Note: The actual lecture contains color images. The colors from one image are di

Note: The actual lecture contains color images. The colors from one image are discussed by the professor. You do not need to see the colors to understand the lecture or to answer the questions.
Astronomy
Narrator
Listen to part of a lecture in an astronomy class.
Professor
Last week, we covered some arguments against going back to the Moon. But there are compelling reasons in favor of another Moon landing, too, um, not the least of which is trying to pinpoint the Moon’s age. We could do this, in theory, by studying an enormous impact crater known as the South Pole-Aitken Basin. Ah, it’s located in the Moon’s south polar region. But, since it’s on the far side of the Moon, it can only be seen from space. Here’s an image of. .. we’ll call it the SPA Basin.
This color-coded image of the SPA Basin—ahh, those aren’t its actual colors, obviously—uh, this image is from the mid-nineties, from an American spacecraft called Clementine. Um, unlike earlier lunar missions, Clementine didn’t orbit only around the Moon’s equator. Its orbits enabled it to send back data to create this topographical map of. .. well, the gray-and-white area toward the bottom is the South Pole. The purples and blues in the middle correspond to low elevations—the SPA Basin itself. Uh, the oranges and reds around it are higher elevations. The Basin measures an amazing 2,500 kilometers in diameter, and its average depth is 12 kilometers. That makes it the biggest known crater in our solar system. And it may well be the oldest.
Y’know, planetary researchers love studying deep craters to learn about the impacts that created them, um, how they redistributed pieces of the planet’s crust. And, in this case, we especially want to know if any of the mantle, the layer beneath the crust, was exposed by the impact. Not everyone agrees, but some experts are convinced that whatever created the SPA Basin did penetrate the Moon’s mantle. And we need to find out, because much more than the crust, the mantle contains information about a planet’s or moon’s total composition. And that’s key to understanding planet formation. Um, Diane?
Female student
So the only way to know the Basin’s age is to study its rocks directly?
Professor
Well, from radio survey data, we know that the Basin contains lots of smaller craters. So it must be really old—about 4 billion years, give or take a few hundred million years. But that’s not very precise. If we had rock samples to study, we’d know whether these small craters were formed by impacts during the final stages of planetary formation, or if they resulted from later meteor showers.
Female student
But if we know around how old the Basin is, I’m not sure that’s reason enough to go to the Moon again.
Professor
Oh, but such crude estimates ... mmm, we can do better than that! Besides, there’s other things worth investigating. Like, is there water ice on the Moon? Clementine’s data indicated that the wall of a south polar crater was more reflective than expected. So some experts think there’s probably ice there. Also, data from a later mission indicate significant concentrations of hydrogen, and by inference, water, less than a meter underground at both poles.
Male student
If there’s water, how’d it get there? Underground rivers?
Professor
We think meteors that crashed into the Moon, or tails of passing comets, may have introduced water molecules. Any water molecules that found their way to the floors of craters near the Moon’s poles, that water would be perpetually frozen because the floors of those craters are always in shadow. Uh, furthermore, if the water ice was mixed in with rock and dust, it’d be protected from evaporation.
Female student
So, are you saying there might be primitive life on the Moon?
Professor
Uh, that’s not my point at all! Um, OK, say there is water ice on the Moon. That would be of very practical value for a future Moon base for astronauts. Uh, water ice could be melted and purified for drinking. It could also be broken down into its component parts—oxygen and hydrogen. Oxygen could be used to breathe. And hydrogen could be turned into fuel, rocket fuel. So, water ice could enable the creation of a self-sustaining Moon base someday, a mining camp, perhaps, or, uh, a departure point for further space exploration.
Male student
But hauling tons of equipment to the Moon to make fuel and build a life-support system for a Moon base ... wouldn’t that be too expensive?
Professor
A permanent base, uh, may be a ways off, but we shouldn’t have to wait for that. The dust at the bottom of the SPA Basin really does have a fascinating story to tell. What I wouldn’t give for a few samples of it!

Narrator
Listen again to part of the lecture. Then answer the question.
Female student
But if we know around how old the Basin is, I’m not sure that’s reason enough to go to the Moon again.
Professor
Oh, but such crude estimates ...
Narrator
What does the professor imply when he says this:
Professor
Oh, but such crude estimates ...