Mars
Polar
Science
White
Paper,
5
August
2019
Page
1
|
16
Mars
and
the
Science
Programme
The
case
for
Mars
Polar
Science
Nicolas
Thomas,
Patricio
Becerra
Space
Research
and
Planetology
Division
Physikalisches
Inst.
University
of
Bern
Switzerland
Email:
nicolas.thomas@space.unibe.ch
Email:
patricio.becerra@space.unibe.ch
Tel:
+41
31
631
4406
Isaac
Smith
Earth
and
Space
Science
and
Engineering
Lassonde
School
of
Engineering
York
University
Canada
Email:
ibsmith@yorku.ca
Tel:
+1
416
‐
736
‐
2100
x
77703
Mars
Polar
Science
White
Paper,
5
August
2019
Page
2
|
16
Abstract
Current
plans
within
ESA
for
the
future
investigation
of
Mars
(after
the
ExoMars
programme)
are
centred
around
participation
in
the
Mars
Sample
Return
(MSR)
programme
led
by
NASA.
This
programme
is
housed
within
the
Human
and
Robotic
Exploration
(HRE)
directorate
of
ESA.
This
paper
focuses
on
the
important
scientific
objectives
for
the
investigation
of
Mars
outside
the
present
HRE
planning.
The
achievement
of
these
objectives
by
Science
Directorate
missions
is
entirely
consistent
with
ESA’s
Science
Programme.
We
illustrate
this
with
a
theme
centred
around
study
of
the
Martian
polar
caps
and
the
investigation
of
recent
(Amazonian)
climate
change
produced
by
well
‐
established
oscillations
in
Mars’
orbital
parameters.
Deciphering
the
record
of
climate
contained
within
the
polar
caps
would
allow
us
to
learn
about
the
climatic
evolution
of
another
planet
over
the
past
few
to
hundreds
of
millions
of
years,
and
also
addresses
the
more
general
goal
of
investigating
volatile
‐
related
dynamic
processes
in
the
Solar
System.
1.
Background
Based
on
the
last
10
years,
ESA’s
science
directorate
can
expect
to
launch
4
‐
5
missions
to
Solar
System
targets
within
the
2033
‐
2050
timeframe
(not
including
Mission
of
Opportunity
contributions
to
missions
of
other
agencies
and
other
directorates
such
as
HRE).
Several
mission
concepts
are
regularly
discussed
as
possible
future
missions
including
an
Ice
Giant
orbiter,
sample
return
missions
to
a
cometary
nucleus,
the
Moon
or
asteroids,
a
Venus
atmospheric
probe,
contributions
to
NASA
missions
to
Titan
and
Enceladus,
and
asteroid/main
‐
belt
comet
landers.
In
this
document
we
wish
to
emphasize
that
the
Science
Programme
has
a
significant
role
to
play
in
the
investigation
of
Mars,
and
that
goals
distinctly
separate
from
those
of
the
HRE
directorate,
can
be
formulated.
We
show
this
by
presenting
a
case
for
a
Mars
Polar
Science
theme
within
Voyage
2050.
The
Science
Programme
of
ESA
(SCI)
was
instrumental
in
placing
Europe
into
the
global
Mars
community
through
Mars
Express.
This
mission
recovered
science
originally
initiated
by
European
investigators
as
part
of
the
Russian
Mars
’96
mission,
and
resulted
in
a
strong
European
presence
in
the
international
community
that
has
continued
until
today.
This
has
led
to
active
instrument
provision
and
participation
in
missions,
the
most
recent
example
being
NASA’s
InSight.
The
European
experiments
on
InSight
had
their
genesis
in
Science
Programme
studies
of
InterMarsnet
(including
studies
before
this
of
NetLander;
Lognonne
et
al.,
2019),
which
ultimately
lost
out
to
Planck
in
the
ESA
SCI
selection
process.
European
participations
in
NASA
missions
such
as
Mars
Pathfinder,
Mars
Polar
Lander,
Phoenix,
and
Curiosity
further
indicate
the
scientific
enthusiasm
for
studying
Mars
through
landed
static
and
mobile
missions.
The
initiation
of
the
ExoMars
programme,
through
studies
carried
out
in
the
late
1990s,
began
a
change
in
direction
within
ESA.
ExoMars
is
part
of
the
optional
programme
and
is
housed
within
the
Human
and
Robotic
Exploration
directorate
(HRE).
This
mission,
which
is
still
intended
to
launch
in
2020,
will
attempt
to
drill
into
the
sub
‐
surface
of
Mars
to
test
for
extinct
and/or
extant
life.
It
is
the
most
comprehensive
astrobiology
mission
to
be
launched
by
any
agency
so
far.
As
part
of
the
programme,
HRE
also
launched
the
Trace
Gas
Orbiter
(TGO).
This
mission
was
primarily
foreseen
as
a
communications
orbiter
for
landed
assets
(including
the
ExoMars
rover)
but
available
mass
was
subsequently
used
to
carry
scientific
payload.
This
included
the
Colour
and
Stereo
Surface
Imaging
System
(CaSSIS)
instrument
led
by
Switzerland,
and
the
NOMAD
spectrometer
led
by
Belgium.
The
Science
Programme
contributes
to
TGO
by
providing
small
but
significant
levels
of
funding
to
support
science
operations.
Together
with
the
continued
operation
of
Mars
Express,
this
is
currently
the
only
Mars
Polar
Science
White
Paper,
5
August
2019
Page
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16
contribution
of
SCI
to
the
scientific
exploration
of
Mars
and,
at
the
time
of
writing,
there
are
no
plans
to
expand
this,
to
our
knowledge.
The
situation
in
Europe
is
thus
one
in
which
HRE
is
now
leading
an
optional
Mars
programme,
and
should
ExoMars
be
successful,
ESA
(through
HRE)
will
seek
to
participate
in
MSR
as
a
joint
international
mission
with
NASA
(and
possibly
other
agencies).
This
mission
could
be
a
precursor
to
the
manned
exploration
of
Mars,
which
would
be
consistent
with
the
perceived
aims
of
the
HRE
directorate.
However,
there
are
numerous
targets
and
investigative
goals
at
Mars
that
are
of
major
scientific
significance
but
that
are
not
easily
coupled
to
MSR,
and
are
unlikely
to
be
a
goal
of
the
HRE
programme.
In
the
following,
we
will
provide
an
example
of
such
targets
and
goals:
the
study
of
the
Martian
Polar
Caps.
Our
aim
here
is
to
voice
the
interest
of
the
community
in
furthering
studies
of
the
Martian
polar
regions,
to
propose
that
the
Science
Programme
remain
open
to
these
ideas
and
to
ensure
that
competitive,
Mars
‐
related,
science
mission
proposals
are
welcome.
In
addition,
Mars
‐
related
missions
may
re
‐
enter
NASA
New
Frontiers
planning
in
the
near
future
through
their
next
Decadal
Survey
(they
are
currently
excluded
from
the
New
Frontiers
‐
class
Announcements
of
Opportunity
following
statements
in
the
previous
Decadal
Survey),
and
an
ESA
SCI
participation
in
a
Mars
‐
related
New
Frontiers
mission
would
be
scientifically
valuable.
2.
Scientific
justification
for
a
Mars
Polar
Science
theme
within
ESA
SCI
In
a
broad
sense,
the
interaction
of
Mars’
polar
regions
(Figure
1)
with
the
Martian
planetary
climate
system
can
be
split
into
three
distinct
timescales.
The
seasonal
polar
caps
are
produced
by
the
annual
mass
transfer
of
CO
2
between
the
atmosphere
and
the
surface.
These
caps
exist
only
during
winter
and
comprise
a
1
‐
2
metre
thick
layer
of
mostly
CO
2
ice.
The
polar
residual
caps
that
remain
in
summer
months
are
composed
of
H
2
O
ice
deposits
in
the
north
(the
north
pole
residual
cap
‐
NPRC)
and
CO
2
ice
in
the
south
(SPRC),
and
they
interact
with
the
current
Martian
climate
on
timescales
of
decades
to
100s
of
years
(Byrne
et
al.
2009),
growing
to
at
most
a
few
metres
in
the
north
and
up
to
ten
meters
thick
in
the
south.
Finally,
the
Polar
Layered
Deposits
(PLD)
are
kilometre
‐
thick
stratified
sheets
of
nearly
pure
water
ice
(Grima
et
al.,
2009),
with
small
amounts
of
dust,
and
they
record
climate
oscillations,
in
an
analogous
way
to
terrestrial
polar
ice
sheets,
from
the
last
few
to
hundreds
of
millions
of
years
of
Martian
history.
The
polar
environment
therefore
comprises
geological
information
on
the
current
state
of
Martian
climate,
as
well
as
its
relatively
recent
history.
Deep
understanding
of
the
connection
between
the
polar
deposits
and
the
Martian
climate
is
necessary
to
understand
the
Martian
climate
system
as
a
whole,
and
is
only
possible
through
missions
that
are
dedicated
to
studying
this
record
in
detail.
a.
The
seasonal
deposits
Mars’
seasonal
polar
caps
buffer
the
CO
2
atmosphere
of
Mars
as
first
described
by
Leighton
and
Murray
(1966).
Surface
pressure
changes
of
around
25%
occur
over
the
annual
cycle
as
a
result
of
mass
transfer
between
the
polar
caps
and
the
atmosphere
with
CO
2
freezing
out
onto
the
polar
caps
in
winter
and
subliming
in
spring
and
summer.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
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16
Figure
1
MOLA
topographic
maps
of
the
polar
regions
of
Mars
with
a
few
important
geographic
locations
indicated.
In
the
south
‐
polar
region,
a
MOC
image
of
the
SPRC
overlays
the
topographic
map.
Parallels
are
drawn
every
5°.
This
leads
to
numerous
small
‐
scale
processes
that
are
active
and
dynamic
on
seasonal
timescales.
Figure
2
shows
an
example
of
the
types
of
activity
commonly
seen
in
the
southern
hemisphere
(Thomas
et
al.,
2010).
During
southern
winter,
CO
2
condenses
slowly
onto
the
southern
polar
cap
and
produces
a
roughly
1
m
thick
layer
of
translucent
CO
2
ice.
As
the
Sun
rises
in
southern
Martian
spring,
sunlight
penetrates
the
layer
and
illuminates
the
surface
below.
The
CO
2
layer
is
opaque
at
infrared
wavelengths,
inhibiting
heat
escape,
which
causes
sublimation
of
the
CO
2
layer
from
its
base,
resulting
in
gas
pressures
that
force
openings
and
cracks
in
the
CO
2
layer
and
release
CO
gas
in
a
geyser
‐
like
process.
Pressure
release
is
accompanied
by
dust
transport
leading
to
dark
deposits
on
the
surface
of
the
ice
(Figure
2
top
left).
This
mechanism
scours
the
surface
beneath
the
ice
layer
and
produces
so
‐
called
araneiform
(spider
‐
like)
surface
structures
(as
seen
in
to
the
top
right
of
the
high
resolution
images.
This
mechanism
(the
Kieffer
hypothesis)
was
first
described
by
Kieffer
et
al.
(2006),
and
modelling
work
has
placed
constraints
on
the
physics
of
the
gas
emission
(e.g.
Thomas
Figure
2
Evidence
of
gas
jet
activity
in
the
"Inca
City"
region
of
the
southern
polar
cap
(~81°S).
The
right
hand
panel
shows
the
full
image
of
the
area
(taken
by
NASA’s
High
Resolution
Imaging
Science
Experiment
(HiRISE).
A
white
box
identifies
the
area
shown
in
more
detail
in
the
left
two
panels.
These
two
images
were
taken
at
different
times.
Black
blotches
and
lineaments
appear
in
spring
as
a
result
of
the
Kieffer
mechanism
for
producing
CO
2
gas
geysers.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
5
|
16
et
al.,
2011).
However,
we
have
never
seen
these
processes
“in
action”
despite
considerable
efforts
to
do
so.
This
may
be
because
the
dust
in
the
jets
is
optically
too
thin,
and
what
we
see
is
the
result
of
continuous
outgassing
with
low
dust
content
over
periods
of
several
hours
or
even
days.
The
whole
process
may
be
analogous
to
that
seen
at
Triton
where
the
volatile
involved
is
probably
nitrogen.
On
Triton,
however,
we
have
seen
active
geysers
indicating
a
higher
non
‐
volatile
content
and
allowing
a
better
assessment
of
their
properties
(Soderblom
et
al.,
1990).
The
CO
2
burden
at
specific
sites
on
the
surface
leads
to
other
phenomena.
Pilorget
and
Forget
(2016)
have
described
how
gully
‐
formation
in
polar
regions
may
be
triggered
by
the
CO
2
sublimation
and
condensation
cycle.
On
the
margins
of
the
Planum
Boreum
dome
of
the
North
polar
cap,
massive
dust
‐
ice
avalanches
could
possibly
be
triggered
by
CO
2
ice
sublimation
(Russell
et
al.,
2008
and
Figure
3)
although
it
is
more
probable
that
the
avalanches
are
triggered
by
thermal
expansion
of
water
ice
when
heated
in
spring.
These
avalanche
events
are
extremely
numerous
and
observations
by
NASA’s
Mars
Reconnaissance
Orbiter
(MRO)’s
High
Resolution
Imaging
Science
Experiment
(HiRISE)
have
frequently
caught
more
than
one
avalanche
within
one
image
swath,
which
typically
take
<10
seconds
to
acquire.
In
Figure
3,
the
bright
surface
to
the
left
is
the
relatively
smooth,
flat
plateau
that
comprises
the
surface
of
the
NPRC.
The
reddish,
fractured
deposits
in
the
middle
of
the
image
are
the
H
2
O
ice
‐
rich
layers
of
the
north
polar
layered
deposits
(NPLD)
which
exhibit
varying
dust
content
with
depth.
At
this
location
(the
margin
of
the
NPLD),
the
icy
layers
exposed
along
this
scarp
can
receive
direct
sunlight
during
summer,
and
therefore
experience
intense
thermoelastic
stresses
leading
to
avalanches
that
initiate
at
various
heights
on
the
scarp
(Russell
et
al.,
2008).
Failure
and
collapse
of
parts
of
many
scarps
along
the
NPLD
margins
have
been
observed
over
annual
timescales
by
HiRISE.
Figure
3
shows
the
base
of
the
scarp
with
a
cloud
of
material
from
one
such
avalanche
captured
at
the
moment
of
imaging.
The
residue
at
the
base
of
the
scarp
is
material
that
has
only
recently
been
exposed.
Studying
this
debris
provides
access
to
the
sub
‐
surface
material
of
the
lowermost
exposed
layers
of
the
NPLD.
Studies
of
the
sublimation
of
the
avalanche
debris
are
also
being
carried
out
(Fanara
et
al.,
2018)
and
indicate
that
the
original
material
is
ice
‐
rich.
However,
it
is
often
difficult
to
tell
where
exactly
along
the
scarp
the
material
came
from,
so
a
detailed
composition
‐
based
stratigraphic
column
is
still
elusive.
Figure
3
HiRISE
image
ESP_016423_2640
showing
an
avalanche
from
a
scarp
that
cuts
into
the
margins
of
the
topographical
dome
of
Planum
Boreum
in
the
north
polar
region
of
Mars
(~83°
N).
Southern
and
northern
winters
are
not
identical
because
of
the
elliptical
orbit
of
Mars,
and
it
is
not
necessarily
the
case
that
processes
operating
in
one
hemisphere
will
be
dominant
in
the
other.
Southern
winters
are
longer
allowing
more
CO
2
to
condense
producing
thicker
ice
layers
(Hansen
et
Mars
Polar
Science
White
Paper,
5
August
2019
Page
6
|
16
al.,
2013).
At
high
northern
latitudes
dynamic
activity
associated
with
sublimation
of
the
seasonal
polar
cap
is
mostly
found
on
dunes
and
araneiform
feature
formation
is
almost
certainly
completely
absent,
whereas
activity
in
the
southern
polar
region
occurs
mostly
on
more
consolidated
substrates
and
spider
‐
formation
is
common.
The
differences
are
such
that
it
is
unclear
what
changes
to
the
simple
application
of
the
Kieffer
mechanism
are
necessary
to
explain
the
observed
phenomena.
The
mass
of
material
condensing
on
the
caps
each
Mars
year
has
been
estimated
by
gravity
field
measurements.
In
combination
with
altimetry
data
from
the
Mars
Global
Surveyor
Spacecraft
the
depth
of
the
precipitation
has
been
measured
to
be
up
to
2
metres
at
each
pole.
However,
the
density
is
suspected
to
be
significantly
greater
in
the
south
than
the
north
indicating
a
different
form
of
precipitation
in
the
south,
probably
direct
condensation
rather
than
snow
or
frost
(Smith
and
Zuber,
2018).
The
evolution
of
the
seasonal
caps
from
year
to
year
allows
knowledge
of
annual
variations
in
the
Martian
atmosphere.
In
addition,
Planet
Encircling
Dust
Events
(PEDE)
are
a
frequent
occurrence
on
Mars,
although
a
reproducible
pattern
to
their
occurrence
has
not
yet
been
established.
Nevertheless,
more
localised
storms
occur
at
predictable
seasons
within
the
Martian
year.
The
dust
elevated
during
storms,
and
from
much
smaller
local
processes,
is
transported
by
atmospheric
circulation,
and
some
of
it
sediments
onto
the
polar
caps.
Upon
sublimation
in
spring,
the
deposited
dust
is
left
behind
as
a
lag
deposit
that
can
be
further
modified
by
surface
winds.
The
influence
and
dynamics
of
dust
transport
on
Mars
and
its
effects
on
the
polar
regions
and
the
Martian
environment
as
a
whole
is
still
the
subject
of
active
research,
given
that
understanding
these
effects
and
their
variability
is
a
key
aspect
in
understanding
Mars’
climate
evolution.
The
principal
questions
that
arise
from
our
current
knowledge
of
seasonal
processes
are
What
is
the
nature
of
the
activity
associated
with
sublimation
of
the
seasonal
polar
caps?
Is
the
density
of
the
CO
2
deposited
on
the
polar
caps
different
between
the
northern
and
the
southern
hemisphere
and,
if
so,
why?
Do
our
current
models,
built
on
the
Kieffer
hypothesis,
correctly
explain
the
behaviour
of
the
surface
activity
in
southern
spring?
What
are
the
current
deposition
rates
of
H
2
O
and
dust
onto
the
polar
caps
as
a
result
of
seasonal
processes?
b.
The
residual
polar
caps
The
residual
caps,
as
their
name
suggests,
are
composed
of
young
ice
that
survives
the
summertime
sublimation
in
each
hemisphere.
The
north
polar
residual
cap
(NPRC),
is
composed
of
water
ice,
and
is
thought
to
be
accumulating
actively
today
(Landis
et
al.
2016).
Its
surface
contains
patterns
and
linear
textures
that
appear
to
be
the
result
of
differential
sublimation
(Figure
4,
left;
Russell
et
al.
2019).
Recent
modelling
studies
suggest
that
these
patterns
reflect
the
conditions
of
accumulation
of
the
ice
of
the
NPRC
(Wilcoski
et
al.
2019),
which
is
likely
to
be
the
top
layer
of
the
NPLD.
Thus,
understanding
the
NPRC
accumulation
provides
a
baseline
with
which
to
understand
the
process
of
accumulation
and
ablation
of
each
NPLD
layer.
However,
understanding
the
properties
of
the
ice
by
in
situ
investigation
may
be
crucial
to
achieving
this
task.
The
south
polar
residual
cap
(SPRC,
Figure
1)
is
composed
entirely
of
CO
2
ice,
and
is
offset
from
the
pole.
According
to
most
general
circulation
models
it
is
highly
unstable
(Byrne,
2009).
It
has
a
unique
geomorphology
that
provides
clues
as
to
how
it
is
even
able
to
survive
in
the
first
place.
Some
of
this
Mars
Polar
Science
White
Paper,
5
August
2019
Page
7
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16
unusual
geomorphology
is
known
as
“Swiss
cheese”
terrain
(Figure
4,
right).
This
name
describes
areas
of
the
SPRC
surface
covered
with
pits
that
are
typically
a
few
hundred
metres
across
and
8
‐
10
metres
deep.
They
have
flat
floors
and
steep
sides.
The
individual
pits
grow
in
size
through
sublimation
at
an
average
rate
of
1
to
3
metres
per
year,
suggesting
that
they
are
formed
in
a
thin
layer
of
CO
2
ice
lying
on
top
of
the
residual
polar
cap.
These
features
are
therefore
modern
and
transient,
reflecting
the
current
orbital
configuration
of
the
Martian
system
(Byrne
and
Ingersoll,
2003).
Underneath
the
SPRC,
older
deposits
of
CO
2
ice
were
discovered
by
MRO’s
Italian
‐
led
Shallow
Radar
experiment
(SHARAD).
These
reservoirs
contain
enough
CO
2
to
double
Mars’
current
atmospheric
pressure
(Phillips
et
al.,
2011,
Bierson
et
al.
2016).
These
units
are
interspaced
with
thinner
layers
of
water
ice,
and
models
of
their
combined
evolution
suggest
ages
of
a
few
hundred
thousand
years
and
an
intricate
interaction
with
the
Martian
atmosphere
over
that
timespan
(Buhler
et
al.
2019).
Figure
4
Left:
False
color
HiRISE
image
(PSP_001922_2680)
of
the
NPRC
showing
its
pitted
texture
and
a
rare
impact
crater.
Right:
An
individual
sublimation
pit
element
of
the
so
‐
called
'Swiss
cheese'
terrain
imaged
by
HiRISE
(PSP_003738_0930).
The
Sun
illuminates
the
scene
from
the
lower
left.
The
feature
is
about
100
m
in
diameter.
The
residual
caps
also
raise
important
questions
directly
related
to
the
Martian
climate
system,
namely
Why
is
there
a
difference
in
composition
between
the
two
residual
caps?
What
are
the
processes
involved
in
the
emplacement
and
removal
of
CO
2
ice
on
the
south
residual
cap?
What
is
the
climate
record
expressed
in
the
PRC’s,
and
can
we
access
it?
c.
The
polar
layered
deposits
The
polar
layered
deposits
(NPLD
in
the
north
and
SPLD
in
the
south)
of
Mars
record
signals
of
climate
over
millions
to
hundreds
of
millions
of
years
of
accumulation.
Deep
troughs
and
marginal
scarps
dissect
the
PLDs
in
a
quasi
‐
spiral
pattern
and
expose
layers
of
ice
and
dust
in
a
way
similar
to
how
the
sedimentary
record
of
the
Earth
is
revealed
in
valleys
and
canyons.
The
alternating
nature
of
the
PLD
structure
is
believed
to
be
caused
by
variations
in
rates
of
ice
and
dust
accumulation
and
the
product
of
oscillations
in
Mars’
orbital
parameters.
This
connection
between
orbital
dynamics
and
geology
is
similar
to
how
Milankovich
cycles
affect
climate,
and
therefore
ice
and
sediment
deposits,
on
Earth.
Observations
of
the
PLD
have
been
obtained
by
several
instruments
over
the
years.
An
example
of
a
marginal
outcrop
of
the
SPLD
taken
by
the
CaSSIS
imager
is
shown
in
Figure
5.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
8
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16
Variations
in
the
orbital
configuration
of
Mars
(obliquity
and
inclination)
dictate
the
amount
of
insolation
received
by
different
latitudes
of
its
surface,
and
therefore
directly
influence
planetary
climate
and
the
locations
where
ice
can
be
stable
on
the
surface.
The
calculated
evolution
Mars’
orbital
configuration
is
mathematically
stable
and
robust
back
to
20
Myr.
Before
this,
the
solutions
are
non
‐
unique
and
Mars’
orbital
motion
cannot
be
constrained
(Laskar
et
al.
2004).
However,
over
the
last
20
Myrs,
the
obliquity
of
Mars
has
varied
periodically
between
15
and
45
degrees
(Laskar
et
al.,
2004),
leading
to
vastly
different
climates
and
to
ice
being
periodically
re
‐
distributed
to
various
regions
of
the
planet.
In
general,
high
values
of
obliquity
mean
that
the
poles
receive
more
sunlight
on
average
than
the
mid
‐
latitudes,
which
leads
to
ablation
of
polar
ice.
Conversely,
low
obliquity
promotes
accumulation
of
polar
ice
(Smith
et
al.,
2018).
Direct
investigation
of
the
internal
structure
of
the
PLD
has
also
been
made
by
subsurface
sounding
radar,
which
showed
the
uniformity
of
PLD
layers
across
the
entirety
of
the
PLD
domes
(Figure
6).
The
data
returned
by
MARSIS
on
Mars
Express
and
SHARAD
on
MRO
(both
Italian
‐
led
experiments)
consist
of
two
‐
dimensional
“radargrams”
that
display
the
returned
power
and
the
time
delay
between
transmission
of
the
radar
signal
and
a
subsurface
return.
Ice
is
relatively
transparent
to
radar
wavelengths,
so
subsurface
returns
occur
when
there
is
a
change
in
permittivity
between
a
less
dusty
layer
to
a
more
dusty
one.
Examples
are
shown
in
Figure
6.
In
the
NPLD,
the
attenuation
of
the
radar
signal
combined
with
permittivity
models
established
a
maximum
bulk
dust
content
of
5%
(Grima
et
al.,
2009).
Similarly,
in
the
SPLD,
the
dust
content
was
found
to
be
around
10
‐
15%
(Seu
et
al.,
2007).
The
relationship
between
permittivity
and
dust
content
is
not
completely
understood
although
experimental
work
in
this
domain
is
becoming
a
European
strength
(Brouet
et
al.,
2019).
The
repeated
fly
‐
overs
resulting
from
the
polar
orbits
of
MEx
and
MRO
have
led
to
3
‐
D
maps
of
the
sub
‐
surface
that
help
establish
a
broad
‐
scale
stratigraphy.
Figure
5
Top:
CaSSIS
colour
image
of
a
bedding
exposure
in
the
SPLD.
Bottom:
CaSSIS
stereo
Digital
Terrain
Model
of
the
same
exposure.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
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16
The
observed
internal
structure
allows
us
to
trace
layers
such
as
those
seen
in
Figure
6
through
the
cap
to
locations
where
exposures
are
absent
(Milkovich
and
Plaut,
2008).
However,
this
has
not
been
fully
achieved
yet,
but
research
is
ongoing
(Christian
et
al.,
2013,
Becerra
et
al.,
2019).
If
this
correlation
is
achieved,
it
could
be
used
to
confirm
the
assumption
made
by
Fishbaugh
and
Hvidberg
(2006),
Fishbaugh
et
al.
(2010)
and
Becerra
et
al.
(2016)
that
the
external
appearance
and
topographic
relief
of
an
exposed
NPLD
layer
at
one
location
can
be
related
to
a
counterpart
exposure
at
a
different
location.
Hence,
a
stratigraphic
column
could
be
produced
to
obtain
a
consistent
view
of
the
deposition
with
depth.
Becerra
et
al.
(2016,
2017,
2019)
have
shown
that
the
photometric
and
topographic
expression
of
layers
can
be
isolated
using
imaging
and
digital
terrain
models
of
the
surface.
These
data
can
studied
with
time
‐
series
(e.g.
FFT
or
wavelet)
to
attempt
to
detect
signals
of
the
climate
variations.
Patterns
and
periodicities
in
the
exposed
layering
can
be
compared
to
the
period
of
the
change
in
obliquity
and
argument
of
perihelion
precession
of
Mars.
Becerra
et
al.
(2017)
found
a
correlation
between
stratigraphic
periodicities
in
the
NPLD
and
orbital
oscillation
frequencies.
The
likely
younger
age
of
the
NPLD
(onset
<5
Ma
due
to
a
decrease
in
average
obliquity
that
allowed
ice
to
deposit
at
the
north
pole),
also
allowed
them
to
make
estimates
of
the
ages
of
individual
layers,
concluding
that
the
top
500
m
of
stratigraphy
accumulated
in
the
last
1
Myrs
of
Martian
history.
The
SPLD
are
older
(perhaps
30–100
Myr;
Koutnik
et
al.,
2002),
which
makes
age
estimation
for
individual
SPLD
beds
impossible
given
the
non
‐
uniqueness
of
the
orbital
solutions
beyond
20
Myr
into
the
past.
Nevertheless,
Becerra
et
al.
(2019)
used
CaSSIS
and
HiRISE
stereo
data
to
study
the
SPLD
and
relate
the
periodicities
in
the
SPLD
stratigraphy
to
the
same
orbital
frequencies
that
forced
the
accumulation
of
the
NPLD.
These
frequencies
should
be
robust
on
much
larger
timescales
than
the
past
20
Myr,
even
though
the
specific
oscillation
solutions
are
not
(Laskar
et
al.,
2004).
It
was
inferred
that
the
water
ice
and
dust
of
the
SPLD
was
deposited
at
variable
rates
of
0.13–0.39
mm/yr,
taking
a
minimum
of
10–30
Myrs
to
accumulate.
The
deposition
and
ablation
of
material
from
the
PLD
is
driven
by
atmospheric
dynamics.
The
global
circulation
models
used
to
study
the
present
behaviour
of
Mars’s
atmosphere
readily
reproduce
the
CO
2
cycle
(e.g.
Haberle
et
al.,
2008),
as
well
as
the
water
cycle
including
cloud
physics
(e.g.
Haberle
et
al.,
2017).
However,
the
interannual
variability
of
the
dust
cycle
remains
difficult
to
predict
with
current
models.
This
shortcoming
is
important,
given
that
varying
dust
deposition
onto
the
caps
is
what
allows
us
to
read
the
signal
of
climate
variation,
whether
it
be
in
the
subsurface
radar
data
or
in
the
images
of
PLD
outcrops.
Nonetheless,
the
predictive
capability
of
Mars
Global
Circulation
Models
(GCMs)
is
improving
and
this
presents
the
possibility
for
studying
Mars’
atmosphere
throughout
the
Amazonian
period
by
using
Laskar
et
al.
(2004)’s
orbital
parameters.
Although
GCMs
are
not
fast
enough
to
study
dynamical
changes
in
climate,
steady
‐
state
solutions
with
past
obliquities
and
inclinations
are
feasible.
The
CO
2
cycle
is
particularly
affected
as
noted
above
and
seasonal
variations
become
more
extreme
as
the
obliquity
increases.
The
regions
on
the
surface
where
water
ice
is
stable
are
also
changed
and
current
models
of
dust
lifting
and
loading
would
predict
enhanced
dust
activity
as
the
obliquity
increases
(Kahre,
2017).
On
the
other
hand,
there
have
been
relatively
few
studies
directly
related
to
the
PLD
–
the
modelling
of
Newman
et
al.
(2005)
is
an
example
although
this
is
now
becoming
somewhat
outdated.
Measurements
of
current
atmospheric
properties
would
provide
additional
constraints
on
models
of
present
day
Mars
and
would
allow
improved
extrapolation
to
earlier
times.
We
note
here
that
GCMs
are
a
strength
of
ESA
and
specifically
the
French
and
British
communities.
Europe
is
the
world
leader
in
studying
atmospheric
dynamics
from
metre
to
global
scales.
These
models
rely
on
validation
and
refinement
from
observations,
which
can
be
performed
in
situ
or
in
orbit.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
10
|
16
The
foregoing
suggests
that
detailed
analyses
of
the
PLD
would
provide
a
unique
opportunity
to
increase
our
understanding
of
the
climate
history
on
Mars
and
test
our
climate
models
on
a
relatively
simple
terrestrial
planet
system.
A
Keck
Institute
for
Space
Studies
(KISS)
workshop
held
in
2017
entitled
“Unlocking
the
Climate
Record
Stored
within
Mars'
Polar
Layered
Deposits”
(see
Smith
et
al.,
2018)
resulted
in
a
set
of
four
questions
that
summarized
the
key
open
issues
to
understanding
and
interpreting
the
climate
record
within
the
PLD:
What
are
present
and
past
fluxes
of
volatiles,
dust,
and
other
materials
into
and
out
of
the
polar
regions?
How
do
orbital
forcing
and
exchange
with
other
reservoirs,
affect
those
fluxes?
What
chemical
and
physical
processes
form
and
modify
layers?
What
is
the
timespan,
completeness,
and
temporal
resolution
recorded
in
the
PLD?
These
issues
include
studies
of
the
atmosphere,
the
surface
‐
atmosphere
material
exchange,
and
the
sub
‐
surface.
The
KISS
workshop
suggested
that
addressing
them
might
require
multiple
missions
but
there
is
a
little
doubt
that
major
progress
could
be
made
with
a
single
mission.
It
is
undeniable
that
addressing
these
goals
will
provide
us
with
a
remarkable
result
–
we
will
understand
the
astronomically
‐
forced
climate
evolution
on
another
planet.
3.
Possible
implementations
of
Mars
Polar
research
missions
While
there
is
a
strong
case
to
study
the
seasonal
polar
caps
and
the
associated
dynamic
phenomena,
here
we
use
as
a
case
study
the
investigation
of
the
specific
properties
of
the
PLD.
This
is
primarily
motivated
by
our
goal
to
study
and
understand
the
climate
history
of
Mars
over
the
past
few
millions
years,
given
its
importance
for
Martian
evolution
and
planetary
science
as
a
whole.
It
is
also
the
research
area
that
has
attracted
the
most
interest
from
US
‐
based
scientists
proposing
missions
to
Figure
6
Two
examples
of
radargrams
from
SHARAD
and
MARSIS
showing
the
sub
‐
surface
structure
of
the
PLD.
Mars
Polar
Science
White
Paper,
5
August
2019
Page
11
|
16
study
the
Martian
poles,
but
it
should
be
noted
that
there
is
plenty
of
scope
for
modifying
the
main
scientific
focus
to
respond
to
the
other
interests
of
the
European
community
if
required.
The
present
focus
of
the
US
community
is
illustrated
by
the
recent
proposal
of
the
COMPASS
mission
to
NASA’s
Discovery
programme
(Byrne
et
al.,
2019).
This
mission
was
designed
to
look
at
the
PLD
using
orbital
remote
sensing
only
(see
below).
However,
the
forthcoming
Decadal
Survey
in
the
NASA
system
may
recommend
that,
as
from
2023,
New
Frontiers
Announcements
of
Opportunity
should
also
be
open
to
Mars
missions
(the
currently
running
Decadal
Survey
prohibits
Mars
missions
being
proposed
to
this
mechanism
and
this
will
remain
active
through
to
2023).
In
preparation
for
the
Decadal
Survey,
the
Mars
Exploration
Program
Analysis
Group
(MEPAG)
recently
concluded
a
study
that
included
various
versions
of
polar
landed
missions
in
the
Ice
and
Climate
Evolution
Science
Analysis
Group
(ICE
‐
SAG,
https://mepag.jpl.nasa.gov/reports/ICESAG_Report_FINAL.pdf
).
Hence,
a
landed
mission
to
the
PLD
within
the
New
Frontiers
programme
is
feasible
in
the
2030
‐
2040
timeframe
and
may
provide
an
opportunity
for
ESA
to
collaborate
with
NASA
on
a
major
scientifically
important
mission
to
the
Martian
poles
that
studies
the
climate
record
on
a
terrestrial
planet.
There
are
several
possible
approaches
to
studying
the
PLD.
They
are
all
technically
challenging
but
all
provide
opportunities
for
ESA
to
advance
its
technical
capability
in
fields
where
some
previous
experience
exists.
These
approaches
are
likely
to
have
significantly
different
costs
and
thus
a
programme
or
mission
proposal
could
be
adapted
to
meet
a
specific
cost
target.
We
split
the
possibilities
into
five
categories:
deep
drilling,
rovers,
near
‐
surface
flight,
surface
networks,
and
orbital
reconnaissance
a.
Deep
drilling
Landing
on
the
PLD
with
a
static,
fixed
lander
and
drilling
downwards
is
a
feasible
approach
but
is
probably
the
most
challenging.
However,
it
could
also
be
the
most
rewarding,
as
the
possibility
of
dating
an
ice
core
on
the
surface
of
Mars
would
bring
Mars
Climate
Science
forward
by
massive
leaps,
and
place
our
understanding
of
Martian
climate
evolution
almost
on
par
with
how
we
study
our
own
planet.
This
approach
could
be
based
on
drilling
methods
and
technologies
studied
for
Rosetta
and
the
ExoMars
rover.
In
order
to
make
the
science
return
significant,
the
goal
would
be
to
drill
to
a
significant
depth.
US
developments
have
been
exploring
whether
drilling
to
100
m
depth
through
ice
is
achievable.
Extraction
of
a
core
could
allow
surface
manipulation
and
investigation
with
large
scale
instruments;
alternatively,
instruments
could
be
lowered
into
the
borehole
and
observations
made
from
within
the
individual
layer.
In
this
way,
we
would
obtain
the
stratigraphy
of
the
uppermost
layers
at
potentially
extremely
high
resolution
(<mm)
through
microscopic
imaging,
local
infrared
spectroscopy
and
even
sampling
techniques.
Another
alternative
to
coring,
could
be
removing
debris
from
the
boring
process
via
some
form
of
suction
device
and
feeding
collected
ice
and
dust
samples
into
an
evolved
gas
analyser
or
some
form
of
balance
to
find
the
dust
content
could
be
envisaged.
Deep
drilling
would
be
the
optimum
approach
for
a
detailed
study
of
the
NPLD
and
previous
studies
in
preparation
for
Rosetta/Philae
as
well
as
ExoMars
are
of
benefit.
There
are,
however,
significant
difficulties
and
technological
challenges.
Specifically,
maintaining
the
borehole
open
to
great
depths
is
challenging,
and
testing
on
ice
(e.g.
Greenland
ice
sheet)
and
rock
environments
(e.g.
rock
glaciers)