Starship Asterisk*

Saturns Moon Helene from Cassini

Explanation: What's happening on the surface of Saturn's moon Helene? The moon was imaged in unprecedented detail last week as the robotic Cassini spacecraft orbiting Saturn swooped to within two Earth diameters of the diminutive moon. Although conventional craters and hills appear, the above raw and unprocessed image also shows terrain that appears unusually smooth and streaked. Planetary astronomers will be inspecting these detailed images of Helene to glean clues about the origin and evolution of the 30-km across floating iceberg. Helene is also unusual because it circles Saturn just ahead of the large moon Dione, making it one of only four known moons to occupy a gravitational well known as a stable Lagrange point.

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Helene is smooth, and according to Wikipedia, one of four Lagrange (trojan) moons in the Saturnian system; the others are Telesto, Calypso, and Polydeuces. Telesto and Calypso are also pretty smooth; couldn't find a picture of Polydeuces which showed its surface. (Likely not imaged yet at a distance/resolution which would show its surface features?)

This has me wondering whether Lagrange objects in general tend to be smooth, darn it; I was so hoping to get some work done today!

owlice wrote:Helene is smooth, and according to Wikipedia, there are four Lagrange (trojan) moons in the Saturnian system; the others are Telesto, Calypso, and Polydeuces. Telesto and Calypso are also pretty smooth; couldn't find a picture of Polydeuces which showed its surface. (Likely not imaged yet at a distance/resolution which would show its surface features?)

This has me wondering whether Lagrange objects in general tend to be smooth, darn it; I was so hoping to get some work done today!

The combination of:
1) a small lunar escape velocity and
2) the gentle "wash cycle" sloshing motion of Lagrange orbits
makes for very gentle collision conditions.

For Helene, both velocities amount to only about 14 m/s each.

This animation shows the path of the 6 Neptune Trojans in a rotating frame with a period equal to Neptune's orbital period.

(Neptune is held stationary.)

But Helene is in Dione's L4 Lagrange point, not Neptune's, in fact it is nowhere close to Neptune. Why Neptune?

I was wondering if, in general, natural satellites in Lagrange points tend to be smooth. I suspect the point of the animation is to show the 'gentle "wash cycle" sloshing motion of Lagrange orbits' that may account for the smoothness of such bodies.

This paper http://www.sciencedirect.com/science?_o ... c839a3f442 found that, "This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith." Which would, I presume, be smooth?

John

As stated in the APOD, there are only four known Trojan moons and of those, Telesto and Calypso appear fairly smooth, as you pointed out. The only other natural Trojan satellites I know of are the Mars and Jupiter Trojan asteroids. Like Polydeuces, I'm not sure there are any definitive photographs of any of the Trojan asteroids.

Nepture has trojans, too, according to this article.

Hmm, interesting. I hadn't known that there were any Neptune Trojans. I suppose those would be captured KBOs or SDOs instead of asteroids. Strange how all of them, at least so far, are in the L4 point.

owlice wrote:I was wondering if, in general, natural satellites in Lagrange points tend to be smooth. I suspect the point of the animation is to show the 'gentle "wash cycle" sloshing motion of Lagrange orbits' that may account for the smoothness of such bodies.

Thank you, owlice. That was precisely the point.

The 14m/s velocities were derived as follows:

• Dione has a radius of 561 km and an surface escape velocity of 510 m/s.
  
  1) assuming that surface escape velocity for a similar density goes linearly with radius
  Helene (with radius of 16 km) has a surface escape velocity of ~14 m/s.
  
  2) Escape velocity above the surface drops inversely with the square root of distance.
  
  Dione's escape velocity at the distance of Helene
  (= Dione's orbital radius of 377, 400 km.)
  is 510 m/s x sqrt ( 561 km. / 377, 400 km.) ~ 20 m/s.
  
  However, since Helene is not escaping from Dione's influence
  the average relative velocity is somewhat less...say ~ 14 m/s.
  
  Total collisional velocities would then average ~ 20 m/s
  and not produce large cratering.

neufer wrote:Total collisional velocities would then average ~ 20 m/s and not produce large cratering.

By the usual terminology, such velocities would not produce any cratering at all, merely impact pits, which are structurally quite different.

OK, I have now qualified the four trojan moons as "Saturnian" so as not to insult the God of the Sea. Also, Emily Lakdawalla has a new post on why Helene is not centered and so I linked to it. Did I mention that I try not to change APODs after they are posted?

I don't know that the change was necessary (though I'm not an astronomer/astrophysicist, nor do I play one on TV); the Space.com article does not refer to the Trojans of Neptune as "moons." Are all natural satellites of a planet moons, or do they have to be a particular size before they are designated as such (in the same way that only some natural bodies in orbit around a star are planets, and others are not)?

In this image it looks very much as though the moon has been carefully wrapped up. Are we sure it is not the latest, most ambitious, art installation by Christo?

I don't think Christo has wrapped any Saturnian moons.

bystander wrote:I don't think Christo has wrapped any Saturnian moons.

Yet.

Bystander and Owlice yea ! How bout this "moon" B a captured astroid or a wandering ring child and being @ a Lagrange point accounts for the smooth, somehow. Ya think ?

I thought I would do a simple calculation of a hypothetical satellite in the Lagrangian point between the two larger bodies. This would start off as F_E- F_M= F_C --or--G Force of Large mass(like Earth) - G Force of smaller mass (like the Moon) = Centripetal force of satellite (satellite masses cancel from each term so becomes acceleration problem). Then using three distances: Earth to satellite , Moon to satellite, and Earth to Moon as R, r, R+r = D, respectively. Next writing the velocity in the centripetal acceleration as a function of the Moon's period T (same as satellite)--and rearranging/canceling a bit I got: M_E/R² - m_M/(D-R)² = 4pi²R/GT_M². Here G is the gravitational constant and T_M is the period of both the Moon and the satellite. I got this far, but I'll be darned if this equation doesn't blow up when trying to solve for R in terms of D, T_M, m_E, and m_M--all of which are given in initial parameters of a problem like this. I don't feel like expanding terms--it makes my brain hurt--and I never know when to do it anyway. Maybe using Lagrangian mechanics this would be ironically easier. Any suggestions--did I miss something obvious?

Helene of Saturnian Troy
Bad Astronomy - 2010 March 11
Phil doesn't really add anything new, I just liked the title.

owlice wrote:I don't know that the change was necessary <snip>
Are all natural satellites of a planet moons, or do they have to be a particular size before they are designated as such (in the same way that only some natural bodies in orbit around a star are planets, and others are not)?

But now I do know it was necessary, as there is no lower limit on size when it comes to moons.

owlice wrote:This has me wondering whether Lagrange objects in general tend to be smooth

Apparently not!

(Thank you, bystander.)