Sky & Telescope - April 2021 - 37

Moon

384,400 km

SIZE CO MPA RISON: N ASA / JPL- CA LTECH / M A LIN SPACE SCIENCE SYSTEMS /
TE X AS A& M UNIV; M OON: N ASA; ORBITS: G REGG DINDER M A N / S&T, SOURCE:
E. ASPH AUG / NATUR E GEOSCIENCE 2016

Out
w ar
d
torq
ue

Ro
ch

Sy
nc
hr
on

bit
or
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ou

Phobos

Ob

e

it
lim

Tidal bulges

Inw
ard
torq
ue

s

Mars

te

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W FROM THE GROUND This composite
shows the Martian and terrestrial moons
at the sizes they'd appear when seen
from the surfaces of their respective planets. Hold the page at arm's length for the
approximate effect.

Deimos

O

per cubic centimeter. This led Rosenblatt to think that Phobos
might have accreted in orbit from chunks of rock blasted to
space by a powerful impact. These irregular rocks would have
left empty voids between themselves as they stuck together,
explaining the moon's low density.
A giant-impact origin instantly solves the main problem
of the capture scenario: The material kicked into orbit by the
impact quickly forms a ring around the equator, where the
moons coalesce shortly after.
" This is a good trick, " Rosenblatt says. " But you also havee
to explain the current distance of the moons to Mars and
their masses. "
The orbits have changed over time due to tidal dissipation.
n.
As the moons raise tides on Mars and vice-versa, they transfer momentum between them. Phobos is below the synchronous orbit, where objects complete a loop around the planet
in the same amount of time that the planet takes to rotate.
The tide that Phobos raises in the Martian ground thus lags
behind the moon, slowing it down. As a result, Phobos loses
velocity and altitude. Deimos is above the synchronous orbit,
traveling slower than the planet's surface. In this case, the
tide gets ahead of the moon, dragging it forward. This boosts
the moon's orbit higher. This is why Deimos is slowly drifting
away from Mars - as our Moon does from Earth.
" We have a very stringent constraint just there: We have
to form a Phobos more massive than Deimos, and the Phobos
has to form below the synchronous orbit and the Deimos
above it, " Rosenblatt explains.
Not knowing how large the protoplanet that hit Mars
might have been, Rosenblatt and his colleagues looked at
Borealis Basin, the lowlands that cover most of the planet's
northern hemisphere. Borealis is thought to have formed
after a giant impactor 2,000 kilometers in diameter - onethird the size of Mars - hit the young Red Planet about 4.5
billion years ago. Such an impact could have blasted 100
quadrillion tons of rock into the Martian orbit, or 10,000
times the mass of Phobos and Deimos combined.
After such an impact, the simulations show, the orbiting
material assembles into a disk around the equator in little

ct

je c

so

ts o

r b it

f a st er t h a n pl a

r b it s

n

lower than pla

r
et

ot

a

a
rot
n et

te s

S SATELLITE ORBITS Deimos orbits just beyond Mars's synchronous
orbit, the distance at which a satellite's orbital period equals the planet's
rotation rate. Inside the synchronous orbit, tides will force massive satellites to spiral in toward Mars; outside it, they'll push the satellite away.
Within the Roche limit, Mars's tidal influence will tear the satellite apart.

more than a day. Most of the material stays below the Roche
limit, an invisible boundary where the tidal influence of the
planet prevents the formation of moons. Random collisions
between particles expand the disk's outer edge beyond the
Roche limit, where a large moon 1,000 times more massive
than Phobos forms.
As the disk continues to expand, it pushes this large moon
outwards, and its gravitational tug produces orbital resonances that " shepherd " the material above it into stable zones
where the debris can concentrate. In Rosenblatt´s simulations, Phobos and Deimos form in these zones.
All the moons, including some smaller ones spawned at
the Roche limit after the large moon moves out, continue
to migrate outwards until the disk runs out of material.
Without a driver for outward migration, the moons begin
to fall back into the planet, dragged by tides. At this point,
only Deimos has been pushed beyond the synchronous orbit,
starting its slow outward drift to its current location. All the
other moons either crash into Mars or disintegrate in orbit,
torn apart by tides.
Phobos could be the last straggler. Formed closer to the
synchronous orbit than its siblings, it has made it to our days,
although its death date is nigh: It has already crossed the
Roche limit, and its cracked surface forecasts its imminent
destruction.
sk yandtelescope.org * APRIL 2 021

37


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Sky & Telescope - April 2021

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