Extending a new Lunar thermal model, Part III: Modelling the Moon at various rotation rates

via Tallbloke’s Talkshop
http://ift.tt/1WIzElD

sun-earth-moon

Overview

More than a year after “Part II” of a guest post from Talkshop contributor ‘Galloping Camel’ on the Moon’s equatorial temperature here is “Part III”. Peter actually sent this to Tim Channon last year, but Tim became to ill to deal with it and forgot to throw it my way. In current discussion of Ned and Karl’s new paper, the issue of planetary surface temperature variation due to speed of rotation arose. Ned thinks it makes no difference. Peter’s model says it does, so now is a good time for discussion, as this impacts theoretical estimates for the temperature of ‘Earth with no atmosphere’.

Modeling the Moon

It has been claimed that the GHE (Greenhouse Effect) is 33 Kelvin because the Earth’s average temperature is 288 K compared to a temperature of 255 K assumed for an “Airless Earth”. The Diviner LRO showed that the Moon’s average temperature is 197.3 K which makes one wonder how an estimate based on impeccable mathematics could be so wrong? Vasavada et al. published a paper in 2012 that mentioned a one-dimensional model of the Moon’s regolith. As I was unable to obtain details of this model I attempted to replicate it using Quickfield, a powerful FEA (Finite Element Analysis) program. Results obtained using my model were published here.

It is now clear that the 255 K estimate was wrong because it made unrealistic assumptions about the thermal properties of planetary surfaces. In contrast Vasavada used properties of lunar regolith measured by Apollo astronauts. There are now several other models that can reproduce the Diviner LRO measurements.

Rate of rotation

Scott Denning, Monfort Professor of Atmospheric Science at Colorado State University has such a model and he sent me temperature data for a wide range of rotation rates. I tried to replicate his results using my FEA model.

gc-fig1

The above graph shows good agreement between our models and supports the idea rotation rate has a significant effect on the average temperature of airless bodies. Here is a graph showing how temperatures should vary with rotation periods ranging from 14 minutes to 7,300 days:

gc-fig2

MOON0.01 shows how temperatures should vary if the rotation period is 0.01 of an Earth day or roughly 14 minutes. At the other extreme MOON7300 corresponds to one rotation every 20 years. This graph shows that daytime temperatures remain constant until rotation rates can be measured in hours, whereas night time temperatures are more sensitive to rotation rates:

gc-fig3