Astrogeology -- Annals of Other Worlds

[Recusant]

Most of this thread is likely to consist of areology aka geology of Mars, but there will be occasional notes of new things learned about the geology of other planets and moons. To start off, a bit of clickbait that leads to a whole "zoo" of curious formations on the surface of Mars.

I can't say I blame them for this, it would be silly to pass it up. Invoking David Bowie for a science story? Certainly. 

"NASA Can Finally Explain Why Creepy 'Spiders' on Mars Keep Appearing" | Science Alert

Araneiform features on the surface of Mars, as imaged by the Mars Reconnaissance Orbiter in 2009.
Image Credit: NASA/JPL-Caltech/University of Arizona.

Mars has spiders unlike anything else in the Solar System. Between the ocher dunes, across the scars of impacts, long-legged shadows appear to scuttle across the dust.

They aren't actual, living spiders. These tendrilled shapes that appear in satellite images of the red planet are made, like almost everything on Mars, from dust. They're known as araneiforms, small systems of dark troughs that appear only in the southern polar region of Mars in the planet's spring.

Exactly how the araneiforms emerge every Martian year has been something of a mystery. Now, scientists on Earth have recreated them in the lab – giving us new insights into the processes that shape the alien landscapes of Mars.

"The spiders are strange, beautiful geologic features in their own right," says planetary scientist Lauren Mc Keown of NASA's Jet Propulsion Laboratory. "These experiments will help tune our models for how they form."

There are many differences between natural phenomena on Earth and Mars. On Mars, conditions get so cold that carbon dioxide freezes into ice – something that doesn't happen in nature here on Earth, where it can be made under special conditions and is known as dry ice.

One interesting thing about carbon dioxide is that it doesn't have a liquid form. Its frozen form sublimes straight into a gas. This is what Mc Keown and her colleagues thought might be responsible for the Martian spiders.

This explanation for araneiforms is known as the Kieffer model, after geophysicist Hugh Kieffer, who described the formation processes in 2006 and 2007. During the cold winter on Mars, carbon dioxide from the atmosphere freezes on the ground. Then, when spring rolls around, and temperatures rise, this carbon dioxide ice returns to its gaseous state.

This, however, can happen from the bottom of a deposit of ice, with darker mars dirt beneath absorbing the heat. This traps the sublimated gas under the slabs of ice above.

With nowhere for the gas to go, pressure builds until the ice cracks in a small explosion; the carbon dioxide gas escapes through these cracks, carrying darker, dusty material with it. Once all the ice has disappeared back into the atmosphere, a dark, spider-like scar is left behind.

To try and recreate the process, Mc Keown and her colleagues used a barrel-sized chamber known as the Dirty Under-vacuum Simulation Testbed for Icy Environments, or DUSTIE. This apparatus is designed to replicate the temperature and atmospheric pressure of Mars.

[Continues . . .]

The paper is open access:

"A Lab-scale Investigation of the Mars Kieffer Model" | The Planetary Science Journal

Abstract:

The Kieffer model is a widely accepted explanation for seasonal modification of the Martian surface by CO₂ ice sublimation and the formation of a "zoo" of intriguing surface features. However, the lack of in situ observations and empirical laboratory measurements of Martian winter conditions hampers model validation and refinement.

We present the first experiments to investigate all three main stages of the Kieffer model within a single experiment: (i) CO₂ condensation on a thick layer of Mars regolith simulant; (ii) sublimation of CO₂ ice and plume, spot, and halo formation; and (iii) the resultant formation of surface features.

We find that the full Kieffer model is supported on the laboratory scale as (i) CO₂ diffuses into the regolith pore spaces and forms a thin overlying conformal layer of translucent ice. When a buried heater is activated, (ii) a plume and dark spot develop as dust is ejected with pressurized gas, and the falling dust creates a bright halo. During plume activity, (iii) thermal stress cracks form in a network similar in morphology to certain types of spiders, dendritic troughs, furrows, and patterned ground in the Martian high south polar latitudes. These cracks appear to form owing to sublimation of CO₂ within the substrate, instead of surface scouring.

We discuss the potential for this process to be an alternative formation mechanism for "cracked" spider-like morphologies on Mars. Leveraging our laboratory observations, we also provide guidance for future laboratory or in situ investigations of the three stages of the Kieffer model.

Figure 1. Some examples of the "zoo" of features proposed to be formed
by seasonal CO₂ sublimation dynamics on Mars.
Image Credit: NASA/JPL-Caltech/University of Arizona

[Link to paper.]

[Tank]

Fascinating.

[Icarus]

I second the fascinating descriptve

[Dark Lightning]

Indeed! Too bad about the canals.

[Tank]

Indeed! Too bad about the canals.

Yes but at least there were no Martians to come and attack us!