Sky & Telescope - April 2025 - 11

INNER PLANETS
Icy Rivers May Have Flowed
on Ancient Mars
MARS IS A FROZEN desert world,
enshrouded in an atmosphere too
rarefied for liquid water to flow on
the planet's surface. Yet the planet's
landscape is replete with ancient river
networks and other signs of water-based
erosion. Peter Buhler (Planetary Science
Institute) explores the connection
between these valley networks and the
polar ice caps in November's Journal of
Geophysical Research: Planets.
Around 3.6 billion years ago, as its
atmosphere thinned, Mars could no
longer efficiently transport heat from
the warmer equatorial regions up to its
poles. Carbon dioxide gas over the poles
froze and fell to the ground, building
up caps of CO2 ice on top of water
ice. Buhler proposes that this carbondioxide
ice acted like a heavy thermal
blanket, trapping the planet's internal
heat and melting the base of the waterIN
BRIEF
Parker's Closest Approach
to the Sun
NASA's Parker Solar Probe flew by the Sun in
the first of three close (and closest) approaches
that will complete the primary mission. The
22nd perihelion on December 24, 2024, took
the spacecraft 6.2 million km (3.9 million miles)
from the solar surface - about nine times the
Sun's radius, breaking previous record approaches.
Since the eighth close pass in April
2021, the mission has truly been " touching the
Sun, " as it flies through the part of the solar
atmosphere dominated by our star's crackling
magnetic field. NASA's STEREO A and
the European Space Agency's Solar Orbiter,
observing from near Earth's vantage point,
provided a wider context for the Parker data.
The mission's perihelia are now occurring
during the peak of activity in Solar Cycle 25,
so the observations provide a comparison for
data collected during the Sun's quieter years.
Project scientist Nour Rawafi (John Hopkins
University Applied Physics Laboratory) says
the team is planning a proposal to extend the
mission for years to come.
¢DAVID DICKINSON
Read more at https://is.gd/ParkerProbe.
ice sheet. The meltwater snuck out in
ice-encased rivers that oozed like solidcrusted
lava flows, Buhler explains.
From the south pole, these rivers
could have wound their way equatorward,
spilling into Argyre Planitia, an
impact basin roughly the size of the
Mediterranean Sea. Excess water overflowed
that ice-covered lake and then
meandered northward again, perhaps
draining into equatorial plains thousands
of kilometers from the pole. This
scenario would explain several geologic
features that scientists have observed in
these parts of Mars, Buhler says.
Anna Grau Galofre (Laboratory of
Planetology and Geosciences, France)
says that, at least as far as the larger
valleys in the south circumpolar region
are concerned, Buhler's scenario is provocative
but plausible. But she's skeptical
of larger-scale implications for valley
networks closer to the equator.
" The strength of this idea is that we
know the phenomenon of atmospheric
collapse occurs, " says planetary geoloProba
3 Launches to Study
Solar Corona
The European Space Agency's Proba 3
rocketed to space on December 5th. Built to
provide views of the solar corona, this is the
third spacecraft of a series designed to test
new technologies while also carrying scientific
instruments. The spacecraft will separate
into two parts, dubbed the Occulter and the
Coronagraph. A 1.4-meter-wide (4.6-foot) disk
on the Occulter will cast a shadow onto the
Coronagraph's instrument package. For this to
work, the two spacecraft must fly 150 meters
(500 feet) apart while maintaining their separation
to within a few millimeters. In this formation,
Proba 3 will image the corona as close
as 1.1 solar radii from the Sun, providing the
closest view we have right now. (The LASCO
C2 camera aboard the Solar and Heliospheric
Observatory can only image as close as 1.5
solar radii.) Proba 3's view won't be constant,
though. The paired craft have an elongated,
19.7-hour orbit and will only conduct observations
during the 6-hour window when they're
farthest from Earth. Proba 3's instruments will
provide insights into the solar corona, solar
wind, and the Sun's total power output.
¢DAVID DICKINSON
Read more at https://is.gd/Proba3.
pAn artist's illustration of an ice-covered river
sourced from meltwater beneath Mars's south
polar cap.
gist James Head (Brown University).
" But can it have such a major effect?
Fortunately, the hypothesis is testable. "
Both Head and Buhler say these tests
include more detailed analyses of geologic
features and computer modeling of
global climate processes - efforts that
could enable scientists to link specific
features to model predictions with more
confidence.
¢CAMILLE M. CARLISLE
To read more, visit https://is.gd/
MarsIceRivers.
Lucy's Second Earth Flyby
NASA's Lucy spacecraft passed by Earth on
December 12-13 during a gravity-assist maneuver
that will help to propel it to the outer
solar system. As part of its mission to explore
11 asteroids, Lucy is primarily intended to
explore the two camps of objects that circle
the Sun in dynamically stable locations on
either side of Jupiter: the Greek camp located
at Jupiter's L4 Lagrangian point (ahead of the
planet on its orbit) and the opposing Trojan
camp at the L5 point behind Jupiter. Getting
there takes some complicated orbital gymnastics,
including three Earth flybys. This one
was the second of those; the first occurred
in 2022. December's flyby increased the
spacecraft's velocity relative to the Sun by 7.3
km/s (16,000 mph). At its closest approach,
the spacecraft was only 360 km (220 miles)
above our planet's surface, whizzing by at
14.8 km/s relative to Earth. This flyby followed
Lucy's visit to 152830 Dinkinesh and its satellite
Selam in November 2023 (S&T: Mar. 2024,
p. 11). Now the spacecraft is headed to the
main-belt asteroid 52246 Donaldjohanson,
named for the discoverer of the " Lucy " hominid
fossil, and will pass 922 km (573 miles)
from it on April 20, 2025.
¢DAVID DICKINSON
Read more at https://is.gd/LucyFlyby2.
sk yand tele scope .o r g * APRIL 2025 11
PETER BUHLER / PSI
https://www.is.gd/marsicerivers https://www.is.gd/marsicerivers https://www.is.gd/ParkerProbe https://www.is.gd/Proba3 https://www.is.gd/LucyFlyby2 http://skyandtelescope.org
Sky & Telescope - April 2025

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