The Incredible 'Caloris Impact Basin' on Mercury...

[t00fri]

Hi all,

Evil Dr. Ganymed asked me a day ago (in my Textures/News on Mercury thread)
about how the amazing "Caloris Impact Basin" would look like on my new
hires Mercury texture. Since this is really interesting stuff, I thought it
might be fun for the "users" department, too...

Reminder:
=========
The first issue concerns the fact that Mariner 10 only observed /half/
of this impact structure that actually involves a fair fraction of
Mercury! The reason is that the Caloris Basin was located half-way in
shadow on the morning terminator when is was discovered. The
incredible hires photo that I quickly colored and shaped displays what
is /actually/ known.



Caloris is Latin for 'heat' and the basin is named so because it is
near the subsolar point (the point closest to the sun) when Mercury is
at aphelion. The Caloris basin is ~1,340 kilometers (~810 miles) in
diameter and is the largest known structure on Mercury. It was formed
from an impact of a projectile with /asteroid/ dimensions. North is
towards the top of this beautiful image.

Here is a short comparison list:

Hellas Basin, Mars - 2000 km
Orientale, Moon - 1300 km
Caloris Basin, Mercury - 1340 km (just the inner crater!)

I think Mercury's impact structure is larger, mainly because it
involves the /whole planet/. The actual crater, Caloris Basin, is
larger than France (too bad, Christophe;-)), with secondary impact
craters extending out hundreds miles past that.

/Directly opposite/ the Caloris Basin (i.e. on the other side of
Mercury!) is an area known as the "weird terrain." It is an 223,000+
sq. mi. area where the ground is strangely broken up. It is believed
to have been caused by /shockwaves/ from the impact radiating around
and though the planet and reenforcing themselves in a super-Mercuryquake!

So far so good, back to Celestia and my new Mercury texture;-)
==============================================================

The latter is now finalized, together with appropriate bump-maps and a
4k spec-map. The existing sizes are 8k, 4k and 1k, besides jpg/png
there are 24bit DXT1c for the main texture and 24bit DXT3 for the
normal-maps. Will soon arrive in the TextureFoundry...

In the upper image below, you see my new 1k /limit-of-knowledge mask/
(with 20pix fuzzy edge) switched ON over the 4k DXT Mercury texture,
displaying the /visible/ part of the Caloris Impact Basin.

The most prominent 'impact halos' in form of rock/crater debris walls
are indicated in purple (cf above hires photo, showing /3rd/
half-circle!) to guide the eye.

[ NB: I always generate the appropriate mask colors by applying a
/large/ Gaussian blur to the actual texture until it appears /monochromatic/].

The lower image has the LOK-mask switched OFF and the Caloris Impact
Basin can be seen in 'full glory' in this 'interpreted' view. This was
actually quite a bit of image manipulation work, since apparently just
flipping the texture does not work (wrong shadows!). As you can easily
see, the 'unknown' side is not just a repetition of the known
one. Clearly such an outstanding structure like the Caloris
Impact Basin only exists once, for example ...

Bye Fridger

[Evil Dr Ganymede]

Not bad at all... thanks!

Though is there any particular reason why the western side of the basin has higher albedo material than the rest of it though? Or is the idea that the eastern side is just as bright, but it's the shadows are covering that material?

Speaking of shadows... ideally the best thing to do would be to get rid of them completely, and make a bump map to produce the shadows (assuming bumpmaps in Celestia do that). I can appreciate that's a huge task though.

[t00fri]

Not bad at all... thanks! :D

Though is there any particular reason why the western side of the basin has higher albedo material than the rest of it though? Or is the idea that the eastern side is just as bright, but it's the shadows are covering that material?

Speaking of shadows... ideally the best thing to do would be to get rid of them completely, and make a bump map to produce the shadows (assuming bumpmaps in Celestia do that). I can appreciate that's a huge task though.

Well, the higher Albedo comes from the procedure of generating the left ''arc'' of debris by flipping and /inverting/ the graymap of the right one. The final gray compensation was not quite perfect yet in the above image. It should be gone now...Some effect also comes from the more intense Sun irradiation on the lhs.

Getting rid of the shadows entirely would certainly be the best for all textures, but it just does not come out satisfactory. Some of us (including myself) have already spent /much/ time experimenting with different techniques without real success...

Bye Fridger

[Don. Edwards]

Hi Fridger,
This impact was a big one. The same thing is believed to have happened on Mars, twice. The first was Hellas. The shock wave is believed to be what broke the ground and forced up the Tharsis rise and created the whole volcanic region. Hellas is directly oposite Tharsis. The other was Argyre which is oposite the Alysium rise. Something similar is believed to of happened on Earth 65 million years ago when the asteriod hit the Yucatan. At that time India was on the other side of the Earth and there was a massive outpuoring of lava in India at the same time the asteriond hit. Mercury probably didn't have a moltem enough mantle to create a volcanic outpouring. So all we see is distored and broken ground. This picture sure does give us an idea of the planets make-up.

Don.