As we learned last week, the surface of the Moon has been subject to billions of years of violent collisions by both small and large meteorites creating thousands of craters. Until the Apollo astronauts brought lunar soil samples back to Earth (1969-72), the belief was that the Moon's dry, nearly airless environment left the soil largely undisturbed. Reality is much harsher. The grains of dirt on the surface of the Moon have been (and continue to be) bombarded with cosmic rays, exposed to solar flares, and battered by micrometeorites--shattered, vaporized and re-condensed countless times over billions of years. Over the years these impacts have pulverized the surface materials forming a fine grained layer of dirt termed “regolith”.
Micrometeorites, many smaller than a pencil point, constantly rain onto the surface at up to 100,000 km/hr (about 62,000 mph), chipping off materials and forming microscopic impact craters. Some actually melt and vaporize the soil which then re-condenses as glassy coats on other specks of dust. These impacts condense debris into "agglutinates”. Complicated interactions with the solar wind convert iron in the soil into myriad of "nano-phase" metallic iron grains just a few nanometers wide.
Below: The lunar surface is exposed to solar wind and constantly pounded by micrometeorites that vaporize and condense to form agglutinates.
Let’s listen to the astronauts describe the soil as they scoop up some samples of lunar dirt (also note the color of their spacesuits – especially around their lower legs – pretty dirty!)
“Regolith” is a Real Giant to Slay
The processes described above form the "regolith" -- Greek for stone blanket (litho + rhegos) – that covers the Moon's surface. The regolith is described as sharp, abrasive, interlocking fragile glass shards and fragments some of which are very finely ground into a powdery “dust”. The Apollo astronauts found that the dust was everywhere and nearly impossible to brush off.
The moon’s regolith has some very interesting characteristics when compared to Earth’s dirt. The particles in the moon’s dust are very small (usually less than 0.1 millimeters across). These tiny particles become electro-statically charged, meaning that they can “stick” to objects like spacesuits, solar cells, equipment and rovers. Lunar dust is almost like tiny fragments of glass or coral -- odd shapes that are very sharp and lock together. The dust can easily become airborne inside equipment grinding machinery and seals. The darker dust particles can even absorb sunlight and heat up whatever they coat.
Right: A speck of Moon dirt. The strange shape tells a tale of the violent collisions that resulted in the welding of rock, mineral and glass by the heat of micrometeoroid impacts.
The depth of the regolith varies from about 2 meters beneath the younger “maria” to up to 20 meters beneath the oldest surfaces of the Moon called “highlands”. The regolith is predominately composed of materials found in the region, but also contains materials ejected by distant impact craters. The term “mega-regolith” is often used to describe the heavily fractured bedrock directly beneath the near-surface regolith layer.
The regolith contains rocks, fragments of minerals from the original bedrock, and glassy particles formed during the impacts. In most of the lunar regolith, half of the particles are made of mineral fragments fused by the glassy particles. The chemical composition of the regolith varies according to its location; the regolith in the highlands is rich in aluminum and silica, just as the rocks in those regions. The regolith in the maria is rich in iron and magnesium and is silica-poor, as the basaltic rocks from which it is formed.
Here is a short NASA video about the characteristics of lunar regolith and the difficulties of digging in Moon dirt:
NO ‘Day at the Beach’!
Neil Armstrong described the Moon as a beach of fine powders. Last week we learned that ancient observers of the Moon believed that the dark features on its surface were oceans or seas thereby giving them the Latin name ‘maria’. However, that is where the similarities to our oceans end. During the Extra Vehicular Activity (EVA) excursions using the Lunar Rover in Apollo Missions, it was reported that the rover kicked-off significant dust (see Figure 1a) and batteries and radiators had to be brushed clean at each stop. However, it was also reported that the conventional “brushing-off” the dust never worked once the dust adhered to the Apollo astronaut's spacesuit. There is some direct evidence in the past Apollo Missions that fine dust affected the performance of instruments and the crew’s health (see Figure 1b). All the lunar astronauts had lung reactions to this dust which was referred to as "lunar dust hay fever." Hope they had lots of tissue!
Figure 1a: The dust plume from the rover wheels.
------------------------------Figure 1b: Apollo 17 commander Eugene Cernan, grimy with lunar soil from three days of exploration.
Let’s listen to a NASA scientist as she tells us about some of the difficulties astronauts had dealing with lunar dust and its possible effect on human health:
Here is a video that demonstrates what Jennifer is talking about – watch for the regolith “rooster tail”:
When removing sand on a beach towel (after a fun day at the beach), we usually first shake it with a large sinusoidal motion. This removes most sand grains with the help of Earth’s significant gravitational pull (remember the Moon’s gravity is only about 1/6 of Earth’s). However, with a closer look at the towel, we notice smaller sand grains embedded in the towel fabric. These are usually removed by washing with the help of surfactant influenced fluid motion (liquid rubbing motion). On the surface of the Moon, we do not have the luxury of using water to wash off dust every time it coats our equipment. Therefore, the question of releasing fine lunar dust from the beach towel (or any other surface) will still remain a problem that we will have to resolve.
For the same reason, building a sandcastle on the Moon would be dangerous and nearly impossible. The lack of liquid water puts a stop to sandcastle construction. On Earth, water sticks to the sand, and causes sand particles to stick to each other. On the moon, however, lunar dust cannot stick to itself because there are no water molecules in the mixture. That is why we could never build a sandcastle on the moon. Lunar dust has some interesting qualities, but it’s clearly not play material and certainly NO “Day at the Beach” to deal with.
As we discovered last week with the other physical properties of the Moon, we must also take into consideration the properties of the lunar regolith as we design our rovers and mission. To maximize mission success, our rovers will have to be designed so that they can function in the lunar environment and we will have to choose our lunar landing site taking into account the terrain and soil content. Of course, we must always be prepared for the unexpected. So, don’t forget the Duct Tape!
