The magnetic memory of Titan's ionized atmosphere

After 3 years and 31 close flybys of Titan by the Cassini Orbiter, Titan was finally observed in the shocked solar wind, outside of Saturn's magnetosphere. These observations revealed that Titan's flow-induced magnetosphere was populated by "fossil" fields originating from Saturn, to which the satellite was exposed before its excursion through the magnetopause. In addition, strong magnetic shear observed at the edge of Titan's induced magnetosphere suggests that reconnection may have been involved in the replacement of the fossil fields by the interplanetary magnetic field.     

Evidence for the exposure of water ice on Titan's surface

The smoggy stratosphere of Saturn's largest moon, Titan, veils its surface from view, except at narrow wavelengths centered at 0.83, 0.94, 1.07, 1.28, 1.58, 2.0, 2.9, and 5.0 micrometers. We derived a spectrum of Titan's surface within these "windows" and detected features characteristic of water ice. Therefore, despite the hundreds of meters of organic liquids and solids hypothesized to exist on Titan's surface, its icy bedrock lies extensively exposed.     

Rapid and extensive surface changes near Titan's equator: evidence of April showers

Although there is evidence that liquids have flowed on the surface at Titan's equator in the past, to date, liquids have only been confirmed on the surface at polar latitudes, and the vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate. We report the detection by Cassini's Imaging Science Subsystem of a large low-latitude cloud system early in Titan's northern spring and extensive surface changes (spanning more than 500,000 square kilometers) in the wake of this storm. The changes are most consistent with widespread methane rainfall reaching the surface, which suggests that the dry channels observed at Titan's low latitudes are carved by seasonal precipitation.     

Widespread morning drizzle on Titan

Precipitation is expected in Titan's atmosphere, yet it has not been directly observed, and the geographical regions where rain occurs are unknown. Here we present near-infrared spectra from the Very Large Telescope and W. M. Keck Observatories that reveal an enhancement of opacity in Titan's troposphere on the morning side of the leading hemisphere. Retrieved extinction profiles are consistent with condensed methane in clouds at an altitude near 30 kilometers and concomitant methane drizzle below. The moisture encompasses the equatorial region over Titan's brightest continent, Xanadu. Diurnal temperature gradients that cause variations in methane relative humidity, winds, and topography may each be a contributing factor to the condensation mechanism. The clouds and precipitation are optically thin at 2.0 micrometers, and models of "subvisible" clouds suggest that the droplets are 0.1 millimeter or larger.     

Planetary science. The weather on Titan

When the Voyager 1 spacecraft returned images in 1980, the dense atmosphere of Saturn's moon Titan was assumed to be bland and featureless. As Lorenz discusses in his Perspective, recent ground-based spectroscopy, and images from the Hubble Space Telescope, are changing this perception. Observations such as the short-lived clouds in Titan's atmosphere reported by Griffith et al. suggest that although average precipitation is likely to be low, individual precipitation events may be heavy enough to cause deep valleys on Titan's surface.     

Evidence for a polar ethane cloud on Titan

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51 degrees to 68 degrees north and all longitudes observed (10 degrees to 190 degrees west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.     

The tides of Titan

We have detected in Cassini spacecraft data the signature of the periodic tidal stresses within Titan, driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of Cassini during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k(2) = 0.589 ± 0.150 and k(2) = 0.637 ± 0.224 (2σ). Such a large response to the tidal field requires that Titan's interior be deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.     

Intensive Titan exploration begins

The Cassini Orbiter spacecraft first skimmed through the tenuous upper atmosphere of Titan on 26 October 2004. This moon of Saturn is unique in our solar system, with a dense nitrogen atmosphere that is cold enough in places to rain methane, the feedstock for the atmospheric chemistry that produces hydrocarbons, nitrile compounds, and Titan's orange haze. The data returned from this flyby supply new information on the magnetic field and plasma environment around Titan, expose new facets of the dynamics and chemistry of Titan's atmosphere, and provide the first glimpses of what appears to be a complex, fluid-processed, geologically young Titan surface.     

Detection of daily clouds on Titan

We have discovered frequent variations in the near-infrared spectrum of Titan, Saturn's largest moon, which are indicative of the daily presence of sparse clouds covering less than 1% of the area of the satellite. The thermodynamics of Titan's atmosphere and the clouds' altitudes suggest that convection governs their evolutions. Their short lives point to the presence of rain. We propose that Titan's atmosphere resembles Earth's, with clouds, rain, and an active weather cycle, driven by latent heat release from the primary condensible species.     

The evolution of Titan's mid-latitude clouds

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal that the horizontal structure, height, and optical depth of Titan's clouds are highly dynamic. Vigorous cloud centers are seen to rise from the middle to the upper troposphere within 30 minutes and dissipate within the next hour. Their development indicates that Titan's clouds evolve convectively; dissipate through rain; and, over the next several hours, waft downwind to achieve their great longitude extents. These and other characteristics suggest that temperate clouds originate from circulation-induced convergence, in addition to a forcing at the surface associated with Saturn's tides, geology, and/or surface composition.     

The process of tholin formation in Titan's upper atmosphere

Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH4 and N2). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (approximately 1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH4 and N2) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (approximately 8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.     

Extreme ultraviolet observations from voyager 1 encounter with saturn

The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.     

Energetic neutral atom emissions from Titan interaction with Saturn's magnetosphere

The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment.     

Titan's magnetic field signature during the first Cassini encounter

The magnetic field signature obtained by Cassini during its first close encounter with Titan on 26 October 2004 is presented and explained in terms of an advanced model. Titan was inside the saturnian magnetosphere. A magnetic field minimum before closest approach marked Cassini's entry into the magnetic ionopause layer. Cassini then left the northern and entered the southern magnetic tail lobe. The magnetic field before and after the encounter was approximately constant for approximately 20 Titan radii, but the field orientation changed exactly at the location of Titan's orbit. No evidence of an internal magnetic field at Titan was detected.     

Geographic control of Titan's mid-latitude clouds

Observations of Titan's mid-latitude clouds from the W. M. Keck and Gemini Observatories show that they cluster near 350 degrees W longitude, 40 degrees S latitude. These clouds cannot be explained by a seasonal shift in global circulation and thus presumably reflect a mechanism on Titan such as geysering or cryovolcanism in this region. The rate of volatile release necessary to trigger cloud formation could easily supply enough methane to balance the loss to photolysis in the upper atmosphere.     

Radar evidence for liquid surfaces on Titan

Arecibo radar observations of Titan at 13-centimeter wavelength indicate that most of the echo power is in a diffusely scattered component but that a small specular component is present for about 75% of the subearth locations observed. These specular echoes have properties consistent with those expected for areas of liquid hydrocarbons. Knowledge of the areal extent and depth of any deposits of liquid hydrocarbons could strongly constrain the history of Titan's atmosphere and surface.     

Planetary science. The glitter of distant seas

It has long been known that Saturn's largest moon, Titan, has a thick nitrogen atmosphere, which obscures the underlying surface. In his Perspective, Lorenz highlights the report by Campbell et al., who have used the giant Arecibo and Green Bank radio telescopes as a radar to probe Titan's hidden surface. The surface appears to be distinct from those of the icy satellites of Jupiter, in both brightness and polarization. The new data show sharp spikes in the reflected microwave spectrum, indicating large, smooth areas of radar-dark material. These features suggest the widespread existence of lakes or seas of liquid hydrocarbons on Titan.     

The Titan -1:0 Nodal Bending Wave in Saturn's Ring C

The most prominent oscillatory feature observed in the Voyager 1 radio occultation of Saturn's rings is identified as a one-armed spiral bending wave excited by Titan's -1:0 nodal inner vertical resonance. Ring partides in a bending wave move in coherently inclined orbits, warping the local mean plane of the rings. The Titan -1:0 wave is the only known bending wave that propagates outward, away from Saturn, and the only spiral wave yet observed in which the wave pattern rotates opposite to the orbital direction of the ring particles. It is also the first bending wave identified in ring C. Modeling the observed feature with existing bending wave theory gives a surface mass density of approximately 0.4 g/cm(2) outside the wave region and a local ring thickness of [unknown]5 meters, and suggests that surface mass density is not constant in the wave region.     

Pioneer saturn infrared radiometer: preliminary results

The effective temperature of Saturn, 94.4 + 3 K, implies a total emission greater than two times the absorbed sunlight. The infrared data alone give an atmospheric abundance of H(2) relative to H(2) + He of 0.85 +/- 0.15. Comparison of infrared and radio occultation data will give a more precise estimate. Temperature at the 1-bar level is 137 to 140 K, and 2.5 K differences exist between belts and zones up to the 0.06-bar level. Ring temperatures range from 60 to 70 K on the south (illuminated) side and from < 60 to 67 K in the planet's shadow. The average temperature of the north (unilluminated) side is approximately 55 K. Titan's 45-micrometer brightness temperature is 80 +/- 10 K.     

Is titan wet or dry?

Titan's dense and cold nitrogen atmosphere contains a small amount of methane under conditions at least approaching those at which one or both constituents would condense. The possibility of methane and nitrogen rain clouds and global methane oceans has been discussed widely. From specific features of radio occultation and other Voyager results, however, it is concluded that nitrogen does not condense on Titan and that Titan has neither global methane oceans nor a global cloud of liquid methane droplets. Certain results indirectly support the conjecture that methane does not condense at any location. However, other considerations favor a methane ice haze high in the troposphere, and liquid and solid methane might exist on the surface and as low clouds at polar latitudes.