The Mole is stuck, but there may not be anything touching it.
NASA and its partner Germany’s DLR are still trying to figure if they can get the inSight Mole unstuck. According to an article published by NASA on 5 July, the team now believes that the Mole has created a cavity so that there is no soil surrounding it. The Mole needs interaction (touching) the Martian soil in order to move forward/downwards. Without the surrounding soil the Mole cannot penetrate further.
The problematic void may be like a cavity formed under a “bridge” similar to a bridge/arch that can form in bulk carrier cargo vessels. Due to normal vibration during the voyage or in the unloading process, a bridge or an arch of the material being transported/unloaded can form across the material. The bridge/arch will not allow the material above it to fall into the cavity created by the unloading process. The bridge/arch may have been formed by the Mole in the Martian soil due to the vibration of the mole mechanism. These vibrations may have caused the material below the bridge/arch to collapse. The bridge/arch does not allow the Martian soil which the Mole has already penetrated to collapse into the cavity. If the material were able to collapse into the cavity perhaps the Mole could progress to is mission depth.
The pictures in this article show that NASA has moved the Mole mechanism from above the mole hole. The hole is indicated by the yellow arrow. The placement of the feet of the mechanism are shown by orange semi-circles. The tether that provides energy to the Mole and which allows data transmission up to the lander is shown by the curved green line. NASA will have to ensure that when it places the mechanism back over the hole that the tether is not crimped or broken.
In my original post on the inSight lander (4/4/2019 – Mars inSight H-P cubed) I asked whether it would be possible to remove the Mole and start at another location. The answer is no; the Mole cannot be extracted. There is no way to extract the Mole and then reload it into its mechanism so it could start again. And even if it did, would it only form another cavity and be in a similar situation?
Links to two articles on bridging are given below. The key to preventing bridging in earth-bound hoppers is control of material flow based on its cohesion and friction on the sides of the container. On Mars, well, it’s different. In the case were a bridge/arch forms, it is generally necessary to shock/vibrate the material so it will start to flow. In the case of the Mole, a shock might allow bridge arch to collapse filling the cavity with soil and the Mole may be able to move forward again.
Right now, according to current thinking, the Mole is probably dangling in this cavity. It needs material around it to move. Maybe if there was a nine-pound hammer on the lander it could give the area close to the hole a good whack. But that is not the case.
We all hope that the NASA/DLR team will be able to develop a means for the mole to move forward again.
The source information and the pictures for this article may be found at https://phys.org/news/2019-07-nasa-insight-mole.html .
Articles on bridging in hoppers on earth: