Juan M. Lora

Possible tropical lakes on Titan from observations of dark terrain

Titan has clouds, rain and lakes—like Earth—but composed of methane rather than water. Unlike Earth, most of the condensable methane (the equivalent of 5 m depth globally averaged) lies in the atmosphere. Liquid detected on the surface (about 2 m deep) has been found by radar images only poleward of 50° latitude, while dune fields pervade the tropics. General circulation models explain this dichotomy, predicting that methane efficiently migrates to the poles from these lower latitudes. Here we report an analysis of near-infrared spectral images of the region between 20° N and 20° S latitude. The data reveal that the lowest fluxes in seven wavelength bands that probe Titans surface occur in an oval region of about 60 × 40 km2, which has been observed repeatedly since 2004. Radiative transfer analyses demonstrate that the resulting spectrum is consistent with a black surface, indicative of liquid methane on the surface. Enduring low-latitude lakes are best explained as supplied by subterranean sources (within the last 10,000 years), which may be responsible for Titan’s methane, the continual photochemical depletion of which furnishes Titans organic chemistry.

GCM Simulations of Titan's Middle and Lower Atmosphere and Comparison to Observations

Abstract Simulation results are presented from a new general circulation model (GCM) of Titan, the Titan Atmospheric Model (TAM), which couples the Flexible Modeling System (FMS) spectral dynamical core to a suite of external/sub-grid-scale physics. These include a new non-gray radiative transfer module that takes advantage of recent data from Cassini–Huygens, large-scale condensation and quasi-equilibrium moist convection schemes, a surface model with “bucket” hydrology, and boundary layer turbulent diffusion. The model produces a realistic temperature structure from the surface to the lower mesosphere, including a stratopause, as well as satisfactory superrotation. The latter is shown to depend on the dynamical core’s ability to build up angular momentum from surface torques. Simulated latitudinal temperature contrasts are adequate, compared to observations, and polar temperature anomalies agree with observations. In the lower atmosphere, the insolation distribution is shown to strongly impact turbulent fluxes, and surface heating is maximum at mid-latitudes. Surface liquids are unstable at mid- and low-latitudes, and quickly migrate poleward. The simulated humidity profile and distribution of surface temperatures, compared to observations, corroborate the prevalence of dry conditions at low latitudes. Polar cloud activity is well represented, though the observed mid-latitude clouds remain somewhat puzzling, and some formation alternatives are suggested.

Titan's asymmetric lake distribution mediated by methane transport due to atmospheric eddies

The hemispheric asymmetry of Titans surface methane has been proposed to be a consequence of orbital forcing affecting Titans hydrologic cycle, but the mechanism behind asymmetrical transport of moisture remains to be examined. Using general circulation model simulations of Titans atmosphere, we show that atmospheric moisture transport by three-dimensional tropospheric eddies is critical in generating Titans surface liquid asymmetry. Comparison of axisymmetric and three-dimensional simulations demonstrates that a significant asymmetry only develops in the latter case. Analysis of the components of the three-dimensional moisture transport reveals that nonaxisymmetric eddies transport methane away from the poles and into the midlatitudes, where they transfer moisture into the cross-equatorial transport by the mean meridional circulation, producing an atmospheric “bucket brigade.” Because these high-latitude, baroclinic eddies are more intense in the south than in the north, the net transport is preferentially northward, with the northern hemisphere gaining surface liquid at the expense of the southern hemisphere.

Abrupt reorganization of North Pacific and western North American climate during the last deglaciation

Dramatic hydroclimate shifts occurred in western North America during the last deglaciation, but the timing and mechanisms driving these changes are uncertain and debated, and previous modeling has largely relied on linear interpolations between equilibrium snapshot simulations. Using a published transient climate simulation and a range of proxy records, we analyze the regions climate evolution in order to identify the mechanisms governing hydroclimate shifts. A rapid loss of ice around 14,000 years ago causes an abrupt reorganization of the circulation, which precipitates drying and moistening of southwestern and northwestern North America, respectively. The atmospheric circulation transitions between two states on a timescale of decades to centuries, during which time the westerly jet shifts north by about seven degrees. In contrast to previous studies, we find that changes in the water budget of western North America prior to this event are not attributable to variations in the position of the jet, but rather to the intensity of moisture transport into the continent.

Abrupt regime shifts in the North Atlantic atmospheric circulation over the last deglaciation

We analyze modeling results of the North Atlantic atmospheric winter circulation from a transient climate simulation over the last 21,000 years. In agreement with previous studies, we find that the midlatitude jet stream assumes a strong, stable, and zonal disposition so long as the North American ice sheets remain in their continent-wide Last Glacial Maximum (LGM) configuration. However, when the Laurentide (LIS) and Cordilleran Ice Sheets separate (ca.14,000 years ago), the jet stream abruptly changes to a tilted circulation regime, similar to modern. The proposed explanation is that the dominant stationary-wave source in the North Atlantic sector changes from the LIS to the Cordilleran mountain range during the saddle collapse. As long as the LIS dominates, the circulation retains the zonal LGM state characterized by prevalent stationary-wave reflection in the subtropical North Atlantic. When the Cordillera takes over, the circulation acquires its modern disposition.

The near-surface methane humidity on Titan

We retrieve vertical and meridional variations of methane mole fraction in Titans lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Adamkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titans lower troposphere is undersaturated at latitudes south of 60N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region.

Titan's Meteorology Over the Cassini Mission: Evidence for Extensive Subsurface Methane Reservoirs

Cassini-Huygens mission, a cooperative endeavor of NASA; ESA; ASI; NASA; Cassini-Huygens grant [NNX13AG28G]; Institut Universitaire de France; UnivEarthS LabEx program of Sorbonne Paris Cite [ANR-10-LABX-0023, ANR-11IDEX-0005-02]; French National Research Agency [ANR-APOSTIC-11-BS56-002, ANR-12-BS05-001-3/EXO-DUNES]