Astrophysicists and planetary geologists need to talk.
There is a connection between things on a galactic scale, and those on a local planetary scale. Today’s optical telescopes have already glimpsed at least four extrasolar planets. With telescopes now under design, humanity may be able to directly measure the spectra of worlds of terrestrial size and smaller. To get the most out of those observations, we must more intensively explore similar worlds closer to home.
Successfully understanding the solar system and its place in the galaxy means developing an integrated, overarching, long-term strategy for exploration.
This strategy should be sufficiently open-ended to let the scientific results we strive for today be applicable to experiments, observations and exploration far into the future.
Below, I suggest three local projects in the inner solar system that would produce exciting results now, while also providing potentially detailed knowledge of conditions elsewhere the galaxy that seems out of reach today.
If the solar system is any guide, most of the bodies around other stars will be Mars-sized or smaller. We are finding gas giant planets close to their primaries because they are relatively easy to find, not necessarily because they are common. As more powerful telescopes are deployed – especially interferometers above the atmosphere in space – we should expect to observe increasingly smaller bodies.
The vast majority of rocky worlds in a galaxy are likely to have surfaces dominated by “regolith,” any lose, unconsolidated material over bedrock. Regolith is commonly defined as dirt without organic materials, or nonliving dirt. Dirt with organic material is called “soil.”
The majority constituent of regolith, and most soil, is shattered or ground up rock – but regolith has none of the rotting leaves, decaying organisms, living microbes and earthworms that make soils alive. Non-living regolith is rare on Earth, but is near-universal on Earth’s Moon, asteroids and other small, airless bodies. It is present over much of the martian surface, albeit heavily modified by water, chemistry and the winds.
Apollo astronauts quickly discovered that lunar regolith is far from the simple material that was expected. Heat generated by impacts melts rock, trapping other rocks into accumulations called breccias, which are shattered in subsequent impacts. Often, beads of glass created in ancient lunar volcanoes, and in more recent impacts, act as a cement, helping additional impacts glue particles together into agglutinates. These types of rocks, or impact-shattered parts of them, are in turn incorporated into new breccias and agglutinates, in every conceivable combination.
NASA and the Japan Aerospace Exploration Agency (JAXA) have sent probes to make detailed observations of two small asteroids, and a number of other spacecraft have taken peeks at other rocks on their ways to someplace else. The small bodies have proven surprisingly complex and widely diverse, quite unlike the lunar surface and with no two alike. Even the tiniest of objects have changing, dynamic surfaces – at least if you are prepared to wait millions of years.
Fortunately, we don’t have to. Geologists visiting the nearby Moon and asteroids will be able to study regolith in place and of many different ages, to learn how it was created, from where it has moved, and attempt to determine why it is so different on seemingly similar worlds.
By comparing the detailed geology of the sun’s small planets and minor bodies to remote observations of those same bodies, we should be able to tie different styles of regolith to their reflectance spectra. Then we can ask, what environmental conditions in different parts of the solar system, past and present, created the very different styles of regolith we are observing in the solar system? In the future, we may be able to use that information to learn something about conditions in alien star systems by observing the spectral signatures of different regolith styles exhibited by the rocky bodies of each system.
To truly understand the galaxy we must understand the majority of its constituent parts, and to do that we must fully understand the styles and behavior of regolith.
David H. Grinspoon, a Venus expert at the Department of Space Sciences at the Denver Museum of Nature and Science, and Mark A. Bulloch of the Southwest Research Institute in
, take another approach to using local information to draw conclusions about remote star systems. In a paper in “Exploring Venus as a Terrestrial Planet,” published by the American Geophysical Union, they point out that the three terrestrial planets in the solar system with atmospheres have evolved in surprising and counterintuitive ways.
Two worlds that at first seem to have almost nothing in common – Mars and Venus – in fact have two great similarities: both have lost much of their water and created atmospheres dominated by carbon-dioxide, while Earth did neither. At the same time, two otherwise very similar worlds that probably started out with similar surface environments – Venus and Earth – evolved in startlingly different directions.
“In the first billion years of Solar System evolution, Venus, Mars, and Earth were all very different from their current states,” the scientists wrote. By studying in detail what these three worlds were like at more than one age, we can effectively increase our accessible sample of terrestrial-type worlds to more than three.
Detailed knowledge of the many ways that planets can express themselves throughout their lives will help us to understand what we see in other star systems.
We can approach the interconnectedness of the local geology with astrophysics from a third direction. One of the most important reasons for returning to Earth’s Moon is to look for fossils.
Impacts on the early Earth should have splashed up bits of the earliest continents, some of which would have struck the Moon – and some of which may contain evidence of the very beginnings of the formation of life. On Earth this vital evidence was destroyed by later geologic activity.
The Moon may also contain samples from further afield. Other star systems are likely to be sculpted by impacts, just like our own. Those with giant planets will sling some of the debris from the larger impacts into interstellar space. A tiny fraction of that material may have passed near our own sun and a tiny bit of that may have struck the Moon.
After its own geologic history ended, the Moon remained a static trap, presumably collecting infalling material for almost four billion years. That is a staggeringly long time, about a third of the estimated age of our galaxy. Thus, our own backyard could contain physical evidence of changing conditions in the galaxy throughout much of its life.
Any samples from other star systems that have found their way to the Moon will be small, extremely rare and probably deeply buried.
Finding them will be exquisitely hard – but undoubtedly easier than interstellar flight.
I have suggested three approaches – regolith studies, ancient planetary geology and direct sampling from the Moon – for using local exploration to measure or understand what we see when we observe smaller extrasolar worlds with the space telescopes now under design.
These three approaches have one thing in common: they require detailed Lewis and Clark-class exploration of large areas on the accessible worlds of the solar system. Geologists on site equipped to do deep drilling at hundreds or thousands of locations and with sophisticated labs generating real-time results to guide further rapid exploration are required to approach the scientific efficiency needed to obtain usable results.
Yes. High risk of failure?Probably. But a successful find or two would be of immeasurable scientific value. For the first time, astrophysics – and, if the fossil search succeeds, the origins of life – could have samples subject to direct observation and manipulation.
Both fields would instantly become true experimental sciences.
Comparing the results of local geological surveys with remote observation of distant star systems could lead to the beginnings of a unified theory of the origins and evolution of planets and life on a galactic scale.
Sending geologists to the small planets and minor bodies of the inner solar system and making the origins of life and astrophysics into experimental sciences is likely to have a more profound impact on our understanding of the universe than any other possible scientific endeavor, bar none.
Donald F. Robertson is a freelance space industry journalist based in