APOD: Crescent Enceladus (2023 Dec 16)

[APOD Robot]

Crescent Enceladus

Explanation: Peering from the shadows, the Saturn-facing hemisphere of tantalizing inner moon Enceladus poses in this Cassini spacecraft image. North is up in the dramatic scene captured during November 2016 as Cassini's camera was pointed in a nearly sunward direction about 130,000 kilometers from the moon's bright crescent. In fact, the distant world reflects over 90 percent of the sunlight it receives, giving its surface about the same reflectivity as fresh snow. A mere 500 kilometers in diameter, Enceladus is a surprisingly active moon. Data and images collected during Cassini's flybys have revealed water vapor and ice grains spewing from south polar geysers and evidence of an ocean of liquid water hidden beneath the moon's icy crust.

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[johnnydeep]

Cassini's camera was pointed in a nearly sunward direction

It looks more like its pointed closer to perpendicularly to the Sun than at it! Wouldn't we see much less of Enceladus illuminated if we were truly pointed toward the Sun?

[Rauf]

Cassini's camera was pointed in a nearly sunward direction

It looks more like its pointed closer to perpendicularly to the Sun than at it! Wouldn't we see much less of Enceladus illuminated if we were truly pointed toward the Sun?

If it was perpendicular, we would see a quarter phase, not a crescent I guess. And the caption says "nearly". So I guess somewhere between perpendicular and sunward direction?

[johnnydeep]

Cassini's camera was pointed in a nearly sunward direction

It looks more like its pointed closer to perpendicularly to the Sun than at it! Wouldn't we see much less of Enceladus illuminated if we were truly pointed toward the Sun?

If it was perpendicular, we would see a quarter phase, not a crescent I guess. And the caption says "nearly". So I guess somewhere between perpendicular and sunward direction?

Yes, but at best, it looks like what we'd see if we were pointed about half way between perpendicular to and straight at the Sun. The qualifier "nearly" doesn't seem accurate.

[Chris Peterson]

It looks more like its pointed closer to perpendicularly to the Sun than at it! Wouldn't we see much less of Enceladus illuminated if we were truly pointed toward the Sun?

If it was perpendicular, we would see a quarter phase, not a crescent I guess. And the caption says "nearly". So I guess somewhere between perpendicular and sunward direction?

Yes, but at best, it looks like what we'd see if we were pointed about half way between perpendicular to and straight at the Sun. The qualifier "nearly" doesn't seem accurate.

I'd put its phase at about 25%, which corresponds to a phase angle of 120° (where 180° would be looking straight into the Sun, with the moon fully in shadow, and 90° would be perpendicular to the Sun, with the moon appearing half lit. So we're 60° from straight into the Sun, and only 30° from perpendicular to the Sun.

[johnnydeep]

If it was perpendicular, we would see a quarter phase, not a crescent I guess. And the caption says "nearly". So I guess somewhere between perpendicular and sunward direction?

Yes, but at best, it looks like what we'd see if we were pointed about half way between perpendicular to and straight at the Sun. The qualifier "nearly" doesn't seem accurate.

I'd put its phase at about 25%, which corresponds to a phase angle of 120° (where 180° would be looking straight into the Sun, with the moon fully in shadow, and 90° would be perpendicular to the Sun, with the moon appearing half lit. So we're 60° from straight into the Sun, and only 30° from perpendicular to the Sun.

Alright. So "nearly perpendicular" is accurate enough I suppose.

[Chris Peterson]

Yes, but at best, it looks like what we'd see if we were pointed about half way between perpendicular to and straight at the Sun. The qualifier "nearly" doesn't seem accurate.

I'd put its phase at about 25%, which corresponds to a phase angle of 120° (where 180° would be looking straight into the Sun, with the moon fully in shadow, and 90° would be perpendicular to the Sun, with the moon appearing half lit. So we're 60° from straight into the Sun, and only 30° from perpendicular to the Sun.

Alright. So "nearly perpendicular" is accurate enough I suppose.

I don't think I'd use "nearly" at all. But it's certainly more nearly perpendicular than it is nearly sunward.

[Roy]

Anyone see anything of interest , besides which way the camera is pointed?

[Chris Peterson]

Anyone see anything of interest , besides which way the camera is pointed?

If we push it hard, we can see a lot of detail in the shadow, and probably some stars (although it's not always clear what is a star and what is a hot pixel or cosmic ray artifact).
_

[johnnydeep]

Anyone see anything of interest , besides which way the camera is pointed?

If we push it hard, we can see a lot of detail in the shadow, and probably some stars (although it's not always clear what is a star and what is a hot pixel or cosmic ray artifact).
_
PIA20522p.jpg

Cool. What does "push it hard" mean?

[Chris Peterson]

Anyone see anything of interest , besides which way the camera is pointed?

If we push it hard, we can see a lot of detail in the shadow, and probably some stars (although it's not always clear what is a star and what is a hot pixel or cosmic ray artifact).
_
PIA20522p.jpg

Cool. What does "push it hard" mean?

Applying a transfer function that brings up the intensity of the darkest pixels.

[johnnydeep]

If we push it hard, we can see a lot of detail in the shadow, and probably some stars (although it's not always clear what is a star and what is a hot pixel or cosmic ray artifact).
_
PIA20522p.jpg

Cool. What does "push it hard" mean?

Applying a transfer function that brings up the intensity of the darkest pixels.

Ok. I played around benightedly in GIMP and brought out some of the detail in the unilluminated portion, but not that much.

[De58te]

Some interesting statistics. NASA states that this photo was taken about 81,000 miles from Enceladus. And also Enceladus orbits about 112,000 miles above Saturn's cloud tops. Plugging in the figures we can imagine that at an angle 120 degrees away behind the camera, the scenery must have been stupendous since Cassini would have been about 31,000 miles above Saturn's cloud tops. That's more than twice closer to Saturn than Cassini was to Enceladus. In fact Cassini was closer to Saturn's cloud tops than the cloud tops were close to Saturn's center of the planet. (A radius of 36,000 miles.)

[Roy]

Surface shows rifts and cracks, leading to the theory of an ice moon with liquid water interior. I looked Enceladus up on Wikipedia.
Picture there shows some cratered terrain in what I assume is the north polar region. The moon is tide locked to Saturn, with an orbital period of 32.9 hours. Minimum temperature is given as -240 C., max -128 C. Gravity is a little over a hundredth earth’s, .0116 g.
I can surmise that impacts send shocks through the ice which open rifts, allowing liquid flows that immediately freeze into the observed terrain. Any vapor and debris from the impact can easily escape due to the low gravity.

[johnnydeep]

Some interesting statistics. NASA states that this photo was taken about 81,000 miles from Enceladus. And also Enceladus orbits about 112,000 miles above Saturn's cloud tops. Plugging in the figures we can imagine that at an angle 120 degrees away behind the camera, the scenery must have been stupendous since Cassini would have been about 31,000 miles above Saturn's cloud tops. That's more than twice closer to Saturn than Cassini was to Enceladus. In fact Cassini was closer to Saturn's cloud tops than the cloud tops were close to Saturn's center of the planet. (A radius of 36,000 miles.)

Why would you suppose Cassini is directly between Enceladus and Saturn?

[Tekija]

If we push it hard, we can see a lot of detail in the shadow, and probably some stars (although it's not always clear what is a star and what is a hot pixel or cosmic ray artifact).
_
PIA20522p.jpg

Cool. What does "push it hard" mean?

Applying a transfer function that brings up the intensity of the darkest pixels.

In the shadow part, all detail is in the upper hemisphere. Is it correct to presume this is because of "Saturnshine" and that the planet would have been roughly in that direction, "above" Enceladus?