Enceladus, object? (APOD 27 Mar 2007)

[Strider2k]

On the left side of the pic, about halfway down, is that Tethys I see? Or am I imagining things?

[Indigo_Sunrise]

It does look like another one of Saturn's moons. I was just wondering the same thing - which one it is. Hopefully someone who knows will be along.

(In the meantime, I'm going to follow some of the links, see if it's described anywhere else.)

[Axel]

What is that dark spot on the picture's main object, Enceladus?

[Writer@large]

Saturn's moon Enceladus appears more a comet than a moon. What are the chances it is a recent capture?

It doesn't appear large enough to sustain mass loss over astronomical time scales.

[rproulx]

My own thoughts run to a distant past capture. When I look at the image I notice that Enceladus is on the inside of the ring, in the portion of thickest apparent density. As you move outwards from there the ring appears to thin out. Is it possible that over long periods of time Enceladus is actually imperceptibly losing speed by colliding against its own debris thereby slowly falling towards Saturn?

[Strider2k]

I think it might simply be the shadow of the moon itself. It appears to also cast a shadow, much like what that other object appears to do.

[Indigo_Sunrise]

Strider2k, you are right, that is Tethys. The first link in the description, (linked as "amazing picture"), says that it is, indeed, Tethys.

And I'm by no means an expert, but Axel, I think you're right: that small dark spot seems like it should be Enceladus, given the angles in which the image was taken. (These are described in several of the links.)

Anyway, great image!

[iamlucky13]

It could be losing energy very gradually that way. If you think about it though, not accounting for Coriolis effect, the ring particles are moving essentially the same orbital velocity around Saturn as Enceladus is.

Writer, Encaladus isn't quite as small as the picture would suggest. It's large enough to maintain a round shape, and is Saturn's 6th largest moon. Also, the E-ring is very diffuse, so it's mass is probably rather small compared to that of the moon. Taking all this into account, I don't think it's necessary that Enceladus be a recent capture, although it doesn't discount it, either.

[Jon Ebert]

Can Encaladus enter Saturns atmosphere? What would it be like to run headlong into a gas giant? I saw something on tv the other day that showed some molten aluminum. I don't remember the program, but the swirling patterns visible reminded me of the swirling patterns that we see on Jupiter and to a lesser extent, Saturn. It got me thinking about the surface tension of gas giants. When Shoemaker-Levy entered Jupiter there were definite signs of a violent end but the angle of attack was large. If Encaladus enters Saturn the angle of attack will be slight which makes me wonder if it wouldn't simply skip-off. What would one encounter if they were dropping into a gas giant, assuming one could withstand the gravity and the temperatures involved. Is the density of the gas great enough to create a hard hit, or would you just sink in?

JE

[BMAONE23]

Enceladus Statistics

Discovered by William Herschel
Date of discovery 1789

Mass (kg) 8.40e+19
Mass (Earth = 1) 1.4056e-05
Equatorial radius (km) 250
Equatorial radius (Earth = 1) 3.9197e-02
Mean density (gm/cm^3) 1.24

Mean distance from Saturn (km) 238,020
Rotational period (days) 1.370218
Orbital period (days) 1.370218
Mean orbital velocity (km/sec) 12.64

Orbital eccentricity 0.0045
Orbital inclination (degrees) 0.02

Escape velocity (km/sec) 0.212

Visual geometric albedo 0.99
Mean surface temperature -201°C
Magnitude (Vo) 11.7

Given that Enceladus is orbiting Saturn at a relative speed of 25,500kph i doubt it would settle in easily it would either bounce off and become the largest comet ever or it would explode into a multitude of fragments and either vaporize in Saturn's atmosphere or fragmentise and send many comets inward toward us. Or if it stayed intact uopn entry, it would likely expolde from pressure at a certain point causing Saturn to be deformed until it could reform and obliterating the Ring system.

[iamlucky13]

It couldn't genuinely skip off Saturn's atmosphere and break orbit, but the upward force of half the planet skimming through progressively denser layers could cause it to skip outward. Since it would've lost some additional orbital velocity due to drag, it would then fall closer in, repeating this process (zero to who knows how many times) until it finally plunged in. On the other hand, an Apollo capsule, for example had enough velocity that it could've skipped off and enter orbit around the sun.

However, given Enceladus' orbital radius (a little smaller than our own moon), it would take a very, very long time for it's orbit to decay that far. As an example, Mars' moons Phobos orbits low enough to have a slight interaction with the martian atmosphere (I think this moons is believed to be a relatively recent capture, BTW), and also experiences tidal drag, but Phobos isn't expected to hit Mars for at least 50 million years.

As it's orbit fell, there is a chance tidal forces could tear it apart, forming a new ring.

If it did hit...dang.
The black spots from Shoemaker-Levy 9 would pale in comparison, but give us some idea of what would happen as a whole. Basically all of its remaining kinetic energy would be converted into heat via drag. If the stress was great enough to fragment the moon, this could happen very quickly (in minutes). The heat would cause the gas giant's atmosphere to expand violently around the plummeting moon and a complex shock wave would probably circle the planet. Heavier, different-colored gas from deep within Saturn's atmosphere would be churned upward forming a distinctive mark, and the bands of clouds would probably be disrupted for years. Fragments of the destroyed moon and pockets of gas from Saturn could be blown out into other regions of the solar system, forming a cloud of debris visible for days or months, and causing Saturn to appear out-of-round shortly after the collision.

Doing some quick and very dirty math with BMAONE23's numbers, the moon currently has a kinetic energy of 1.6 E18 Joules. If that remained within a factor of 10 when it entered, the effect would be equivalent to a quintillion of the largest nuclear bombs ever made going off.

If any large pieces, or even the whole core of Enceladus survived the initial entry, I believe they would gradually collapse under their own weight to smaller sizes as they sank towards the center of Saturn. Depending on their density and the interior composition of Saturn, they could remain floating at various levels in Saturn's interior.

Actually, as I think about it more, Enceladus may actually gain orbital velocity from Saturn due to tidal forces, thus it would never fall inward. That would be a question best left to someone more initimately familiar with the Saturn system.