Thursday, September 6, 2012

The Little Robot That Could... Visits Miranda!

Note: This wedge has now been accreted at En Tequila Es Verdad.

This month's Accretionary Wedge (#49) theme is "Out of this World," hosted by Dana Hunter at En Tequila Es Verdad. A similar theme was done four years ago in September of 2008. We've tried to avoid repeating themes that are similar, but this one troubles me not at all, for several reasons. First, it's a little startling to realize just how much has been done and accomplished in four years- that's four extra years of Spirit and Opportunity tooling around in the Martian desert, Curiosity is getting geared up, calibrated, and checked out for its own Martian odyssey, Dawn exploring one asteroid and preparing a voyage to a second, four more years of Cassini exploring the Saturnian neighborhood and moons, and Messenger arriving, doing initial higher orbit observations of Mercury, then descending and starting higher resolution observations in a lower orbit. And THOSE are just the big name probes. Second, the geoblogosphere has seen quite a few new members (including Dana herself), so at this point, I have no problem with repeating or doing similar AW editions to earlier ones. New members, new perspectives. Third, the earth is plenty big, but the solar system is inconceivably huger. There are quite a number of bloggers out there who post almost exclusively on planetary geology (as I grew up calling it) or exogeology (as it seems to be most commonly called now). (Emily Lakdawalla at the Planetary Society Blogs is my favorite.) So it's not as if one AW post on Out of This World Geology was going to put more than a scratch on the countless subjects that could be addressed.

In my post four years ago, my first participation with the AW, I addressed the broad topic of cryogeology, and the idea of how alien it is to us to think of ices as lithic material. While it is quite literally alien from our perspective to think of water and other ices as making up a large or even dominant portion of a large solid body, that is in fact exactly the composition of many bodies in the mid- to outer portion of the Solar System. I mentioned Miranda in that post, and I featured that Uranian moon in my first Moonday post (and the APOD upon which that post was based), but it's my favorite known moon, so forgive me for returning to it yet again.

35 years ago yesterday (September 5, 1977), Voyager 1 was launched into space. Oddly, Voyager 2 had been launched on August 20- if I recall correctly, the reason they were numbered "backwards" was that Voyager 1's path would quickly put it ahead of Voyager 2, even though the latter had a 16-day head start. Voyager 1 was thus able to scout the way at Jupiter and Saturn, and give scientists a better idea of how to target their observations when Voyager 2 passed through those systems later. At launch, Voyager 2 was expected to visit Jupiter and Saturn; extending its planetary mission was contingent on further money being budgeted by Congress, always iffy. However, the budget was extended, and Voyager 2 was pushed deep into Uranus's gravity well, to give a final gravity assist "kick" to also intercept Neptune.

The dive in to Uranus meant that some of the outer moons, which initially were more interesting to planetary scientists, would not be as well observable as had been hoped. And since there was no "scouting trip" by Voyager 1, there would be one chance and one chance only to see what that planetary system had offer. Miranda was not expected to be as interesting as other targets might be, but the path of the probe, and sun/moon illumination aspects would give ample opportunity to study that object. Good thing. Here's a whole-disc image:
At some point in its past, Miranda must have been one of the most geologically active bodies in the solar system. Early speculation was that the moon had been severely impacted, perhaps a number of times, with enough force to disaggregate it, but not enough to cause the pieces to fly apart. That was the story I repeated for roughly 15 years. Then I saw somewhere a new theory that it was impacts and melting, which didn't make as much sense- complete melting would allow segregation of lithics/silicates from ices- and at a density of ~1.2 grams per cubic centimeter, Miranda looks to be a mixture of both. In reading up for this post, I find a much more appealing explanation, which is that Miranda went through a period of orbital resonance with Umbriel, which could have warmed its interior enough to trigger diapirs to develop in the equivalent of its mantle. This would involve solid ice deforming plastically, not melting to liquid. But whatever the explanation, the sense I've always had is that we're seeing enormous depth, compared to the radius of this body, turned over on its side. That fascinates me.

Central in the image above is the feature referred to as the "chevron," and on the right, structures that, as far as is known, are unique to Miranda and Venus, coronae. Looking at the heavily cratered terrain in the upper portion of the image, and down the lower left middle, one can infer you are seeing an ancient, mostly undisturbed surface- much at odds with the dynamism apparent on so much of this small moon. At less than 500 km (300 mi), Miranda would fit easily within the borders of a number of US states.
Above, from a NASA image gallery, shows three different terrain types, from left to right, ancient cratered terrain, linearly grooved terrain- an example of a corona- and complex terrain, in which grooves and folds abruptly terminate into each other. Speaking of abrupt, the transition from one terrain type to another is strikingly sudden. Another striking feature pointed out in the Moonday and APOD posts linked in the second paragraph above is the tallest known cliff in the solar system: Verona Rupes, a 20 km fault scarp. At more than 8% of Miranda's radius, an equivalent cliff on Earth would be over 300 miles tall: bigger than Miranda itself! I think the APOD is a zoom and crop of this image, which shows how completely disrupted the terrain is in the neighborhood of that enormous cliff.
I commented to Dana in a recent note, that in each page I had found worthwhile reading while brushing up on this topic, I was amused to see some variation of the comment, "...but, really, no one knows for sure." When I was young, in middle school, I was under the impression that essentially all the questions in science had been answered. And while I loved science, I didn't really grasp what scientists *did,* beyond just "knowing things." Without that context, I don't suppose you'd be able to grasp the childlike glee that the phrase "...but, really, no one knows for sure," gives rise to in my heart. Knowing is wonderful, but coming to know is even more wonderful. And recognizing what one does not know is the first step in that process.

New Horizons is on its way to Pluto, with passage through that system expected in 2015. Based on the Voyager missions, as well as innumerable other planetary science missions over the last few decades, I have no doubt whatsoever that there will be things there that will blow me out of the water. But I will always have a special fondness for Miranda, a tormented child of the the solar system. There are no further missions planned to Uranus, and it's unlikely any great strides in understanding exactly what caused that chaos will occur during my lifetime. I'm okay with that. I wouldn't want to live in a universe where's there's nothing to point at, and with a note of wonder and awe, say, "...but, really, no one knows for sure."