Some water falls from above the clouds
When the Earth was formed, it was a big ball of molten rock.
There was no water on the surface, since it would have immediately evaporated. Furthermore, most of the water vapor in the atmosphere was lost into space due to the high temperatures.
So, how did the Earth’s oceans form?
The water came in part from the continuous bombardment of asteroids, which were plentiful in the early solar system. Those asteroids contained ice. The Earth was like a gravitational vacuum cleaner, pulling in stray material as it circled in orbit. Over time, enough water accumulated to fill the oceans.
Today, the region of space near Earth’s orbit is different. The Earth has sucked in most of the ice and rock that has crossed its path over billions of years.
But that space still isn’t entirely empty. Some asteroids are brushed aside in the wake of other planets as they orbit, and occasionally one is pulled in by Earth, seen as a meteor.
In addition, the Earth continually pulls in small particles, space dust, which contain trace amounts of ice. Although estimates vary, about 40,000 tons of space dust is accumulated every year.
The bottom line is that water can accumulate on Earth due to bombardment from space. Even now, the Earth accumulates some water from space dust. Of course, space dust can also be captured by the moon.
The difference is that the moon doesn’t have an atmosphere, due to its weaker gravity. Any ice delivered to its surface would melt in the sunlight and escape back into space. So, it was a bit of a surprise when it was announced last week that NASA scientists had measured the signature of water on sunny parts of the moon.
The new evidence was reported by the SOPHIA project, using an infrared light telescope aboard a 747-jet flying in the upper atmosphere. It’s necessary to go above the atmosphere to get enough sensitivity for the infrared measurements. A surface infrared telescope would be obscured by water vapor in Earth’s atmosphere.
The NASA scientists think that the moon’s water might be trapped inside glass beads, which could have formed by impacts of micrometeorites, or possibly formed ages ago when larger meteorites struck the moon creating the many craters we see today.
The real question is whether this water can be harvested by astronauts living at a future moon base. We already know that ice exists at the moon’s pole, in the shadow of craters where sunshine can’t reach. That polar ice would be easier to harvest.
Although the SOPHIA results are of academic interest, it may not be a practical source of water to astronauts of the future. But you never know where the science may lead.
hicks@ohio.edu