Venus Earth Mars - Earth and Space Sciences

Venus Earth Mars

Mean Orbital Radius: 0.72 AU
Equatorial Radius: 0.95xEarth Radius
Mean Surface Temperature: 462C (864F)
Mean Surface Atmospheric Pressure: 92 bar, and parched

Mean Orbital Radius: 1.00 AU
Equatorial Radius: 6378 km
Mean Surface Temperature: 15C
Mean Surface Pressure: 1 bar, swimming in water

Mean Orbital Radius: 1.50 AU
Equatorial Radius: 0.53x Earth Radius
Mean Surface Temperature: -63C (-81F)
Mean Surface Atmospheric Pressure: 0.006 bar, and parched

Venus was supposed (ca. mid-20 th century) to be an exotic,
steamy jungle beneath its clouds.
Mars was supposed to be Barsoom, or at least “Taos without
the tourists” (Burkhard Bilger, “The Martian Chroniclers”, The
New Yorker, 22 April 2013, http://www.newyorker.com/reporting/
2013/04/22/130422fa_fact_bilger)

Mars was an unfortunate runt (maybe prevented from growing
by Jupiter – Walsh et al., “A low mass for Mars from Jupiter’s
early gas-driven migration”, Nature 475, 206-209, 2011).
It got cold early, lost it’s early dynamo and associated magnetic
field.
The Sun blew away it’s atmosphere and with it a lot of its water.
All this happened by ~ 3.5 billion years ago – maybe a billion
years before widespread oxygenic photosynthesis on Earth.

Formation of Glaciers
by Atmospheric
Precipitation at High
Obliquity, Forget et al.,
Science 311, 2006.
Buried ice on Mars:
Radar evidence for
remnant glaciers in
the southern midlatitudes,
Holt, et al.,
Science, 2008.
.
Relict ice deposits, geological evidence of
flowing glaciers in past

Dynamic Ice Reservoirs
• On 100,000 to 5M yr. (obliquity)
time scales (J.W. Head III et al.,
“Recent Ice Ages on Mars”,
Nature 426, 2003)
• Perhaps on 25,000 yr.
(precessional) time scales (R.M.
Haberle, Eos Trans. AGU 84(46),
Fall Meeting Suppl. Abstract
P42C-04, 2003.
• Decadale-scale variation in global
temperature driven by dust-related
albedo change (Fenton,Geissler
and Haberle, Nature 446, 2007)

What Can We Learn from Mars’ Polar Ice Caps
Dale P. Winebrenner 1, Michelle Koutnik 1 , Edwin D. Waddington 1 , Asmin V.
Pathare 2 , Shane Byrne 3 , and Bruce C. Murray 4
1
University of Washington
2
Planetary Science Institute
3
University of Arizona
4
California Institute of Technology

Stratification exposed on scarps -- layers of differing dustiness
deposited in differing climate episodes

Figure from Clifford et al. (2000) Icarus 144, 210-242.
ca. 600 km across, on exceptionally flat
northern plains
Up to 3 km thick, but low elevation wrt
global mean geoid (base ca. -5100 m)
Volume approx 1.5x10 6 km 3 (about half
that of Greenland, 1/20th that of
Antarctica)
Mean annual T s : 162K (-111C), basal heat
flux unknown (30 mW/m 2 )
Water ice exposed on young surface
Some evidence for underlying, dustcovered
deposit (of ice)
Incision by strange troughs, and an
enormous chasma (Chasma Boreale)
Note Titania Lobe/Gemina Lingula

(Bamber, 2004)
On Earth:
• Long, near-linear ridge lines with near-uniform gradient along their
length are characteristic of flow divides on deforming ice sheets

Standard shadedrelief
MOLA DEM
Shaded to make
high elevations
with low slopes
stand out
• Resemblance is not identity, but let’s test.
• Feature is cut relatively little by troughs.
• Troughs apparently expose internal layers.

On Earth, flow thins layers at depth,
so “reading” stratigraphy requires
correction for strain.
Do Martian ice caps flow
• Of course! How could they not
• The question is: How fast (in
comparison to rates of other processes
that might sculpt the caps)

Ice always “flows”…
We can consider if flow controls the surface shape relative to other processes.
Flow number:
Surface mass exchange
with the atmosphere
Move ice over the length
of the glacier by flow
F > 1: flow controls surface shape
Winebrenner et al. (2008)

Estimation of Pre-Trough Surface Topography
1) Exclude places where
slope exceeds 0.015
radians.
2) Exclude places in bottoms
of troughs where slope is
near-zero.
3) Bridge
troughs by
linear
interpolation.
4) Smooth.

Surface with Troughs Interpolated and Smoothed
Aspect
Contours

Candidate Flow Lines AS IF Troughs Were Filled with Ice
Aspect
Contours

Simple, Steady-State Ice-Flow Model for Topography
• steady-state flow equilibrates surface mass fluxes
• flat bed (at -5100 m), no sliding (cold)
• 2 spatial dimensions
• incompressible, non-Newtonian fluid – flow law exponent n.
• temperature constant over depth
• neglect longitudinal stresses and vertical velocities
• works surprisingly well in East Antarctica

Steady-State Ice-Flow Model for Topography
• steady-state flow equilibrates surface mass fluxes
• flat bed (at -5100 m), no sliding (cold)
• 2 spatial dimensions
• incompressible, non-Newtonian fluid – flow law exponent n.
• temperature constant over depth
• neglect longitudinal stresses and vertical velocities
accumulation, c
⎛⎛
⎜⎜
⎝⎝
h
H
2+
⎞⎞
2 n ⎛⎛ c ⎞⎞
⎟⎟ + ⎜⎜ 1+ ⎟⎟
⎠⎠ ⎝⎝ a⎠⎠
1
n
⎛⎛ x ⎞⎞
⎜⎜ ⎟⎟
⎝⎝ L⎠⎠
1+ 1 n
=1
H
x = 0
h(x)
equilibrium line
€
x = L
x = R €
ablation, a
⎛⎛
⎜⎜
⎝⎝
h
H
⎞⎞
⎟⎟
⎠⎠
2
⎛⎛
= 1+ a ⎞⎞ n +1⎛⎛
x ⎞⎞
⎜⎜ ⎟⎟ ⎜⎜ 1− ⎟⎟
⎝⎝ c ⎠⎠ ⎝⎝ L⎠⎠
Flux balances integrals of mass
balance, so shape insensitive to
details of spatial pattern
1

Fits in Places with Seemingly Ablated Topography Don’t Fit
a Flow model (one of the 11, not one of the 40)
RMS Difference 44 m (and greater)

But Everywhere with Extensive ‘Relict’ Topography Does
Fit, With Consistent (Inferred) Parameters

Again

And Again

Inferred Terminus Positions and Even Equilibrium
Line Positions Tell a Consistent Story

The shape of an F~1 ice mass is generic.
Gemina Lingula
East Antarctica: F~1
Gemina Lingula: F~1
Meighen Ice Cap, Arctic Canada: F

Interpretation
Climate
Change
1. Flowing Ice Sheet
with Accumulation
and Ablation Zones
(Sometime in the
Past)
2. Flow (and Accumulation)
Largely Stopped

3. Trough Incision in Stopped Ice Sheet Produces Present-Day
Topography of Titania Lobe

Relict Surface
Claim: The present-day topography of Titania
Lobe preserves evidence of flow prior to trough
formation.

In the Accumulation Zone: NO intersection of subsurface
layers with surface ALONG FLOW LINES
In the Ablation Zone: Layers expected to intersect surface
ALONG FLOW LINES

23
25

Relict
topography
near
terminus

PRESENT DAY
MOLA along
Gemina Lingula
Surface in
steady state with
present-day conditions

PRESENT DAY
Surface temperature = 170 K:
Ablation rate = 8 × 10 -6 mm/yr
Volume response time = 232 Gyr
!!!
Ablation rate = 0.2 mm/yr:
(Pathare and Paige, 2005)
Surface temperature = 228 K
Volume response time = 9.5 Myr

Phillips et al., Science 2008
• Almost impossible to warm base of NPLD to 240K via surface
temperature increase, even at high obliquity (A .V. Pathare, pers. comm.)
• Lack of isostatic compensation of NPLD indicates a very cold, stiff
mantle (Phillips, Zuber et al., Science, 2008), indicating small geothermal
heat fluxes
SO HOW DID THE ICE GET WARM ENOUGH TO FLOW

Credit: C.A. Raymond

€

€

Figure 2 of Holt et al., Nature 465, 27 May 2010|, doi:10.1038/nature09050

‘proto-GL’
600-800 m
thick beneath
GL dome
€
extends with
thicknesses
< 400 m into
‘flowtopography’
region (1600m
dividethickness)
Figure S4, Holt et
al. (2010)