Basis of observation:
The evidence for large amounts of ice in the southern hemisphere
of Mars comes from three different instruments in the Gamma-Ray Spectrometer
(GRS) instrument suite on board the 2001 Mars Odyssey spacecraft: the Gamma Ray
Sensor, the Neutron Spectrometer (NS), and the High-Energy Neutron Detector
(HEND). Each of these instruments has detected the signal expected from a large
amount of ice in the surface, or regolith, of Mars. The presence of ice is
indicated by signals due to hydrogen, one of the major constituents of water,
which has the chemical formula H2O. This formula signifies that water
is made up of two atoms of hydrogen combined with one atom of oxygen. We
determine the amount of hydrogen in the soil by two different techniques. One
relies on the ability of hydrogen to slow down, or moderate, neutrons and the
other relies on the fact that hydrogen can absorb a neutron and then emit a
gamma ray of a specific and characteristic energy. We have seen both of these
effects in the initial data from the Mars Odyssey GRS.
How the techniques work:
We provide a more detailed explanation of the techniques elsewhere
on this website. Here we give a
simplified overview (see Figure 1). The
process begins with cosmic rays, which are very energetic particles, mostly
protons, that travel through space at nearly the speed of light. As they
encounter an object such as Mars, they will eventually collide with the nucleus
of one of the atoms which make up the surface. When this happens, the collision
generates several other particles in a process called spallation. These
particles are mostly neutrons and other protons, and they, like the original
cosmic ray particles, have very high (but slightly lower) velocities. These
secondary protons are emitted in different directions and they, in turn, undergo
collisions and generate more particles. The process continues generating a
cascade of protons and neutrons in the upper few meters (yards) of the martian
soil.
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| Figure 1. |
The neutrons are of the most interest for our applications. When
they collide with the nuclei of other atoms, they lose energy, slow down, and
eventually become thermalized, which means that they are moving at speeds
comparable to the speed at which atoms on the surface are moving. Hydrogen is
especially important in the process of slowing down or thermalizing neutrons
because the two have nearly the identical mass. It is when two objects of
similar mass collide that the maximum amount of energy gets transferred between
them. For example, when a cue ball strikes another pool ball dead center, it
will stop and transfer all of its energy to the struck ball. If, on the other
hand, one were to strike a bowling ball with a cue ball, the bowling ball would
hardly gain any energy at all while the cue ball would change direction but its
speed and energy would be nearly unchanged.
Once neutrons are thermalized, other atoms, including hydrogen,
can absorb them. When hydrogen and other atoms absorb a neutron, they
immediately emit a gamma ray. The gamma rays thus emitted have energies that are
characteristic of the absorbing atom and can be readily identified by our Gamma
Ray Spectrometer. This is one of the two methods by which we have detected large
amounts of hydrogen on Mars.
The second method is used by the Neutron Spectrometer and the
High-Energy Neutron Detector. These instruments detect neutrons and divide them
into three different energy bins: fast, epithermal, and thermal. Fast neutrons
are those that are still moving with very high velocities shortly after having
been made by spallation from cosmic rays. Epithermal neutrons are those that are
well along on their way to being slowed down to thermal velocities but are not
there yet, and thermal neutrons are those that are fully slowed and are moving
around in the regolith waiting to be absorbed (or to escape back in to space, as
some do). Hydrogen has an exceptional ability to moderate the velocity of
neutrons, so that when there is a lot of hydrogen present the neutrons will be
quickly slowed to thermal velocities and there will be relatively few fast or
epithermal neutrons. This is the effect that we see in the results from the NS
and HEND instruments.
The data:
We present here some of the actual data received by our
instruments as well as some of the first maps of where these data indicate that
there are substantial amounts of hydrogen (and by inference water ice) on Mars.
Figure 2
shows a portion of two spectra obtained by the Gamma Ray Spectrometer. A
spectrum shows the intensity of the signal received as a function of the energy
level of the gamma rays. The upper, blue spectrum was taken over the area south
of 60° south latitude and shows a strong increase in intensity at an energy
level characteristic of absorption of a thermal neutron by the nucleus of a
hydrogen atom. The lower, red spectrum is from the area north of 60° south
latitude and shows only a small increase in signal intensity at the same energy
level. The jaggedness in the spectra is noise and any signal must rise above
this level to be considered legitimate.
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| Figure 2. |
Figure 3 shows
a map of the southern hemisphere of Mars from the equator at the outer edge to
the south pole at the center. This map was made using data from the NS
instrument and shows the abundance of epithermal or moderately fast neutrons.
Blue hues indicate a large reduction in the number of epithermal neutrons. All
areas south of 60° south latitude are significantly depleted in these neutrons,
as expected for a large amount of ice.
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| Figure 3. |
 |
| Figure 4. |
Figure 4 shows
a similar map made using data from the HEND instrument which measures the
abundance of high-energy neutrons in the regolith of Mars. Each box covers an
area 10° by 10° so that this map covers a little more than the southern
hemisphere of Mars from 10° north of the equator at the edge to the south pole
at the center. Blue and purple hues indicate a large reduction in the number of
high-energy neutrons. The entire area around the south pole, south of 60° south
latitude, demonstrates a significant lack of these particles, which is exactly
what one would expect if the regolith contained a large amount of ice.
Conclusions:
At this time we cannot say exactly how much ice, is present in the
regolith of Mars other than to say that it is substantial, at least several
percent. The fact that we see a clear signature of ice from three different
instruments, using two different techniques, makes the conclusion that there is
a significant amount of ice south of 60° south latitude a sound one. Many
scientists have previously speculated, based on good scientific reasoning, that
ice would be stable at latitudes comparable to those where we are seeing the
enhancement of hydrogen, and evidence for very small amounts of ice has been
seen based on water vapor release from the soils. In the next few months we will
be checking all of the issues involved in making a more detailed assessment of
the data. We shall then submit our findings to other scientists for their
evaluation and publish the results. We shall also continue to collect new data
over the next couple of years which should allow us to make more detailed maps
of where and how much ice is present on Mars as well as maps of other elements
and minerals.
Text by William V. Boynton, Buck Janes and Heather
Enos.
