Photometric and spectral analysis of the distribution of crystalline and amorphous ices on Enceladus as seen by Cassini

doi:10.1016/j.icarus.2007.04.019

Icarus

Volume 193, Issue 2, February 2008, Pages 397-406

https://doi.org/10.1016/j.icarus.2007.04.019 Get rights and content

Abstract

Photometric and spectral analysis of data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) has yielded significant results regarding the properties and composition of the surface of Saturn's satellite Enceladus. We have obtained spectral cubes of this satellite, containing both spatial and spectral information, with a wavelength distribution in the infrared far more extensive than from any previous observations and at much higher spatial resolution. Using a composite mosaic of the satellite, we map the distribution of crystalline and amorphous ices on the surface of Enceladus according to a “crystallinity factor” and also the depth of the temperature- and structure-dependent 1.65 micron water-ice band. These maps show the surface of Enceladus to be mostly crystalline, with a higher degree of crystallinity at the “tiger-stripe” cracks and a larger amorphous signature between these stripes. These results suggest recent geological activity at the “tiger stripe” cracks and an intriguing atmospheric environment over the south pole where amorphous ice is produced either through intense radiative bombardment, flash-freezing of cryovolcanic liquid, or rapid condensation of water vapor particles on icy microspherules or on the surface of Enceladus.

Introduction

The Cassini mission has yielded impressive observations of Saturn's icy satellites, specifically of the currently active moon Enceladus. In addition to the improved spectral resolution of many of Cassini's remote sensing instruments, the spacecraft has obtained data at new phase angles and longitudes and at spatial resolution over an order of magnitude better than that of Voyager. This new Cassini data makes it possible to model Saturn, its rings, its magnetosphere and its satellites with much greater accuracy, depth and totality than ever before.

Enceladus is one of Saturn's most intriguing icy satellites. It is the brightest object in the Solar System with a geometric albedo of greater than 1.0, indicating a highly backscattering surface (Buratti and Veverka, 1984, Verbiscer et al., 2005). Observations from ground-based telescopes and satellites such as Voyagers 1 and 2 and Cassini have shown the surface to be covered in almost pure water ice (Brown et al., 2006, Buratti and Veverka, 1984, Cruikshank et al., 2005). It also has smooth, craterless regions, implying resurfacing of the satellite. It is located in the middle of a region of maximum concentration of Saturn's E-ring and shows no leading/lagging hemispheric variation (as do all the other satellites in the E-ring), suggesting that Enceladus could be the source of the E-ring material (Buratti, 1988).

In February and March of 2005, the Cassini Magnetometer found a perturbation in Saturn's magnetic field at Enceladus and enhanced ion cyclotron wave activity, indicating a localized atmospheric anomaly (Dougherty et al., 2006). This discovery resulted in the decision to bring the July 14, 2005 targeted encounter of Enceladus down to 168 km at closest approach. This encounter revealed many interesting features of the satellite (Fig. 1) that were never seen before, such as the “tiger stripes” that flank the south pole and a circular ridge that completely encloses the region (Porco et al., 2006). In addition, an unexpected hot spot was found on the south pole by the Composite Infrared Spectrometer (CIRS) in the shape of this ridge (Fig. 2a). This area is 20 K hotter than expected from models of temperature distribution on the satellite, which predict a temperature of 68 K given Enceladus' albedo (Spencer et al., 2006). Higher resolution temperature maps (Fig. 2b) show that the extra heat is coming from the tiger stripe cracks, as the variation in temperature between the cracks and the region in between them is upwards of 17 K with a maximum surface temperature of at least 145 K at the cracks (Spencer et al., 2006).

The Cassini Ultraviolet Imaging Spectrograph (UVIS) conducted a stellar occultation on this flyby with egress over the south pole, and detected a plume of water vapor and micron-sized water ice particles emanating from that region (Hansen et al., 2006). The Cassini Ion and Neutral Mass Spectrometer (INMS) also detected this plume and the Plasma Spectrometer (CAPS) found an enhanced plasma ion flux, further confirming the existence of the atmospheric plume (Waite et al., 2006, Tokar et al., 2006). The mass supply rate of this plume would be sufficient to produce and maintain the E-ring, depleting a significant amount of Enceladus' ice mass over billions of years (Hansen et al., 2006).

Detection of the hot spot and plume fits in context with previous observations of Enceladus, which suggest an active, evolving surface (Smith et al., 1982). The cratering record and distribution of geologic features on Enceladus indicate long-term or episodic resurfacing and cryovolcanism, which is consistent with endogenic heating and an active plume (Kargel and Pozio, 1996, Porco et al., 2006). These processes could also explain the special location of Enceladus at a maximum concentration in the E-ring and its high albedo, which suggests the surface is covered in fresh ice particles.

The Visual and Infrared Mapping Spectrometer (VIMS) also found evidence of activity at Enceladus' south pole. Using VIMS data to identify the structural type of water-ice, it was determined that the south polar region of Enceladus is covered with ice of a high amorphous quality and the tiger stripes are composed of ice more crystalline in nature (Brown et al., 2006). This result implies geologic activity at the stripes and either radiation damage, cryovolcanic flash-freezing, or rapid condensation of water vapor at Enceladus' south pole. The aim of this paper is to further examine the distribution of crystalline and amorphous ices on the surface of Enceladus using various analytic techniques, specifically the determination of a ‘crystallinity factor’ in order to better understand these processes.

Section snippets

Solar System ices

Crystalline and amorphous ices are formed and maintained under different conditions. Additionally, external or internal forces, such as ion bombardment and internal heating, can cause transitions between these ice types. It is important to examine the properties of these ices and the conditions in which they can transform to better understand the processes occurring on Enceladus.

Data and analysis

We examine the distribution of crystalline and amorphous ices on the surface of Enceladus using Cassini VIMS cubes, which contain both spatial and spectral information about the surface of a target. By analyzing the shape of the 3.1 micron Fresnel reflection peak in Enceladus' water-ice spectra, we determine a “crystallinity factor” and map this parameter on an Enceladus mosaic. In addition, we also map the relative crystallinity of the surface of Enceladus by examining the depth of the 1.65

Ratioed spectra

We initially investigate the ice structure of specific regions of Enceladus by examining ratios of different ice spectra. In Fig. 8 we compare ratioed spectra from several models with the ratio of the tiger stripe spectrum to the spectrum of the region in between the stripes. These two Enceladus spectra were made by coadding the spectra of pixels in each of these two regions from the Enceladus mosaic. Fig. 8a contains five different models which show the ratioed spectra for different ice grain

Discussion and conclusions

The discovery of hot material under the surface of Enceladus' south pole is made only more intriguing by the presence of amorphous ice adjacent to the hot crystalline cracks. There are several possible mechanisms that could explain the distribution of this amorphous ice, including high-energy ion bombardment primarily at the south pole, flash freezing of cryovolcanic liquid within the south polar plume, or rapid condensation of water vapor particles on icy microspherules or directly on the

Acknowledgments

This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration. We thank Janice Gepner, Eric Newman, Richard Binzel, Jeffrey Kargel and an anonymous reviewer for their careful reviews of this paper and thoughtful comments. S. Newman would also like to thank Eric and Mark Newman for technical assistance with analyzing and imaging of this data.

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Photometric and spectral analysis of the distribution of crystalline and amorphous ices on Enceladus as seen by Cassini

Abstract

Introduction

Section snippets

Solar System ices

Data and analysis

Ratioed spectra

Discussion and conclusions

Acknowledgments

Icarus

Icarus

Icarus

Icarus

J. Mol. Spectrosc.

Icarus

Icarus

Icarus

Spectrochim. Acta A

Icarus

Icarus

Continuous random network model for amorphous solid water

Science

The 3.1 micron feature in ion-irradiated water ice

Astron. Astrophys.

Infrared spectra of ice Ih and Ic in the range 4000 to 350 cm−1

J. Chem. Phys.

Absorptivity of Ice I in the range 4000–30 cm−1

J. Chem. Phys.

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) investigation

Space Sci. Rev.

Composition and properties of Enceladus' surface

Science

Structure, spectra, and mobility of low-pressure ices: Ice I, amorphous solid water, and clathrate hydrates at T<150 K

J. Geophys. Res.

Identification of a dynamic atmosphere at Enceladus with the Cassini magnetometer

Science

The infrared spectral properties of frozen volatiles

Hydrogen peroxide production by ion irradiation of thin water ice films

Astron. Astrophys.

The temperature-dependent near-infrared absorption spectrum of hexagonal H2O ice

J. Geophys. Res.

Amorphous and crystalline ice on the Galilean satellites: A balance between thermal and radiolytic processes

J. Geophys. Res.

Enceladus' water vapor plume

Science

Infrared spectra of ice Ih and Ic in the range 4000 to 350 cm⁻¹

Absorptivity of Ice I in the range 4000–30 cm⁻¹

Structure, spectra, and mobility of low-pressure ices: Ice I, amorphous solid water, and clathrate hydrates at $T < 150 K$

The temperature-dependent near-infrared absorption spectrum of hexagonal H₂O ice