Cassini to Collect Halloween Treats

The Cassini spacecraft will be trick-or-treating at Saturn‘s moon Enceladus this Friday, swooping in to snap as many images as possible of the unusual features known as tiger stripes that slash across the moon’s south pole.

The maneuver builds on a very close flyby Cassini did earlier this month, which sent it deep into the plume of ice and gas that seems to be coming from the tiger stripe fissures.


Cassini near Enceladus’s icy geysers, as seen in an artist’s rendering

—Image copyright 2008 Karl Kofoed

At just 16 miles (25 kilometers) above the surface, that October 9 trip marked the closest Cassini had ever been to Enceladus’s southern surface.

But the focus of the previous trip was on the plume’s composition, not on taking pictures.

This time the craft is staying a bit farther away and will work its cameras and other sensors to build on visual data about the moon’s plume.

As a kind of preview for the Halloween flyby, Cassini scientists released today a shot of Enceladus occulting.


—Image courtesy NASA/JPL/Space Science Institute

This is not as spooky as it sounds: Enceladus is simply being partly covered by another Saturnian moon, Dione, as seen from the spacecraft’s point of view.

Looking at the two moons in this position gives astronomers a better idea of how bright they are in comparison with each other, given the same amount of light coming at them from the same angle.

Although Enceladus is farther away in this image, it’s brighter and smoother, because its icy plume coats the moon in fresh, highly reflective material.

Dione, meanwhile, is also covered in water ice but is duller and heavily cratered, suggesting its visible surface is much less fresh.

Images like this give scientists a baseline for using reflectivity to show changes on a given moon even when you can’t see other moons in the same frame.

For example, other images show Dione with bright, wispy stripes across its surface. Cassini shots from 2005 revealed these to be canyon walls. The cliffs appear brighter because material occasionally drops away, exposing new ice.

Scientists think Dione’s canyons could have been formed by tectonic activity, and may even be more mature versions of the tiger stripes on Enceladus.

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