Messenger Discovers "Spider" Crater on Mercury

I Don't Believe in Imaginary Property brings us a Washington Post story which discusses how scientists are finding surprises among the pictures sent back from Mercury by the Messenger spacecraft. In particular, images depicting a crater with over 100 troughs radiating out from it are stumping researchers. The crater is referred to as 'The Spider', and it occupies a basin that has turned out to be larger than once thought. NASA also has a discussion of the crater. The Messenger craft began taking the up-close photos earlier this month. From the Post: "Scientists were also surprised by evidence of ancient volcanoes on many parts of the planet's surface and how different it looks compared with the moon, which is about the same size. Unlike the moon, Mercury has huge cliffs, as well as formations snaking hundreds of miles that indicate patterns of fault activity from Mercury's earliest days, more than 4 billion years ago."

Re:Faults from extreme tides, etc

[TripMaster Monkey]

I wouldn't exactly call it "insane"...the Sun's tidal effects on Mercury are only about 17% greater then the Moon's tidal effects on Earth.

Re:Faults from extreme tides, etc

[kmac06]

Also Mercury is tidally locked with the sun, so even if there are huge forces, they are constant, not varying.

Re:Faults from extreme tides, etc

[nusuth]

That is incorrect, Mercury is not tidally locked with Sun. It is in 3:2 spin resonance with Sun, therefore the forces vary slowly (change direction twice for every three orbits) but they are not constant.

Tidally Locked?

[DeeVeeAnt]

Oh no it isn't!
http://en.wikipedia.org/wiki/Tidal_locking#Planets [wikipedia.org]
"Until radar observations in 1965 proved otherwise, it was thought that Mercury was tidally locked with the Sun. Instead, it turned out that Mercury has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it were totally locked."

Re:Probably Moon was formed later

[spacemandave]

Actually, you have that backwards. The Moon is covered in volcanic features. The dark "seas" are actually huge lava flood plains formed by volcanoes that were active for about a billion years after the Moon's formation. Mercury lacks these extensive volcanic features, likely because Mercury's crust is under compression making it harder for magma to break through and reach the surface. The compression is likely due to Mercury's massive iron core, which shrunk slightly as it cooled shortly after the planet formed.

Reporter not paying attention

[L. J. Beauregard]

Scientists were also surprised by evidence of ancient volcanoes on many parts of the planet's surface and how different it looks compared with the moon, which is about the same size.

FAIL. Mercury has about 1.4 times the Moon's radius and 4.5 times its mass.