Tuesday, August 30, 2005

Tiger Stripes on Enceladus found to very young and active



At a press conference in the UK at the Cassini PSG (Project Science Group) meeting, additional details from the July flyby of Enceladus were revealed. The briefing focused on the cryovolcanic activity observed in the south polar region observed by a number of Cassini's instruments, including ISS, VIMS, CIRS, INMS, UVIS, and CDA. There is a lot of ground to cover on this and I will be posting several times today on this story.

First, INMS and CDA observed asymmetries in their data that have been plotted on a graph as well as shown on a polar projection map of Enceladus with the ground-track and the locations of the CDA and INMS peaks, and the CIRS hotspot, indicated. Previously, CDA reported that their data was consistent with impact-generated dust, not from endogenic activity. If you look at the graph, it is pretty close to a bell-shape curve, which is what you would see in an impact-generated dust scenario. However, the curve is shifted in time from closest approach, with the peak occuring 70 seconds before C/A. In the impact-generated dust scenario, the peak would be right at closest approach, so this shift would not be consistent with that scenario, but with one where dust is ejected up to a certain height from a volcanic vent. So it now appears that the south polar region of Enceladus is the source of the very small particles in the south polar region. UVIS data from two star occultations in February and July also seem to suggest that the "atmosphere" produced by the venting of water vapor and micron-sized dust is not global and localized to the south polar region.

VIMS was also at the press conference, showing a view of the surface of Enceladus taken by their instrument. This image shows the distribution of crystalline ice on the surface, which under Enceladus conditions quickly degrades to amorphous ice. VIMS found that there was quite a bit of crystalline ice in the area surrounding each fracture collectively known as the tiger stripes (because of their appearance and the way they standout in the normally very bright south polar region). Considering the conditions at Enceladus, crystalline ice is expected to convert to amorphous ice over a period of several decades so geologic activity along the stripes must have occured between 10 and 1000 years ago (though more recent episodes are certainly possible, and likely, as I will discuss in a post later today).

In my next post, I will discuss the ISS releases as well as the possible models for activity.

6 Comments:

Anonymous Anonymous said...

>> At 7/29/2005 06:22:50 PM, Jason said...
>> The "blue" colored ice has been determined to be caused by coarse-grained water ice.

Wha? I specifically asked whether hi-res spectral analysis had been performed and I took your answer as meaning "yes it has and there is no evidence for crystalline ice."

Would you care to comment on what might have been going on here? Was the radar team obfuscating or were the early off-the-cuff assessments of radar data overturned by more careful analysis? Or were the early assessments based on something besides the radar data which only became available later? Or was there some other explanation
for the mistake?

I know you were just quoting the information that emerged from the project leaders, but jeez that response was brisk, uninformative and, now we find out, wrong! I am very curious about what transpired.

8/30/2005 11:50:00 AM  
Blogger Jason said...

?? Not sure what this about, but remember that data analysis has been going on for the last month and a half and I am not prevy to everything other teams are doing. The blue-green material we seen along fracture walls and surrounding the tiger stripes is likely due to course-grained ice. VIMS is seeing the stripes are associated with crystalline ice while the rest of Enceladus has amorphous ices.

8/30/2005 12:08:00 PM  
Blogger Jason said...

Okay, now that I have read the exchange you are refering to, I should have specifically stated that I wasn't aware of any news from VIMS beyond their party line - nothing detected besides water ice - bit. I apolgize for the confusion. The only spectral analysis we can say with ISS data is that outcrops and the tiger stripes contained material that was dark in the IR3 filter (at 930 nm). Combined with what we knew from VIMS at the time (remember, nothing but water ice), we felt this was likely due to grain size differences.

8/30/2005 12:17:00 PM  
Anonymous Anonymous said...

Many thanks. That makes sense and fits my guess for what was going on. I will take this up further with the VIMS team.

The reason that this is interesting to me is that we are apparently seeing a process that is also occuring on several KBOs and perhaps other solar system objects as well. Understanding the commonalties and differences amongst the KBOs may shed light on the formation of the solar system.

(Sorry for saying "radar" in my post above. Bad brain fade on my part. I was thinking "infrared" and somehow it came out wrong.)

Thanks again for your excellent site.

8/30/2005 02:33:00 PM  
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