Research project

Slides of ground or rock material are common on Earth. Snow or mudslides are particular types of runoff. Under weak gravitational field conditions, as on the surface of comets, landslides were considered unlikely or impossible. However, in recent years space missions have identified possible landslides on the surfaces of small comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko with effective diameters of several kilometres [5,7], as well as on the surfaces of asteroids Ceres and Vesta [3,4]. In the case of comet 9P/Tempel 1, the possibility of floating of surface layers of material was demonstrated by computer simulations [1] only 4 years after the existence of a structure with an appearance suggestive of formation by runoff was found.

On small space bodies, where the existence of any liquid on the surface is impossible, the mechanisms of material flow must be different than on Earth. Granular material can start to flow when it is saturated with liquid, as happens on Earth, but also when gas outflow from deeper layers results in a mixture of gas and solid grains. Initiation of flow can occur as a result of seismic shocks, but also as a result of meteoroid impacts. Few meteoroids reach the Earth's surface, but this is not the case for comets and other cosmic bodies that lack an atmosphere (Moon) or have a very rarefied atmosphere (Mars). Thus, there are some analogies between runoff processes on Earth and on small cosmic bodies, but the dramatic differences in physical conditions must be taken into account when analysing the processes responsible for the generation of instabilities and the initiation of runoff, as well as the deposition of flowing material.

The computer simulations described in [1] deal with the large-scale runoff of a layer tens of metres thick, liquefied by the rapid release of carbon monoxide gas during the strongly exothermic crystallization of amorphous water ice. Unfortunately, the presence of amorphous ice inside comet nuclei is uncertain. Thus, there is a need to study other causes of instabilities as well as small-scale processes.

In the planned project we intend to study the conditions necessary for the dry flow of granular material on the inclined surfaces of comets and asteroids. The main problem is the source of energy needed to increase the gas pressure under the material layer. The surfaces of comet nuclei are covered by a layer of dust, sand and coarser fractions of material, traditionally called the dust mantle. Deeper down is a mixture of grains of non-volatile material, water ice and other compounds of varying volatility. This material can be heated by absorption of solar radiation as well as by heat release at greater depth. This can be caused by tidal interactions as well as by the decay of radioactive elements. Both processes can only be relevant in the interiors of large asteroids, moons and large comets. On a small comet, it is possible to heat the surface and release heat in the interior due to phase transitions (crystallization of amorphous ice). The heat released can cause sublimation of water ice, strongly dependent on the presence of dopants in the ice [2]. If the sublimation is intense enough, liquefaction of the dust mantle can occur.

We plan laboratory measurements, in a vacuum and under normal pressure, and computer simulations.

(1) Experiments conducted in a vacuum chamber can show: how the presence of chemical compounds in water ice affects the rate of gas release during sublimation, and how the granularity of the material covering the ice and the slope of the surface affect the sliding of the material and the uncovering of the ice.

We plan to study 2 situations:

• granular non-volatile material (dust, sand) with ice, on non-volatile material, and
• granular non-volatile material on ice, or material containing ice.

We are also planning larger-scale experiments under normal conditions.

(2) Computer simulations may indicate places on the surface of the analysed space body where sublimation is likely to cause material runoff and the extent of the runoff. The consequences of the discovery of ice for the course of geological processes can also be analysed.

We intend to perform laboratory measurements and computer simulations with our own equipment.