Have you ever wondered what the Universe looks like from elsewhere in it? What the Solar System looks from elsewhere in it? I have considered that for Saturn’s largest moon Titan, and I will describe what I’ve found in my next few posts here.
I will start with what one can learn without looking upward.
Titan’s surface gravity is about 1/7 of the Earth’s, a bit less than the Moon’s at 1/6. So it should be easy to jump upward one’s own height, at least in a shirtsleeves environment. But Titan’s surface temperature is around 95 K (-188 C, -289 F), and its surface atmospheric pressure about 1.5 bar (the Earth’s is 1.013 bar). This implies a column density 11 times the Earth’s. It is almost entirely nitrogen with some methane and some other gases.
So one would need the sort of pressurized and temperature-controlled environment maintained in manned spacecraft and space stations, the sort of environment proposed for colonizing the Moon and Mars.
Titan’s size and shape
Let’s say that we are living on Titan and we want to map the moon’s surface without looking upward. We construct a grid of reference points as we go outward, a grid constructed with triangles. As we go farther and farther outward, we start noticing that the circumference of our mapped region is increasing more slowly than its radius. This can be apparent if we try to fit flat-surface locations to our survey grid. As we go farther and farther out, the more and more it becomes evident that it is not measurement error, and we eventually conclude that Titan is approximately spherical with a radius of about 2575 km, bigger than the Moon.
Eventually, some of us try the ultimate test: going around Titan while going in a locally straight line. That’s going in a great circle.
At first, it may seem obvious that one’s residence is not rotating. But one might want to check to see that it isn’t. So one does Foucault’s pendulum experiment, and one gets better and better upper limits on one’s residence’s rotation until one gets to some rotation rate somewhat less than once every 16 days.
One can repeat this experiment elsewhere and also get a rotation rate — if far enough away, a noticeably different one. From several such observations, one then concludes that Titan is rotating with a period of about 16 (Earth) days, and one also gets the direction of the moon’s spin axis relative to its surface.
(Foucault rotation rate) = (planet rotation rate) * sin(latitude) = (planet rotation vector) . (observer direction vector)
So if one is near the equator, one will not see much effect, while it is largest at the poles, and with opposite directions at opposite poles.
Filed under: Sciences |