What does Titan look like?

Cassini unmasks Titan

Figure 1: Image of Titan taken by the Voyager ship when going through the system. The cover of clouds and dust frequently does not reveal the surface. (Credit: NASA)

Before the Cassini probe began its journey through the Saturn system, little was known about Titan (one of the many moons of Saturn). When Voyager It happened near Titan in 1981, it obtained a brownish and diffuse image of this moon (see Figure 1). For the next 24 years, we are left with the question: what could be happening on the surface of Titan, obscured by its dense atmosphere?

The answer to this question came in 2004, when Cassini, equipped with instruments capable of measuring different wavelengths, arrived at the Saturn system. Using multiple colors, it was able to take images of different layers of Titan's atmosphere and at certain wavelengths, it was even possible to reveal its surface. In doing so, it was discovered that Titan is, in many ways, very similar to Earth, albeit with a lower temperature, of 100 K. One of the most striking similarities is that the methane cycle on Titan it is almost equivalent to the terrestrial hydrological cycle. Astronomers have observed giant methane storms in the atmosphere, methane lakes near the poles and arid regions of dunes likely due to some type of frozen methane or hydrocarbon "sand" in the equatorial zone. These dune fields are the subject of the work we describe today, as the authors have noticed that unusually bright spots have appeared in this region three times throughout Cassini's 13-year visit.

The dates corresponding to the appearance of this phenomenon were June 7, 2009, January 12, 2010 and June 21, 2010. Figure 1 shows several images of Titan taken by the VIMS instrument (Visual and Infrared Mapping Spectrometer, for its acronym in English) on board Cassini. The different colors represent different wavelengths, with red corresponding to an average intensity that falls between 5 and 5.07 microns and green, between 2 and 2.78 microns. The location of the bright spot is represented by an arrow and tends to appear pink on this scale.

Figure 2: Images of Titan taken by VIMS in different approaches. The colors represent different intensities at different wavelengths. Red corresponds to the range between 5 and 5.07 microns and green, to the range between 2 and 2.78 microns. The bright spot near the dune fields is indicated by a white arrow and appears pink compared to the darker dunes on the surface. (Figure 1 of the article.)

These three events lasted between 11 and 14 hours, the duration of Cassini's near passage by Titan. However, as Cassini had other tasks beyond monitoring, they were not observable until the next probe approach, 4-5 Earth weeks later (3 days on Titan), by the time they were gone. The moment in which the events occurred is striking: they all took place during the equinox on Titan, when the Sun was directly above the equator. Since it takes Titan (and Saturn) 30 Earth years to orbit the Sun, the equinox lasts for several Earth years and these three events occurred during a single equinox. Whatever the cause, the authors suggest that it is in some way correlated with solar heating in the region.

Cloudy with the possibility of methane?

The bright areas in the VIMS images are not uncommon. The presence of large clouds of methane on Titan is correlated with methane storms, which appear bright in both the infrared and optical. Some of these storms have been observed even in the equatorial region, although they are rare. However, after discovering that these events disappeared at wavelengths less than 1.6 microns, the authors ruled out the possibility of a methane cloud. His decision was validated when considering atmospheric models to determine the object's altitude. They concluded that the events should occur at an altitude of less than 14 km, much less than that of most methane clouds observed. Moreover, by making certain approximations about the relative humidity of Titan and the convective nature of a cloud, the authors calculated that a methane cloud should at least extend to a minimum height of 25 km. So, whatever is causing this polishing phenomenon, it is not related to a typical methane cloud.

A cryovolcanic eruption (an icy volcano)?

Infrared waves can also be a good temperature indicator. A bright spot usually represents a hot spot. The authors investigated the hypothesis that this event is correlated with some type of hot lava emitted by a cryovolcano. However, a comparison of the surface just before and just after the appearance of the bright zone indicated that the temperature at the surface remained constant. It would be difficult for the lava to cool sufficiently fast by the time of Cassini's new appearance. Although the idea of cryovolcanism is attractive, it is not the cause of the observed phenomenon.

Organic dust storms?

The authors propose the theory that Titan, like Earth and Mars, is susceptible to dust storms in arid areas. As in the Earth and Mars, the dunes are usually present in arid regions. The difference is that the "dust" on Titan would be composed of some kind of micron-size organic particles (tolines) that would be easy to lift with a relatively small wind speed. In fact, when the probe Huygens landed on Titan, detected a very thin layer of material that could be the culprit! The authors extended their atmospheric models to simulate different wind speeds and particle sizes and were successful in creating a dust cloud that adjusts the signal observed in the VIMS images (see Figure 3). However, the winds required to raise and suspend the particles in the atmosphere for a long time must be stronger than those typically observed on the surface of Titan (from several m / s to tens of m / s). The authors propose that these winds could be caused by a methane storm. This process is similar to that of beginning of some storms on EarthWhen the pressure drops, the temperature goes down and the wind raises its speed. As only a few equatorial storms have been observed on Titan, this could explain why these dust storms are uncommon. One problem with this theory is that there do not appear to be methane storms occurring at the same time as dust storms, although this observation could be due to the limited time of Cassini observation during each approach. Still, the authors suggest that if the dust storm theory becomes successful, Titan would join the set of objects in the solar system, which include Earth and Mars, where there is a eolic erosion active

Figure 3: The brightness of one of the three events as a function of wavelength. The data is plotted in yellow and the curves represent three different atmospheric models. The black line assumes that there is no cloud; the blue assumes the presence of a methane cloud with a total height of 13 km; and the red, the presence of an organic cloud of dust at a height of 12 km. While the model that best fits the data is that of the dust cloud ("Tholin cloud" in the legend), the authors do not rule out the possibility of a methane cloud based on its fit. They say, however, that no methane cloud could exist at such a low height, indicating that the model in blue does not have a good physical sustenance. (Part c of figure 3 of the article.)

The legacy of Cassini

After making his 20 year visit to the Saturn system, the Cassini mission was immersed in the atmosphere of the gas giant in September 2017. However, his legacy still lives, as evidenced by today's publication. The data taken by Cassini years ago continue to be analyzed and re-analyzed, leading to new surprises, discoveries and also mysteries about Saturn and its moons. The dust storms were discovered based on images taken 8 years ago. Who knows what the next step in the archives is for us!

Cover image: IPGP / Labex UnivEarthS / University Paris Diderot - C. Epitalon & S. Rodriguez