Sky & Telescope - May 2019 - 8

NEWS NOTES

Lunar Craters Reveal Recent Impact History
RESEARCHERS USING A NEW method

to estimate ages of lunar craters have
found that the rate of large impacts -
on both the Moon and Earth - nearly
tripled 290 million years ago.
In the January 18th Science, Sara
Mazrouei (University of Toronto) and
colleagues came up with a new way
of dating craters, based on the fading warmth of their impact debris.
Younger craters, they realized, will be
surrounded by larger rocks than older
craters are, because space weathering
grinds down debris with time. Since
larger rocks take more time to cool during lunar night, younger craters must
also appear warmer.
Using the Diviner thermal radiometer onboard NASA's Lunar Reconnaissance Orbiter, the team measured the

p A team of scientists used NASA's Lunar
Reconnaissance Orbiter to estimate the ages
of 111 of the Moon's craters. The circles are
scaled by size and color-coded by age (blue
indicates those younger than 290 million years).
Young craters dominate the lunar surface.

temperatures, and thus ages, of 111 craters. The researchers limited their study
to craters larger than 10 kilometers
across and less than a billion years old,
because smaller craters wouldn't have
excavated enough material from the
lunar bedrock, and older craters' debris
aprons would already have disintegrated
under the solar wind.
The crater ages revealed a surprising
trend: The rate of impacts increased 290
million years ago by a factor of 2.6. That
goes against the typical assumption that
the ﬂux of impactors has been constant

COSMOLOGY

Quasar "Standard Candles" Shed Light on Dark Energy
ASTRONOMERS HAVE FOUND a way to

use quasars to measure the evolution of
dark energy, the repulsive pressure that
accelerates our universe's expansion.
The results, appearing January 28th in
Nature Astronomy, have potentially farreaching implications for cosmology.
Cosmology is based on accurate
gauges of distance, and detonating
white dwarfs known as Type Ia supernovae have long been the standard candle
8

M AY 2 0 1 9 * S K Y & T E L E S C O P E

of choice. Their intrinsic luminosities
are known, so their distances are, too.
With these objects astronomers have
probed the universe at a time when dark
energy began to dominate its expansion.
To see even earlier times, before dark
energy took over, Guido Risaliti (University of Florence, Italy) and Elisabeta
Lusso (Durham University, UK) turned
to quasars, gas-guzzling supermassive
black holes that are brilliant enough to

■ JAVIER BARBUZANO

be seen when the universe was less than
a billion years old.
Risaliti and Lusso made use of a
well-studied relation, which shows that
quasars that are intrinsically brighter at
visible wavelengths emit relatively fewer
X-rays. The researchers argue that this
relationship is based on quasar physics - an accretion disk emits visible
light, while a hot, gaseous corona emits
X-rays - and therefore should remain
stable over cosmic time.
Previously, contaminants had muddied the relation, so for this study

N ASA / LRO / USGS / UNIV ERSIT Y OF TORON TO

THE MOON

for the last 3 billion years, says coauthor
Rebecca Ghent (also at University of
Toronto). The team plans to investigate
whether the change was transient or
has persisted until today.
The researchers also found evidence
in Earth's impact history that appears
to support the trend. Scientists already
knew that there is an overabundance
of terrestrial craters younger than 290
million years, but they thought it was
because erosion had wiped out the older
craters. The team's analysis, however,
suggests the uptick is real. The researchers focused on regions that have been
relatively stable over the last 650 million years by looking for kimberlite pipes.
These are extinct, carrot-shaped volcanoes that are buried several kilometers
below Earth's surface and are often
mined for diamonds. If a region contained preserved kimberlites, the team
reasoned, then the surrounding area
must be stable enough to have preserved
old, large craters as well.
The search uncovered intact kimberlites over 11% of Earth's surface. In
these regions, the team ﬁnds the same
impact rates as observed on the Moon.
Conﬁrmation on Earth may not even
be necessary, says Peter Schultz (Brown
University), who was not involved in the
study. "Whatever happened on Earth, to
me there are too many ways of dodging
the bullet scientiﬁcally," he notes. "But
there is no way to dodge that bullet on
the Moon, because that is the record -
it is our mirror."

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019