Geysers on Enceladus, seen by Cassini. © NASA/JPL

Spatial variations of Enceladus’ plumes

Hi there! I guess most of you have heard of Enceladus. This mid-sized icy satellite of Saturn arouses the interest of planetologists, because of its geological activity. Permanent eruptions of plumes, essentially made of water ice, have been detected at its South Pole, by the Cassini spacecraft. The study I present you today, Spatial variations in the dust-to-gas ratio of Enceladus’ plume, by M.M. Hedman, D. Dhingra, P.D. Nicholson, C.J. Hansen, G. Portyankina, S. Ye and Y. Dong, has recently been published in Icarus.

The South Pole of Enceladus

Enceladus orbits around Saturn in one day and 9 hours, at a mean distance of 238,000 km. It is the second of the mid-sized satellites of Saturn by its distance from the planet, and is in an orbital 2:1 resonance with Dione, i.e. Dione makes exactly one revolution around Saturn while Enceladus makes 2. This results in a slight forcing of its orbital eccentricity, which remains anyway modest, i.e. 0.005. Like our Moon and many satellites of the giant planets, Enceladus rotates synchronously.

Interestingly, the Cassini spacecraft detected geysers at the South Pole of Enceladus, and fractures, which were nicknamed tiger stripes. They were named after 4 Middle East cities: Alexandria, Cairo, Baghdad, and Damascus.

The South Pole of Enceladus. We can see from left to right the famous tiger stripes, i.e. Alexandria, Cairo, Baghdad and Damascus sulci. © NASA/JPL/Space Science Institute/DLR
The South Pole of Enceladus. We can see from left to right the famous tiger stripes, i.e. Alexandria, Cairo, Baghdad and Damascus sulci. © NASA/JPL/Space Science Institute/DLR

These 4 fractures are 2km-large and 500m-deep depressions, which extend up to 130 km. The plumes emerge from them. Interior models suggest that the source of these geysers is a diapir of water, located at the South Pole.

Analysis of these plumes require them to be illuminated, and observed with spectroscopic devices. This is where the instruments UVIS and VIMS get involved.

The instruments UVIS & VIMS of Cassini

The study I present you today presents an analysis of VIMS data, before comparing the results of the same event given by UVIS.

UVIS and VIMS are two instruments of the Cassini mission, which completed a 13-years tour in the system of Saturn in September 2017 with its Grand Finale, crashing in the atmosphere of Saturn. It was accompanied by the lander Huygens, which landed on Titan in 2005, and had 12 instruments on board. Among them were UVIS and VIMS.

And then, you wonder, dear reader, whether I will introduce you UVIS and VIMS, since I mention them since the beginning without introducing them. Yes, this is now.

UVIS stands for Ultraviolet Imaging Spectrograph, and VIMS for Visible and Infrared Mapping Spectrometer. Their functions are in their names: both analyze the incoming light, UVIS in the ultraviolet spectrum, and VIMS in the visible and infrared ones. And the combination of these two spectra is relevant in this study: the analysis in the ultraviolet tells you one thing (quantity of gas), while the analysis in the infrared gives the quantity of dust. When you compare them, you have the dust-to-gas ratio. Of course, this is not that straightforward. First you have to collect the data.

Analyzing a Solar occultation by the plumes

As I said, the plume needs to be illuminated. And for that, you have to position the spacecraft where the plumes occult the Sun. So, this could happen only during a fly-by of Enceladus, which means that it was impossible to have a permanent monitoring of these plumes. Moreover, from the geometry of the configuration, i.e. location of the plume, of the Sun, of the spacecraft,… you had the data at a given altitude. It is easy to figure out that the water is more volatile than dust, is ejected faster, and higher… In other words, the higher is the observation, the lower the dust-to-gas ratio.

The studied occultation happened on May 18, 2010, and lasted approximately 70 seconds, during which the illuminated plumes originated from different tiger stripes. This means that a temporal variation of the composition of plumes during the event means a spatial variation in the subsurface of the South Pole. The altitude was 20-30 km.

But detecting a composition is a tough task. Actually the UVIS data, i.e. detection of water, were published in 2011, and the VIMS ones (detection of dust) only in 2018, probably because the signal is very weak. The authors observed a Solar spectrum in the infrared, and at the exact date of the occultation, a slight flux drop occurred, which was the signature of a dusty plume. For it to be exploitable, the authors had to treat the signal, i.e. de-noise it.

After this treatment, the resulting signal was an optical depth in 256 spectral channels between 0.85 an 5.2 microns. You then need to compare it with a theoretical model of diffraction by micrometric particles, the Mie diffraction, to have an idea of the particle-size distribution. Because the particles do not all have the same size, of course! Actually, the distribution is close to a power law of index 4.

Spatial variations detected

And here is the results: at an altitude of around 25 km, the authors have found that the material emerging from Baghdad and Damascus are up to one order of magnitude, i.e. 10 times, more particle-rich than the ones emerging from Alexandria and Cairo sulci.

It is not straightforward to draw conclusions from this single event. Once more, a permanent monitoring of the plumes was impossible. Spatial variations of the dust-to-gas ratio at a given altitude could either mean something on the variations of the dust-to-gas ratio in the subsurface diapir, and/or something on the spatial variations of the ejection velocities of dust and gas. Once more, the ratio is expected to decline with the altitude, since the water is more volatile.

We dispose of data from other events, for instance a fly-by, named E7, which occurred in November 2009, of the South Pole at an altitude of 100 km, during which the Ion and Neutral Mass Spectrometer (INMS) analyzed the plumes. The data are pretty consistent with the ones presented here, but the altitude is very different, so be careful.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

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