Sky & Telescope - April 2021 - 36
weak to cause fractures. Instead, observations show that
internally it resembles a rubble pile, a collection of jumbled
materials of varied sizes. Such interiors deform easily under
pressure, stressing the mildly cohesive regolith layer and
eventually breaking it.
Deimos, on the other hand, looks like a quieter place. It
has the overall lumpy and irregular shape of Phobos and is
equally covered in craters, but they appear more subdued,
suggesting that the regolith layer might be thicker than on
its neighbor. It lacks any massive impact feature or signs of
structural collapse, but it has two large craters named Swift
and Voltaire, two authors that anticipated the existence of
Martian moons in their literary work.
Not Asteroids
The origin of Phobos and Deimos is still unclear. More than
four decades after the Mariner and Viking probes returned
images and spectrographic observations of the moons, scientists are still considering two possibilities.
The spacecraft revealed that the moons are among the
darkest known solar system objects. They didn't show spectral
signatures that could give away any minerals on their surfaces. They also appeared to be porous, either hollow or filled
with water. These characteristics, along with their battered,
irregular shapes, led scientists to believe that they could be
captured asteroids.
" Those sorts of spectra are usually found in primitive
meteorites, the kind of things we think Bennu and Ryugu
are made of, " says Andrew Rivkin (Johns Hopkins University
Applied Physics Laboratory), who has thoroughly studied the
moons' spectra.
However, their looks could also correspond to basaltic
rocks that have been sitting out in space, bombarded by
micrometeorites over time. That would imply a radically different origin, likely a giant impact that blasted Martian material into space, Rivkin explains.
" For quite some time we thought that it was more likely
that they were like the carbonaceous-chondrite meteorites,
primitive bodies that would have been captured by Mars, "
Same Ingredients, Different Results
But scientists are puzzled. How could the same process that
made Earth's Moon also create two satellites so different
from our own?
In 2016, Rosenblatt, along with Sébastien Charnoz (Paris
Institute of Earth Physics, France) and other colleagues,
published the result of a series of computer simulations that,
for the first time, showed that Phobos and Deimos could
have formed from a giant impact. Rosenblatt got his inspiration after the European Space Agency's Mars Express orbiter
approached Phobos in March 2010, confirming that the small
moon is barely twice as dense as water - roughly two grams
Phobos
MARTIAN CLOSE-UP
9,380 km
Deimos orbits 15 times closer to
Mars than the Moon does to Earth.
Phobos is even closer: Its orbit is 40
times tighter than the Moon's. Phobos and Deimos's sizes are exaggerated here for visibility.
MARS
Deimos
23,460 km
EARTH
36
Rivkin says. However, " the people who study the orbits, the
dynamicists, have always said there's no way you can capture
these into an orbit like Phobos's. "
Captured asteroids should wind up in highly elliptical
orbits and randomly oriented around the planet's spin axis.
Instead, Phobos and Deimos describe neatly circular orbits,
right above the equator. " Unless [an asteroid] has engines
and fires them exactly in the right way, you are not gonna get
that, " says Matija C′ uk (SETI Institute), an expert in orbital
dynamics. " It just looks much less likely than kicking stuff
out in an impact. "
But the giant-impact theory wasn't seriously considered
for the Martian moons until very recently, even after being
widely accepted for our own Moon decades ago.
" If you look at the terrestrial planets, you have a big
moon only around Earth, you don't have any moons around
Mercury, neither around Venus, and you have two very small
moons around Mars, " says Pascal Rosenblatt (University of
Nantes, France). " No one was convinced of the idea that
[giant impacts] could be a universal process to form moons
around terrestrial planets. "
While still on the table, the orbital evidence paints a bleak
picture for the capture scenario. " It's a best-of-seven series,
and I think the impact theory is up three games to one, "
Rivkin says. " It's not quite done, but it's looking like it's going
to be the impact theory to me. "
APRIL 2 021 * SK Y & TELESCOPE
PHOBOS: ESA / DLR / G. NEUK U M (FU BERLIN); DEIM OS: N ASA / JPL- CA LTECH / UNIV. OF
A RIZON A; E A RTH: RE TO STÖCK LI / N ASA E A RTH OBSERVATORY; M A RS: ASU / N ASA; M OON:
N ASA / GSFC / A RIZON A STATE UNIV.; INFOG R A PHIC: TERRI DUBÉ / S&T
Oddball Satellites
Sky & Telescope - April 2021
Table of Contents for the Digital Edition of Sky & Telescope - April 2021
Contents
Sky & Telescope - April 2021 - Cover1
Sky & Telescope - April 2021 - Cover2
Sky & Telescope - April 2021 - 1
Sky & Telescope - April 2021 - Contents
Sky & Telescope - April 2021 - 3
Sky & Telescope - April 2021 - 4
Sky & Telescope - April 2021 - 5
Sky & Telescope - April 2021 - 6
Sky & Telescope - April 2021 - 7
Sky & Telescope - April 2021 - 8
Sky & Telescope - April 2021 - 9
Sky & Telescope - April 2021 - 10
Sky & Telescope - April 2021 - 11
Sky & Telescope - April 2021 - 12
Sky & Telescope - April 2021 - 13
Sky & Telescope - April 2021 - 14
Sky & Telescope - April 2021 - 15
Sky & Telescope - April 2021 - 16
Sky & Telescope - April 2021 - 17
Sky & Telescope - April 2021 - 18
Sky & Telescope - April 2021 - 19
Sky & Telescope - April 2021 - 20
Sky & Telescope - April 2021 - 21
Sky & Telescope - April 2021 - 22
Sky & Telescope - April 2021 - 23
Sky & Telescope - April 2021 - 24
Sky & Telescope - April 2021 - 25
Sky & Telescope - April 2021 - 26
Sky & Telescope - April 2021 - 27
Sky & Telescope - April 2021 - 28
Sky & Telescope - April 2021 - 29
Sky & Telescope - April 2021 - 30
Sky & Telescope - April 2021 - 31
Sky & Telescope - April 2021 - 32
Sky & Telescope - April 2021 - 33
Sky & Telescope - April 2021 - 34
Sky & Telescope - April 2021 - 35
Sky & Telescope - April 2021 - 36
Sky & Telescope - April 2021 - 37
Sky & Telescope - April 2021 - 38
Sky & Telescope - April 2021 - 39
Sky & Telescope - April 2021 - 40
Sky & Telescope - April 2021 - 41
Sky & Telescope - April 2021 - 42
Sky & Telescope - April 2021 - 43
Sky & Telescope - April 2021 - 44
Sky & Telescope - April 2021 - 45
Sky & Telescope - April 2021 - 46
Sky & Telescope - April 2021 - 47
Sky & Telescope - April 2021 - 48
Sky & Telescope - April 2021 - 49
Sky & Telescope - April 2021 - 50
Sky & Telescope - April 2021 - 51
Sky & Telescope - April 2021 - 52
Sky & Telescope - April 2021 - 53
Sky & Telescope - April 2021 - 54
Sky & Telescope - April 2021 - 55
Sky & Telescope - April 2021 - 56
Sky & Telescope - April 2021 - 57
Sky & Telescope - April 2021 - 58
Sky & Telescope - April 2021 - 59
Sky & Telescope - April 2021 - 60
Sky & Telescope - April 2021 - 61
Sky & Telescope - April 2021 - 62
Sky & Telescope - April 2021 - 63
Sky & Telescope - April 2021 - 64
Sky & Telescope - April 2021 - 65
Sky & Telescope - April 2021 - 66
Sky & Telescope - April 2021 - 67
Sky & Telescope - April 2021 - 68
Sky & Telescope - April 2021 - 69
Sky & Telescope - April 2021 - 70
Sky & Telescope - April 2021 - 71
Sky & Telescope - April 2021 - 72
Sky & Telescope - April 2021 - 73
Sky & Telescope - April 2021 - 74
Sky & Telescope - April 2021 - 75
Sky & Telescope - April 2021 - 76
Sky & Telescope - April 2021 - 77
Sky & Telescope - April 2021 - 78
Sky & Telescope - April 2021 - 79
Sky & Telescope - April 2021 - 80
Sky & Telescope - April 2021 - 81
Sky & Telescope - April 2021 - 82
Sky & Telescope - April 2021 - 83
Sky & Telescope - April 2021 - 84
Sky & Telescope - April 2021 - Cover3
Sky & Telescope - April 2021 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202412
https://www.nxtbook.com/nxtbooks/aas/st_202411
https://www.nxtbook.com/nxtbooks/aas/st_202410
https://www.nxtbook.com/nxtbooks/aas/st_202409
https://www.nxtbook.com/nxtbooks/aas/st_202408
https://www.nxtbook.com/nxtbooks/aas/st_202407
https://www.nxtbook.com/nxtbooks/aas/st_202406
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
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/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