Simple NonFluvial Models of Planetary Surface Modification

1
Simple Non-Fluvial Models of Planetary Surface
Modification

• Alan D. Howard
• Department of Environmental Sciences
• University of Virginia

2
Adding and Subtracting Material to/from a Surface

• Two very simple models involve addition or
  removal of material uniformly over a surface
• Vertical sedimentation
• Uniform accrescence or decrescence (e.g.,
  chemical precipitation or dissolution)

3
(No Transcript)
4
Vertical Sedimentation

• The landform morphology does not change through
  time, although the surface becomes increasingly
  buried
• Not a particularly realistic model, because most
  sedimentary surfaces tend to become smooth during
  deposition, either because of effects of
  currents, etc. on local deposition rate, or
  subsequent redistribution processes

5
Uniform Accrescence
6
(No Transcript)
7
Uniform Decrescence
8
(No Transcript)
9
Uniform Addition or Removal

• For a planetary surface, the rate of vertical
  build up for uniform addition (accrescence) is
  proportional to the deposition rate divided by
  the cosine of the surface gradeint.
• For uniform addition, projections become rounded
  and valleys remain, or become, sharply indented
• For uniform removal, projections remain or become
  sharply pointed, and indentations become rounded

10
Uniform Accretion of a Cratered Surface
11
Donut-type crater rims might be an indication
of uniform accrescence.
12
Uniform Decrescence of a Cratered Surface
13
(No Transcript)
14
Six Years of Uniform Decrescence
15
Accresence and Decrescence are not Reversible
16
Heuristic Model of Eolian Sedimentation

• Wind- or current-induced sedimentation typically
  favors accumulation in low areas and less
  sedimentation, or even erosion, on exposed ridges
  and summits
• The propensity for erosion or deposition is
  characterized by an exposure index, I, which is
  an inverse distance-weighted sum of the
  gradients, S, measured from the horizontal,
  between a given location and nearby locations in
  all directions
• ISSe-kd / Se-kd ,
• where d is the distance to a surrounding
  point

17
Deposition or Erosion rate as a Function of
Exposure Index
18
Modeled Eolian Infilling of a Single Crater
19
(No Transcript)
20
Martian Eolian Deposition
21
Diffusional Mass Wasting

• Creep traditionally has been modeled as a linear
  process whose mass flux, q, is governed by a
  diffusivity, K, and the downslope gradient, S
• q K S
• Recently, with the recognition that rates of mass
  wasting increase dramatically as a limiting slope
  steepness is approached, non-linear creep
  relationships have been proposed by Howard and by
  Roering et al.

22
Non-linear Mass Wasting Howard

• q K1 S K2 / (1-(S/Sc)n) 1 ,
• where Sc is the critical slope gradient.
• This is strictly a heuristic relationship

23
Non-linear Mass Wasting Roering

• Roering et al. suggest that mass wasting is
  caused by disturbances of equal magnitude in the
  upslope and downslope direction. In the upslope
  direction the movement is impeded by both
  friction and gravity, and downslope it is also
  impeded by friction but is aided by gravity. The
  net flow is the addition of upslope and downslope
  displacements. A simple analysis yields the
  relationship
• q K S / 1 (S/Sc)2

24
Effect of Non-linearity

• In steady-state landscapes (constant erosion rate
  due to steadily falling base level) the effect of
  non-linearity is strong in rapidly eroding
  landscapes, creating straight rather than rounded
  slope profiles

25
Mass Wasting without Fluvial Erosion

• On planetary landscapes diffusion can occur
  without fluvial erosion, for example by
  freeze-thaw induced creep or by micrometeorite
  impacts
• In such circumstances both linear and non-linear
  mass wasting produce similar results a
  landscape that looks like an out-of-focus
  photograph

26
Effect of Mass Wasting
27
Other Models of Mass Wasting

• In addition to models of linear or non-linear
  creep dependency upon slope gradient, there may
  be circumstances where diffusivity, K, is a
  function of position within the landscape
• One type of position dependency could be similar
  to that for the heuristic eolian deposition
  model, in which diffusivity depends upon how
  exposed or sheltered the location is
• E.g., summits and crater rims are exposed, crater
  floors and stream valleys are sheltered.

28
Exposure-disadvantaged Creep
Exposure-enhanced Creep

• These model results resemble, respectively, the
  eolian deposition and uniform accrescence models
  presented earlier, but unlike these, they imply
  no net addition or removal of material

29
Emplacement of Lava Flows

• Lava flows are generally depositional rather than
  erosive, so that their effects are broadly
  diffusive, with the exception that flows derive
  from a few rather than widely distributed
  sources.
• A heuristic model is briefly discussed

30
A Heuristic Model

• The rules-based model incorporates the following
  properties
• Flows originate from one or more sources
  (specified boundary conditions in the model)
• Flows persist stochastically with a fixed
  probability of the source becoming exhausted
• If a flow is interrupted, it starts anew from the
  source
• Flows are primarily fed by lava tubes, so that
  they have a strong probability of continuing in
  the same direction, but with some chance of flow
  blockage and redirection
• The probability of flow in a given direction
  increases with gradient, but lava can flow across
  level surfaces.
• Flow thickness is small compared to flow length

31
A Single Source
32
Multiple Sources
33
Comparing Flow with Flooding
34

• Some fairly characteristic features of
  mid-latitude terrain
• Donut craters (an issue is that the size of the
  donut is generally proportional to crater size,
  which would not be expected unless larger craters
  are always younger)
• Smooth surface
• Indistinct broad valleys with sharper narrow
  channels.
• Also round-edged craters with no exterior rim.
  Cookie-cutter-like.

35
Cookie-cutter craters, some hint of donut rings,
also flat-floored depression at bottom of image
36
Intersting combination of ghost craters, which
could be buried or eroded, raised circular
platforms that look like remnants of floors of
eroded craters, rimless craters that look buried,
and generally smooth terrain that looks either
like strong diffusional creep or deep deposition.
The odd feature out in this image are the
raised circular platforms.
37
Another contrast between rimless crater and
raised crater floor. Presumably lots of
secondaries as well.
38
Roundec crater wall, rimless craters, and
possible buried ghost crater. Mostly suggestive
of deposition. Some interesting
compressional-ridge-like features at floor of
inner crater wall.
39
Fairly typical fluvial features of mid latitudes.
Some valleys fresh looking, others indistinct,
either due to deposition or creep. Rounded
valley walls suggestive of creep, but possibly
also uniform accrescence. Channels floors
sometimes have lineations. Donut craters.
40
Good example of rounded valley walls. Presumably
the craters are secondaries
41

• The rim of a large crater. Small superimposed
  crater has donuting. There is the hint of
  erosional attack of the inner and outer walls of
  the largest donut by a sapping-like process.
  Smooth interior crater floor
• Smooth, convex divides.
• Channel at bottom starts imperceptibly small but
  with well-defined banks Channel widens
  abruptly into wide, flat-floored valley with
  rounded walls.

42

• Interior crater wall. Sparse drainage network
  with well-defined, rounded valley walls
• Note flat surface at that looks like an
  alluvial flat eroding headwards into the crater
  wall, but crater wall retains a rounded
  appearance.
• Some hint that the drainage network may have
  been more elaborate at an earlier time, but
  either deposition or creep has obliterated the
  earlier network.
• Smooth, rounded slopes (depositon? Creep?)

43
Donut crater with some hint of erosional lateral
attach of the crater rim both from the interior
and exterior side. Crater to right has possible
erosional attack only on the inner side. Not
narrow but well defined channel at bottom of
image.
44
This inner crater wall looks like there has been
fairly late-stage rapid erosion of the seeper
crater rim, formation of alluvial fans, and a
flat-topped deposit exposing benches (layers?
Eroded shorelines?) in the central parts of the
crater (central peak at top).
45
Good example of a rounded crater rim modified by
local fluvial incision. Flat areas look
likealluvial surfaces or possibly ice creep
features that have eroded laterally into the
rounded terrain. Notice sharp transition and
possible raised ridge at the contact with the
crater floor.
46
Pretty much the same type of landscape as
previous one
47
This is also pretty common, a broadly rounded
valley with a fairly narrow, and fresh-looking
late-stage inner channel or valley. Some dom
48
Really nice smooth, flat valley floor features
(e.g. ), a few sharply-edged but sometimes
discontinous fluvial channels and, at top of
image, two level of benches in the center of the
crater, the top one exposing layering or
erosional beveling.

49
An example of the enigmatic mid-latitude
channels/valleys
50
This crater wall has some of the classic gullies
and ramparts, as well as concentric crater fill
51
Ditto, probably a combination of fluvial and ice
sculpting
52
Here what may have been a flat-floored alluvial
surface or ice feature appears to have been
preferentially eroded sublimation with a high
ice content?
53
A late-stage fracture along the inner crater
wall. Wrinkle ridge? Slump features (sense seems
wrong)?