Mar 17 2008
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Cassini’s dive through the plume
Last week, the NASA spacecraft Cassini dove through the water and dust plumes erupting from Saturn’s moon Enceladus. It was a success! As Carolyn Porco said,
Yesterday’s grazing flyby over the equatorial regions of Enceladus and through its south polar plume went spectacularly well. Several of the Cassini instruments successfully collected valuable information about the south polar surface and the gaseous/particulate environment above it, and we imaging scientists didn’t do too badly ourselves.
She’s not kidding; the images are incredible. Emily, of course, has already been posting about them; see here and here. My favorite so far is this one:
It shows the north pole of Enceladus, which is heavily cratered. Like Jupiter’s moon Europa, Enceladus is encrusted in water ice, which is why it is so white and why there are large smooth regions; cracks in the surface can let water pour through and refreeze. It’s thought the moon may have an undersurface ocean like Europa does as well.
You can read more about the Cassini dive in their press release.
All these worlds are yours, except Europa. Attempt no landing there…oh, and maybe Enceladus, too.
- Jack
Hey Phil,
Did you see userfriendly.org’s Sunday comic? http://ars.userfriendly.org/cartoons/?id=20080316
Assuming the geysers aren’t from some Enceladian water fun park it will be interesting see what heats up the moon enough to cause the geysers. Cool how the geysers spurt water at faster than escape velocity too, even for a such a tiny moon.
Pretty little thing too.
That’s odd. I read somewhere that there was a software glitch during the encounter that led to much of the particle data not being collected, and that they would try again in a later encounter.
YEs, but the CDA instrument’s issues at Closest Approach didn’t cause problems with the other instruments. Our images came back just fine. The other instruments, particularly CIRS, INMS, and RPWS, all got great data. They are still sorting it all out and will hopefully post some preliminary results soon.
Glad you liked the mosaic, Phil. Obviously, this region (with the exception of the area on the right) has not been reworked as extensively as the south polar region, where the original cratered terrain is completely unrecognizable.
Good to know, thanks.
Question about ice and meteor impacts? How big would an impact have to be to melt (and thus smoothe out) a significant chunk of terrain? I remember reading that on Earth, large impacts would heat the atmosphere to frying temperature, and boil oceans to their basins.
I ask because I’ve NEVER seen anyone suggest that such external energy inputs could be a cause of smoothness of ice on outer moons, rather than gravitational tugs or natural radiation, etc.
Smaller impacts of course leave craters- so, HOW BIG DOES BIG NEED TO BE?
http://ciclops.org/view.php?id=4858
On this raw, it looks like there are lights on the surface of Enceladus, reminiscent of city lights on Earth. Are these geysers or specks of dust on the camera lens, or is there some other cause?
http://www.planetary.org/image/W00043237_labeled.png
I really like that bit… So the moon wasn’t even touched by direct sunlight? Sweeetttt
Let’s see if a big asteroid impact would boil the oceans. I’ll refrain from considering asteroids “the size of Texas” that are suddenly on a collision course with the Earth and restrict myself to one 10 km in diameter.
Radius is 5000 m, so volume is 5.2 x 10^11 m^3. Let’s make it a stony iron, so the density is midway between 2800 kg m^-3 for stone and 7800 for iron — 5300 kg m^-3, which gives our asteroid a mass of 2.8 x 10^15 kg.
Let’s impact it at escape velocity, 11,200 m s^-1 or so. KE = (1/2) m v^2, so kinetic energy comes out as 1.8 x 10^23 joules. Wow!
Now, the ocean has a mass of 1.39 x 10^21 kg, and I don’t know the specific heat for seawater, so I’ll use the freshwater figure of 4814 J/K/kg. Our asteroid the raises the temperature of the ocean by 0.027 K.
This is, of course, a world average. Things will be much worse at impact point, fading out to not so bad at the antipodes. A lot of the ocean probably will boil, assuming an ocean strike, which has a 70.8% probability. The tsunami will be enormous, the atmosphere at impact will flame and emit everything up to X-rays, and life within a thousand km or so, maybe even 10,000, will not seem worth living.
I am continually amazed at just how DARK space really is when these pictures get published.
Gary, those are most likely cosmic ray hits on the detector, which causes pixels to light up. We see them all the time in space-based images and they’re a pain in the butt to deal with. Another cause of spots like those are “hot pixels”, pixels that are more sensitive than others and so they light up. Those can be processed out of the images with pretty good accuracy, but they make the raw data look ugly.
That picture has been my desktop wallpaper for a couple of days. It complements my all Black/White theme well.
i realize i had spacephobia !!!
Also, most of those bright spots in the eclipse images are background stars.
Michelle:
What was the context of that graphic? If it was presented pretty much as-is, it’s wrong. The Sun was definitely illuminating Enceladus during the fly-by. That’s the bit in yellow in that graphic. Saturn contributed almost nothing to that effect because Saturn was almost eclipsing the Sun at the time. (We were therefore seeing the unilluminated side of the planet.) I’m pretty sure that the rings will do very little as well: Enceladus orbits basically in the ring-plane. Scattering of light at such an extremely low angle off the rings in the forward direction is pretty weak. You’re looking at multiple-bounces to get anything at that phase and each bounce knocks down the brightness by a factor of two, in addition to the low probability of scattering toward Enceladus at all.
Dione and Tethys might conceivably contribute to the illumination discernibly, but I expect it to be a small effect. They’re 140,000 km and 260,000 km from Enceladus and at a quarter phase at this point. In their favor, their albedoes are reasonably high: roughly 0.6-0.8 (although I think that those were measured from Earth at very low phase; at higher phases like this they might not extrapolate well). Rhea, on the other hand, is pretty much right out: it’s a slim crescent from Enceladus’s vantage point at this time, as well as being 640,000 km away.
Apologies, I hadn’t realized that was an eclipse image. That is actually light being refracted through Saturn’s atmosphere, apparently. This also means that the light is very dim, so the shine from the moons may matter more (relatively) than when we’re dealing with sunlight.
http://www.planetary.org/blog/article/00001360/
John: That image comes from this link.
Barton, it’s good you figured the energy contribution from a 10km asteroid rather than a Texas-sized one, because the Texas-sized one could never hit us. Bruce Willis would save us from it at the last minute.
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I didn’t realize a subsurface ocean had been confirmed on Europa. When did that happen?