How Planetary Exploration is Helping Understand Earth a Bit Better
“We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.”
— TS Eliot
After surveying our solar system, as well as thousands of others beyond our own, we can now look back at our home planet with a new perspective, that of a planetary geologist. One of four rocky planets in the inner solar system, ours sits in the “Goldilocks Zone” — neither too hot nor too cold. Also known as “Terra,” it’s better described as the water planet as it’s the only known body with abundant surface water.
Viewing the Earth from the perspective of a planetary geologist allows us to compare geological processes that occur here with those we find on other planets and moons, a subject called comparative planetology. If we take a very broad view of the Earth, drain the oceans, and examine its topography, we see that the Earth is unique in that it has many lows (the ocean basins) and many highs (the continents), with not much elevation in between.
We now know that this unique “signature” is caused by plate tectonics, the discovery in the 1960s that Earth’s continents are fairly rigid plates floating and drifting on a semi-molten mantle. The crust beneath the oceans turns out to be denser volcanic rock formed at volcanoes along mid-ocean ridges. Iceland lies on the ridge of volcanoes running up the middle of the Atlantic Ocean.
In my discipline of geomorphology, or the science of landscapes, scientists try to explain how the landforms around us came to be and continue to evolve as the product of competing geological processes such as earthquakes that build up mountain ranges and erosion that tears them down. The landscape we see is then a product of the tug of war between these opposing forces.
In the western U.S. for example, earthquakes are relatively common (anyone who’s lived here long enough has probably experienced one), but the climate, especially in the southwest, is dry and has been for the last 10,000 years or so. Thus, we have taller, more rugged mountains than the eastern U.S., where earthquakes are few and the climate wetter. Of course, we also know that Earth’s climate has varied over tens of thousands of years as ice ages have come and gone, and we see evidence of that in remnant landforms that no longer “belong” in a particular area.
These kinds of processes also operate on other planets, so we can use our understanding of how they work here at home to explain the landforms we find there.
For example, ancient landforms on Mars indicate to us that water once flowed and formed landforms typically found on Earth in wet environments like ocean shorelines and deltas. We also found dust-covered glaciers that hearken back to ice ages Mars went through. On Saturn’s moon, Titan, we can find river channels that meander across an uplifted terrane, similar to what the Colorado River makes in the Grand Canyon. Sand dunes found on Venus, Mars, and Titan can all be compared to Earth dunes to learn more about the direction and strength of the local winds as well as the nature of the sand making up the dunes.
Earth’s dynamic atmosphere is one of its defining characteristics, and very important to us as well. And while I’m not an atmospheric scientist, I’m well aware that our understating of how our atmosphere circulates, how clouds and aerosols move around, and its composition has helped us understand the atmospheres of other planets like Mars, Venus, Titan, and the gas giants of the outer solar system. It also points the way toward the highest priority observations to make of planets outside our solar system, especially if we can make a measurement that will tell us if a planet hosts life.
When NASA’s Galileo spacecraft was working its way to Jupiter, a close Earth flyby was used to slingshot it out toward Jupiter. As a member of the science team, Carl Sagan took the opportunity to turn its instruments on Earth and perform an experiment: Could we detect signs of life with the limited array of instruments available? As reported in an article in the journal Nature, the team detected oxygen, methane, chlorophyll, and AM radio waves: all indicative of life, possibly intelligent life.
Observing from Above
Spacecraft monitoring Earth also notice the circulation of the atmosphere as clouds stream across its surface and, longer term, a green wave propagating north in the Northern Hemisphere summer, while moving south in the Southern Hemisphere summer.
More subtle is the circulation of the oceans which can be read in thermal images and changes in wave patterns. And while no known extraterrestrial body has such a large amount of surface water, new discoveries at the moons of Jupiter and Saturn indicate that some may have oceans below a thick ice crust. These discoveries came about at nearly the same time as polar scientists on Earth discovered lakes at the bases of some of the ice sheets in Antarctica. The Antarctic lakes are now being investigated both for their implications for life on Earth and as analogs of the ocean worlds now known to be out there.
Terrestrial analogs of extraterrestrial landforms and geological processes must always be taken with a grain of salt. It’s often difficult to take into account the environmental differences between Earth and another planet. When I’ve led field trips to the Mojave Desert or Death Valley for planetary geologists and astronauts, I’ve always emphasized that fact.
For example, Mars has both less gravity and much lower atmospheric pressure than Earth, but we still find dunes. Experiments in low-pressure wind tunnels at Ames Research Center in the Bay Area showed that dunes can form in Mars’ current environment, but that the sizes of the grains must be smaller than on Earth. On Titan, there’s enough atmosphere for sand to blow around, but we still haven’t figured out what the material is, only that it has to be sand sized to form dunes.
So, while comparative planetology has led to a much better understanding of the planets in our own solar system, as well as beyond, it has also given us new insights into our own planet, as we have begun to view the operation of our planet from a broader systems perspective. These systems, like the atmosphere, oceans, and biosphere, interact in complex ways we’re slowly beginning to appreciate.