Mars Odyssey: Observation of large amounts of ice on Mars

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
[ link ].
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.

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, yellow 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, blue 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.

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.

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 have 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.