Transcription
Hello, I'm Rebecca Barnes, and welcome to the Science EA vcast. In this episode, we'll take a look at the Earth's moon and Titan, two very different natural satellites in our solar system, and find out about the two ESA missions that have explored them.
The eight planets orbiting the Sun are accompanied by over 150 natural satellites. These moons, orbiting their companion planets, are worlds in their own right, with a wide variety of shapes, sizes, and landscapes. Some moons have active processes; others preserve a record of the Solar System's history; and some even have atmospheres.
Let's focus on two very diverse natural satellites: the Earth's moon and Titan, Saturn's largest moon. We'll start with Earth's only natural satellite, the one with which we are most familiar and has been well explored since the dawn of the Space Age. The Moon is an airless world with an ancient and heavily cratered surface that has changed very little in the last 4 billion years. It bears scars of the events that have shaped the rocky planets of the inner solar system. As Earth's closest celestial neighbor, it is within our reach to observe and explore. The moon's surface has two types of terrain: the relatively younger, dark-colored regions of the Maria and the primitive light regions of the Highlands.
The samples of rock returned to Earth by the Apollo astronauts and Russian landers have been thoroughly studied. They have provided a template for dating the surfaces of the inner planets and insight into how the moon formed. It is thought that about 4.5 billion years ago, a Mars-sized object smashed into the Earth, leaving a cloud of debris that accreted to form the Moon. The energy released in this process created an object with seas of magma. As the Moon cooled and started to solidify, it was bombarded with asteroids, some so large that their impacts stripped and split the crust. These craters later flooded with lava, forming the Maria. Despite the many missions to the Moon and the intensive study of the collected rock samples, scientists have barely scratched the surface of understanding this nearby celestial object.
In 2003, Europe launched SMART-1, the first small mission for advanced research in technology—Europe's first mission to the Moon. This marked the beginning of a series of missions designed to prepare the technologies needed for future exploration of the solar system. This mission aimed to voyage to the Moon, testing a new propulsion system along the way and new instrument technologies. Once it arrived, it was a masterpiece of miniaturization, with everything needed for propulsion, communications, and instrumentation packed into a cube just 1 m across.
SMART-1 took a leisurely route to the Moon, powered by a solar electric propulsion system. Within the system, an ion engine made use of the Hall effect, where a current flowing across a magnetic field induces an electromotive force at right angles to both the current and the magnetic field. SMART-1's propulsion system was powered by the spacecraft's solar panels, and the induced electromotive force was used to accelerate Xenon ions that provided the thrust. This type of engine releases its ionized gas propellant at a much greater velocity than traditional chemical rockets. This means that they are extremely fuel-efficient. By conservation of momentum, an ion engine can deliver about 10 times more thrust per kilogram compared to a chemical rocket. Ion engines may take longer to accelerate, but they can provide a sustained thrust for a long period of time and potentially reduce the journey time over long distances.
The testing of the ion engine was extremely successful; it worked flawlessly. SMART-1's unconventional amble to the Moon took 16 months, but it used just 60 L of fuel. This amount would enable a car to travel just a few hundred kilometers. After launch, the ion engine was fired intermittently to gradually expand the spacecraft's elliptical orbit around the Earth. At around 60,000 km from the Moon, the gravitational effect of the Moon and the Earth balanced at the first Lagrange point, L1. Here, SMART-1 was captured by the Moon's gravitational field, and the ion engine was used to propel the spacecraft even closer to the Moon.
When SMART-1 entered into orbit around the Moon, the mission's next objective was to perform science experiments with three novel instruments. The Demonstration of a Compact X-ray Spectrometer, or D-CIXS, used brand-new technology in an experiment to detect X-rays emitted by chemical elements excited by incoming high-energy solar radiation. D-CIXS made the first detection of calcium from lunar orbit and looked at the differences in the amount of titanium, magnesium, and silicon between the near and far sides of the Moon. Information about the abundance of these elements on the lunar surface will help to determine if a large impact on Earth did lead to the creation of the Moon.
The Advanced Moon Microimager experiment, AMIE, was a pocket-sized, high-resolution camera. This experiment imaged the lunar surface from different angles under different light conditions. These images were combined to produce maps and a geological context for the planetary processes that are shaping the lunar landscape, such as impact craters, tectonic wrinkles, volcanic features, and space weathering. The illumination of these regions could be important for future explorers.
Another compact experiment, the SMART-1 Infrared Spectrometer, SIR, was the first instrument of its kind to be used in a lunar mission. SIR employed new technology to map the distribution of minerals in the lunar rocks. The composition of these minerals reflects the physical and chemical conditions under which the rocks were formed. Determining what minerals are found and their location on the Moon provides information about the Moon's origin, the evolution of the crust, and the time of key events such as volcanism and asteroid bombardment.
After spending 18 months orbiting the Moon, the SMART-1 mission came to an end in September 2006 when the spacecraft made a controlled impact into the surface of the Moon. When SMART-1 visited the Moon, it was the only spacecraft orbiting the Earth's natural satellite. It was therefore able to support countries that were planning new lunar missions by helping to test communication systems. This collaboration saw the next generation of science instruments from Europe—SMART-1—fly in 2008 on Chandrayaan-1, India's first mission to the Moon. Europe has also provided spacecraft and ground operations support services for the first Chinese lunar mission, Chang'e-1. The Chinese National Space Administration and ESA have shared the data returned by this mission.
Many moons in the solar system are airless worlds similar to our Moon. However, there are some that are very different. Titan, Saturn's largest moon and the second largest in the solar system, is strikingly different from the Earth's only natural satellite. Titan is the only moon in the solar system with a significant atmosphere. It is a curious world that appears to bear similarities to the Earth, to other icy moons in the outer solar system, and has some features that are unique.
In 1997, NASA, ESA, and the Italian Space Agency launched Cassini-Huygens, a mission designed to explore the entire Saturnian system, including the planet itself, its magnetosphere, the famous rings, and many of its satellites, in particular Titan and a number of the icy moons. Cassini-Huygens is the most complex interplanetary spacecraft ever built and is made up of the NASA Cassini orbiter and the ESA Huygens probe. After launch, the spacecraft needed to gain sufficient velocity to reach Saturn, first swinging by Venus twice, then past the Earth, and on to Jupiter for its final push to the ringed planet. This epic journey to Saturn lasted nearly 7 years. In July 2004, Cassini-Huygens became the first spacecraft to enter into orbit around Saturn.
On the 14th of January 2005, the European Huygens probe made a historic journey of approximately 2 1/2 hours through Titan's hazy atmosphere. At around 160 km above the surface, the probe's scientific instruments were exposed to the atmosphere and started to transmit data up to the Cassini orbiter. The entire Huygens mission was designed to take place during the descent through the atmosphere because it was not known if the probe would survive the impact or what kind of terrain it would land on. However, communication with the orbiter was designed to be maintained for some time after landing in case the probe survived after impact.
The Huygens probe carried six scientific instruments that were designed to provide detailed measurements of the properties, composition, and dynamics of Titan's atmosphere and surface. Traveling at a speed of just 18 km per hour, the Huygens probe touched down gently on the surface of Titan, marking the first and so far only landing of a man-made probe in the outer solar system. The Huygens probe exceeded expectations and survived on the surface for more than 3 hours. Now that Huygens has accomplished its mission on the surface, the Cassini orbiter continues to probe Titan by performing regular flybys as part of its ongoing investigation of the Saturnian system.
The direct measurements taken by the Huygens probe, combined with those made remotely by Cassini, provide an invaluable wealth of data about one of the Solar System's most intriguing moons. As Huygens traveled through Titan's alien atmosphere, it was carried by the wind. This induced a frequency shift on Huygens's radio signal that provided information about Titan's winds. Following Huygens, the Cassini orbiter performed a large number of measurements of the wind higher up in the upper stratosphere. Computer modeling of the atmosphere based on the winds measured by the orbiter revealed that it is like a giant conveyor belt circulating from north to south and south to north. Measurements of these winds by the orbiter were made possible by studying how the molecules present in the moon's atmosphere fare move. This was achieved by following the faint infrared light they emit.
Cassini-Huygens confirmed that nitrogen is the main component of the atmosphere and that it also contains a lot of methane, which is the driving force of the chemistry and weather. Methane is destroyed by interaction with solar ultraviolet light, and it should not last for long on geological time scales in the atmosphere. The large amounts found mean there must be a process replenishing it. One possibility is that it is being released from the interior of the Moon.
Titan has a hazy, thick atmosphere, which Huygens found to be even hazier than expected due to the presence of aerosols all the way down to the surface. Measurements of the atmosphere confirm that complex organic compounds—the building blocks of the amino acids necessary for life—were present in both gas and solid phases. Despite the thick atmospheric haze, the cameras on board the Huygens probe were able to take clear images of Titan from about 45 km above the surface. The first images of the surface showed a world that resembled the Earth in many ways, with evidence that a liquid, possibly methane, had flowed on the surface, carving flood plains and river channels into the landscape. Other than the Earth, Titan is the only world in our solar system that is known to have an active liquid cycle. On Titan, methane plays many of the same roles that water plays on Earth. At the landing site of the probe, the surface was found to have the consistency of loose wet sand that was mostly made up of dirty water ice pebbles. The surface measurements detected compounds that had not been found higher up in the Titan atmosphere, including carbon dioxide. The Huygens probe also measured the weather conditions at the surface, detecting light winds, a temperature of -180° C, and an atmospheric pressure 50% higher than Earth's.
Before the Cassini-Huygens mission, very little was known about how the Saturnian moon formed and the composition of its internal structure. The mission has spurred new models of Titan to be made. It is now thought to have a silicate core with a water-rich mantle, and in addition, an underground ocean may exist a few tens of kilometers below the icy surface. Evidence of this comes from measurements made by the Huygens probe of electric conductivity in the atmosphere.
The landing of the Huygens probe on a small body so far from Earth was a truly momentous achievement. The huge amount of data taken by the probe is still being analyzed thoroughly. With this and further data taken by the Cassini spacecraft during its primary and extended missions, scientists will continue to unravel more and more about the past and present workings of Titan. Cassini-Huygens is one of the most ambitious and successful missions thus far in the history of solar system exploration.
As a desire to venture further into the solar system grows, new and more efficient methods of exploration must be developed and tested. It is through missions such as SMART-1 that this can be achieved. Advanced propulsion, as tested during the SMART-1 mission, may provide an efficient way to return to Titan or visit other outer solar system moons in the future.
The solar system's planets and moons are within our reach to directly explore. Their diversity provides a remote laboratory from which a profound understanding of the origins of the Earth and the solar system can be developed. We just need to get there.
I'm Rebecca Barnes. Thank you for watching the Science EA podcast.