**SMALL-SCALE SLOPES AT CANDIDATE MER LANDING SITES**

Ross Beyer and Alfred McEwen, LPL/UofA

**What good is Photoclinometry (PC)?**

Has a bad rap \-- because errors in topographic profiles are cumulative.

But, we just want to measure slopes, for which PC errors are not
cumulative

It\'s **much** better than just qualitatively attempting to judge slopes
in MOC images

(or it least it is a good lesson in humility)

Variety of illumination angles (with seasons, latitude)

Each is typically stretched according to content of each image;

hard stretch (low albedo contrast) \--\> rugged-looking topo

light stretch (high albedo contrast) \--\> smooth-looking topo

We have created a set of slope-normalized images:

slope -30 to black, +30 to white

allows normalized qualitative comparisons

**Quantitative slope information can be extracted, but errors must be
understood**

1\. Calibration \-- excellent relative cal., all that matters to us.

2\. Choice of photometric function and parameters

for sunangle variations on surface, any simple function is fine

as long as the phase angle is not near zero. (MOC

equatorial typically 30 deg.)

emission angles near zero, avoids high-e problems except over

extremely steep slopes.

Errors no more than a few degrees.

3\. Atmospheric Haze (and offset calibration errors)

Big source of uncertainty

Hazy image w/o correction gives very low slopes

We subtracted the lowest reflectance value in each image

from all pixels, a good upper limit to haze, resulting

in upper limits to slopes.

4\. Albedo variations

Large-scale (\>300 m) albedo variations normalized

via filtering (also normalized slopes more than

300 m long, but MOLA is a better way to measure these)

Where small-scale albedo contrasts exceed contrast from topo shading,

PC is SOL.

but, the eye-brain is remarkably talented at seeing the distinction,

so our slope-normalized images still improve qualitative

evaluations.

Residual small-scale albedo interpreted as topography, so again our

results give upper limits to the roughness.

Results from major sources of error (haze and albedo): we can derive
**upper limits**

to the roughnesses.

5\. Strike of the slope:

We measure slope in downsun direction, so if strikes are

uniform then slopes should be increased by \~1.4 1/cos(45)

But this may not apply to steepest slopes (\> 15 degrees)\--

steep slopes we can measure typically strike about

normal to the illumination.

If trends are systematic, apply correction

**What have we done?**

Create images of slopes in downsun direction

all 6 m/pixel for now (no extended-mission images)

Extracted statistics for portion of each image in a candidate landing
ellipse.

avoiding obvious problem areas

Results:

Landing Site **Upper Limit** to % area steeper N images

than 15 deg over 6 m scale

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Pathfinder 2.5 6

Viking 1 2.1 6

Viking 2 0.8 4

Ares Valles 0.6 1

Athabasca Valles 0.6 7

(Elysium flow)

Coprates 5.0 2

EP69A crater 3.1 1

Eos Chasm 0.5 4

Gale Crater 1.3 4

Gusev Crater 1.4 5

Isidis Basin 5.1 1

Isidis 2.7 2

Melas Chasm 17.7 6

(Melas Chasm no haze) 0.2 6

Sinus Meridiani 8.3 9

Terra Meridiani 19.0 3

VM outflow 5.9 1

Hematite Regions:

TM22B & TM11A 5.8 11

TM9A 0.7 3

TM19B 4.2 2

TM10A 2.3 4

TM20B 2.7 6

TM21B 1.2 1

**Summary and Conclusions**

We can be reassured that several candidate sites are no more hazardous
than Pathfinder and Viking 1.

We cannot demonstrate that any site has significant areas steeper than
15 degrees,

but maybe this can be accomplished via independent data to constrain the
haze

(value needed to match MOLA or stereo slopes)

We will place all slope images and statistics on a web site.