Long-Term Obliquity Variations of a Moonless Earth

Jason W. BarnesAssistant Professor of Physics

University of Idaho

Planetary Obliquities are Chaot

icFrequency map analysis

gives chaotic region for Earth w/o Moon, 0º - 85º. For Mars, 0º - 60º (Laskar & Robutel 1993).

• Uncertainties grow exponentially with time in chaotic zone.

Average Yearly Flux changes as a function of Obliquity

Look What Happened to Mars!

What might the obliquity evolution

of a Moonless Earth be like?

Timescales

What might the obliquity evolution

of a Moonless Earth be like?

Changing Axis Angle

Chaos of Obliquity Evolution

Obliquity Range for different planetary rotation periods

Retrograde Rotators are More Stable

Changing Rotation, Initial Axis Azimuth

Chaos as a function of Initial Axis Azimuth

Conclusions• Obliquity changes of Earth without the Moon are chaotic

• Frequency-map analysis shows moonless-Earth 0-85o allowed• Real Mars 0-60o allowed – Mars can access nearly this full range

• Current prevailing conventional wisdom in astrobiology is that moon is necessary for obliquity & climatic stability• Assumes solar system• Assumes Earth’s present rotation & obliquity• Assumes 0-85o obliquity without Moon

• We use explicit, brute-force numerical integrations to explore the obliquity evolution of Moonless Earths (using modified J. Chambers’ mercury)• Typical integrations show variations +/- 10 degrees – survivable.• Some show the predicted large excursions• Retrograde rotators are totally stable

• Conclusion: Moons are NOT necessary for climatic stability. They help in some cases, and hurt in others. Need to know full system parameters.• Greatly increases estimates for number of habitable worlds in galaxy