Tethys Images are up. Haven't seen this angle before.

Melanthius is the large crater. Looks like a double crater if you ask me!? Is it possible the orginal rock that hit it broke apart before impact?

http://saturn.jpl.nasa.gov/multimedia/imag...0/N00110914.jpg

Bit of a play about...

Click to view attachment

Nice, that's the first I can recall seeing the whole of Melanthius and Dolius (to the N) in one view. Fairly face on to boot with Melanthius. Should help with image navigation since before it was always piecemeal views near the terminator or limb. Dolius is near the limb, though at least we can see all of it at once.

The shape of Melanthius has seemed a bit odd, so with the clearer view Decepticon has a good point about the double crater shape (with elongated central peak[s]).

It might be a fragmented impactor ("decapitated" by the extremely low impact angle) or a binary asteroid impact.

There were Janus images too.

Phil

Apparently there's been something happening at Mars???....meanwhile out at Saturn, some nice images of Janus (~180k range) and Prometheus (535k range with Saturnshine) have been posted, along with a handful of Enceladus and Tethys shots from further out. These were not certain to be received due to conflicts with the Phoenix landing DSN requirement, so these are a nice bonus

Those DSN guys are scheduling wizards, aren't they? Impressive that they got these down!

Prometheus is a bit overexposed in this image, but, wow, check out the perspective on the F-ring!

That's indeed a crazy perspective, the gravitational ripple in the ring makes it look like it curves around in a funky way. On a side note, I always wondered what it was about the GRN filter that caused it to create ghost images of objects to the upper right in the NAC. I've seen it many times in brightened imagery, but this is the best example I've seen so far: http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=155860

Is this one of the highest resolution resolution views of Prometheus by Cassini?

Yes, the view shows I think, a 'streamer' drawn out of the inner F-ring material rather than a view across the end of the ring ansa.
These are indeed some of the best shots yet of Prometheus - the closest images so far are around 380,000km I believe.
Hoping for some better views later - there are six sub-100,000km passes before the end of the XM.

http://saturn.jpl.nasa.gov/multimedia/imag...0/N00111364.jpg


A cool new image of Prometheus casting its shadow onto the F ring. This shows clearly the relationship between the moon and the cusp in the ring. I can't recall seeing another picture of a shadow on the F ring.

Phil

Rhea in approx. natural color on June 10, range 420 000 km:

Mimas from recent periapse pass

The Looking Ahead article for Rev74 has been delayed until Monday because the webmaster is on vacation. In the mean time, here is a quick overview of the observations planned of Saturn's icy sats:

* Two reasonably close non-targeted occur on June 30. There is an Enceladus encounter at a distance of 84,255 km over the sub-Saturn hemisphere. About 12 minutes later, Cassini passes by Janus at a distance of 30,975 km.
* Because the encounters are so close in time, the science teams had to prioritize the two encounters. Cassini will observe Janus at its closest approach to that moon, which is the closest encounter during the primary mission. Enceladus will be observe on either side of the Janus sequence. The first covers the Northern sub-Saturn hemisphere at around 1 km/pixel. The second covers the southern sub-Saturn hemisphere at around 600-700 meters/pixel.
* Mimas' southern sub-Saturn hemisphere will be covered later in the day from a distance of 193,000 km.
* There will be a series of distant observations of Dione, Rhea, and Tethys on June 29.

Thanks for that VP.
I've been keenly anticipating this reasonably close Janus flyby. Any idea of the phase angle range during C/A? I'm hoping for a fairly low angle, as I'm intrigued as to whether some of those craters do have some dark stuff at the bottom..

The phase angle will be between around 130 and 70 degrees.
Unfortunately Janus will be in shadow just after closest encounter.
By the time Janus gets out of Saturn's shadow Cassini is already 166,000 km away.

At the closest encounter the phase angle is about 120.

Hmmm it's a shame the lighting angle isn't ideal then for albedo studies.
The topography though should show up in awesome detail at the range stated (similar to the best Epimetheus shots, but then Janus should practically fill the NAC field of view)
Looking forward to this!

There should be some illumination of Janus from light scattered off the rings, so a well planned longish exposure should give extra detail of the shaded side of Janus. I hope that will happen. The geometry isn't very good but it might help a bit.

Phil

This is totally arm-waving here as I haven't seen the actual trajectory, but if prolonged exposures are used in the NAC at C/A, they're bound to cause serious motion blurring so that's probably going to be of limited use.

Given the distance and the results so far in the mission, Cassini may be able to do some rock-solid tracking.

I'm not talking about target tracking, I'm talking primarily about radial motion blur - moving principally toward or away from an object. There's not much to be done in this regard. Again, I don't really know the trajectory and whether it will be more "sideways" or "head-on". Even with precise tracking and going sideways, there can still be substantial parallax blurring on an object as well.

Remember the last Enceladus flyby and the wide-angle outbound shots that were blurred radially and this is with a more sensitive camera (well, more photons per pixel per sec anyway) than NAC.

Those Enceladus shots were from much closer in.

http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=146114

A bit closer in:
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=146109

The first one is binned 2x2 for a reason. Whereas the narrow angle camera has 10x the angular resolution meaning any similar amount of motion blur at the same distance will be spread over 10x as many pixels. For equal amounts of illumination this is further worsened by the fact the narrow angle camera needs something like 5x longer exposures (OTOH) for the same S/N ratio.

Unless there's some good saturnshine on Janus, I'm not expecting very impressive results. Since Janus is said to enter eclipse around the time of C/A, I don't expect any reasonable illumination apart from other moons which is considerably weaker.

I guess a major issue is how long an exposure we are talking about, and, in addition to the direction, the relative velocity is important. If conditions are bad, a good idea (but a memory hog) would be to take multiple shots at shorter exposures and then stack them after correcting them as much as possible for the geometric differences.

Rev74 article up now: http://ciclops.org/view/5048/Rev_74

The exposure times aren't too long. There are the usual long exposure UV3 frames (~12 sec), but most are below 1 sec.

Those Enceladus images were taken when that moon was in eclipse so the exposure times were absurdly long, generally between 50-220 seconds.

Yes, but we were discussing the potential of seeing something on the night side, but there are no good candidate light sources. Judging from that Celestia snapshot the crescent view won't be half that bad, though.

Ugordan, I totally missed the fact that you were talking about the nightside.

I know. But someone brought up Range-to-target smear, and I wanted to point out that exposure times this time around are much lower, so that kind of blurring should be quite low.

What I am most concerned about is noise. The Epimetheus images taken last year were VERY noisy:

http://saturn.jpl.nasa.gov/multimedia/imag...5/N00098342.jpg

You have no idea how much trouble I went through to clean that up to produce this:

http://photojournal.jpl.nasa.gov/jpeg/PIA09813.jpg

I guess I will be spending this afternoon digging up my procedures from when I did that. Or I will just work on Enceladus images tomorrow. Those should be less noisy... Darn you G-ring!!!!

EDIT: I forgot, the original caption does describe some of the trouble I went through: http://ciclops.org/view/4656/Epimetheus_Revealed

Oh, I hear you. I did some cleaning up on PDS images as well of inner rocks, it's a nightmare.

I have idea. What you think about ?
http://members.tripod.com/petermasek/example17.html
http://members.tripod.com/petermasek/example18.html

Janus from 32,000 km:

http://saturn.jpl.nasa.gov/multimedia/imag...1/N00114756.jpg

G-ring: I want you to die. Now.

Enceladus from 164,000 km:

http://saturn.jpl.nasa.gov/multimedia/imag...1/N00114735.jpg

Compare to the Voyager 2 view:

http://www.ciclops.org/view/3183/Voyager_2..._Full_Disk?js=1

Terribly noisy images - exactly how does the G ring contribute to image noise?

Janus looks rather smooth to me, especially compared to something like Hyperion (which is only about two times 'longer') or even Mimas. Effects from dust?

Not sure how. All I know is that when ever we take images of the inner moons in the range of the G-ring, we get very noisy images.

Some weird enhancement (amplification?) of Saturnian radio emission in that region interfering with the camera electronics ?

Or something more (bwa, ha, ha, ha, ha!) interesting ?

Like it ain't noise but actual debris particles in line of sight ? Streaks and pops in image are specular reflections from polished facety ringy stuff ?


(everyone loves a mystery, work with me here!)

No, it has to be something like a radiation belt at this distance from Saturn.

No, you don't.

Seriously, this is pretty noisy, but it's not exactly unusable. Some clever filter operations can make much of this go away. It only helps if you have a bland target (color-wise) to begin with.

<speculation>
Regarding the radiation, could it be UV light ionizing the stuff in the G ring and producing a load of electrons which are then picked up by the magnetic field? </speculation>

Yep... peak radiation belt particle fluxes are at 2.66 Saturn radii, just inside the G ring.

Oh, another radiation belt.

How scintillating.

Nice images of Janus, but let me get this straight. G-Ring particles strike Cassini’s antenna creating other particles that penetrate the imaging system causing the noisy images? That is just cool. I always assumed the noise was due to higher radiation levels close to the planet.

Okay, I have learned that it isn't the G-ring's fault. Basically, there is nothing substantial between Mimas and Janus to soak up energetic particles (which show up in images as noise hits, bright pixels or streaks), so as ynyralmaen said, radiation fluxes peak at around the distance of the G-ring. If the G-ring were a bit larger (like the E-ring), it could soak up those energetic particles.

So, G-ring, I'm sorry.

Group hug!



-the other Doug

These are the two Janus views just taken. I've done a quick clean-up, but obviously a lot more could be done (and will be by Vp, I think). On the highest resolution image, I have arrowed a place which seems to have a dark patch in a depression, very much like Epimetheus in our best images of that moon.

Phil

Click to view attachment

Click to view attachment

Some Mimas images from yesterday... map-gap filling.
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=164636
etc.

Huh. Dark, apparently deposition patches on yet another moon (another inner one to boot)?

Something interesting is going on (or did, more likely). I'll reserve future observations for the Iapetus thread. Really an unexpected find, but of course that's why we're there in the first place.

I don't know if I'm in the right thread for this one, but what's going on here:
http://saturn.jpl.nasa.gov/multimedia/imag...iImageID=164650

It looks like an eclipse. I'm pretty new to Celestia, but if I'm running it right, I think Saturn is eclipsing the sun here. Anyone know if I'm right?

Nice catch. I haven't yet taught myself how to use Celestia, which would probably be a good way to answer these questions. Instead, I use the Solar System Simulator. Here's the view of the Sun from Mimas at 9:00, and again at 9:30. Looks like Mimas came out of eclipse again at about 11:45. Since there was a long series of shots before the weird one I assume we're looking at Mimas before the eclipse. Just to play, here's the view of Mimas from Cassini at 9:30 and again at 11:45. I can sorta convince myself that craters on the immediately pre-eclipse view from Cassini match the craters in the solar system simulator view from 9:30.

--Emily

Wow. That's an eclipse alright, but it's not Saturn's shadow cast on Mimas... it's the A ring's! Very cool.

Thanks, Emily. I should have thought of that. The SSS is an amazing resource. Celestia did one interesting thing though: instead of looking at the view from Mimas, I watched the surface of Mimas from a distance, and when the eclipse occurred, Celestia actually rendered the shadow crossing its face. Then I was able to just 'turn around' in real time and look to see what was casting the shadow. I don't think the simulator does that kind of thing, or does it and I just don't know how?

Anne - wow, can the F ring even cast a shadow that dark? I do see it crossing the sun right before the sun goes behind Saturn.

No, the SSS doesn't render shadows, so the only way to catch eclipses is to do what I did -- ask for the view of the Sun from the point of view of the body that's passing in to shadow. If something big is between you and the Sun, you're in eclipse

--Emily

Upon closer examination, it does indeed look like this shadow is cast by the A ring, not the F ring.... but still definitely not by Saturn.