Smooth Sailing by Rhea and Helene

Amanda Hendrix
Amanda Hendrix

Cassini’s closest-ever flyby of Saturn’s moon Rhea went quite smoothly and teams are busy checking out their data! These flybys never fail to amaze me. And the raw images — which give us an unprocessed first look — are really cool!

Raw image N00152150 gives us a view of part of the bright, fractured terrain we refer to as “wispy terrain” from about 14,000 kilometers (8,900 miles) away. We know that Rhea’s albedo overall is quite high. (When I say “albedo,” I basically mean “brightness” or “reflectivity.” Studying the albedo can tell a lot about surface composition, geologic processes, and interactions with external environment.) But this image demonstrates how bright these cracks are since they are so shiny that the surrounding terrain looks quite dark. There are also some interesting apparent albedo variations seen in this image, which are really intriguing.

The camera was pointing toward RHEA.
The camera was pointing toward RHEA at approximately 38,532 kilometers away, and the image was taken using the CL1 and CL2 filters. This image has not been validated or calibrated. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

The camera was pointing toward HELENE.
The camera was pointing toward HELENE, and the image was taken using the CL1 and CL2 filters. Image credit: NASA/JPL/Space Science Institute.
› Full image and caption

This raw image (N00152175) from Cassini’s narrow-angle camera image was taken about 40 minutes after closest approach. The image shows a region adjacent to the wispy terrain –craters, craters everywhere! And wow, are those crater rims bright compared to the surrounding terrain.

Cassini captured a full portrait of the serene moon with its wide-angle camera (raw image W00063107) on the outbound leg of the flyby, about 1.25 hours after closest approach. Keep in mind that the phase angle is quite low here (only about 2.5 degrees), meaning that the sun is almost directly behind Cassini and Rhea is nearly fully illuminated — so there are no shadows. Large-scale albedo variations are apparent across the surface.

The spacecraft also obtained a cool image of little Helene with raw image N00152211. We’re basically looking at the night side of the body — but it doesn’t appear very dark, because it’s illuminated by sunlight reflecting off Saturn. During the later image sequence of Helene, this small moon was transiting Saturn - so you can see Saturn in the background. Sometimes,pointing at these little guys can be very tricky, especially so close after a targeted flyby. It can be difficult (or impossible!) to get the positions of the spacecraft, the moon and the instruments all lined up — but boy are these close-up Helene images incredible! The detail on the surface is tremendous, and should go a long way to informing geologists about surface properties and processes.

As the imaging team is taking a closer look at images such as these, other instrument teams — including those for the radar instrument, composite infrared spectrometer, visual and infrared mapping spectrometer and the ultraviolet imaging spectrograph (the instrument I work on) — are also busy processing their data. At a science meeting Friday, we talked about a few of the preliminary results. Some of the magnetospheric and plasma science instruments teams reported that they’re seeing some really interesting and surprising results! So stay tuned to hear more about those!

Of course, after one successful flyby, we get right to work on another. Coming up next: Dione on April 7!


    7 Responses to “Smooth Sailing by Rhea and Helene”

  1. SSS383 Says:
    March 10th, 2010 at 6:48 am

    Spelling Correction

    Second Paragraph, 1st sentence, “approacha” should be “approach”

    Good to hear about this flyby, I’ve always wanted to see more of Rhea.

  2. Andreas Trupp Says:
    March 11th, 2010 at 5:45 am

    As regards Helene, I would like to know whether you guys have been collecting data with respect to what could turn out to be an even greater surprise than were the geysirs of Enceladus (given the very small size of Helene). Look for yourself what was announced at a conference in early 2009:

    Detection of Plasma from Dione and Tethys and Their Trojans

    J. L. Burch, J. Goldstein, D. T. Young, and A. D. DeJong Southwest Research Institute, San Antonio TX, USA ( 210-522-2526)

    Analysis of Cassini Plasma Spectrometer data indicates that Saturn’s moons Dione, Tethys, Helene, Telesto, and Polydeuces are significant sources of plasma. Observations suggest that interchange injections transport plasma from these moons outward through Saturn’s magnetosphere. Unlike Enceladus, none of these moons produces a fully developed plasma torus partly because the sources are weaker but also because the plasma is transported efficiently outward through the magnetosphere by interchange. Efficient ejection of plasma from these icy moons may exceed expectations from sputtering. One possibility is that these bodies possess active outgassing as is known to be the case for Enceladus.

    Amanda Hendrix responds:

    The fields and particles instruments are taking data nearly constantly, and these data sets can eventually be used to investigate and understand potential activity on Saturn’s moons. The CAPS results are certainly intriguing!

  3. johnny Says:
    March 16th, 2010 at 12:03 am

    what do these moons tell us about our solar system and its evolution?

    Amanda Hendrix responds:

    At this point, it’s difficult to say specifically what the study of Rhea and Helene tell us about our solar system and its evolution. I think this will take some time to really understand. Eventually, after putting all of the data together, we’ll be able to study processes of solar system formation, accretion and cratering, among others.

  4. NHamateur Says:
    March 23rd, 2010 at 2:32 pm

    Any idea of what the long nearly opposite arcs are that cross the surface of RHEA?

    Amanda Hendrix responds:

    The long arcs on Rhea’s surface are fractures or cracks in the surface, likely the result of some past tectonic action. Fractures such as these are seen on Rhea and some of the other moons as well. They often appear brighter than the surrounding terrain, probably because of compositional and/or grain size differences between the material inside the cracks and the surrounding material.

  5. John Says:
    March 26th, 2010 at 8:40 pm

    Any further thoughts regarding the possibility of Rhea rings? Is there conclusive evidence of rings?

    Amanda Hendrix responds:

    The signatures measured by fields and particles instruments at Rhea looked different on Orbit 127 than on previous flybys, but scientists are still scratching their heads over what it all means.

  6. Matt Says:
    May 16th, 2010 at 9:40 am

    Thanks for the blog! I just discovered it, and enjoy the various blogs dedicated to specifics in astronomy and other areas. I am an analyst with the fire service, and in using GPS and GIS to develop new pre-arrival firefighting tactics, have developed an interest in astronomy and quantum physics as a by-product. If no one ever mentions it, all of your endeavors are appreciated by many, many folks. Keep up the interesting work. “M”

  7. ernie Says:
    November 30th, 2010 at 12:32 pm

    WOW!! what is the find??? is there life??? I really hope that is what has been discovered.! darn I cant wait til thursday……

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