Sky & Telescope - October 2019 - 12

NEWS NOTES

Huge Mass Found Under
Moon's Largest Basin
NEW RESEARCH SHOWS that a dense
mass lies buried under the South PoleAitken Basin, hidden from view on the
lunar farside. It might be the remains of
the impactor that formed the basin.
Even though the South Pole-Aitken
Basin spans roughly 2,400 km (1,500
miles), it isn't particularly obvious in
spacecraft photos. Topographically,
though, it's an unmistakable pit up
to 13 km deep. Geologists have long
suspected that the basin formed some 4
billion years ago in a slow, shallow, and
oblique collision with an object roughly
200 km across.
Now a team led by Peter B. James
(Baylor University) has identiﬁed a
huge mass buried underneath the
basin, based on analysis of topographic
maps from NASA's Lunar Reconnaissance Orbiter and gravity data from

NASA's Gravity Recovery and Interior
Laboratory (GRAIL) mission. The team
suggests that the primordial impactor's
iron-rich core could have dispersed
itself into the Moon's upper mantle.
Alternatively, the dense mass could
come from oxides left behind when the
Moon's magma ocean solidiﬁed. The
results appear in the May 28th Geophysical Research Letters.
Similar mass concentrations are
found under many other lunar basins,
but they usually have a bull's-eye
structure, with a deeply buried ring
of lower-density material sandwiched
between a denser core and outer ring.
But the massive blob buried at least
300 km under the South Pole-Aitken
Basin has a different gravitational
ﬁngerprint. James and colleagues conducted simulations to show that, after
striking the Moon with a glancing blow,
the impactor's metallic core could have
ended up as a lump beneath the basin
- concentrated enough to create the

EXOPLANETS

Youngest Conﬁrmed Planet Discovered
OBSERVATIONS OF THE YOUNGEST

conﬁrmed exoplanet, the young hot
Jupiter known as CI Tau b, suggest that
it formed via gravitational collapse.
Christopher Johns-Krull (Rice University) and Lisa Prato (Lowell Observatory) presented the results of a new
study led by Laura Flagg (also at Rice) at
a meeting of the American Astronomical Society. Their work appears in the
June 20th Astrophysical Journal Letters.
q An artist's concept shows several gas-giant
planets orbiting near a young star surrounded
by a protoplanetary disk.

OCTOBE R 2 019 * SK Y & TELESCOPE

Scientists have proposed two mechanisms to explain how large planets
form: core accretion, in which a rocky
core slowly attracts gas around it, and
gravitational instability, in which portions of the disk collapse directly and
rapidly into a planet. In the latter process, planets have higher temperatures
and therefore glow more brightly at
infrared wavelengths, so it's also known
as the hot start model.
Planet formation scenarios can
affect how astronomers interpret
measurement of the planets themselves.
For example, a young planet's infrared
emission comes from the heat left over
from its formation. Determining its
mass requires knowing how quickly the
planet came together.
The CI Tauri system, which is only
2 million years old, offers the opportunity to break this degeneracy. By
combining the host star's radial velocity
wobbles with a direct measurement of

p A dashed circle shows the location of the
huge mass buried under the South Pole-Aitken
Basin in this topographical map of the Moon.

gravity anomaly recorded by GRAIL, but
dispersed enough that it didn't sink en
masse toward the lunar core.
The impact that created the largest
and deepest basin on the Moon also
redistributed the lunar crust, created
magnetic anomalies all over the surface, and might even have affected the
Moon's rotation. Dynamicists will now
be investigating what role this massive, newly discovered lump might have
played in altering lunar history.
■ J. KELLY BEATTY

the planet's spectrum, the team was
able to peg the planet's mass at 11.6
Jupiters without making any assumptions about the planet's formation
scenario.
Between 2014 and 2018, the astronomers observed the system using the
high-resolution Immersion Grating
Infrared Spectrograph on telescopes at
the McDonald and Lowell Observatories. In addition to detecting the chemical ﬁngerprint of carbon monoxide
from the planet, the team also measured its infrared brightness. The latter
can be converted to a mass, depending
on how quickly the planet formed.
By comparing the model-independent mass with the infrared brightness,
the astronomers determined that the
planet formed in a hot-start scenario.
The next question is, did this hot
Jupiter migrate inward or form where
it is now? The star's young age suggests
the answer: "It's such a young system,
it's hard to believe that it migrated in
that quickly," says Prato.
■ STEVE MURRAY

SPA BASIN: N ASA / GODDA RD SPACE FLIG H T
CEN TER / UNIV. OF A RIZON A; PL A NE TS
FOR MING: N ASA / JPL- CA LTECH / T. PY LE

MOON

Sky & Telescope - October 2019

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