# Presentation: 571732

## Lunar Borehole Summary

- 1. Two scenarios of the Sun’s luminosity (TSI) differing by  2 W/m2 over 300 years can be distinguished by the lunar regolith temperature profiles that they produce, with peak difference  10 mK at depth  10 m. Paper in GRL, available at: *GRL* _**35**_, L02716, doi:10.1029/2007GL032171, online at:
**	****	****http://climate.gsfc.nasa.gov/****viewPaperAbstract.php?id=1098**
- 2. The Moon’s surface is NOT in radiative thermodynamic equilibrium during day or night. ”Turning off” the Sun in a time dependent thermal model demonstrates that it would take  1000 years to reach a nearly constant equilibrium temperature of about 24-38 K.  However, equilibrium *may* be a good first-order approximation to surface temperature in a permanently shadowed region.

## 300 years

**What is the recent history of our Sun’s TSI?**

**Sun’s TSI *****highly uncertain***** due to **_**unknown**_** faculae!**

## Earth’s Moon is an ideal place to discover the history of the Sun’s luminosity or TSI

**Earth’s Moon is an ideal place to discover the history of the Sun’s luminosity or TSI**

## Two widely used TSI scenarios

**Two widely used TSI scenarios**

## Apollo Heat Flow Experiments (HFEs)

**Apollo Heat Flow Experiments (HFEs)**

- Heat Flow Experiments (HFEs) from Apollo 15 & 17 show very small thermal diffusivity of lunar regolith 10-8m2/s, 100 X smaller than that of Earth’s crust.

## Scale Analyses

- 1. Diffusion depth (Ze) of a periodic surface variation is proportional to the square root of diffusivity, K, times the period of the variation, T.  Ze  (KT), where K  10-8 m2/s, T  300 π 107 s, Ze  10 m.
- 2. Relaxation time scale   Ze2/K  500 - 1600 years for a regolith layer of  10 m thick, and K  2-7x109 m2/s.

- e-folding penetration depth as a function of period of surface temperature variation

## Energy Budgets of Lunar Surface Climate

**Energy Budgets of Lunar Surface Climate**

- 1. Daytime absorption of solar radiation is the driving force.
- 2. Daytime absorbed solar (542W/m2) is not balanced by emitted infrared (536W/m2).
- 3. Daytime net flux at the surface is stored as heat (6W/m2). This energy storage is released during lunar night.
- 4. Terrestrial radiation from the Earth (0.09W/m2). is about two orders of magnitude less than the heat storage (6W/m2), and can be ignored at Apollo sites

## Model Results vs Observations at the surface @ Apollo 15 latitude

**Model Results vs Observations at the surface @ Apollo 15 latitude**

## Temperature anomalies as response to two scenarios of reconstructed TSI at the equator, mid-latitude and near south pole.

## 2. Simple radiative equilibrium (e.g. Huang 2007) is inappropriate to relate Apollo-observed nighttime temperature to Earth’s radiation budget. 

- 1. ”Turning off” the Sun in the time dependent model shows that it would take  1000 years to reach a nearly constant equilibrium surface temperature in the range 24-38 K.
- 2. Simple radiative equilibrium (e.g. Huang 2007) is inappropriate to relate Apollo-observed nighttime temperature to Earth’s radiation budget.
- 3. Thermal equilibrium is a good first-order approximation for temperature in permanently shadowed regions where terrestrial radiation from the Earth does become important.

## Lunar Borehole Summary

- 1. Two scenarios of the Sun’s luminosity (TSI) differing by  2 W/m2 over 300 years can be distinguished by the lunar regolith temperature profiles that they produce, with peak difference  10 mK at depth  10 m. Paper in GRL, available at: *GRL* _**35**_, L02716, doi:10.1029/2007GL032171, online at:
**	****	****http://climate.gsfc.nasa.gov/****viewPaperAbstract.php?id=1098**** **
- 2. The Moon’s surface is NOT in radiative thermodynamic equilibrium during day or night. ”Turning off” the Sun in a time dependent thermal model demonstrates that it would take  1000 years to reach a nearly constant equilibrium temperature of about 24-38 K. However, equilibrium *may* be a good first-order approximation to surface temperature in a permanently shadowed region.

## Workplan

## Lunar Borehole Model (Keihm 1984)

- Paper in press in GRL & available at:
**http://climate.gsfc.nasa.gov/****viewPaperAbstract.php?id=1098**