Geoscience Reference
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10
Earth
1
Solar system
Habitable
zone
0.1
Tidal lock radius
0.01
0.01
0.1
1
10
100
Distance, AU
Figure 1.1. Habitability zone as determined by Jim kasting and colleagues including the place-
ment of the eight planets (plus Pluto) of our solar system. One AU is one earth distance
from the sun. the vertical axis shows the ratio between the mass of a star and the mass of
the sun. At distances less than the tidal lock radius from a star, planets become locked in
rotations around their axis with small integer values relative to the time scale of the planet's
orbit around the star (mercury rotates 3 times on its axis for every 2 orbits around the sun).
In some cases a planet can orbit with a 1/1 rotation to orbit ratio, with the same side of the
planet always facing the star. Planets within the habitable zone of small stars are within the
tidal lock radius. From kasting (2010).
4.5 million miles closer to the Sun than we are. The results of Jim's cal-
culation are presented in igure 1.1 , and by all accounts we are lucky;
Earth sits snugly within the habitable zone of the Sun.
If this is true, why do we keep entertaining the possibility for life on
Mars? Consistent with Jim's habitable zone arguments, there's no evi-
dence for continuously standing surface water on Mars, at least not
now. But during decades of satellite and surface exploration, including
the recent, highly successful rovers, Spirit and Opportunity of the Mars
Exploration Rover Mission (MER), and the THEMIS (high resolution
thermal imaging system) imager onboard the Mars Odyssey orbiter,
water has flowed and still does occasionally flow on Mars. This is evi-
denced by all sorts of channels, ditches, pools, and sedimentary rocks,
 
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