Star Gazing

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory

E-mail: ken.tapping@nrc-cnrc.gc.ca

Among the various space missions going on at the moment there are two of particular interest.

The first is the Japanese spacecraft Hayabusa2, which visited the surface of the asteroid Ryugu, about 300 million kilometres from Earth, took samples of its surface rocks, and brought them back to Earth.

The sample capsule landed successfully and has been retrieved. The second is the Chinese spacecraft, Chang'e 5, which landed on the Moon, took rock samples, which are now, at the time of writing, on their way back to Earth.

Why are these missions of particular interest?

Apart from our wanting to better understand asteroids, these objects are of particular interest because many of them are time capsules. They are lumps of material left over from the construction of the Solar System, 4.5

billion years ago. Some asteroids visit the inner Solar System, where they get warmed or even roasted by the Sun.

This evaporates ice, drives off volatile materials and produces chemical changes. What is left over is no longer a good sample of that building material.

On the other hand, asteroids with orbits keeping them far from the Sun stay frozen, with all their ingredients intact.

If we want a sample of this primordial Solar System building material, these are the objects to visit. In addition, a small body, with weak gravity would be a good choice, because it would be very easy to land on, and its surface material might not be as firmly consolidated as the material making up the surface of the Earth or Moon, and therefore easier to sample.

To be most confident our samples are of the primordial building material of the Solar System, we would prefer them to come from beneath the surface, where they have not been exposed to space for billions of years, which might have changed in their nature or composition.

So why would we like more rocks from the Moon? There are two reasons. Firstly, since scientists have been working on the rock samples brought back by the Apollo astronauts back in the 1970s, a lot of science has been done, and new questions have arisen which are not answered by those samples.

Secondly, an important consideration in sending astronauts to the Moon is minimizing risk. There are places on the Moon that, until we develop improved technologies, remain risky places to attempt manned landings.

One of the big outstanding questions regarding the Moon is when volcanism eventually ended. The Earth is larger, cooling more slowly and is still hot enough to be volcanically active. The Moon cooled much faster and volcanic activity stopped. The rocks retrieved by the Apollo missions and by unmanned sampling missions by the Soviet Union are old, three billion years or more. This suggests that by the time the Moon was at most a billion years old, volcanism had ceased. Any younger lava flows are probably due to meteoric impacts.

However, observations by ground-based telescopes of the Mare Procellarum (Ocean of Storms) suggest something different. This area is a large, dark lava flow on the left side of the lunar disc as we see it in the sky. The observations suggest the surface rocks in that region solidified as recently as one billion years ago.

These are the rocks sampled by the Chinese lander.

Probably the most stunning thing that has happened over the last decade or two is that we can now buy real textbooks about the geology of other planets. This not only tells us more about those other worlds, but also how our Earth fits into the cosmic scheme of things.

Saturn and Jupiter lie extremely close together, low in the southwest just after dark. Mars is high in the southeast. Venus lies low in the dawn glow. The Moon will be Full on the 29th. I would like to take this opportunity to wish you all a wonderful Christmas and a 2021 better than 2020 has been.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton.