Book Mania

Monday, May 12, 2014

Petavius, A Lunar Crater Of Wonder

Crescent Moon
I was finally blessed with a clear, relatively calm night recently, so I took advantage and went out to observe Jupiter and Mars.  Mars was still available in this 2014 opposition, and I wanted to get in as many observing sessions as I could while it was still of sufficient angular size.

While my telescopes were acclimating, I could't help but admire the crescent Moon that was still up. I decide to start by viewing some lunar craters, since Mars was still a bit low in the East.

The Moon was in about the phase shown in the image at left. Mare Crisium  was the most prominent feature, surrounded north and south by typical rugged terrain. Since the moon in this early phase is to a direction where I tend not to get great views, I've not often observed the features highlighted in this lunar phase.
60mm x 1000mm

I started my observations with my trusty 60mm x 1000mm Refractor. My pipe tripod mounted 60mm is much like the one in the Orion 24445 Lunar Explorer Telescope Bundle. It's an ideal instrument for this type of viewing, big enough to produce some great images, and small enough to be generally little affected by atmospheric turbulence. Plus, it is ready to go in a hurry, being very portable and needing very little cool-down time.

So I and my 60mm began our investigation of the eastern limb of the Moon. Since the Mare Crisium feature was so prominent, I started my lunar examination with it. I enjoyed perusing the periphery of the mare, looking for craters and mountains that lead into the interior. I also enjoyed the view of the frozen "waves" that stretch across portions of the mare floor. Here and there were small pockmarks that I could make out.

But then, as I explored the more rugged regions along the lunar terminator, I was totally taken aback by a large crater some distance south from Mare Crisium. The crater looked normal enough for the most part, like many of the larger craters in the 60 to 100 mile diameter range. And like many such expansive craters, this crater had a small jumble of hills at its center. But running straight out from the central peaks to the rim of the crater was a thin, straight line, like a spoke. It ran for miles, and was easily noticeable in my 60mm telescope.

I couldn't imagine what it was. It was far too long to be a shadow cast by a central peak, yet so straight it seemed that it must be a shadow. So I changed to my other scope for the evening, my planet busting 6 inch Dobsonian. I could, as I expected, make out a bit more detail, but not enough to tell me precisely what I was seeing. The only explanation seemed to be a fault line, like the Straight Wall, or a deep rille. But it was so straight, it still amazed me.

After viewing that puzzle for awhile, I decided to get to my main objectives for the evening, Jupiter and Mars. Jupiter was interesting as a comparison object on that night, showing mostly the major cloud bands, with a bit of mottling in the NEB. The mottling was just visible in the 60mm, and of course more so in the 6 inch f/10. But there weren't, on that evening, any moon transits or Great Red Spot to examine, and the atmosphere wasn't giving up any more tantalizing details, so I moved on.

My next stop was Mars. On that evening, Mars was only at about an apparent size of 14 arc-seconds, not a very big target. It looked approximately like the image at left through my 60mm, though smaller. That is, what's shown is approximately the level of detail that I saw. Not a lot, but I could easily see the dark feature at the right of the image, which was Syrtis Major. I didn't feel too bad about being able to see features like Syrtis Major on a 14 arc-second Mars with my 60mm telescope.

I was using a 17mm Plossl eyepiece and my 3x Barlow, yielding a magnification of about 180x. I've found that on Mars and Jupiter, being that they are so bright, I can often push the 60mm telescope up to around 200 times magnification.

After viewing Mars for about a half hour, I put up my equipment, and retired for the evening (not enough of a trooper to stay up for Saturn). I'd seen the targets I'd set out to see, and while most anticipating the view or Mars, it was the mysterious crater on the Moon that had provided the most excitement.

Crescent Moon
Since that evening, I've had a chance to go back and research the enigmatic crater with the single linear spoke. Using the moon map included with the Xephem Planetarium planetarium program, I was able to locate the crater I'd seen.

At left is an image showing about the phase of the moon on the night I was observing. I've marked the feature most observers are familiar with, Mare Crisium, the large, dark circular patch near the bottom of the image. The crater I'd ran across that had the strange linear feature was the crater Petavius, which I've also marked.

You may well be familiar with Petavius, but I have to tell you, I was not. And I was definitely not familiar with the odd feature that I saw that evening. It turns out, according to the Wikipedia Petavius Crater article, that the odd, radial spoke feature is a large rille. Apparently, lunar orbiter pictures show the floor of Petavius crater to be full of odd cracks and unusual scars.
NASA/GFSC/Arizona State University Photo

At right is a clear image of the spoke feature I saw. This is a satellite photo, taken from a near overhead position. Viewed through a telescope from Earth, the crater appears more elongated, given that it's seen from a more peripheral angle.

This photo, and others, are featured at Lunar Networks. You can see other strange features in this crater, like the grove that rings the inside left rim of the crater. Another, more sinuous rille is seen moving upward from the center peaks.

So for me it's a mystery solved, but also a reminder that many exciting lunar features lurk near the moons periphery. Features that I've tended to overlook.

The cause of the many odd features on the floor of Petavius remain a mystery for scientists who study the moon. Usually, impacts like that which likely caused Petavius, break the lunar surface and allow lava to flow and cover the basin floor. For some reason, with Petavius this lava coating was incomplete, leaving a major break in the crater floor exposed.

If you're already familiar with Petavius, you're probably laughing at my naivete. But if you're not familiar with Petavius, try looking at that region of the Moon shortly after the lunar phase that exposes it. See if the linear feature of the rille jumps out at you like it did me. I'll certainly be looking for it again. But note that like the Straight Wall, the Petavius rille is a feature that's only easily visible when the Sun angle throws a shadow on the bottom of the rille. If you observe Petavius much after the Moon's crescent phase, that area will be too evenly illuminated to reveal the rille.

And one final point. I remind you that I was able to pick out the illusive rille with my modest 60mm refractor. So if you have a telescope, even a small one, you can likely see the Petavius rille -- when the time is right.

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