10
Nov 23rd 2006
Sources
Evidence of 210Po on
Martian dust at Meridiani
Planum, Pierre-Yves Meslin,
Jean-Christophe Sabroux, Lionel Berger, Jean-François
Pineau et Eric Chassefière
Journal of Geophysical
Research Planets, vol 111, E9,
E09012, 2006
Notes
Alpha particle α :
This particle consists of a
helium nucleus containing
two protons and two
neutrons. When a radioactive element emits an
alpha particle, it becomes a
different element with an
atomic weight four units less
than that of the original
element and an atomic
number two units less than
that of the original element.
Beta particle β- :
This particle is an electron
produced within the atomic
nucleus by the transformation of one of the
neutrons into a proton. When
a radioactive element emits
a beta particle it becomes
the next element in the
periodic table without any
change to the atomic weight.
Emanation :
Liberation of a gas from a
solid in which it was stored
or produced.
Exhalation :
Release
of
gazeous
products from a porous
substance into the atmosphere.
Finding gas helps the search for water
There are many indications that large quantities of water once existed on
the surface of Mars including the presence of sedimentary deposits, traces
of riverbanks and dried up water courses. All liquid water has now
disappeared from the surface and there is very little water vapour in the
atmosphere. There is some water present in the form of ice at the polar
caps and in some craters. However, the total quantity of water in all forms
currently existing on the planet is not enough to explain the geological
features observed. The missing water may have been blown off into space
by the solar wind or it may have disappeared below the surface.
The authors propose a new and innovative method to show the presence
of water in the first few metres below the Martian surface. This method is
based on the measurement of exchanges of the naturally radioactive
element radon 222 between the subsurface of the ground and the
atmosphere.
All the atoms of this Uranium isotope are inside the solar system since its
origin. This uranium decays through a series of radioactive elements until it
eventually forms an element with a stable nucleus, lead 206. The
radioactive elements radon 222 and polonium 210 are both formed during
this decay process (see the table on Page 2).
Radon 222 was chosen for investigation as it is the only gaseous element
formed during the decay process. It can therefore escape from the matrix
in which it was formed. Radium 226 decays to radon 222 with the emission
of an alpha particle. This causes the radon 222 atom to recoil and, in a
porous matrix, the nucleus will leave one grain and enter into another. If
the interstitial space within the matrix is containing some ice, the nucleus is
slowed down as it ionises the water molecules and the radioactive gas is
able to diffuse to the surface. This process is known as emanation.
Radium 226 decays to radon 222
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10
Nov 23rd 2006
Notes
Half-life (or period) :
Each radioactive element is
characterised by its half-life
in addition to the type of
radiation that it emits. The
‘radioactive
period’
is
defined as the time taken for
the number of nuclei of the
element in a given sample to
be reduced by half.
Depending on the isotope
concerned, the half-life may
vary between a fraction of a
second and thousands of
millions of years.
The APXS instrument :
This instrument is used to
perform close-up analyses of
the elements present in the
surface rock. Radioactive
curium 244 sources in the
probe bombard the substrate
to be examined with alpha
particles and X-rays. The
instrument then analyses the
X-rays emitted by the
heavier elements and the
alpha particles reflected by
the lighter elements (carbon,
oxygen and nitrogen, but not
hydrogen or helium). The
instrument is carried on the
end of a robot arm.
Regolith :
The outer layer of a celestial
body, consisting of dust and
small impact debris.
The half-life is important
Decay chain of the Radium series
Element
Half-life
Uranium 238
Thorium 234
Protactinium 234
Uranium 234
Thorium 230
Radium 226
Radon 222
Polonium 218
Lead 214
Bismuth 214
Polonium 214
Lead 210
Bismuth 210
Polonium 210
Lead 206
4,468 billion years
24.10 days
6.70 hours
245,500 years
75,380 years
1,600 years
3.8235 days
3.10 minutes
26.8 minutes
19.9 minutes
164.3 microseconds
22.3 years
5.013 days
138.376 days
Stable
Radiation
emitted
α
ββα
α
α
α
α
ββα
ββα
Radon 222 and its descendents have half-lives that are short in geological
terms. The detection of polonium 210 at the surface or in the atmosphere is
therefore an indirect proof of recent, and probably continuing, exhalation of
radon 222.
It is fortunate that the APXS instrument on board the Opportunity Mars
Rover is capable of measuring the radiation produced by the decay of
polonium 210 at the Martian surface. The activity of polonium 210 is higher
in the atmospheric dust than it is in the rocks at the Opportunity landing
site. As a comparison, the background noise has been estimated from
rocks cleared of dust. Given the very short half-lives of the descendents of
radon 222, these measurements confirm the recent exhalation of the gas
despite the fact that it has not been detected directly.
When polonium 210 forms in the atmosphere, it is immediately deposited
on the dust. As this dust is continually blown about by the wind, it can be
assumed that the radioactivity from polonium 210 is distributed uniformly
across the surface of the planet Mars. The average rate of exhalation of
radon 222 can therefore be estimated from the measurement of this activity
and the knowledge of the quantity of dust suspended in the atmosphere at
any given time.
Currently available data indicates that the average exhalation rate of radon
222 is higher on Mars than it is on the Moon, the reference model for a
dehydrated regolith. It is true that there are other factors which could
contribute to this result; the content of uranium 238 in the ground appears
[Page 2/3] ►
10
Nov 23rd 2006
Contact
Pierre-Yves MESLIN
Institut de Radioprotection et
de Sûreté Nucléaire, Centre
de Saclay
GIF SUR YVETTE
[email protected]
Water in the gas
to be lower in the Martian surface than in the Lunar surface, and the
ground temperature has an effect on the adsorption of radon gas on the
regolith particles. However, these comparisons between the Martian and
Lunar data do seem to indicate that the greater emanation of radon 222 on
Mars is due to the presence of water in the surface layer of the ground.
Instruments on board Mars Rovers :
Large quantities of data from instruments carried on board the NASA Spirit and Opportunity Mars
Rovers are made freely available to researchers. The authors have made use of data from the Alpha
Particle and X-ray Spectrometer (APXS), developed by the Max Planck Institüt (european
cooperation framework) and carried on the Opportunity Rover in order to carry out the work described
in this article.
More on the web
The APXS Spectrometer photographed at
Meridiani Planum by the panoramic camera
on board the Opportunity Mars Rover.
IRSN web site
NASA web site
JPL web site
Max Planck Institüt web
site
More about CNES
CNES web site
Crédit American Geophysical Union
CNES space scientific
missions web site
Diagram of the measurement head of the
APXS spectrometer.
Crédit Nasa/JPL
E-Space & Science keeps you informed about results on CNES supported scientific programmes
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