APOD: Stickney Crater (2023 Jul 08)

[APOD Robot]

Stickney Crater

Explanation: Stickney Crater, the largest crater on the martian moon Phobos, is named for Chloe Angeline Stickney Hall, mathematician and wife of astronomer Asaph Hall. Asaph Hall discovered both the Red Planet's moons in 1877. Over 9 kilometers across, Stickney is nearly half the diameter of Phobos itself, so large that the impact that blasted out the crater likely came close to shattering the tiny moon. This enhanced-color image of Stickney and surroundings was recorded by the HiRISE camera onboard the Mars Reconnaissance Orbiter as it passed within some six thousand kilometers of Phobos in March of 2008. Even though the surface gravity of asteroid-like Phobos is less than 1/1000th Earth's gravity, streaks suggest loose material slid down inside the crater walls over time. Light bluish regions near the crater's rim could indicate a relatively freshly exposed surface. The origin of the curious grooves along the surface is mysterious but may be related to tidal stresses experienced by close-orbiting Phobos or the crater-forming impact itself.

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[VictorBorun]

skiing across Stickney crater mind the speed limit of 8 m/s unless you feel like launching yourself to an orbit

[Roy]

1/1000 g = 1 millimeter /second squared, so striations are not from material interference. Electrical maybe?

[Chris Peterson]

1/1000 g = 1 millimeter /second squared, so striations are not from material interference. Electrical maybe?

10 mm/s². Certainly enough to result in enough force to cause material deformations in a rubble pile.

[Guest]

Late night,dropped one too many decimal points - still, doesn’t look like a rubble pile to me.

[orin stepanek]

I was looking closely; and to me it looks like a message inscribed on
this moon! I know it is just imagination; but sometimes that is fun
also!

[Chris Peterson]

Late night,dropped one too many decimal points - still, doesn’t look like a rubble pile to me.

With a bulk density of well under two, it pretty much has to be a rubble pile. Asteroids of this size commonly acquire a thin regolith or crust. And as close as this body is to Mars, and in a mildly eccentric orbit, you'd expect tidal forces to disrupt the body and the surface, which may well explain the visible structure.

[De58te]

That is some hi-res hirise camera. Stickney crater is some 9 kilometers wide and it fills the photo frame. You feel like you see it from some 9 kilometers away. Yet we are told the camera was some 6,000 kilometers away!
That would be like photographing the city of Washington DC from London England, which are some 6,000 kms apart. (disregarding the curvature of the Earth which would hide a direct view line.)

[Roy]

Looked up the averaged diameter and the estimated mass - my back of the envelope calculation is about 2 grams / cc. That is hardly a thick gumbo. There must be some way they are estimating.mass that I am missing? I suppose it could be like limestone, but granite is over 2.5 g/cc. What say you, Chris?

[Chris Peterson]

Looked up the averaged diameter and the estimated mass - my back of the envelope calculation is about 2 grams / cc. That is hardly a thick gumbo. There must be some way they are estimating.mass that I am missing? I suppose it could be like limestone, but granite is over 2.5 g/cc. What say you, Chris?

I think that the density is about 1.8. The mass is accurately calculated by analyzing the gravitational influence of Phobos on orbiting probes. There could be a lot of ice, but I think the best evidence suggests a lot of empty space.