Just two days ago, the Cassini spacecraft flew by Saturn's tiny moon Helene at a distance of only 7000 km (4300 miles). As it swept past, Cassini's first view was of the dark, unilluminated portion of the moon, getting this lovely, moody shot:
[Click to embiggen.]
Helene is tiny, only 36 x 32 x 30 km (22 x 19 x 18 miles) in size. That's far too small to shape itself into a sphere by gravity, so it's lumpy and irregular. You can see a large flattened region on the left lit at low angle by the Sun, and a depression at the bottom which appears to be lit by reflected light from Saturn itself.
One thing I noticed after the beauty of this picture gave way to scientific curiosity is that there aren't any smaller craters in that lit area. Most of Saturn's moons are covered in craters, so what's going on? The mystery deepens a bit in the next picture, captured by Cassini as it moved on, the viewing angle changed, and it saw the sunlit part of the moon more fully:
Again, the lack of smaller craters is pretty obvious on the moon. At least, on this part of it. Helene orbits Saturn well beyond the rings and spins once every orbit, keeping the same face toward Saturn. The side of the moon facing Saturn has lots of small craters, while the side facing away does not.
There could be many reasons for this; perhaps small chunks of ice from the direction of Saturn get launched outward by gravitational interactions from the inner moons, making small craters on the Saturn-facing side of Helene (that seems unlikely to me; that's not seen on other outer moons). Maybe there's dust that falls in toward Saturn that sand-blasts the outward-facing half (again, why don't we see this on the other moons?). Maybe it's something else entirely... Helene's sister moon Dione also shows such a two-faced personality, and there's evidence a big impact may have spun Dione around 180°! It could be that's what happened to Helene as well, with the cratered side facing Saturn and the smooth side facing away. I'm guessing here, but hopefully images like these will give the real experts some ideas on how this happened.
Emily Lakdawalla at the Planetary Society Blog has an atlas of Helene created using images from a previous Cassini encounter, and a more thorough discussion of this weird little moon and this crater dichotomy.
I've said it before, and as more data come in I'll say it again and again: Saturn's moons are damn weird. We sent Cassini to investigate these moons, as well as Saturn itself, its atmosphere, rings, and space environment. I wish we had a thousand such spacecraft orbiting Saturn... but, as you can see here, Helene has a face -- two faces, in fact -- that make it worth launching at least one ship.
Tip o' the nose cone to Cassini Imaging Team Leader Carolyn Porco.