenz, and J. I. Lunine, 2018. A post-Cassini view of Titan’s methane cycle, Nature Geoscience, 11, 306-313 [2] Lorenz, R. .