The Moon, as well as the sun, is an astronomical body that we are most familiar with. While the Moon has long been deeply associated with humankind's lives and cultures, it is also a key to answering one of the most fundamental questions that have been asked over many years: How was the Earth born? Two planetary scientists, Assistant Professor Makiko Ohtake and Associate Senior Researcher Hiroyuki Sato, talk about a profound link between the Moon and Earth and a lunar exploration project currently underway.
The Moon and Earth share the same history as
astronomical bodies formed about the same time
—First of all, please tell us about how the Moon was born.
Ohtake:
According to the giant-impact hypothesis, which is considered the most likely explanation for the lunar formation, soon after the Solar System was built, the Earth, which was still in its infancy, was hit by a large body approximately the same size as Mars. This collision produced a great deal of debris that eventually clumped together to form the Moon. In other words, it is believed that the Moon and Earth were formed about the same time. So the Moon is not just a body close to the Earth in distance but also in inseparable relations with Earth as they share the same history of formation side-by-side.
—As known widely, the ebb and flow of the tide is caused by the gravitation of the Moon. As this example shows, the environment of Earth would have been totally different from the current one without the Moon.
Ohtake:
Most likely. Did you know that the Moon is moving away from the Earth by several centimeters a year? So the further back in time, the closer the Moon was to the Earth, and, therefore, Earth must have been subject to a greater influence from the Moon than now.
—Studying the Moon leads to studying the Earth.
Sato:
The starting point of planetary scientists' research is to pursue answers to such fundamental questions for humankind as "How did we come to exist on Earth?" and "How was the Earth born?" I feel that I'm learning a lot about how the Earth was built through research on the Moon. For instance, the Moon didn't become an Earth-like body because its mass is not large enough. Like this I compare Earth with the Moon and various other bodies to narrow down the conditions required for the Earth to exist. Furthermore, for planetary scientists, the Moon is easier to be studied compared to most of other bodies.
—What do you mean?
Sato:
The Moon is one fourth the size of the Earth thus compact, and is not affected by active weather or erosion like Earth because it has no atmosphere or water. There were times back in the past when igneous activity on the Moon caused lava to spurt up and cover a part of its surface, bringing considerable changes to it. However, the Moon currently experiences almost no changes, except in occasional cases when a meteorite hits its surface and creates a crater. Because craters remain almost intact as they were created for many years, the Moon is a very good subject for study to know, for instance, how craters are formed, compared with other bodies including the Earth.
—Your remark reminds us of what JAXA Researcher Risa Miyazaki told us in the interview for the last issue. She said, "The crust of the Moon cooled and solidified quickly, so unlike Earth, the Moon preserves information about the distant past."
Ohtake:
That's why we often call the Moon a time capsule of the Earth. We know Earth was born 4.6 billion years ago, but because of plate tectonics and other activities, there are almost no rocks remain that were formed more than 3. something billion years ago. On the other hand, on the Moon where the crust that cooled and solidified in the early stage of its formation, rocks that were formed more than 4 billion years ago can be seen everywhere on the surface. This is why knowing about the Moon may be a shortcut to knowing about Earth.
—So your interest in lunar exploration derived from your interest in knowing more about the Earth?
Ohtake:
That's right. Earth is the source of life. This environment could not have been developed by water or atmosphere alone. How universal can this environment be in the space?—this is the theme that I've been interested in. So I wanted to know how the Earth was born and research rocks from the oldest chapter of Earth's history—as a matter of fact, I was determined to go anywhere in the world if such a rock exists!—but the oldest rocks that I was able to get were ones from about 3.8 billion years ago. I was unable to study the first 800 million years. This led me to focus on the Moon.
Sato:
I also at first wanted to know how Earth's land, on which I live, had been formed. I used to climb mountains to collect stones and draw geological maps, which illustrate the distribution of stone types and geological layers under the surface soil. Then, I took interest in the inner structure of astronomical bodies other than Earth and began to study the geology of Mars. I chose Mars because the Selenological and Engineering Explorer (SELENE) "KAGUYA" project didn't exist at that time. But before long, I was hired by the lunar exploration satellite project in the United States, and this marked the beginning of my research on the Moon.
The SLIM's mission is to land on the Moon based on lunar data collected by KAGUYA
—JAXA's lunar exploration projects include the KAGUYA lunar explorer satellite, which completed its mission in 2009, and the Smart Lander for Investigating Moon (SLIM), a small unmanned lunar lander, which is slated to be launched in FY2021.
Ohtake:
KAGUYA was a mission that investigated the entire Moon for the first time in the world to collect 15 types of observation data, including elemental and mineralogical distribution, gravitational field, and morphology. The mission helped us, among other things, obtain a great deal of high-resolution spectroscopic observation data on the lunar surface and locate the distribution of peridotite rocks containing a lot of olivine, which provides a key to understanding what the inside of the Moon is like. Thanks to these observations by KAGUYA, we can now determine which rock was discharged from which crater, rather than "from somewhere on the Moon." The SLIM project will make use of these data collected by KAGUYA to land a lunar lander exactly in the planned location where peridotites are concentrated and make scientific observations on the lunar surface.
The SLIM's targeted landing point determined based on the data from KAGUYA. It is near the Sea of Nectar situated in a low-latitude region.
Sato:
As the resolution of lunar surface observation data is becoming increasingly higher, we can now find craters that couldn't be spotted before. Most of these craters have no names yet, and the crater on which the SLIM is scheduled to land also didn't have the name. So the project team named and registered the crater.
—What role are you in charge of in the SLIM project?
Ohtake:
I'm the payload manager for the mission. I'm responsible for "matchmaking" between the lunar lander and a spectroscopic camera that is to be loaded. The SLIM, whose main mission is to demonstrate a high-precision landing technology, is—as its name suggests—much smaller in size and lighter in weight than existing probes. But because it became possible to load only one instrument, we discussed and decided to load a spectroscopic camera. In addition to demonstrating pinpoint landing, we will also take this precious opportunity to make observations as well using this camera.
Smart Lander for Investigating Moon (SLIM)
Sato:
My main role is to develop a focusing system for the spectroscopic camera. The camera that can be loaded in the SLIM is very small. To observe crystals on the surface of a peridotite lying 30 meters away, for instance, a focusing system is essential. Focusing must also be automatic because we can't focus manually each time. To achieve this autofocus function, we need a sophisticated algorithm. When everything is done well, we may be able to observe, among other things, the size of crystals that constitute a peridotite.
—Our expectations have become even higher for the SLIM mission as we have been listening to you.
Ohtake:
To ensure its slow descending to and soft landing on the lunar surface, the SLIM needs a great amount of fuel. Even though small in size, the Moon has gravity of its own. So strict control is required to prevent the SLIM from descending too fast. Through the SLIM mission, we are also intending to build up technologies necessary for future Mars exploration, but it is said that landing on the Moon may be more difficult than landing on Mars in some aspects. This is because, although Mars has gravity, it also has a little atmosphere that the Moon doesn't have, and the atmosphere is expected to act as a brake to control the descending of a spacecraft. Pinpoint landing on the Moon is really challenging. I'm so excited.
Profile
Makiko Ohtake
Assistant Professor
Department of Solar System Sciences
Institute of Space and Astronautical Science (ISAS)
Born in Hyogo Prefecture. Her research includes the tracing of the origin and evolution of the Moon through the analysis of spectroscopic observation data on the lunar surface collected by the KAGUYA lunar explorer satellite as well as the development of spectroscopic observation equipment for future lunar landing exploration missions. She is also a member of the Lunar and Planetary Exploration Data Analysis Group. Her hobby is mountain-climbing. It is not rare that she stops on her way up to the summit every time she finds an interesting stone.
Hiroyuki Sato
Associate Senior Researcher
Institute of Space and Astronautical Science (ISAS)
Mr. Sato is also a member of the Lunar and Planetary Exploration Data Analysis Group. His hobbies include camping, hiking, and surfing. After enjoying hiking in the great nature of Arizona for several years, now he enjoys the beach in Japan.
