Review/Discussion by Sam Schon
An Intense Terminal Epoch of Widespread Fluvial Activity on Early Mars: 2.
Increased Runoff and Paleolake Development.
Irwin et al. (2005): JGR, doi: 10.1029/2005JE002460
Big Idea: “Highland gradation was prolonged, but generally slow and possibly
ephemeral during the Noachian Period, and that the immature valley networks
entrenched during a brief terminal epoch of more erosive fluvial activity in the late
Noachian to early Hesperian.”
Background:
 Valley networks were developing during Noachian
 Heavy impact flux disrupts drainage basins during formation
 Superposing crater populations indicate that valley network development
slows during LN/EH [ref. to Tanaka 1986, Irwin & Howard, 2002, Irnwin et al.
2004]
 Fundamental differences in crater degradation on Mars compared to
Mercury or the Moon (e.g., more than just mass wasting).
 Drainage densities are lower than terrestrial values (see Hynek & Phillips,
2003)
 Low-order tributaries have unpredictable junction angles, which suggests
pre-existing topography is a primary control (immature drainage basin
development – a recurring theme in this field).
 Discussion of early Mars climate, faint sun, etc. [this paper is pre-Halevy et al.
(2007) on sulfur-powered greenhouse…]
Discussion Question Posed in Text: “How the Noachian craters became so deeply
degraded while the valley networks remained poorly developed?” [p.5]
Breaching of Enclosed Basins: [what about subsequent re-surfacing?]
1. Pristine Entrance Breaches [Gale, Fig. 3], Gale (summary of events on p.9)
is the best example presented for the hiatus and late-stage epoch; the
argument turns on the interpretation of mound deposits, their origin, and
modification.
2. Pristine Exit Breaches [terraced crater in Memnonia, argued as stable
baselevel – but Irwin et al. argue for “late-stage breach – how do you support
stable baselevel in a closed system, implies delicate balance (?)]  Idea that
upstream integration of previously enclosed basins dramatically changes the
water budget…
3. Entrenchment with Terraces [Unnamed crater in Terra Sirenum]; Evidence
for lake stand at 1040-m, then subsequent incision and erosion to 900-m
level; outlet channel is entrenched. The volumes implied, lead Irwin et al. to
conclude that drawdown of a paleolake is the most likely water source. No
delta or coherent terraces indicate the 1040-m level. [Variable lake level, etc.]
Review/Discussion by Sam Schon
Irwin et al. discusses the presence of multibasin valley networks (p.18), but not in
detail. More recent work (Fassett & Head, 2008) has presented abundant evidence for
these types of systems. What implications do this level of integration have for Irwin et
al.’s concept of a terminal epoch of fluvial activity? Resurfacing seems common and
delta/fan deposits rare – how does this limit our understanding of these systems’
development?
p.22, “Taking this reclassification of water sources [putative paleolakes sourced by
valley networks], into account, Cabrol and Grin [1999], reported 65 deltas within
equatorial highland (30S – 45N) impact craters that are breached by valley
networks. Of these, 50 show no indication of a delta at THEMIS visible or infrared
resolutions (18 and 100 m/pixel respectively). In these 50 cases the contributing
valley simply terminates at the crater floor, and no elevated deposit is present.”
“Only 33 positive-relief deposits are known at valley mouths.”
Table 1; Fig. 13-18



Scarp-fronted Deposits: rounded, triangular, or projecting in planform; no
evidence of channelization, trenching, or dissection; abrupt scarp
Scarp-fronted Fans: e.g., Eberswalde; channels; distinct scarp
Stepped Deposits: two or more scarps, continuous around cone-shaped
surfaces; no dissection
Interior Channels
 Channel vs. Valley issue
 Estimate of lake development (e.g., Eberswalde – why no outlet)
 Terraces and resurfacing