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#BAFact: To the accuracy with which we can measure them, both Mercury and Venus are perfect spheres.

I already posted in previous #BAFact s that Saturn is the flattest planet, and Earth is very nearly spherical; just 0.3% off round. But the roundest planets are Mercury and Venus: to the accuracy we can measure them, they are both perfectly spherical!

Most likely they are not precisely perfect spheres, though. The Sun has a tidal force on Mercury that is 17 times bigger than the Moon's tides on the Earth, so Mercury is stretched out along its equator by many meters at least. However, that's far too small to measure with current technology. The Same goes for Venus; both planets are essentially the ball bearings of the solar system.

Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
John Naddaf's profile photoDavid Foster's profile photoChris Lutz's profile photoBill Klumpp's profile photo
For Venus, are they talking about the surface or the atmosphere? I thought there were volcanoes all over venus, I would have expected less uniformity. 
Thanks. Why are Mercury and Venus so round like perfect spheres?
Yes, what about the mountains on Venus? I know there are some tall ones. 
Because their days are so long, so there's not a lot of centrifugal force.
Thank you, +Philip Plait, for posting this.  It answers the question I asked in your post yesterday.
I was expecting this from Venus it is a solid planet who rotates really slowly, but Mercury I am surprise. +Philip Plait the stronger magnetic field  of Mercury is given by a Dynamo effect. which also means the the Mercury core is fluid. Even if Mercury rotates slowly it should get a deformation anyway
+Philip Plait what is the accuracy of our measurements? You said a few meters is too small to measure...but what about a hundred or thousand meters? Curious as to the exact numbers :)

Also, do we have any idea what the density is like on either planets...I'm guessing that's certainly non-uniform.
Would earth be a perfect sphere if it were measured with the same precision?

Or how much more precision we have when we measure earth?
Is there any reason the MESSENGER probe hasn't been able to give us a more accurate picture of Mercury's shape?
Alright! Chock up two more for the interstellar marbles collection.
I'm sure Mercury's shape data includes current data from MESSENGER and Venus will be left to a certain degree of error considering the planets climate plus the accuracy of the probes currently there and no hiccups in data transmission back to Earth. Hopefully, Juno will be able to improve the data received from Galileo when it sets up shop around Jupiter. We'll hopefully get a better idea of how many more probes we need to send to the Kuiper Belt and Pluto from New Horizons in 2015. And I'm sure there's talk somewhere about going to Uranus and Neptune based on what Cassini has shown us at Saturn.
that because they are two of the more rocky solid planets,so shape holds better after being rounded by its gravity?
I'm sure gravity plays a part, but also its rotation and how fluid its core is also plays a part in its orbital path and magnetic field. At least, this might be true with Venus. With Mercury, there might be evidence in data that is not publicly available yet about how a rocky body, so close to the Sun or any Sun-type star, is able to maintain its shape without being ripped apart from gravitational forces being exerted on it constantly by both the Sun and the rest of the bodies in its system.. 
+Bill Klumpp Mercury is tidally-locked to the sun. Consequently, the tidal forces exerted by the sun's gravity do not have as drastic an effect as one might imagine on the planet's structure (it isn't being flexed the way the sun's and moon's tidal forces flex the earth as it rotates, because it isn't rotating relative to the sun). The fact that mercury is a small, dense, rocky body is exactly why it is perfectly capable of existing at its orbital distance. Now, if it was a gaseous planet, or had a discernible atmosphere, there would be some explanation necessary.
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