Tap. Tap. Tap.
Then – no further.
One of the dedicated science instruments on the Mars InSight lander has had to pause during its deployment. The instrument is officially known as the Heat Flow and Physical Properties Probe, which according to the Launch Press Kit is abbreviated HP3 (pronounced “H-P cubed”). Its mission is to take the temperature of Mars. It will determine the amount of heat that is escaping from the interior of the planet. Knowing this heat flow will help us better understand the evolution of the Martian interior – and the rate at which Mar’s internal core energy is diminishing.
The instrument includes a probe that is being hammered into the Martian soil to a depth between ten and sixteen feet (3 – 5 meters). But the probe, which is called the “Mole” by NASA, has met significant resistance at a much shallower depth. NASA is trying to determine if the resistance is coming from a rock or a gravel layer. Then they will need to decide how best to get beyond the obstacle. Can it be penetrated, or will another method be necessary?
As a gardener, or I should say as the shovel-man for a gardener, I know what it is to hit a resistive object while digging a hole. For me the resistance is often a stone of small to moderate size or perhaps a root of a nearby tree. Sometimes I can remove the impediment, but sometimes I must shift the location of the hole. Removing the impediment is not an option on Mars. There is no gardener on Mars that can kneel and sweep out the rubble with their gloved hand. *
The Mole is about 16 inches long and approximately an inch in circumference. The exterior of the Mole is aluminum. It is attached to a flexible tether that carries information to the instrument package that is attached to the deck of the lander. The hammer that provides the driving force is built into the Mole. There is no outside hammer at the surface level to drive the Mole into the soil. This means that there is no device, like the claw on the back of a hammer, that can extract the Mole so it can be placed in a different location.
Going back to my gardening efforts, I have often driven spikes for mats or pegs for garden borders into the ground. I have also driven steel rods to a depth or 18 to 24 inches to support a structure or a wall. In these cases, if I hit a rock or a root that I cannot penetrate I may be able to slightly reorient the item and try to slide past the obstacle. But that may not to be an option for the Mole on Mars. For clarity, I will have to ask NASA.
The Mole’s internal hammer was designed to enable the tip of the Mole to penetrate objects up to a certain hardness. This can be understood from the description of the operation in the mission Launch Press Kit which describes the process. The information in these documents states that it is expected that the hammer will be dropped between 5,000 and 20,000 times to penetrate the soil to its planned depth. The number of hammer blows required depends on the density and hardness of the soil matrix.
Currently the Mole has met an object of significant hardness. The hammer not only has to overcome the hardness of the material it has to penetrate, but it also has to overcome the friction of the sides of the Mole as it is driven through the obstacle plus the added friction of the flexible tether as it is dragged down the Mole’s hole.
NASA will determine the best course of action to allow the Mole to penetrate beyond its current depth. On March 21, the NASA Mission page stated that many ideas are being considered to free the Mole from the obstacle, and that the ideas will require “at least several more weeks of careful analysis.”
Stay tuned. Perhaps we will be able to slide past the obstacle and reach the appropriate depth.
Information for this article is taken form the NASA Mars InSight Launch Press Kit. The documents may be found at https://www.jpl.nasa.gov/news/press_kits/insight/ .
Picture based on NASA map in Launch Press Kit.
*I always wear gloves because there can be glass from an old bottle or a rusted nail dropped during construction.