July 2019, ScientificAmerican.com 79evidence has inspired new ideas, such as the “synestia”
model that planetary scientists Simon J. Lock and Sar-
ah T. Stewart describe in “Origin Story,” on page 68.THE STORY OF THE SOLAR SYSTEM
studigs of lunar samplgs have also informed us about
other planetary bodies. Perhaps the most significant
result is the Nice model (so named because it was cre-
ated in Nice, France) of the evolution of the solar sys-
tem. According to this model, the giant planets of the
outer solar system initially formed close together.
After several hundred million years, their orbits
became unstable, causing Saturn, Uranus and Nep-
tune to rapidly migrate to their present-day orbits,
which are much farther away from the sun. The move-
ment of the giant planets pulled material from the
outer solar system—the Kuiper belt—inward, where it
collided with planets and moons
and caused general chaos through-
out the solar system.
This model may sound far-
fetched, but it elegantly explains a
number of seemingly unrelated ob-
servations about our cosmic neigh-
borhood. For instance, by dating
Apollo samples and analyzing im-
pact craters, scientists concluded
that there was a cataclysmic spike
in impacts on the moon about 700
million years after the planets formed, referred to as
the “late heavy bombardment.” Initially there was no
easy explanation for why the number of impacts would
have suddenly jumped at this time. Yet the chaotic peri-
od of impacts predicted in the Nice model provides a
source of impactors during the exact era in question.
In addition to telling us about the evolution of the so-
lar system, the lunar samples have also allowed scien-
tists to investigate the chemical evolution of planetary
surfaces. “Space weathering” is a process that describes
the physical and chemical erosion on bodies with no at-
mosphere. Studies of Apollo soils scooped from the sur-
face showed that they contain agglutinates, welded glass
and mineral fragments created by the impact of micro-
scopic grains of dust. These agglutinates accumulate
over time and can make up 60 to 70 percent of mature
regolith samples. Tiny spheres of elemental iron called
nanophase iron are also produced by space weathering
and build up on the outer rims of certain soil grains,
causing surfaces to become darker over time. We now
know that solar radiation, large temperature fluctua-
tions and the constant bombardment by tiny microme-
teorites are some of the sources of space weathering.SAMPLES FOR THE FUTURE
this is an gxciting timg in lunar science: this year caches
of samples will be released that have remained un-
opened since they were collected almost 50 years ago on
the moon. When the rocks were collected, nasa inten-
tionally left a portion sealed to wait for technology to ad-vance beyond the capabilities of the Apollo era. In March
the Apollo Next Generation Sample Analysis (ANGSA)
program selected nine research teams to receive un-
opened, vacuum-sealed samples from Apollo 15, 16 and- The opportunity to study “new” lunar samples will
likely lead to more fundamental discoveries about the
formation and evolution of our natural satellite.
As much as we have learned from the Apollo sam-
ples and surface experiments and as much as we will
undoubtedly learn from the new caches, we desperate-
ly need more samples. For instance, we have no recog-
nized samples from the lunar far side, the polar regions
or the deep interior. Two samples I would particularly
like to have are material from the South Pole–Aitken
Basin, on the lunar far side, and ice from a polar crater.
The South Pole–Aitken Basin is the largest recognized
impact basin on the moon—and one of the largest in
the solar system—and its interior could contain materi-
al from the moon’s lower crust and even its mantle.
Studying the South Pole–Aitken Basin would also help
us understand how extremely large basins shape the
surfaces and interiors of planetary bodies. Returning a
sample of lunar polar ice would tell us about the age
and origin of lunar water—which, in turn, could clarify
where Earth’s water originated.
These wish-list specimens could come from human
exploration or robotic missions: there is no consensus
among planetary scientists that either is best. Many
experts argue, rightly, that robotic missions are cheaper,
safer and can last longer than human missions. On the
other hand, humans are more likely than robots to pick
out a wider variety of unusual specimens, as evidenced
by the diversity of the Apollo sample suite (rock, scooped
and sieved soils, boulder chips, drill cores), sample vol-
ume and sample geology (composition, rock type, age).
The Apollo missions represent a singular accomplish-
ment that fundamentally altered our view of the solar
system. While we celebrate the 50th anniversary of
humanity’s giant leap, no human has set foot on another
planetary body since Harrison “Jack” Schmitt and the
late Gene Cernan departed from the lunar surface, dur-
ing the Apollo 17 mission, on December 14, 1972. As a sci-
entist deeply inspired by those missions, I am actively
working toward creating my generation’s Apollo
mo ment: to see humans (people of color and of all gen-
ders) land on the surface of the moon, fueled by ingenu-
ity, perseverance and a drive to explore the unknown.This year caches of samples will
be released that have remained
unopened since they were
collected almost 50 years ago.
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