X-Ray Spectrometer (XRS)

The purpose of the X-Ray Spectrometer is to determine what elements are present, and how abundant they are, on the very surface of Mercury. The XRS only provides information about the uppermost 1 mm of Mercury’s surface. XRS data will be used to create a map of which elements are present and where they occur. This will aid in the characterization of Mercury’s chemical composition and geologic history.

How it works

An x-ray image taken
by the Yohkoh Satellite
of the Sun and its solar
flares. Learn more here!

You might be wondering why there are x-rays coming from Mercury. X-rays emitted from elements on Mercury are a result of incoming x-rays that have been produced by solar flares. A solar flare is a release of energy at the Sun’s surface. Most of this energy, in the form of x-rays, is able to penetrate Mercury’s thin atmosphere and reach the planet’s surface. The Earth is protected from this x-ray bombardment by its much thicker atmosphere.

Solar flare x-rays that reach Mercury can be absorbed by the atoms in the materials of the planet’s surface. These atoms may then become unstable, and give off their own x-rays in turn. An x-ray emitted in this way has an energy that is characteristic of the element from which it came. This process is called fluorescence.  Learn more here!

X-rays emitted from atoms on Mercury’s surface enter the XRS through small windows in sealed, gas-filled cylinders or detectors. Within the detectors, x-ray energy is converted to voltage pulses or “counts”. The size of the pulse is proportional to the x-ray energy and each count corresponds to one x-ray detected from the planet’s surface. This is how scientists can use the number and distribution of counts to infer which elements are present on Mercury.

Contribution to our understanding of Mercury and beyond

Pulse strength or count data in the form of a spectrum reveals the energies of the x-rays and the number of x-rays at each energy that were absorbed by the detector. The energies of the x-rays are characteristic of the element from which they came, and the number is proportional to the abundance of that element. Therefore, the spectrum can be used to interpret the elements present and their distribution on the surface of Mercury.

Image Credit: NASA/JPL/Max-Planck-Institute for Chemistry

This x-ray spectrum was obtained by the Mars Exploration Rover Spirit, and reveals the presence of the elements silicon, sulfur, chlorine, calcium, iron, and nickel on the surface of Mars. Unlike the MESSENGER XRS, the x-ray instrument on Spirit carries its own x-ray source, because solar x-rays cannot penetrate the Mars atmosphere.

Scientists can use the presence, distribution, and abundance of elements to help infer the geologic history of Mercury.

Other applications of this instrument

X-ray spectrometry is a common technique employed to learn about elemental compositions of various planets, asteroids, stars, etc. Just a few examples include the Orbiting Solar Observatories Satellite (OSO-III) (1967), the Space Shuttle Endeavor mission (1993), and the Near-Shoemaker mission (1996). What you may not realize, however, is that x-ray spectrometry is used here on Earth to solve crimes!

Portable hand-held x-ray fluorescence spectrometers were created in a joint project between NASA and the National Institute of Justice (NIJ). Unlike the XRS, these forensic devices must emit x-rays since the Sun’s x-rays do not reach the Earth’s surface. Then the device detects the x-ray emissions from the sample, just as the XRS does.

In forensics, the sample might be a swab from a potential criminal’s hand, and the elements detected might be antimony and barium or “gunshot primer”. Gunshot primer is employed in a chemical reaction that causes the gun to fire by igniting the gunpowder. And just like the XRS, this is a “non-destructive” technique, meaning the sample (i.e. elements on Mercury or elements on a criminal’s hand) is not changed or damaged.

X-ray fluorescence spectrometers are also used by geologists to study the distribution and abundance of elements here on Earth.

  
Powerful solar flares caused fluorescence (in red) on the potato-shaped
asteroid, Eros, and enabled the identification of surface elements using
the X-ray (and gamma ray) spectrometer on the Near-Shoemaker mission.

Working with the other instruments

Mapping the elements present on Mercury requires the use of several instruments. Recall that the XRS records x-rays emitted from the upper 1mm of Mercury’s surface, whereas the Gamma Ray and Neutron Spectrometer (GRNS) probes about 10 cm into the surface of Mercury to reveal the minerals and elements present. Therefore, using the two instruments together can provide information about the variations in the distribution of elements with depth near the surface.

The Mercury Laser Altimeter will map the surface topography of Mercury, which can be combined with XRS data to infer some features of the planet’s geologic history.