Tetsuya Tokano's research while affiliated with University of Cologne and other places

Publications (59)

Article
Titan’s paleoclimate in the presence of hypothetical hydrocarbon oceans 1 Gyr ago is simulated by a global climate model with a slab ocean model. Simulations are run for two different ocean compositions and with and without continents. The climate over ethane-rich oceans is dry with respect to methane but is windier than over methane-rich oceans. C...
Article
Full-text available
Titan's paleoclimate after the onset of the putative last major methane outgassing event 700 Myr ago is simulated by a global climate model. If the atmosphere was methane‐depleted prior to outgassing, outgassed methane initially causes warming due to increased greenhouse effect. Further outgassing leads to methane snowfall, which in turn cools the...
Article
Full-text available
NASA’s Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan’s prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based “life as we know it” (as might occur in the interior mantle ocean, potential cryovolcanic flows, an...
Article
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A general circulation model with geography constrained by Cassini is used to predict how ethane precipitation in Titan’s lower stratosphere varies with latitude, season, and orbital forcing over the past 100 kyr. Ethane precipitation is generally more prevalent near the winter pole, where stratospheric ethane is transported downward toward the cold...
Article
The temporal variation in the size of Ontario Lacus, the largest lake in the southern polar region of Titan, over the past 45 kyr caused by seasonal and orbital forcing is simulated by a lake balance model along with the meteorological input from paleoclimate simulations. Orbital forcing can change the lake volume and lake area by a factor of three...
Article
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Plain Language Summary Saturn's largest moon Titan has many hydrocarbon lakes in the moist polar region and dunes in the dry equatorial region. However, one of the imaging instruments of Cassini possibly saw tropical lakes, while other imaging instruments did not. If true, the presence of tropical lakes on Titan is surprising at a glance, as one ex...
Article
Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan's atmosphere and climate, and several three-dimensional general circulation models (GCMs) now exist that simulate Titan with a high degree of fidelity. However, subs...
Article
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Seasonal variations in lake levels of Titan's hydrocarbon seas/lakes are predicted by an ocean circulation model in an effort to understand the observed temporal changes in lake size or lack thereof. Three different ground permeabilities are assumed so as to change the relative importance of precipitation, evaporation, river runoff, and groundwater...
Article
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The entry, descent and landing of Schiaparelli, the ExoMars Entry, descent and landing Demonstrator Module (EDM), offered a rare (once-per-mission) opportunity for in situ investigations of the martian environment over a wide altitude range. The aim of the ExoMars AMELIA experiment was to exploit the Entry, Descent and Landing System (EDLS) enginee...
Article
Previously, seasonal asymmetry in Titan's climate caused by Saturn's orbital eccentricity was proposed as a possible external cause of the asymmetric polar lake distribution on Titan. However, climate studies of other planets indicate that seasonal asymmetry can also be caused by a hemispheric asymmetry in geography such as the ocean-continent dist...
Article
The strong, nearly wavelength-independent absorption cross section of aerosols produces featureless exoplanet transmission spectra, limiting our ability to characterize their atmospheres. Here we show that even in the presence of featureless spectra, we can still characterize certain atmospheric properties. Specifically, we constrain the upper and...
Article
Temperature profiles of Titan’s atmosphere obtained by Cassini radio occultations approach the nitrogen condensation temperature curve at some places. This raises the question as to whether nitrogen, Titan’s main atmospheric constituent, might condense in some seasons and areas contrary to previous perception. To address this question, possible are...
Article
Titan experiences solar eclipses by Saturn on ~20 consecutive orbits around equinox for durations of up to ~6 h. The impact of these eclipses on Titan׳s surface, lower atmosphere and middle atmosphere is investigated by a global climate model. When an eclipse commences, the surface temperature on the subsaturnian side drops by up to 0.3 K, so that...
Article
Wind-blown dunes are a record of the climatic history in Titan’s equatorial region. Through modeling of the climatic conditions associated with Titan’s historical orbital configurations (arising from apsidal precessions of Saturn’s orbit), we present evidence that the orientations of the dunes are influenced by orbital forcing. Analysis of 3 Titan...
Article
Density-driven circulation in Titan's seas forced by solar heating and methane evaporation/precipitation is simulated by an ocean circulation model. If the sea is transparent to sunlight, solar heating can induce anti-clockwise gyres near the sea surface and clockwise gyres near the sea bottom. The gyres are in geostrophic balance between the radia...
Conference Paper
Full-text available
Dune fields on Titan cover more than 17 % of the moon's surface, constituting the largest known surface reservoir of organics. Their confinement to the equatorial belt, shape, and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply. Herein, we present a comprehensive analysis of Titan's dun...
Article
The availability of liquid water on the surface on Earth's continents in part relies on the precipitation of water. This implies that the habitability of exomoons has to consider not only the surface temperature and atmospheric pressure for the presence of liquid water, but also the global precipitation climatology. This study explores the sensitiv...
Article
Circulation in Titan's seas forced by wind is simulated by an ocean circulation model using surface wind data predicted by a global circulation model. Wind-driven circulation is insignificant throughout much of the annual cycle, but becomes significant from late spring to late summer, when the wind stress becomes strong. The large-scale circulation...
Article
Tides and tidal currents in Titan’s hydrocarbon seas are numerically simulated by a 3-dimensional ocean circulation model using a bathymetry map constrained by Cassini. These predictions are used to calculate the tidally induced variations of the oceanic angular momentum of the seas. The tides behave as a quasi-standing wave with anti-nodes at the...
Article
Full-text available
Dune fields on Titan cover more than 17% of the moon's surface, constituting the largest known surface reservoir of organics. Their confinement to the equatorial belt, shape, and eastward direction of propagation offer crucial information regarding both the wind regime and sediment supply. Herein we present a comprehensive analysis of Titan's dune...
Article
Fundamental questions involving the origin, evolution, and history of both Titan and the broader Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. Observations from an orbit-platform ca...
Article
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We present a radar map of the Titan’s seas, with bathymetry estimated as proportional to distance from the nearest shore. This naïve analytic bathymetry, scaled to a recent radar sounding of Ligeia Mare, suggests a total liquid volume of ∼32,000 km3, at the low end of estimates made in 2008 when mapping coverage was incomplete. We note that Kraken...
Article
The global distribution of atmospheric pressure retrieved from Cassini radio occultations is reanalyzed and interpreted in a meteorological context. The retrieval of Titan’s atmospheric pressure profile from radio occultations is ambiguous because it depends on the methane mole fraction, which is not precisely known in the troposphere beyond the en...
Article
The centrifugal force associated with the superrotation in Venus' and Titan's middle atmosphere reduces the effective gravity and thereby modifies the shape of the geopotential surface, which manifests itself as an equatorial bulge. General circulation models (GCMs) based on the hydrostatic primitive equations cannot correctly represent this dynami...
Article
While extratropical cyclones cannot be expected in Titan’s barotropic troposphere, tropical cyclones which gain their energy from the latent heat of sea evaporation cannot be entirely dismissed over Titan’s polar hydrocarbon seas. The most essential condition for the genesis of tropical cyclones on Titan is a methane-rich composition of the polar s...
Article
Titan's polar regions, and its hydrocarbon lakes in particular, are of interest for future exploration. The polar conditions have considerable seasonal variation and are distinct from the equatorial environment experienced by Huygens. Thus specific environmental models are required for these regions. This paper, informed by Cassini and groundbased...
Article
Previously, the large angular momentum of Titan's superrotating atmosphere was simulated and discussed by global climate models that assumed a globally flat surface. This assumption is no longer valid since Titan lacks a global ocean on the surface and instead possesses a diverse surface with topography of various scales. This study investigates ho...
Article
We use two independent General Circulation Models (GCMs) to estimate surface winds at Titan’s Ligeia Mare (78°N, 250°W), motivated by a proposed mission to land a floating capsule in this ∼500km hydrocarbon sea. The models agree on the overall magnitude (∼0.5–1m/s) and seasonal variation (strongest in summer) of windspeeds, but details of seasonal...
Article
An exotic arrow-shaped cloud was discovered in the atmosphere of Saturn's moon Titan last year. Numerical modelling shows how a large-scale atmospheric wave can naturally shape tropical clouds to such an arrow.
Article
Titan's atmosphere possesses an equatorial component of angular momentum, which can be transferred to the surface and can excite polar motion of Titan. The atmospheric excitation of Titan's polar motion is calculated using the wind and pressure data prediction from a general circulation model. The polar motion equation is solved considering Titan's...
Article
The climate of planets is often described in terms of surface temperature only. In contrast, the widely used climate classification of Köppen (1) considers not only the surface temperature but also precipitation. Saturn's moon Titan is a rare extraterrestrial body where a similar concept could be applied to describe the climate because there is a m...
Article
Numerous linear dunes with eastward streamline pattern on Titan have mostly been interpreted as evidence of predominantly westerly (eastward) equatorial surface winds, although such winds are meteorologically hard to understand. Simulation with a general circulation model shows that the observed dune alignment nearly parallel to the equator maximiz...
Article
Numerous hydrocarbon lakes have recently been detected on Saturn’s largest moon Titan, representing the only known large bodies of liquids on a planetary surface outside the Earth. In the context of comparative oceanography, tides and tidal currents in two representative lakes on Titan (Kraken Mare and Ontario Lacus) are simulated by a three-dimens...
Article
The atmospheres/exospheres of icy satellites greatly vary from one to the next in terms of density, composition, structure or steadiness. Titan is the only icy satellite with a dense atmosphere comparable in many ways to that of the Earth’s atmosphere. Titan’s atmosphere prevents the surface from direct interaction with the plasma environment, but...
Article
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The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the...
Article
The orientation of the atmospheric angular momentum vector of Titan and its temporal variation predicted by a general circulation model are analysed and interpreted. The atmospheric angular momentum vector is tilted by a few degrees from the polar axis and the vector rotates (precesses) westward with a constant period of 1 Titan day. The fast westw...
Article
Some 20% of Titan’s surface is covered in large linear dunes that resemble in morphology, size and spacing (1–3 km) those seen on Earth. Although gravity, atmospheric density and sand composition are very different on these two worlds, this coincident size scale suggests that the controlling parameter limiting the growth of giant dunes, namely the...
Article
The detection of large hydrocarbon seas/lakes near the poles by the Cassini spacecraft raises the question as to whether and how polar seas affect the meteorology on Titan. The polar meteorology and methane hydrological cycle in the presence of seas are investigated by a three-dimensional atmospheric general circulation model coupled to a one-dimen...
Article
Full-text available
Saturn’s satellite Titan is a particularly interesting body in our solar system. It is the only satellite with a dense atmosphere, which is primarily made of nitrogen and methane. It harbours an intricate photochemistry, that populates the atmosphere with aerosols, but that should deplete irreversibly the methane. The observation that methane is no...
Article
Cassini radar recently detected several putative liquid hydrocarbon lakes in the polar region of Saturn's moon Titan. Such lakes may contain organic sediments deposited from the atmosphere that would promote prebiotic-type chemistry driven by cosmic rays, the result of which could be the production of more complex molecules such as nitrogen-bearing...
Article
While the Voyager mission could essentially not reveal the dynamics of Titan's troposphere, useful information was obtained by the Cassini spacecraft and, particularly, by the Huygens probe that landed on Titan's surface; this information can be interpreted by means of numerical models of atmospheric circulation. The meridional circulation is likel...
Article
Numerous extended dunes on Saturn's moon Titan detected by the Cassini RADAR constrain the long-term pattern of surface winds. We analyse the statistics of surface wind speed and direction and their spatial and temporal variability predicted by a general circulation model (GCM) in order to constrain surface wind predictions of this GCM by dune obse...
Article
This study aims at interpreting the zonal and meridional wind in Titan's troposphere measured by the Huygens probe by means of a general circulation model. The numerical simulation elucidates the relative importance of the seasonal variation in the Hadley circulation and Saturn's gravitational tide in affecting the actual wind profile. The observed...
Article
On January 14, 2005, the Huygens probe descended on the surface of Titan, the largest moon of Saturn and the only moon in the Solar system with a substantial atmosphere. After the deployment of the main parachute and the release of the heat shield at an altitude of about 150 km, the local pressure and temperature were measured by HASI (Huygens Atmo...
Article
Huygens Atmospheric Structure Instrument (HASI) for the first time performed an in situ measurement of the thermal structure in Titan's atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer (PBL). The vertical potential temperature profile reveals the presence of a weakly convective PBL, with a surface layer thick...
Article
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Saturn's moon Titan shows landscapes with fluvial features suggestive of hydrology based on liquid methane. Recent efforts in understanding Titan's methane hydrological cycle have focused on occasional cloud outbursts near the south pole or cloud streaks at southern mid-latitudes and the mechanisms of their formation. It is not known, however, if t...
Article
Possible trajectories of passive balloons in Titan's troposphere are simulated with the instantaneous wind field predicted by a GCM (general circulation model). In most areas the basic motion of a balloon is a predominantly eastward or westward drift, depending on altitude, latitude and season of the balloon release point. Some meridional oscillati...
Article
Possible hydrocarbon lakes on Titan’s surface, as seem to be present according to recent radar observations, may play an important role in Titan’s chemical evolution in that they would protect organic material from photochemical destruction and may enable the production of new organic species therein. The lake temperature and its spatial and season...
Article
Titan's substantial obliquity and the global extent of the Hadley circulation give rise to seasonal variation in the mean zonal wind speed and direction in the geostrophic lower troposphere, causing an exchange of a substantial amount of angular momentum between the surface and atmosphere. The wind-induced seasonal length-of-day variation calculate...
Article
Full-text available
Recent experimental studies have suggested that zeolites, if present on the Martian surface, could undergo a strong diurnal cycle of hydration and dehydration with possible impact on the atmosphere. This study evaluates this possibility using a global model of hydration/dehydration of two different zeolites (clinoptilolite and chabazite) assuming a...
Article
The latitudinal profile of near-surface air temperature on Titan retrieved by Voyager 1 has been difficult to understand and raised several speculations about possible exotic processes that might be occurring near Titan's surface, while the thermal properties of the surface itself are unknown. This study systematically investigates the seasonal and...
Chapter
The global water cycle on Earth constitutes one of the most relevant components of the terrestrial ecosystem. While the vast majority of terrestrial water is stored in the world oceans, the perpetual cycle of water between ocean, atmosphere and land in all three phases is recognised as one basic feature that characterises the Earth, and is contrast...
Article
In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface–atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution i...
Article
A linear correlation analysis between the Mars Odyssey neutron fluxes and various surface parameters indicates that the annual maximum surface temperature is the most important factor controlling the soil water content in the surficial (upper few tens of centrimetres) layers of the Martian soil. This is likely to be associated with the higher entha...
Chapter
A milestone in the understanding of the Martian hydrological evolution and water cycle is being marked by the Gamma-Ray Spectrometer (GRS) onboard Mars Odyssey ([1], [2], [4]). These gamma rays are supposed to be produced during inelastic collision and/or capture of secondary neutrons which were produced in the soil by cosmic rays. The signals are...
Article
The influence of Saturn's gravitational tide on the atmosphere of Titan is investigated by means of a three-dimensional general circulation model. Titan's orbital eccentricity of 0.0292 gives rise to time-dependent radial and librational tide whose potential circles eastward on Titan. Unlike atmospheric tides on terrestrial planets, Saturn's tide o...

Citations

... Dragonfly, an astrobiological drone mission to Titan is scheduled to launch in 2027 (Barnes et al., 2021) with arrival planned for 2036. The proposed landing site, on the edge of the Shangri-La Sand Sea near Selk Crater, is not close to any known mapped karstic landscapes or SAPs (Malaska et al., 2016. ...
... The predicted time series of hemispheric ethane precipitation (Figure 9(b)) implies that one hemisphere receives up to 2 × 10 13 kg, or 30 km 3 more ethane precipitation than the opposite hemisphere over a period of ∼20 kyr. If all ethane precipitation in the southern hemisphere drains to Ontario Lacus, whose volume varies between 250 and 500 km 3 owing to secular variation in methane precipitation (Tokano 2021), the predicted variation in ethane precipitation would change the ethane content at most by 10% over the course of the Croll-Milankovitch cycle. In reality, the catchment area of Ontario Lacus covers 11% of the southern hemisphere (Dhingra et al. 2018), which would reduce the ethane input to Ontario Lacus by one order of magnitude and reduce the secular variation in ethane content of this lake to 1%. ...
... The studied parameters were the initial relative humidity of the lowest atmospheric layer, the subsurface temperature boundary condition, the initial lake surface temperature, the lake mixed layer depth, the background wind speed, the solar longitude (season), and the latitude. Seas and lakes on Titan are mostly found at high latitudes (Punga Mare at 85°N, Ligeia Mare at 79°N, Jingpo Lacus at 73°N, Ontario Lacus at 72°S, and Kraken Mare at 68°N), although some are observed at midlatitudes (Hammar Lacus at 49°N, Sionascaig Lacus at 42°S, and Urmia Lacus at 39°S; Griffith et al. 2012;Vixie et al. 2015;Tokano 2020). As the scope of this paper is to study the effect of 2 radiation on the evaporation of lakes, and as the solar radiative forcing is higher at lower latitudes, we performed most simulations at the lowest latitude at which lakes are found: 42°. ...
... We assess the aeolian transport potential of Titan's prevailing circulation by implementing the proposed mass flux parameterization (Equation 6) in the TAM (Lora et al., 2015(Lora et al., , 2019 accounting for the effect of large-scale topography (Corlies et al., 2017) (see Section S8 in Supporting Information S1 for additional detail). We perform runtime calculations of the wind-driven saltation mass flux for 30 Titan years, using surface friction velocities and intermittency factors computed at the model time step of 10 min. ...
... Here, we propose a radar system that is able to provide three-dimensional high resolution bathymetric maps of Titan seas and lakes at multiple frequencies, allowing for the investigation of any possible spatial [32,33] or seasonal [34] variation in composition of the liquid and/or sea floor properties. The system will additionally enable the investigation of any suspended particulates-material, density heterogeneities of the liquid column within a resolution of few meters. ...
... Broader reaching questions on how to deal with incomplete EDL data, how to best use and design FADS instrumentation, and how to develop a comprehensive suite of reconstruction methodology, will arise spontaneously and are addressed in the course of this work. The Ph.D. research was performed in the scope of the AMELIA scientific research group, selected by ESA and responsible for the atmospheric reconstruction of Schiaparelli (Ferri, 2019). Due to limited funding for post-flight analysis after the mission, the work by AMELIA and the present research are significant contributions to the mission outcome in terms of its lessons learned. ...
... Other factors are also very likely to influence or even drive the accumulation of moisture at the North pole, like enhanced precipitations driven by the global circulation (Aharonson et al. 2009;Schneider et al. 2012;Lora et al. 2014;Lora & Mitchell 2015;Tokano 2019). However, cloud observations ) and recent models (Lora et al. 2022) do not completely support this, and more in detail investigation is needed to understand the effect of lake-induced circulation on deep convection and precipitation. ...
... Furthermore, this spread does not seem to be directly correlated with the amount of irradiation they receive from the host star. The origin of this phenomenon is unclear and several factors are likely to play a role, such as protoplanetary nebula lifetime (e.g., , mass accretion time (e.g., Schiller et al. 2018), migration history, collisions (e.g., Bonomo et al. 2019), received amount of high-energy irradiation (e.g., Fossati et al. 2017;Owen and Wu 2016;Jin and Mordasini 2018;Kubyshkina et al. 2018bKubyshkina et al. , 2018c, possible biases in the measurement of planetary radii due to the presence of high-altitude aerosols (e.g., Cubillos et al. 2017aCubillos et al. , 2017b. ...
... The partial pressure of N 2 ( 2 , ) is thus held fixed at the present value (1.441 bar), which is determined from the atmospheric surface pressure (1.467 bar) measured by the Huygens Probe (Fulchignoni et al., 2005) and the vertical profile of atmospheric composition measured by the Huygens Probe (Niemann et al., 2010). Liquid N 2 cloud formation is possible in Titan's troposphere under cold conditions (Charnay et al., 2014;McKay et al., 1993;Tokano, 2017). However, this effect is neglected since liquid N 2 is radiatively inactive and the scattering greenhouse effect by N 2 cloud particles has a generally minor influence on the surface temperature (Charnay et al., 2014). ...
... However, present-day climatic conditions are incompatible with stable liquid surface water (Chevrier et al., 2020). The evolution and contemporary distribution of water is one of the most important open questions in the Martian geoscience because of its significance for the planet's climate evolution, potential habitability, and human exploration (Baker, 2004;Tokano, 2004). As a result, there has been great interest, excitement, and debate surrounding the paper by Orosei et al. (2018) reporting evidence for liquid water beneath Mars south polar ice cap. ...
Citing chapter