Sky & Telescope - December 2021 - 9

ESA / HRSC / DLR
MARS
Gale Crater: Ancient Lake
or Puddles?
WHEN NASA SENT the Curiosity rover
to Gale Crater in 2012, scientists
thought the site looked like a massive,
ancient lakebed. But now, an independent
analysis of both Curiosity data
and recent imagery taken from orbit
suggests that the rock layers previously
thought to be deposits laid down by
water might actually come from wind
and chemical weathering instead.
In the August 6th Science Advances,
Jiacheng Liu (University of Hong Kong)
and colleagues point out that even
though early observations indicated
that Gale Crater once held a large body
of water, few of the rocks Curiosity has
examined actually look like lake deposits.
At the very beginning of the journey,
scientists identified mudstone at the
bottom of Gale Crater, which likely
formed when mud settled out of calm
water. But since then, as the rover has
traveled up to the base of Mount Sharp,
it has seen few lake-formed rocks.
The team thinks that this discrepancy
is best explained if the crater only
ever held a smattering of shallow ponds.
In that case, the majority of the ground
FAST RADIO BURSTS
New Observations Challenge Radio Burst Model
MOST ASTRONOMERS AGREE that
the brief flashes known as fast radio
bursts (FRBs) probably come from
highly magnetized neutron stars (aka
magnetars; S&T: Sept. 2020, p. 10). But
astronomers can't yet explain why a
minority of these sources flare repeatedly,
even though most FRBs appear
to be one-off events. A new study of
a known repeating FRB published in
the August 26th Nature " adds a new
piece to the puzzle, " says Victoria Kaspi
(McGill University, Canada).
Astronomers have watched FRB
20180916B, located in a galaxy some
460 million light-years away, produce
multiple bursts over a days-long " window, "
which recurs about every 16 days.
That 16-day pattern was thought to be
the orbital period of the magnetar and
a companion star, explains first author
Inés Pastor-Marazuela (University of
Amsterdam). If the companion powered
a thick, radio-absorbing wind, then the
magnetar's bursts would only be visible
when it was on " our " side of the orbit.
However, simultaneous observations
of the source by the International
Low-Frequency Array (LOFAR) and the
14-dish Westerbork Synthesis Radio
Telescope (WSRT) in the Netherlands
challenge the binary-wind scenario.
Stellar winds should absorb more lowerfrequency
radio waves than higher-frequency
ones, so astronomers expected
that the bursts LOFAR observed (down
Large lake model
Small lakes model
pThese images of Gale Crater show two models, one in which it once hosted a large lake (left) and
one in which only smaller, shallow ponds ever existed on the crater floor (right). A star and oval mark
the rover's landing site and landing ellipse.
explored to date would be basalt sand
and silt. The layers of different kinds of
rock, which scientists thought were left
behind by an ancient lake, could instead
have been deposited by wind in a caustic
atmosphere.
Data from Curiosity indicate that
elements that stay put under chemical
weathering, such as aluminum, become
more prevalent at higher altitudes.
Meanwhile, other elements that are
more easily dispersed, such as iron,
become scarcer.
This pattern is similar to what we see
on ancient, exposed rock formations
on Earth, which formed under a very
different atmosphere than exists today.
The researchers argue that a low-oxygen
greenhouse atmosphere containing
methane and hydrogen gas chemically
weathered the rocks in Gale Crater.
" This new paper says you don't really
need much water at all, " says Bradley
Thomson (University of Tennessee,
Knoxville), who was not involved in the
study. " It points out a possible ambiguity
in the data that only more data - a
lot more samples or even sample return
- could resolve. "
Perseverance Rover will collect
multiple samples for eventual return to
Earth, and it's possible that those rocks
could provide more definitive clues
to Mars's water history. Meanwhile,
scientists will continue to analyze data
coming in from Curiosity's slow trek up
Mount Sharp.
¢ARWEN RIMMER
to 110 megahertz) would be visible over
a narrower time window than the ones
WSRT witnessed (around 1.4 gigahertz).
" We found the exact opposite, " says
team member Joeri van Leeuwen (University
of Amsterdam). Moreover, the
peak in the number of high-frequency
bursts preceded the low-frequency peak
by a few days, which the binary model
also doesn't predict.
" I agree that the observations are
challenging for the model, " says Kaspi,
who has studied the LOFAR data for
this object but was not involved in the
current study. However, she's not ready
to rule out the binary idea. " We need
more sources and better statistics. "
Even if this particular source is
special, it could shed more light on the
properties of FRBs in general.
¢GOVERT SCHILLING
sk yand tele scope .o r g * DECEMBER 2021
9
http://Skyandtelescope.org

Sky & Telescope - December 2021

Table of Contents for the Digital Edition of Sky & Telescope - December 2021

Contents
Sky & Telescope - December 2021 - Cover1
Sky & Telescope - December 2021 - Cover2
Sky & Telescope - December 2021 - 1
Sky & Telescope - December 2021 - Contents
Sky & Telescope - December 2021 - 3
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