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telfer
2007-Aug-01, 09:20 AM
I've just listened to the last Astronomy Cast podcast about tidal forces as was fascinated by how Mercury rotates around the sun and that we originally thought that it always kept the same face to the sun.

This got me thinking how hot does Mercury get on the "day" side and does the temperature plummet drastically on the "night" side (given how close it is to the sun?)

Van Rijn
2007-Aug-01, 09:56 AM
From here:

http://hypertextbook.com/facts/2000/OlesyaNisanov.shtml

The temperature of the surface of Mercury changes from day to night. Before the sunrise the temperature on the surface of Mercury is as low as 100 K (-170 C) and by noon the temperature on the surface of Mercury rises to about 700 K (400 C).

Keep in mind, though, that Mercury has no atmosphere, so we are talking about the surface specifically, and "noon" here is in reference to a Mercury day, not an Earth one.

grant hutchison
2007-Aug-01, 10:05 AM
Mercury has a significantly elliptical orbit, so the daytime temperature also depends on its current distance from the sun. Noon during closest approach will produce the hottest temperatures.
Because of Mercury's spin-orbit resonance, if one point on its surface gets the "hottest noon" during one orbit, a point exactly on the opposite side of the planet will get the hottest noon next time around. And then it cycles back to the first point again.
So Mercury has a couple of "hot poles" on its equator, which alternately get the full heat of the sun at its closest. One of those hot poles lies in the Caloris Basin (which got its name from the Latin calor, "heat").

Grant Hutchison

telfer
2007-Aug-01, 10:45 AM
Thanks for the replies. It's pretty astounding that Mercury goes through such temperatures. Picking up on the point about Mercury having no atmosphere I have another question. Does having an atmosphere mean that in theory a planet would not have such temperature differences? As in an does having atmosphere mean that the day side of the planet get protected from over-hot temperatures and the atmosphere traps the heat on the surface of the night side. I know that this can obviously depend on distance from the sun and the composition of the atmosphere.

John Mendenhall
2007-Aug-01, 02:43 PM
Thanks for the replies. It's pretty astounding that Mercury goes through such temperatures. Picking up on the point about Mercury having no atmosphere I have another question. Does having an atmosphere mean that in theory a planet would not have such temperature differences? As in an does having atmosphere mean that the day side of the planet get protected from over-hot temperatures and the atmosphere traps the heat on the surface of the night side. I know that this can obviously depend on distance from the sun and the composition of the atmosphere.

The extra solar 'hot Jupiters' where the atmospheres can be observed (at least one, so far) apparently have terrific winds around the planet. So an atmosphere redistributes heating. Venus is a good example, also. The anomalous glow reported on the night side of Venus is believed to be heat glow from the atmosphere.

EDG
2007-Aug-02, 05:02 AM
Does having an atmosphere mean that in theory a planet would not have such temperature differences? As in an does having atmosphere mean that the day side of the planet get protected from over-hot temperatures and the atmosphere traps the heat on the surface of the night side. I know that this can obviously depend on distance from the sun and the composition of the atmosphere.

If it's thick enough then yes. With no atmosphere the surface just sits there absorbing all the heat it can during the day and radiating it off into space at night with nothing reflecting it back. With an atmosphere the clouds can reflect it, the air can circulate and move the heat from the dayside to the darkside (warming that up) and from the darkside to the dayside (cooling that down), and the air and clouds can also trap heat on the darkside too. Of course that can go too far as well, look at Venus - the atmosphere is so thick that it traps a heck of a lot of heat and does it so well that the dark side isn't actually any cooler than the dayside (even though the night is very long).

Jens
2007-Aug-02, 05:35 AM
By the way, what would you use the measure the temperature on Mercury? :)

Romanus
2007-Aug-02, 04:33 PM
The preferred instrument would probably be a radiometer, IIRC.

neilzero
2007-Aug-10, 07:12 PM
Even at sea level on Earth, measuring temperature leads to some uncertanty. I've heard that the sun's temperature is a million degrees hotter a few thousand miles above the photophere. This extreemly hot corona has less matter per cubic meter than a good laboritory vacuum. Please comment. Neil

Aiwe
2007-Aug-10, 07:32 PM
Even at sea level on Earth, measuring temperature leads to some uncertanty. I've heard that the sun's temperature is a million degrees hotter a few thousand miles above the photophere. This extreemly hot corona has less matter per cubic meter than a good laboritory vacuum. Please comment. Neil

The temperature of the corona plasma is on the order of a fantastic millions of Kelvin, while the photosphere is just six thousand degrees Kelvin and a bit above that, when first entering the sun's atmosphere, it drops to only four thousand K.
The plasma in the low corona is about 10^14 particles per meter cubed, to 10^16 per meter cubed. For comparison, at sea level on earth the particle density is at least a good nine orders of magnitude higher, at 10^25 particles per meter cubed.
The density in the vacuum chamber I use for Argon plasma probably has about 10^19 to 10^20 particles per meter cubed on average.