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which are the oldest exposed part of the Figure 3. (A) Obliquity of Mars at 2–6 Ma as cal- 50°
martian crust, contain abundant evidence culated by Touma and Wisdom (1993). Note that A
of erosion by water. Mars is freezing cold obliquity exceeded 40° at 5–6 Ma. (B) Probability
of reaching high obliquities during the chaotic
now and, with similar surface and atmo- obliquity evolution of Mars over a range of time 40°
spheric conditions, should have been even periods, with initial 25° obliquity (from Laskar et
al., 2004). (C) Total annual insolation versus lati-
colder at 3–4 Ga. Some drainages are tude for obliquity variation of 0°–90°. Insolation Obliquity
thousands of kilometers long and were units are relative to the solar constant at 1.52 AU 30°
(from Ward, 1974). (D) Normalized density func-
fed by numerous tributaries that reached tion for chaotic eccentricity variation for Mars
drainage divides in headwater regions (from Figure 18d of Laskar et al., 2004). At pres- 20°
(Howard et al., 2005; Hynek et al., 2010). ent eccentricity, solar insolation at perihelion
(orbital point closest to the Sun) is 45% greater
Many rivers flowed into or through crater than at aphelion.
lakes, and some left delta deposits (Irwin 10° 6 5 4 3 2
et al., 2005; Fassett and Head, 2008b; Age (Ma)
Goudge et al., 2016). Calculations based obliquity is currently 23°, but because of
on canyon width and depth indicate that stabilizing tidal forces associated with the 100
canyons reflect ~10 –10 years of erosion Moon, obliquity varies over geologic time B
3
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and were not incised by catastrophic out- by <± 2° (Laskar et al., 1993). Mars, with 80
flows (Barnhart et al., 2009; Hoke et al., current obliquity of 25°, does not have a
2011; Rosenberg and Head, 2015). massive moon, and its obliquity is not 60
Precipitation, drainage incision, and similarly stabilized. Because of tidal Probability (percent)
crater-lake filling are inferred at ca. forces exerted on Mars by the Sun and 40 50 Ma
3.3–3.9 Ga based on crater density in planets, obliquity varied chaotically over 25 Ma 100 Ma
affected terrains (Fassett and Head, millions of years to >60° (Figs. 3A and 20 10 Ma 250 Ma
2008a; Hoke and Hynek, 2009) (Fig. 2E). 3B) (Touma and Wisdom, 1993; Laskar
Several factors would have supported et al., 2004). At obliquities >~45°, polar 0 5 Ma
warmer Noachian environmental condi- regions receive more sunlight than equa- 40° 50° 60°
tions, although maybe not enough for pre- torial regions, potentially resulting in sea- Maximum obliquity (degrees)
cipitation and flowing water. The pressure sonal sublimation and evaporation at high
of the modern martian atmosphere, at 6–10 latitudes and snow at low latitudes at 0.5
millibars, is ~1% that of Earth, but the times near summer solstices (Fig. 3C) C
ancient atmosphere was much thicker. The (Ward, 1974; Jakosky and Carr, 1985; 0.4
size and abundance of the smallest martian Wordsworth, 2016). High-latitude evapo- 75° 90°
impact craters can be used to determine ration would be especially effective if the 0.3
60°
atmospheric pressure because the smallest summer solstice coincided with greater Annual insolation 60°
45°
meteorites are slowed or destroyed during proximity to the Sun during a period of 0.2 45°
30°
passage through the atmosphere and so do high orbital eccentricity, which also 30°
not create impact craters. Size-frequency varies chaotically (Fig. 3D). 0.1
15°
distributions for craters in fluvial deposits Favorable obliquity and eccentricity, 15°
near Gale crater indicate that Noachian and a thick CO atmosphere, may have 0.0 0° 0°
2
atmospheric pressure was in the range of been adequate for evaporation and subli- 90° 75° 60° 45° 30° 15° 0°
~1–2 bars during heavy Noachian bom- mation of ice at low elevations and accu- Latitude
bardment (Kite et al., 2014). Atmospheric mulation of snow and ice at high eleva-
pressure greater than a few hundred mil- tions, but warmer conditions are needed D
libars results in a vertical temperature pro- to melt snow and ice at high elevations 10 present
file that approximates an adiabatic gradi- and produce runoff to carve river valleys Probability
ent (Wordsworth, 2016). Under such and fill lakes in the Noachian highlands 5
conditions, surface temperatures are lower (Forget et al., 2012). Global climate mod-
at higher elevation, with potential accumu- els indicate that 1%–10% hydrogen and 0 0.0 0.05 0.10 0.15
lation of snow and ice at high elevations. methane in a thick CO atmosphere could Eccentricity
2
Even if the atmosphere was pure CO , have elevated temperatures sufficiently to
2
however, this would not be adequate to melt ice at high elevations (Wordsworth
warm early Mars to the point of supporting et al., 2017). These reduced gases are
running water, especially in highland highly effective at absorbing infrared produce a weak bond in which the two
regions (Kasting, 1991; Forget et al., 2012). radiation that would otherwise leave the gas molecules can absorb infrared radia-
Orbital factors relevant to early Mars planet because of a process called “colli- tion that would not be absorbed by the
climate are the variable tilt of its spin axis sion-induced absorption.” In this process, individual gas molecules. Collision-
relative to the normal to the orbital plane extremely brief (~10 s) electrostatic induced absorption with these gases can
–13
(the obliquity) and the variable eccentric- interactions between colliding gas mol- potentially produce an early Mars atmo-
ity (ellipticity) of the orbit. Earth’s ecules (CO -H and CO -CH in this case) sphere warm enough to cause melting and
2
2
2
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