Titan is a moon shrouded in mystery. Despite multiple flybys and surface exploration conducted in the past few decades, this Cronian moon still manages to surprise us from time to time. In addition to having a dense atmosphere rich in hydrocarbons, which scientists believe may be similar to what Earth’s own atmosphere was like billions of years ago, it appears that methane is to Titan what water is to planet Earth.
In addition, methane fog was also observed by the Cassini space probe back in 2009 as it conducted a flyby of Titan. But recent findings by a team of researchers from York University indicates that the Huygens lander also detected fog during its descent towards the surface in 2005. This evidence, combined with the data obtained by Cassini, have helped to shed light on the weather patterns of this mysterious moon.
In ancient Greek lore, the Titans were giant deities of incredible strength who ruled during the legendary Golden Age and gave birth to the Olympian gods we all know and love. Saturn‘s largest moon, known as Titan, is therefore appropriately named. In addition to being Saturn’s largest moon – and the second-largest moon in the Solar System (after Jupiter’s moon Ganymede) – it is larger by volume than even the smallest planet, Mercury.
Beyond its size, Titan is also fascinating because it is the only natural satellite known to have a dense atmosphere, a fact which has made it very difficult to study until recently. On top of all that, it is the only object other than Earth where clear evidence of stable bodies of surface liquid has been found. All of this makes Titan the focal point of a great deal of curiosity, and a prime location for future scientific missions.
Discovery and Naming:
Titan was discovered on March 25th, 1655, by the Dutch astronomer Christiaan Huygens. Huygens had been inspired by Galileo’s improvements in telescopes and his discovery of moons circling Jupiter in 1610. By 1650, he went about developing a telescope of his own with the help of his brother (Constantijn Huygens, Jr.) and observed the first moon of Saturn.
In 1655, Huygens named it Saturni Luna (Latin for “Saturn’s moon”) in a tract De Saturni Luna Observatio Nova (“A New Observation of Saturn’s Moon”). As Giovanni Domenico Cassini discoveries four more moons around Saturn between 1673 and 1686, astronomers began to refer to them as Saturn I through V (with Titan being in the fourth position as Saturn IV).
After William Herschel’s discovery of Mimas and Enceladus in 1789, which are closer to Saturn than any of the larger moons, Saturn’s moons once again had to be re-designated. Thenceforth, Titan status became fixed as Saturn VI, despite the discovery of several smaller moons that were closer to Saturn since then.
The name Titan, along with the names for all the seven major satellites of Saturn, were suggested by William Herschel’s son, John. In 1847, John Herschel published Results of Astronomical Observations Made at the Cape of Good Hope, in which he suggested that the moons be named after the mythological Titans – the brothers and sisters of Cronus, who is the Greek equivalent to Saturn.
In 1907, Spanish astronomer Josep Comas i Solà observed limb darkening of Titan. This effect, where the center part of a planet or star appears brighter than the edge (or limb), was the first indication that Titan had an atmosphere. In 1944, Gerard P. Kuiper used a spectroscopic technique to determine that Titan had an atmosphere composed of methane.
Size. Mass and Orbit:
With a mean radius of 2576 ± 2 km and a mass of 1.345 × 1023 kg, Titan is 0.404 the size of Earth (or 1.480 Moons) and 0.0225 times as massive (1.829 Moons). Its orbit has a minor eccentricity of 0.0288, and its orbital plane is inclined 0.348 degrees relative to Saturn’s equator. It’s average distance from Saturn (semi-major axis) is 1,221,870 km – ranging from 1,186,680 km at periapsis (closest) to 1,257,060 km at apoapsis (farthest).
Titan takes 15 days and 22 hours to complete a single orbit of Saturn. Like the Moon and many satellites that orbit the other gas giants, its rotational period is identical to its orbital period. Thus, Titan is tidally-locked and in a synchronous rotation with Saturn, which means one face is permanently pointed towards the planet.
Composition and Surface Features:
Though similar in composition to Dione and Enceladus, Titan is denser due to gravitational compression. In terms of diameter and mass (and hence density) Titan is more similar to the Jovian moons of Ganymede and Callisto. Based on its bulk density of 1.88 g/cm3, Titan’s composition is believed to consist half of water ice and half of rocky material.
It’s internal makeup is likely differentiated into several layers, with a 3,400-kilometre (2,100 mi) rocky center surrounded by layers composed of different forms of crystalized ice. Based on evidence provided by the Cassini-Huygens mission in 2005, it is believed that Titan may also have a subsurface ocean which exists between the crust and several deeper layers of high-pressure ice.
This subsurface ocean is believed to be made up of water and ammonia, which allows the water to remain in a liquid state even at temperatures as low as 176 K (-97 °C). Evidence of a systematic shift of the moon’s surface features (which took place between October 2005 and May 2007) suggests that the crust is decoupled from the interior – possibly by a liquid layer in between – as well as the way the gravity field varies as Titan orbits Saturn.
The surface of Titan is relatively young – between 100 million and 1 billion years old – despite having been formed during the early Solar System. In addition, it appears to be relatively smooth, with impact craters having been filled in. Height variation is also low, ranging by little more than 150 meters, but with the occasional mountain reaching between 500 meters and 1 km in height.
This is believed to due to geological processes which have reshaped Titan’s surface over time. For example, a range measuring 150 km (93 mi) long, 30 km (19 mi) wide, and 1.5 km (0.93 mi) tall has been potted in the southern hemisphere, composed of icy material and covered in methane snow. The movement of tectonic plates, perhaps influenced by a nearby impact basin, could have opened a gap through which the mountain’s material upwelled.
Then there is Sotra Patera, a chain of mountains that is 1000 to 1500 m (0.62 and 0.93 mi) in height, has some peaks topped by craters, and what appears to be frozen lava flows at its base. If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, tidal flexing caused by Saturn’s influence, or possibly the interaction of Titan’s subsurface ice layers.
An alternate theory is that Titan is a geologically dead world and that the surface is shaped by a combination of impact cratering, flowing-liquid and wind-driven erosion, mass wasting and other externally-motivated processes. According to this hypothesis, methane is not emitted by volcanoes but slowly diffuses out of Titan’s cold and stiff interior.
The few impact craters discovered on Titan’s surface include a 440 km (270 mi) wide two-ring impact basin named Menrva, which is identifiable from its bright-dark concentric pattern. A smaller, 60 km (37 mi) wide, flat-floored crater named Sinlap and a 30 km (19 mi) crater with a central peak and dark floor named Ksa have also been observed.
Radar and orbital imaging has also revealed a number of “crateriforms” on the surface, circular features that may be impact related. These include a 90 km (56 mi) wide ring of bright, rough material known as Guabonito, which is thought to be an impact crater filled in by dark, windblown sediment. Several other similar features have been observed in the dark Shangri-la and Aaru regions.
The presence of cryovolcanism has also been theorized based on the fact that there is apparently not enough liquid methane on Titan’s surface (see below) to account for the atmospheric methane. However, to date, the only indications of cryovolcanism are particularly bright and dark features on the surface and 200 m (660 ft) structures resembling lava flows that were spotted in the region called Hotei Arcus.
Titan’s surface is also permeated by streaky features (aka. “sand dunes“), some of which are hundreds of kilometers in length and several meters high. These appear to be caused by powerful, alternating winds that are caused by the interaction of the Sun and Titan’s dense atmosphere. Titan’s surface is also marked by broad regions of bright and dark terrain.
These include Xanadu, a large, reflective equatorial area that was first identified by the Hubble Space Telescope in 1994 and later by the Cassini spacecraft. This region (which is about the same size as Australia) is very diverse, being filled with hills, valleys, chasms and crisscrossed in places by dark lineaments – sinuous topographical features resembling ridges or crevices.
These could be an indication of tectonic activity, which would mean that Xanadu is geologically young. Alternatively, the lineaments may be liquid-formed channels, suggesting old terrain that has been cut through by stream systems. There are dark areas of similar size elsewhere on Titan, which have been revealed to be the patches of water ice and organic compounds that darkened due to exposure to UV radiation.
Titan is also home to its famous “hydrocarbon seas”, lakes of liquid methane and other hydrocarbon compounds. Many of these have been spotted near the polar regions, such as Ontario Lacus. This confirmed methane lake near the south pole has a surface area of 15,000 km² (making it 20% smaller than its namesake, Lake Ontario) and a maximum depth of 7 meters (23 feet).
But the largest body of liquid is Kraken Mare, a methane lake near the north pole. With a surface area of about 400,000 km², it is larger than the Caspian Sea and is estimated to be 160 meters deep. Shallow capillary waves (aka. ripple waves) that are 1.5 centimeters high and moving at speeds of 0.7 meters per second have also been detected.
Then there is Ligeia Mare, the second largest known body of liquid on Titan, which is connected to Kraken Mare and also located near the north pole. With a surface area of about 126,000 km² and a shoreline that is over 2000 km (1240 mi) in length, it is larger than Lake Superior. Much like Kraken Mare, it takes its name from Greek mythology; in this case, after one of the sirens.
It was here that NASA first noticed a bright object measuring 260 km² (100 square miles), which they named “Magic Island”. This object was first spotted in July 2013, then disappeared later, only to reappear again (slightly changed) in August 2014 . It is believed to be inked to Titan’s changing seasons, and suggestions as to what it might be range from surface waves and rising bubbles to floating solids suspended beneath the surface.
Although most of the lakes are concentrated near the poles (where low levels of sunlight prevent evaporation), a number of hydrocarbon lakes have also been discovered in the equatorial desert regions. This includes one near the Huygens landing site in the Shangri-la region, which is about half the size of Utah’s Great Salt Lake. Like desert oases on Earth, it is speculated that these equatorial lakes are fed by underground aquifers.
Overall, the Cassini radar observations have shown that lakes cover only a few percent of the surface, making Titan much drier than Earth. However, the probe also provided strong indications that considerable liquid water exists 100 km below the surface. Further analysis of the data suggests that this ocean may be as salty as the Dead Sea.
Other studies suggest methane rainfall (see below) on Titan may interact with icy materials underground to produce ethane and propane that may eventually feed into rivers and lakes.
Titan is the only moon in the Solar System with a significant atmosphere, and the only body other than Earth who’s atmosphere is nitrogen-rich. Recent observations have shown that Titan’s atmosphere is denser than Earth’s, with a surface pressure of about 1.469 KPa – 1.45 times that of Earths. It is also about 1.19 times as massive as Earth’s atmosphere overall, or about 7.3 times more massive on a per-surface-area basis.
The atmosphere is made up of opaque haze layers and other sources that block most visible light from the Sun and obscure its surface features (similar to Venus). Titan’s lower gravity also means that its atmosphere is far more extended than Earth’s. In the stratosphere, the atmospheric composition is 98.4% nitrogen with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%).
There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene and propane; as well as other gases such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon and helium. The hydrocarbons are thought to form in Titan’s upper atmosphere in reactions resulting from the breakup of methane by the Sun’s ultraviolet light, producing a thick orange smog.
Energy from the Sun should have converted all traces of methane in Titan’s atmosphere into more complex hydrocarbons within 50 million years—a short time compared to the age of the Solar System. This suggests that methane must be replenished by a reservoir on or within Titan itself. The ultimate origin of the methane in its atmosphere may be its interior, released via eruptions from cryovolcanoes.
Titan’s surface temperature is about 94 K (-179.2 °C), which is due to the fact that Titan receives about 1% as much sunlight as Earth. At this temperature, water ice has an extremely low vapor pressure, so the little water vapor present appears limited to the stratosphere. The moon would be much colder, were it not for the fact that the atmospheric methane creates a greenhouse effect on Titan’s surface.
Conversely, haze in Titan’s atmosphere contributes to an anti-greenhouse effect by reflecting sunlight back into space, cancelling a portion of the greenhouse effect and making its surface significantly colder than its upper atmosphere. In addition, Titan’s atmosphere periodically rains liquid methane and other organic compounds onto its surface.
Based on studies simulating the atmosphere of Titan, NASA scientists have speculated that complex organic molecules could arise on Titan (see below). In addition, propene – aka. propylene, a class of hydrocarbon – has also been detected in Titan’s atmosphere. This is the first time propene has been found on any moon or planet other than Earth, and is thought to be formed from recombined radicals created by the UV photolysis of methane.
Titan is thought to be a prebiotic environment rich in complex organic chemistry with a possible subsurface liquid ocean serving as a biotic environment. Ongoing research of Titan’s atmosphere has led many scientists to theorize that conditions there are similar to what existed on a primordial Earth, with the important exception of a lack of water vapor.
Numerous experiments have shown that an atmosphere similar to that of Titan, with the addition of UV radiation, could give rise to complex molecules and polymer substances like tholins. In addition, independent research conducted by the University of Arizona reported that when energy was applied to a combination of gases like those found in Titan’s atmosphere, many organic compounds were produced. These includes the five nucleotide bases – the building blocks of DNA and RNA – as well as amino acids, which are the building blocks of protein.
Multiple laboratory simulations have been conducted that have led to the suggestion that enough organic material exists on Titan to start a chemical evolution process analogous to what is thought to have started life here on Earth. While this theory assumes the presence of water that would remain in a liquid state for longer periods that have been observed, organic life could theoretically survive in Titan’s hypothetical subsurface ocean.
Much like on Europa and other moons, this life would likely take the form of extremophiles – organisms that thrive in extreme environments. Heat transfer between the interior and upper layers would be critical in sustaining any subsurface oceanic life, most likely through hydrothermal vents located at the ocean-core boundary. That the atmospheric methane and nitrogen might be of biological origin has also been examined.
It has also been suggested that life could exist in Titan’s lakes of liquid methane, just as organisms on Earth live in water. Such organisms would inhale dihydrogen (H²) in place of oxygen gas (O²), metabolize it with acetylene instead of glucose, and then exhale methane instead of carbon dioxide. Although all living things on Earth use liquid water as a solvent, it is speculated that life on Titan could actually live in liquid hydrocarbons.
Several experiments and models have been constructed to test this hypothesis. For instance, atmospheric models have shown that molecular hydrogen is in greater abundance in the upper atmosphere and disappears near the surface – which is consistent with the possibility of methanogenic life-forms. Another study has shown that there are low levels of acetylene on Titan’s surface, which is also in line with the hypothesis of organisms consuming hydrocarbons.
In 2015, a team of chemical engineers at Cornell University went as far as to construct a hypothetical cell membrane that was capable of functioning in liquid methane under conditions similar to that on Titan. Composed of small molecules containing carbon, hydrogen, and nitrogen, this cell was said to have the same stability and flexibility as cell membranes on Earth. This hypothetical cell membrane was termed an “azotosome” (a combination of “azote”, French for nitrogen, and “liposome”).
However, NASA has gone on record as stating that these theories remain entirely hypothetical. Furthermore, it has been emphasized that other theories as to why hydrogen and acetylene levels are lower nearer to the surface are more plausible. These include a as-of-yet unidentified physical or chemical processes – such as a surface catalyst accepting hydrocarbons or hydrogen – or the existence of flaws in the current models of material flow.
Also, life on Titan would face tremendous obstacles compared to life on Earth – thus making any analogy to Earth problematic. For one, Titan is too far from the Sun, and its atmosphere lacks carbon monoxide (CO), which results in it not retaining enough heat or energy to trigger biological processes. Also, water only exists on Titan’s surface in solid form.
So while the prebiotic conditions that are associated with organic chemistry exist on Titan, life itself may not. However, the existence of these conditions remains a subject of fascination among scientists. And since its atmosphere is thought to be analogous to Earth’s in the distant past, researching Titan could help advance our understanding of the early history of the terrestrial biosphere.
Titan cannot be spotted without the help of instrumentation, and is often difficult for amateur astronomers because of interference from Saturn’s brilliant globe and ring system. And even after the development of high-powered telescopes, Titan’s dense, hazy, atmosphere made observations of the surface very difficult. Hence, observations of both Titan and its surface features prior to the space age were limited.
The first probe to visit the Saturnian system was Pioneer 11 in 1979, which took images of Titan and Saturn together and revealed that Titan was probably too cold to support life. Titan was examined in 1980 and 1981 by both the Voyager 1 and 2 space probes, respectively. While Voyager 2 managed to take snapshots of Titan on its way to Uranus and Neptune, only Voyager 1 managed to conduct a flyby and take pictures and readings.
This included readings on Titan’s density, composition, and temperature of the atmosphere, and obtain a precise measurement of Titan’s mass. Atmospheric haze prevented direct imaging of the surface; though in 2004, intensive digital processing of images taken through Voyager 1‘s orange filter did reveal hints of the light and dark features now known as Xanadu and Shangri-la.
Even so, much of the mystery surrounding Titan would not begin to be dispelled until the Cassini-Huygens mission – a joint project between NASA and the European Space Agency (ESA) named in honor of the astronomers who made the greatest discoveries involving Saturn’s moons. The spacecraft reached Saturn on July 1st, 2004, and began the process of mapping Titan’s surface by radar.
The Cassini probe flew by Titan on October 26th, 2004, and took the highest-resolution images ever of Titan’s surface, discerning patches of light and dark that were otherwise invisible to the human eye. Over the course of many close flybys of Titan, Cassini managed to detect abundant sources of liquid on the surface in the north polar region, in the form of many lakes and seas.
The Huygens probe landed on Titan on January 14th, 2005, making Titan the most distant body from Earth to have a space probe land on its surface. During the course of its investigations, it would discover that many of the surface features appear to have been formed by fluids at some point in the past.
After landing just off the easternmost tip of the bright region now called Adiri, the probe photographed pale hills with dark “rivers” running down to a dark plain. The current theory is that these hills (aka. “highlands”) are composed mainly of water ice, and that dark organic compounds – created in the upper atmosphere – may come down from Titan’s atmosphere with methane rain and become deposited on the plains over time.
Huygens also obtained photographs of a dark plain covered in small rocks and pebbles (composed of water ice) that showed evidence of erosion and/or fluvial activity. The surface is darker than originally expected, consisting of a mixture of water and hydrocarbon ice. The “soil” visible in the images is interpreted to be precipitation from the hydrocarbon haze above.
Several proposals for returning a robotic space probe to Titan have been made in recent years. These include the Titan Saturn System Mission (TSSM) – a joint NASA/ESA proposal for the exploration of Saturn’s moons – that envisions a hot-air balloon floating in Titan’s atmosphere and conducting research for a period of six months.
In 2009, it was announced that the TSSM lost out to a competing concept known the Europa Jupiter System Mission (EJSM) – a joint NASA/ESA mission that will consist of sending two probes to Europa and Ganymede to study their potential habitability.
There was also a proposal known as Titan Mare Explorer (TiME), a concept under consideration by NASA in conjunction with Lockheed Martin. This mission would involve a low-cost lander splashing down in a lake in Titan’s northern hemisphere and floating on the surface of the lake for 3 to 6 months. However, NASA announced in 2012 that it favored the lower-cost InSight Mars lander instead, which is scheduled to be sent to Mars in 2016.
Another mission to Titan was proposed in early 2012 by Jason Barnes, a scientist at the University of Idaho. Known as the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR), this unmanned plane (or drone) would fly through Titan’s atmosphere and take high-definition images of the surface. NASA did not approve the requested $715 million at the time and the future of the project is uncertain.
Another lake lander project known as the Titan Lake In-situ Sampling Propelled Explorer (TALISE) was proposed in late 2012 by the Spanish-based private engineering firm SENER and the Centro de Astrobiología in Madrid. The major difference between this and the TiME probe is that the TALISE concept includes its own propulsion system, and would therefore not be limited to simply drifting on the lake when it splashes down.
In response to NASA’s 2010 Discovery Announcement, the concept known as Journey to Enceladus and Titan (JET) was proposed. Developed by Caltech and JPL, this mission would consist of a low-cost astrobiology orbiter that would be sent to the Saturnian system to asses the habitability potential of Enceladus and Titan.
In 2015, NASA’s Innovative Advanced Concepts (NIAC) awarded a Phase II grant to a proposed robotic submarine in order to further investigate and develop the concept. This submarine explorer, should it be deployed to Titan, will explore the depths of Kraken Mare to investigate its makeup and potential for supporting life.
The colonization of the Saturn system presents numerous advantages compared to other gas giants in the Solar System. According to Dr. Robert Zubrin – an American aerospace engineer, author, and an advocate for the exploration Mars – these include its relative proximity to Earth, its low radiation, and its excellent system of moons. Zubrin has also stated that Titan is the most important of these moons when it comes to building a base to develop the system’s resources.
For starters, Titan possess an abundance of all the elements necessary to support life, such as atmospheric nitrogen and methane, liquid methane, and liquid water and ammonia. Water could easily be used to generate breathable oxygen, and nitrogen is ideal as a buffer gas to create a pressurized, breathable atmosphere. In addition, nitrogen, methane and ammonia could all be used to produce fertilizer for growing food.
Additionally, Titan has an atmospheric pressure one and a half times that of Earth, which means that the interior air pressure of landing craft and habitats could be set equal or close to the exterior pressure. This would significantly reduce the difficulty and complexity of structural engineering for landing craft and habitats compared with low or zero pressure environments such as on the Moon, Mars, or the Asteroid Belt.
The thick atmosphere also makes radiation a non-issue, unlike with other planets or Jupiter’s moons. And while Titan’s atmosphere does contain flammable compounds, these only present a danger if they are mixed with sufficient enough oxygen – otherwise, combustion cannot be achieved or sustained. Finally, the very high ratio of atmospheric density to surface gravity also greatly reduces the wingspan needed for aircraft to maintain lift.
Beyond this, Titan presents many challenges for human colonization. For starters, the moon has a surface gravity of 0.138 g, which is slightly less than that of the Moon. Managing the long-term effects of this presents a challenge, and what those effects would be (especially for children born on Titan) are not currently known. However, they would likely include loss of bone density, muscle deterioration, and a weakened immune system.
The temperature on Titan is also considerably lower than on Earth, with an average temperature of 94 K (-179 °C, or -290.2 °F). Combined with the increased atmospheric pressure, temperatures vary very little over time and from one local to the next. Unlike in a vacuum, the high atmospheric density makes thermoinsulation a significant engineering problem. Nevetherless, compared to other cases for colonization, the problems associated with creating a human presence on Titan are relatively surmountable.
Titan is a moon that is shrouded in mystery, both literally and metaphorically. Until very recently, we were unable to discern what secrets it held because its atmosphere was simply too thick to see beneath. However, in recent years, we have managed to pull back that shroud and get a better look at the moon’s surface. But in many ways, doing this has only confounded the sense of mystery surrounding this world.
Perhaps someday we will send astronauts to Titan and find life forms there that completely alter our conception of what life is and where it can thrive. Perhaps we will find only extremophiles, life forms that live in the deepest parts of its interior ocean huddled around hydorthermal vents, since these spots are the only place on Titan where lifeforms can exist.
Perhaps we will even colonize Titan someday, and use it as a base for further exploration of the Solar System and resource extraction. Then, we may come to know the pleasures of looking up at a ringed planet in the sky while sailing on a methane lake, the hazy light of the Sun trickling down onto the cold, hydrocarbon seas. One can only hope… and dream!
When you’re walking around on soft ground, do you notice how your feet leave impressions? Perhaps you’ve tracked some of the looser earth in your yard into the house on occasion? If you were to pick up some of these traces – what we refer to as dirt or soil – and examine them beneath a microscope, what would you see?
Essentially, you would be seeing the components of what is known as regolith, which is a collection of particles of dust, soil, broken rock, and other materials found here on Earth. But interestingly enough, this same basic material can be found in other terrestrial environments as well – including the Moon, Mars, other planets, and even asteroids.
The term regolith refers to any layer of material covering solid rock, which can come in the form of dust, soil or broken rock. The word is derived from the combination of two Greek words – rhegos (which means “blanket”) and lithos (which means “rock).
On Earth, regolith takes the form of dirt, soil, sand, and other components that are formed as a result of natural weathering and biological processes. Due to a combination of erosion, alluvial deposits (i.e. moving water deposing sand), volcanic eruptions, or tectonic activity, the material is slowly ground down and laid out over solid bedrock.
It can be made up of clays, silicates, various minerals, groundwater, and organic molecules. Regolith on Earth can vary from being essentially absent to being hundreds of meters thick. Its can also be very young (in the form of ash, alluvium, or lava rock that was just deposited) to hundreds of millions of years old (regolith dating to the Precambrian age occurs in parts of Australia).
On Earth, the presence of regolith is one of the important factors for most life, since few plants can grow on or within solid rock and animals would be unable to burrow or build shelter without loose material. Regolith is also important for human beings since it has been used since the dawn of civilization (in the form of mud bricks, concrete and ceramics) to build houses, roads, and other civil works.
The difference in terminology between “soil” (aka. dirt, mud, etc.) and “sand” is the presence of organic materials. In the former, it exists in abundance, and is what separates regolith on Earth from most other terrestrial environments in our Solar System.
The surface of the Moon is covered with a fine powdery material that scientists refer to it as “lunar regolith”. Nearly the entire lunar surface is covered with regolith, and bedrock is only visible on the walls of very steep craters.
The Moon regolith was formed over billions of years by constant meteorite impacts on the surface of the Moon. Scientists estimate that the lunar regolith extends down 4-5 meters in some places, and even as deep as 15 meters in the older highland areas.
When the plans were put together for the Apollo missions, some scientists were concerned that the lunar regolith would be too light and powdery to support the weight of the lunar lander. Instead of landing on the surface, they were worried that the lander would just sink down into it like a snowbank.
However, landings performed by robotic Surveyor spacecraft showed that the lunar soil was firm enough to support a spacecraft, and astronauts later explained that the surface of the Moon felt very firm beneath their feet. During the Apollo landings, the astronauts often found it necessary to use a hammer to drive a core sampling tool into it.
Once astronauts reached the surface, they reported that the fine moon dust stuck to their spacesuits and then dusted the inside of the lunar lander. The astronauts also claimed that it got into their eyes, making them red; and worse, even got into their lungs, giving them coughs. Lunar dust is very abrasive, and has been noted for its ability to wear down spacesuits and electronics.
The reason for this is because lunar regolith is sharp and jagged. This is due to the fact that the Moon has no atmosphere or flowing water on it, and hence no natural weathering process. When the micro-meteoroids slammed into the surface and created all the particles, there was no process for wearing down its sharp edges.
The term lunar soil is often used interchangeably with “lunar regolith”, but some have argued that the term “soil” is not correct because it is defined as having organic content. However, standard usage among lunar scientists tends to ignore that distinction. “Lunar dust” is also used, but mainly to refer to even finer materials than lunar soil.
As NASA is working on plans to send humans back to the Moon in the coming years, researchers are working to learn the best ways to work with the lunar regolith. Future colonists could mine minerals, water, and even oxygen out of the lunar soil, and use it to manufacture bases with as well.
Landers and rovers that have been sent to Mars by NASA, the Russians and the ESA have returned many interesting photographs, showing a landscape that is covered with vast expanses of sand and dust, as well as rocks and boulders.
Compared to lunar regolith, Mars dust is very fine and enough remains suspended in the atmosphere to give the sky a reddish hue. The dust is occasionally picked up in vast planet-wide dust storms, which are quite slow due to the very low density of the atmosphere.
The reason why Martian regolith is so much finer than that found on the Moon is attributed to the flowing water and river valleys that once covered its surface. Mars researchers are currently studying whether or not martian regolith is still being shaped in the present epoch as well.
It is believed that large quantities of water and carbon dioxide ices remain frozen within the regolith, which would be of use if and when manned missions (and even colonization efforts) take place in the coming decades.
Mars moon of Deimos is also covered by a layer of regolith that is estimated to be 50 meters (160 feet) thick. Images provided by the Viking 2 orbiter confirmed its presence from a height of 30 km (19 miles) above the moon’s surface.
Asteroids and Outer Solar System:
The only other planet in our Solar System that is known to have regolith is Titan, Saturn’s largest moon. The surface is known for its extensive fields of dunes, though the precise origin of them are not known. Some scientists have suggested that they may be small fragments of water ice eroded by Titan’s liquid methane, or possibly particulate organic matter that formed in Titan’s atmosphere and rained down on the surface.
Another possibility is that a series of powerful wind reversals, which occur twice during a single Saturn year (30 Earth years), are responsible for forming these dunes, which measure several hundred meters high and stretch across hundreds of kilometers. Currently, Earth scientists are still not certain what Titan’s regolith is composed of.
Data returned by the Huygens Probe’s penetrometer indicated that the surface may be clay-like, but long-term analysis of the data has suggested that it may be composed of sand-like ice grains. The images taken by the probe upon landing on the moon’s surface show a flat plain covered in rounded pebbles, which may be made of water ice, and suggest the action of moving fluids on them.
Asteroids have been observed to have regolith on their surfaces as well. These are the result of meteoriod impacts that have taken place over the course of millions of years, pulverizing their surfaces and creating dust and tiny particles that are carried within the craters.
NASA’s NEAR Shoemaker spacecraft produced evidence of regolith on the surface of the asteroid 433 Eros, which remains the best images of asteroid regolith to date. Additional evidence has been provided by JAXA’s Hayabusa mission, which returned clear images of regolith on an asteroid that was thought to be too small to hold onto it.
Images provided by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) cameras on board the Rosetta Spacecraft confirmed that the asteroid 21 Lutetia has a layer of regolith near its north pole, which was seen to flow in major landslides associated with variations in the asteriod’s albedo.
To break it down succinctly, wherever there is rock, there is likely to be regolith. Whether it is the product of wind or flowing water, or the presence of meteors impacting the surface, good old fashioned “dirt” can be found just about anywhere in our Solar System; and most likely, in the universe beyond…
It’s almost exactly 10 years ago that humanity parachuted a spacecraft into Titan, that moon of Saturn that could hold chemistry similar to what sat on Earth before life arose. Called Huygens, the probe survived for just about an hour on the surface on Jan. 14, 2005, transmitting information back about conditions there and on the way down.
Huygens is long dead, but its carrier craft is doing just fine. On Dec. 10, Cassini will make the 107th close pass by Titan to learn more about the moon’s atmosphere. Although Huygens made it to the surface fine, showing at least a basic understanding of how a parachute behaves on Titan, there’s still so much more we need to learn.
Specifically, Cassini’s different instruments have been coming up with different answers for Titan’s atmospheric density, so this flyby is hoping to resolve some of that. In part, they hope to get more accurate measurements by measuring how much drag the spacecraft experiences when it flies past the moon.
When Huygens probed the atmosphere on its way down, scientists figured that its measurements agreed in many ways with those taken by the flying-by Voyager 2 spacecraft previously. That said, the probe also discovered “a significant correspondence of wind shear and buoyant stability structures” in the stratosphere and lower tropopause of Titan, according to a 2006 presentation on Huygens results.
Correction, 11:33 a.m. EST: The University of Central Florida’s Phil Metzger points out that the image composition leaves out Eros, which NEAR Shoemaker landed on in 2001. This article has been corrected to reflect that and to clarify that the surfaces pictured were from “soft” landings.
And now there are eight. With Philae’s incredible landing on a comet earlier this week, humans have now done soft landings on eight solar system bodies. And that’s just in the first 57 years of space exploration. How far do you think we’ll reach in the next six decades? Let us know in the comments … if you dare.
More seriously, this amazing composition comes courtesy of two people who generously compiled images from the following missions: Rosetta/Philae (European Space Agency), Hayabusa (Japan Aerospace Exploration Agency), Apollo 17 (NASA), Venera 14 (Soviet Union), the Spirit rover (NASA) and Cassini-Huygens (NASA/ESA). Omitted is NEAR Shoemaker, which landed on Eros in 2001.
And remember that these are just the SURFACES of solar system bodies that we have visited. If you include all of the places that we have flown by or taken pictures from of a distance in space, the count numbers in the dozens — especially when considering prolific imagers such as Voyager 1 and Voyager 2, which flew by multiple planets and moons.
To check out a small sampling of pictures, visit this NASA website that shows some of the best shots we’ve taken in space.
It was eight years ago on January 14, 2005 that the Huygens spacecraft descended through Titan’s murky atmosphere and touched down – if a bit precariously – by bouncing, sliding and wobbling across the surface of Saturn’s largest moon Titan. This was the first time a probe had touched down on an alien world in the outer Solar System.
But that surface wasn’t quite what we expected.
While earlier studies of data from Huygens determined the surface of Titan to be quite soft, scientists now think the surface consisted of a hard outer crust but is soft underneath, so that if an object put more pressure on the surface, it sank in significantly.
“It is like snow that has been frozen on top,” said Erich Karkoschka, a co-author of a paper published in October 2012. “If you walk carefully, you can walk as on a solid surface, but if you step on the snow a little too hard, you break in very deeply.”
The scientists think that Huygens landed in something similar to a flood plain on Earth, but that it was dry at the time. The analysis reveals that, on first contact with Titan’s surface, Huygens dug a hole 12 cm deep, before bouncing out onto a flat surface.
The probe, tilted by about 10 degrees in the direction of motion, then slid 30–40 cm across the surface.
A new animation. top of the event has been created using real data recorded by Huygen’s instruments, allowing us to witness this historical moment as if we had been there.
The animation takes into account Titan’s atmospheric conditions, including the Sun and wind direction, the behaviour of the parachute (with some artistic interpretation only on the movement of the ropes after touchdown), and the dynamics of the landing itself.
Even the stones immediately facing Huygens were rendered to match the photograph of the landing site returned from the probe, which is revealed at the end of the animation.
Split into four sequences, the animation first shows a wide-angle view of the descent and landing followed by two close-ups of the touchdown from different angles, and finally a simulated view from Huygens itself – the true Huygens experience.
Also, a ‘fluffy’ dust-like material – most likely organic aerosols that are known to drizzle out of the Titan atmosphere – was thrown up and suspended for around four seconds around the probe following the impact. The dust was easily lifted, suggesting it was most likely dry and that there had not been any ‘rain’ of liquid ethane or methane for some time prior to the landing.
Huygens was released from the Cassini spacecraft on Christmas Day 2004, and arrived at Titan three weeks later. The probe began transmitting data to Cassini four minutes into its descent and continued to transmit data after landing at least as long as Cassini was above Titan’s horizon, for about 90 minutes, and radio telescopes on Earth continued to receive Huygen’s signal well past the expected lifetime of the craft.
Cassini was supposed to receive Huygen’s signal over two channels, but because of an operational commanding error, only one channel was used. This means that only 350 pictures were received instead of 700 that were expected. All Doppler radio measurements between Cassini and Huygens were lost as well; however, Doppler radio measurements of Huygens from Earth were made, though not as accurate as the expected measurements that Cassini would have made. But when added to accelerometer sensors on Huygens and VLBI tracking of the position of the Huygens probe from Earth, reasonably accurate wind speed and direction measurements could still be derived.