Thursday, September 14, 2017

Dear NASA and ESA: Thanks for Cassini-Huygens

Seeing Saturn as a child through a reasonably good amateur telescope was one of many formative experiences in my life. When the planetary 'seeing' was good (a still atmosphere is more important than how dark or clear the skies are when looking at anything in the solar system) and Saturn was well above the horizon, a 6 inch f8 Newtonian reflector showed the three main rings, some detail in the atmosphere, and as many as half a dozen moons. Though, really, the moons were just Titan and assorted specks.

Then, for the last 20 years, we've had Cassini-Huygens. Here's a composite image of Saturn eclipsing the Sun. It's one of my favorites, and definitely worth a click.


All day today NASA TV will be rerunning Casssini programming. At 7 a.m., Friday, September 15 (that is Eastern daylight, UTC -0400) they start End of Mission Commentary, etc. There will be new content appearing for a while, then they'll start rerunning it. The schedule is at https://www.nasa.gov/multimedia/nasatv/schedule.html.

It's amazing how much we're learned about the Saturn system my childhood days. There's a good summary of what NASA judges to be the Top 10 at https://saturn.jpl.nasa.gov/news/2892/cassini-10-years-at-saturn-top-10-discoveries/. Be sure to look at some of the links below the subsections. For instance, under #7 'Vertical structures in the rings imaged for the first time', there's a link to https://saturn.jpl.nasa.gov/news/2442/cassini-reveals-new-ring-quirks-shadows-during-saturn-equinox/. From that link,
The chunks of ice that make up the main rings spread out 140,000 kilometers (85,000 miles) from the center of Saturn, but they had been thought to be only around 10 meters (30 feet) thick in the main rings, known as A, B, C, and D. 
140E6 meters in diameter to 10 meters in thickness, or 14E7 to 1. That is a remarkable ratio. To look at 14 million to 1 on a more human scale, pack that ring diameter into the distance from Los Angeles to New York City, which is 3940 km or 2448 miles, as the crow flies. That gives us a reduction of 140,000 / 3940, a factor of about 35.5. Our thickness then becomes 10 / 35.5, or about .281 m, or 281 mm, or 11 inches. That seems pretty thin!

If it's the best data available, I would imagine a lot of people spent time arguing about the physics of how such a structure might have evolved and survived. But then came Cassini-Huygens, and an explosion of new knowledge about a dynamical system. From that same page,
In the new images, particles seemed to pile up in vertical formations in each of the rings. Rippling corrugations -- previously seen by Cassini to extend approximately 804 kilometers (500 miles) in the innermost D ring -- appear to undulate out to a total of 17,000 kilometers (11,000 miles) through the neighboring C ring to the B ring.
The heights of some of the newly discovered bumps are comparable to the elevations of the Rocky Mountains. One ridge of icy ring particles, whipped up by the gravitational pull of Saturn's moon Daphnis as it travels through the plane of the rings, looms as high as about 4 kilometers (2.5 miles). It is the tallest vertical wall seen within the rings. 
It seems likely to provide important clues to our understanding proto-planetary disks.  Exploring those links will be time well spent. Unfortunately for me, lunch is now over. But the yield from Cassini-Huygens is very far from over; these data are going to be the basis for important research results for decades. I look forward to it being an enormous time-sink over the coming years. More immediately, I'm going to catch some of that NASA TV coverage tomorrow, cup of coffee in hand. Because it will be 0400 here on the West coast, at the start of what's going to be a busy day.


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