Sky & Telescope - June 2020 - 11

t Arrokoth's deep red color and its unmottled
and craterless surface support the notion of a
gentle formation and a peaceful existence.

SOLAR SYSTEM

Arrokoth Details Reveal
How Planets Form
FOURTEEN MONTHS AFTER New

ASA / JHU A PL / SWRI

Horizons flew by Arrokoth (originally
designated 2014 MU69), mission scientists have concluded that the peanutshaped object came together more gently than previously thought. The team
presented the findings at the annual
meeting of the American Association
for the Advancement of Science and
published three papers in the February 28th Science.
New Horizons data show that Arrokoth has remained largely untouched
since its formation 4.5 billion years
ago. The spacecraft's cameras reveal
a uniform, deep-red surface that's
relatively unmarred by craters. Spectra of reflected infrared light show the
chemical fingerprints of methanol ice,
a molecule commonly found in protoplanetary disks.
Most surprising, though, are multiple lines of evidence that Arrokoth's two
lobes came together exceedingly gently.
"There is no evidence that the merger
of these two lobes was at all violent,"
says William McKinnon (Washington
University in St. Louis).

from the first two lunar days (59 Earth
days) in the February 26th Science
Advances. The data show fine materials
down to a depth of 12 meters (39 ft),
with occasional larger rocks mixed in.
Beneath that, down to 24 meters, the
number of boulders increase. Further
still, to a limiting depth of 40 meters,
the regolith changes again to alternating layers of fine and coarse materials. Impact ejecta helped create these
multiple layers, while other geological
processes also played a role.
The Yutu rover of the 2013 Chang'e 3
mission (see page 34) also carried
ground-penetrating radar, but it could
only see down to a depth of about
10 meters. The more transparent farside
suggests that its geological history differs from the Chang'e 3 landing site.

McKinnon and colleagues ran a
series of computer simulations to test
this peaceful scenario. To reproduce
Arrokoth's appearance and characteristics, they found that the two bodies must have initially been orbiting
each other and slowly merged, joining
together at less than 4 m/s (9 mph) and
likely at a slanting angle.
Arrokoth also sheds light on how
planetesimals formed. In the classic hierarchical model of planet formation, dust
gloms together into pebbles and planetesimals collide to make planets. But
scientists have had difficulty explaining why pebble-size objects would fuse
together rather than ricocheting off
each other.

The current study only considers the
first two lunar days of the Chang'e 4
mission, and also only deals with radar
experiments conducted at 500 MHz.
The team still needs to analyze data
taken at the same frequency from further along the rover's path. Additional
data taken at 60 MHz, which ought to
penetrate deeper, remain difficult to
analyze because of interference from
the rover's metallic body.
With its recent missions, China
has now established its expertise in
ground-penetrating radar, according to Clive Neal (University of Notre
Dame). Understanding the subsurface
will inform not just the science of the
Moon's structure and formation, but
also its resource potential.

More recently, scientists have proposed that the streaming instability, in
which gas in the protoplanetary disk
would drag on pebble-size particles,
could help bridge that gap. Like a line of
cyclists riding against the wind, some
pebbles plowing into the gas would drop
behind, where they'd feel less drag. The
resulting pebble "streams" would be
gravitationally unstable, quickly collapsing into multiple massive objects
- typically binaries or contact binaries,
like Arrokoth.
Arrokoth's pristine nature represents an important piece of evidence
supporting the streaming instability's
role in planet formation, says New
Horizons principal investigator Alan
Stern (Southwest Research Institute): "I
believe this is a game-changer."
■ JAVIER BARBUZANO

IN BRIEF
Bennu Features Named
The International Astronomical Union
(IAU) has bestowed official names on
12 features on the asteroid 101955
Bennu. Team members on NASA's
OSIRIS-REX mission (S&T: May 2020,
p. 14), who have been mapping Bennu
in detail over the past year, proposed
the names to the IAU, the international
authority for naming celestial bodies and their surface features. As the
asteroid's name was itself inspired by
the ancient Egyptian deity Atum, who
is often depicted as the Bennu bird,
the features' names are likewise related
to mythological birds and bird-like
creatures. The large boulder jutting out
21.7 meters (71 feet) from the asteroid's southern hemisphere is the most
prominent feature to receive an official
designation: Benben Saxum. The name
comes from the primordial hill that
first rose out of the dark waters in an
ancient Egyptian creation myth. Atum,
in the form of the Bennu bird, flew over
the primordial waters before settling
upon Benben to create the world.
■ MONICA YOUNG

■ ANDREW JONES
sk yandtelescope.org * JUNE 2 02 0

11


http://www.skyandtelescope.org

Sky & Telescope - June 2020

Table of Contents for the Digital Edition of Sky & Telescope - June 2020

Contents
Sky & Telescope - June 2020 - Cover1
Sky & Telescope - June 2020 - Cover2
Sky & Telescope - June 2020 - 1
Sky & Telescope - June 2020 - Contents
Sky & Telescope - June 2020 - 3
Sky & Telescope - June 2020 - 4
Sky & Telescope - June 2020 - 5
Sky & Telescope - June 2020 - 6
Sky & Telescope - June 2020 - 7
Sky & Telescope - June 2020 - 8
Sky & Telescope - June 2020 - 9
Sky & Telescope - June 2020 - 10
Sky & Telescope - June 2020 - 11
Sky & Telescope - June 2020 - 12
Sky & Telescope - June 2020 - 13
Sky & Telescope - June 2020 - 14
Sky & Telescope - June 2020 - 15
Sky & Telescope - June 2020 - 16
Sky & Telescope - June 2020 - 17
Sky & Telescope - June 2020 - 18
Sky & Telescope - June 2020 - 19
Sky & Telescope - June 2020 - 20
Sky & Telescope - June 2020 - 21
Sky & Telescope - June 2020 - 22
Sky & Telescope - June 2020 - 23
Sky & Telescope - June 2020 - 24
Sky & Telescope - June 2020 - 25
Sky & Telescope - June 2020 - 26
Sky & Telescope - June 2020 - 27
Sky & Telescope - June 2020 - 28
Sky & Telescope - June 2020 - 29
Sky & Telescope - June 2020 - 30
Sky & Telescope - June 2020 - 31
Sky & Telescope - June 2020 - 32
Sky & Telescope - June 2020 - 33
Sky & Telescope - June 2020 - 34
Sky & Telescope - June 2020 - 35
Sky & Telescope - June 2020 - 36
Sky & Telescope - June 2020 - 37
Sky & Telescope - June 2020 - 38
Sky & Telescope - June 2020 - 39
Sky & Telescope - June 2020 - 40
Sky & Telescope - June 2020 - 41
Sky & Telescope - June 2020 - 42
Sky & Telescope - June 2020 - 43
Sky & Telescope - June 2020 - 44
Sky & Telescope - June 2020 - 45
Sky & Telescope - June 2020 - 46
Sky & Telescope - June 2020 - 47
Sky & Telescope - June 2020 - 48
Sky & Telescope - June 2020 - 49
Sky & Telescope - June 2020 - 50
Sky & Telescope - June 2020 - 51
Sky & Telescope - June 2020 - 52
Sky & Telescope - June 2020 - 53
Sky & Telescope - June 2020 - 54
Sky & Telescope - June 2020 - 55
Sky & Telescope - June 2020 - 56
Sky & Telescope - June 2020 - 57
Sky & Telescope - June 2020 - 58
Sky & Telescope - June 2020 - 59
Sky & Telescope - June 2020 - 60
Sky & Telescope - June 2020 - 61
Sky & Telescope - June 2020 - 62
Sky & Telescope - June 2020 - 63
Sky & Telescope - June 2020 - 64
Sky & Telescope - June 2020 - 65
Sky & Telescope - June 2020 - 66
Sky & Telescope - June 2020 - 67
Sky & Telescope - June 2020 - 68
Sky & Telescope - June 2020 - 69
Sky & Telescope - June 2020 - 70
Sky & Telescope - June 2020 - 71
Sky & Telescope - June 2020 - 72
Sky & Telescope - June 2020 - 73
Sky & Telescope - June 2020 - 74
Sky & Telescope - June 2020 - 75
Sky & Telescope - June 2020 - 76
Sky & Telescope - June 2020 - 77
Sky & Telescope - June 2020 - 78
Sky & Telescope - June 2020 - 79
Sky & Telescope - June 2020 - 80
Sky & Telescope - June 2020 - 81
Sky & Telescope - June 2020 - 82
Sky & Telescope - June 2020 - 83
Sky & Telescope - June 2020 - 84
Sky & Telescope - June 2020 - Cover3
Sky & Telescope - June 2020 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/newtrack/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com