| LATE NOACHIAN CLIMATE OF MARS: CONSTRAINTS FROM VALLEY
NETWORK SYSTEM FORMATION TIMES AND THE INTERMITTENCIES
(EPISODIC/PERIODIC AND PUNCTUATED).
J. W. Head, Brown University, Providence, RI 02912 USA (james_head@brown.edu)
Formation of Late Noachian-Early Hesperian (LN-EH) valley network systems (VNS)
signaled the presence of warm/wet conditions generating several hypotheses for
climates permissive of these conditions. To constrain options for the ambient Noachian
climate, we examine estimates for time required to carve channels/deltas and total
duration implied by plausible intermittencies. Formation Times for VN, OBL, Deltas,
Fans: A synthesis of required timescales show that even with the longest estimated
continuous duration of VN formation/intermittencies, total time to carve the VN does
not exceed 106 years, <∼0.25% of the total Noachian. Intermittency/episodicity
assumptions are climate-model dependent (e.g., most workers use Earth-like fluvial
activity and intermittency). Noachian-Early Hesperian Climate Models: 1) Warm and
wet/semiarid/arid climate: Sustained background MAT >273 K, hydrological system
vertically integrated, and rainfall occurs to recharge the aquifer. Two subtypes:
a) “Rainfall/Fluvial Erosion-Dominated Warm and Wet Model”: “Rainfall and
surface runoff” persist throughout Noachian to explain crater degradation, and a
LN-EH short rapidly ending terminal epoch. b) “Recharge Evaporation/Evaporite
Dominated Warm and Wet Model”: Sustained period of equatorial/mid-latitude
precipitation and a vertically integrated hydrological system driven by evaporative
upwelling and fluctuating shallow water table playa environments account for sulfate
evaporate environments at Meridiani Planum. Sustained temperatures >273 K are
required for extended periods (107-108 years). 2) Cold and icy climate: Sustained
background temperatures extremely low (MAT ∼225 K), cryosphere is globally
continuous, hydrological system is horizontally stratified, separating groundwater
system from surface; no combination of spin-axis/orbital perturbations can raise
MAT to 273 K. Adiabatic cooling effects transfer water to high altitudes, leading to
“Late Noachian Icy Highlands Model”. VNS cannot form in this nominal climate
environment without special circumstances (e.g., impacts or volcanic eruptions elevate of
temperatures by >∼50 K to induce melting and fluvial/lacustrine activity). 3) Cold and Icy
climate warmed by greenhouse gases: The climate is sustained cold/icy model, but
greenhouse gases of unspecified nature/amount/duration elevate MAT by several
tens of Kelvins (say 25 K, to MAT 250 K), bringing annual temperature range into
the realm where peak seasonal temperatures (PST) exceed 273 K. In this climate
environment, analogous to the Antarctic Dry Valleys, seasonal summer temperatures above
273 K are sufficient to melt snow/ice and form fluvial and lacustrine features, but
MAT is well below 273 K (253 K). Fluvial systems driven by episodic/periodic
intermittency typically involve short intermittency time-scales (10-106 years) but require a
warm climate (MAT >273 K) to be sustained for >0.4 x 109 years. Fluvial systems
driven by punctuated intermittency typically involve short duration time-scales
(10-105 years) but only require a warm climate (MAT >273 K) for the very short
duration of the climatic impact of the punctuated event (102-105 years). We conclude
that a cold and icy background climate with punctuated intermittency of warming
and melting events is consistent with: 1) the estimated durations of continuous VN
formation (<105 years) and 2) VN system estimated recurrence rates (106-107 years). |