Quick! Which of these two moons was closer to the Cassini spacecraft when it took this image?
Hard to tell, isn’t it? Actually, it’s impossible to tell without knowing what’s what.
The gray moon to the upper right is Dione, and the blindingly white moon to the lower left is Enceladus (famed for its geysers of water erupting from its south pole). When I look at this picture, I can mentally swap the two moons in distance, making one seem farther away, then closer. It’s a bit like the Necker cube illusion.
But here’s a big hint: Dione is Saturn’s third largest moon at 1123 km (698 miles) across. Enceladus is comparatively small at 504 km (313 miles) in diameter. Since Dione is not twice the size of Enceladus in the picture, it must be farther away. In fact, Cassini was 510,000 km (317,000 miles) from Enceladus when it took this picture, but Dione was a more distant 830,000 km (516,000) — more than twice as far as our own Moon is from Earth.
Note that Dione is much duller and dimmer than Enceladus; its surface is rocky while that of Enceladus is highly reflective water ice. That’s why the smaller moon’s image is washed out; the exposure was set for darker objects. Both moons look slightly non-circular, but that’s because the Sun was very slightly off to the side when this picture was taken, so neither moon is quite "full".
Engineers and scientists know precisely where Cassini, Saturn, and its moons are at any given time by first knowing celestial mechanics: the math and physics of orbital motion. But they also get telemetry from Cassini that gives them its position. Physics is all well and good, but nothing beats a measurement in the field. And that’s how they know how distant those moons were when the shot was taken. Without that knowledge, these pictures would still be amazing and important to planetary astronomy, but would hold a lot less scientific value.
As usual, you have to be careful when looking at pictures of objects in space. Your brain is accustomed to all sorts of cues for judging size and distance like sunlight angle, amount of detail seen, the amount of haze in the air… all of which are totally absent in space. You might think you’re not easily fooled, but your brain has other ideas.
Image credit: NASA/JPL/Space Science Institute
When you look at the Moon, you see a surface covered in craters. Yet the Earth, which is bigger and has more gravity — and therefore, you’d think, be hit more often than the Moon — hardly has any craters!
The difference, of course, is that the Earth has weather and tectonic activity. Craters erode, and over time go away*. But not all of them do. Some are so big they take hundreds of millions of years to erode, while others are in dry climates where erosion is limited… like, say, in Algeria. Where the Tin Bider crater lies!
This picture, from the Earth Observatory-1, shows the roughly 6 km (4 mile) wide crater, located in the high desert of northern Africa. It has a complicated terraced structure, indicating that the rock inside may have slumped after impact — a common feature in larger craters. It has undergone some erosion, too… not surprising, given its age of about 70 million years!
There’s an interesting thing about this crater. North is up in the picture, so the sunlight is coming from the south, from the bottom of the picture. Regular readers of this blog know that this induces the well-known crater/dome illusion (another example can be found here). Our brains expect light to come from above, so when it comes from below the shadows send mixed signals to our brains, and we interpret craters as domes and vice-versa.
And if you like that, you’ll love these:
I keep thinking there’s nothing new under the Sun– or on it. With SOHO, and SDO, and a thousand other telescopes pointed at it, it would take something pretty freaking cool to surprise me.
Well then. Surprise!
Holy solar retinopathy! That’s the Sun?
Yup. But this is not a space-based image from some bazillion dollar observatory! This phenomenal picture was taken by astrophotographer Alan Friedman with this relatively small (but very, very nice) ‘scope. He shot it on October 20th, and it shows our nearest star in the light of hydrogen, specifically what astronomers call Hα (H-alpha). I’ll get to that in a sec…
In this picture you can see sunspots, giant convection cells, and the gas that follows magnetic loops piercing the Sun’s surface. When we see them against the Sun’s surface they’re called filaments, and when they arc against the background sky on the edge of the Sun’s disk they’re called prominences.
The image he took is amazingly high-resolution! He has two closeups, one of the filament and sunspot near the edge of the disk on the left, and the other of prominences leaping up off the edge and silhouetted against the sky: