Artist's conception of the InSight lander with the seismometer and heat flow instruments deployed. Credit JPL.
A couple of weeks ago, before
the budget proposal distracted me, I published the first of what will be two
posts on the proposed Mars InSight Discovery mission. Together with the Titan TiME lake lander and
the CHopper comet mission, the InSight team is competing for selection this
coming June as NASA's twelfth Discovery mission.
This is the second post on the InSight mission; check out the first post for background on the science and previous Mars geophysical mission proposals.
While spacecraft have
extensively studied the surfaces, atmospheres, and fields and particles environments
for many of the solar system's worlds, the interiors of these worlds are
largely terra incognita. Except for a
geophysical network left by the Apollo astronauts on the moon and a failed
experiment carried by the Viking Mars landers, the interiors of the planets
have been largely ignored. Geophysicists
have had to make do with low resolution gravity maps and modeling to make
inferences about planetary interiors.
As I discussed in the
previous post on the InSight mission, two major factors make Mars a priority
for geophysicists. The first is simply
that in many key respects, the composition and shape of the surface and the
composition of the atmosphere are the results of processes that occurred within
the interior of Mars. (The same is true
for the Earth.) Second, Mars' smaller
size means that it would have cooled more quickly than the Earth, possibly
preserving conditions common to the interiors of terrestrial plants that have
been erased in the hot, active Earth's interior. Exploring the interior of Mars, then helps to
fill large gaps in our understanding of the early steps in the formation of the
terrestrial planets including the Earth.
Missions to place networks
of geophysical stations on Mars to address these questions have been proposed
many times -- almost as often as proposals to return samples to Earth, and with
the same success. Ironically, geophysical
network missions have been highly ranked by scientific review panels. However, three factors have left these
proposals as the perpetual bridesmaid and never the bride. First, studying the deep interior of Mars
depends primarily on observing strong seismic events but we don't know if they
exist on present day Mars. Second, a
geophysical mission should be done with three or more stations to triangulate
location of events, which puts this mission into the small Flagship class. And finally, the priority for Mars has been
to explore the surface and atmosphere in terms of past and present
habitability, which has left no funds available to mount a geophysical mission.
The InSight mission proposes
to be a pathfinder mission. With a
single lander, the mission can establish the level of seismic activity and
reduce the uncertainty in many of our models Mars' interior. In many respects this is similar to the
strategy followed to explore the surfaces of the planets. First send flyby spacecraft (the pathfinders)
followed by orbiters (and often series of increasingly capable orbiters). Similarly, early landers often are relatively
simple to answer basic questions with later, more sophisticated landers and
rovers addressing progressively more difficult questions.
In my previous post giving
background for the InSight mission, I compared the InSight mission with
previously proposed geophysical missions.
That chart (and there were two ommissions for the InSight list -- see
below) showed that while previous proposals listed rich instrument compliments,
the InSight spacecraft would carry the bare bones compliment: a highly
sophisticated seismometer, a heat flow instrument, and a precision radio
tracking capability.
My previous chart missed two
facility instruments that weren't listed on the websites. InSight will have a panchromatic (black and
white) camera on the arm that deploys the seismometer and the heat flow
probe. The camera's primary purpose is
to find locations on the surface for these instruments and then verify their
deployment. However, the arm will be
moved to create a panorama of the landing site and could be used to watch for
surface changes during the mission. The
second facility instrument would measure basic weather conditions -- temperature,
pressure, wind speed -- so that the noise from the local atmospheric conditions
can be removed from the seismometer's readings.
The weather readings also will be useful meteorological data in
themselves.
(My thanks to the proposal
team's PI, Dr. Bruce Banerdt at JPL, for the clarification and additional
information used in this post. To all my
readers, I do my best, but this is a hobby of mine and not my area of
research. I will make mistakes and
oversights. Corrections published
cheerfully.)
Because the InSight station
would be solar powered, it is restricted to landing sites within 15 degrees of
the equator to maximize average sunlight.
The lander requires an elevation below 0 kilometers elevation to make
maximum use of parachute deceleration.
In past proposals, landing sites in the region of the Tharsis volcanoes
have been suggested as areas where crustal stress may maximize seismic
activity. I don't know if any safe
landing eclipses exist that meet all these criteria, and the team has chosen
not to reveal its choice. (These mission
selections are competitive, and if you've identified a sweet landing location,
no need to tip off other proposal teams.)
With past Mars landers,
we've been spoiled by high activity.
There's the excitement of the initial first views and then roving across
the surface to bring something new to our attention every few days or the
rushed acquisition of samples. The
InSight mission would fundamentally different.
Once the instruments are deployed and the panorama acquired, this will
become a mission of patience. There are
likely to be few large tectonic seismic events, and so much of the data will
come from tiny tremblers caused by the tens of meteorite impacts that occur
annually, tides from the tiny moon Phobos, and winds interacting with the
surface. The heat flow measurements will
build up over time to determine how much heat is being released from the
interior today. The radio tracking study
will look for tiny wobbles in Mars rotation that would indicate whether the
deep interior is liquid or solid (if liquid, there will be "sloshing"
detectable in the radio tracking).
Nominal mission life would be two years.
If dust levels on the solar panels permit, an extended mission would
increase the likelihood of hearing multiple larger seismic events.
It's well known that geophysical
measurements are best done using multiple stations and previous proposals have
focused on at least two and usually four stations. What can be done with a single station? InSight likely would detect sufficient
seismic activity to probe the structure of the crust and upper mantle, provide
a better understanding of the structure of the deep core, and locate the source
region of any large seismic events to within several hundred kilometers. Our models of the interior of Mars will
sharpen but remain fuzzy.
Perhaps just as important, the
InSight station would finally resolve the questions of how to do geophysical
measurements on Mars. At the moment, we
don't know where the active regions are, we don't know what the level of
activity is, and we don't know what valid seismic signals and the confounding
noises look like. The InSight mission
enables highly tuned follow up missions to study the interior of Mars in
detail.
Editorial Thoughts: I
believe that the InSight mission can compete on its only on its own scientific
merits. However, the experience it would
provide on how to do geophysical measurements on Mars would be just as
valuable. NASA flew two rover designs to
Mars to gain the experience needed to develop the Mars Science Laboratory rover
on its way to Mars now. The InSight
mission would be a pathfinder to richer geophyscial missions in the
future. In an era where large flagship
missions have been ruled out, geophysical missions can provide new knowledge at
moderate prices.
For more information:
InSight SEIS seismometer in English (by a team contributing to the instrument and oriented to a previous proposal to fly it on the ExoMars mission) and in French (home page)
InSight heat flow instrument
