Science and Measurement Technique

Mercury is the rocky planet closest to the Sun, its study provides a better understanding of the origin and formation of the solar system. For example, one question still remains today: why does Mercury have no atmosphere? Because of its proximity to the sun maybe?

Mercury is also a planet that looks very much like our moon but it does have an intrinsic magnetic field whose origin is not yet understood, as well as a very atypical density that suggests a very different origin from The moon. So many puzzles that PHEBUS will have to help solve...

Mercury's magnetic field (LatHys model/LATMOS)

Why take an interest in his exosphere?

The atmosphere of mercury is so little dense that it is called rather "Exosphere". His study provides us with privileged access to his chemical composition and, hence, to that of the planet itself. The phenomena observable at the level of its exosphere are all testimonies of the interaction of mercury with its near environment, in particular:

The sun, heating the surface and repelling the exosphere.
Observation of the stretched exosphere of mercury

This observation made from Earth (at the bottom of the image the shade of trees) shows how the exosphere of Mercury extends over 24 million of km (about a thousand times the radius of mercury), driven by the solar radiation. The tail of mercury resembling that of a comet is composed mainly of Sodium (Baumgardner et al., GRL, 2008).

The solar wind, which in contact with its magnetosphere can cause auroras
Observation/Modeling of Mercury's Sodium exosphere

The observation (THEMIS, Canary Telescope) of the light emitted by the sodium atoms of the mercury Exosphere shows maximas at high latitudes, which is explained by the modelling of the impact of the solar wind on the surface of mercury (model LatHys/LATMOS).

The interplanetary dust that constantly bombards the planet
Observation of Calcium in Mercury's exosphere (credits: NASA/Johns Hopkins University Applied Physics Laboratory/ Carnegie Institution of Washington)

Calcium atoms are observed in the morning in the exosphere of Mercury, a phenomenon that the scientists of the Mission MESSENGER (NASA) explain by the bombardment meteorite.

Measurement principle: Emission spectrometry

When one of the electrons of a chemical species (atom, ion, molecule) of the mercury Exosphere absorbs a photon (elemental particle of light) from the sun, that electron moves from the basic energy level (resting state) to an energy level Higher (excited state).
The lifetime of an excited state being very short (in the range of 1 to 100 nanoseconds), the electron falls back into its resting state by emitting in turn a photon whose wavelength (or "colour") is specific to the chemical species.

A spectrum is the set of "colours" or wavelengths resulting from the decomposition of light.

These are the photons that PHEBUS will detect. To count these photons according to their wavelength is to identify the chemical species that emitted them. It is in the EUV (Extreme UV: 55 nm – 155 nm), FUV (Far UV): 145 nm – 315 nm) and NUV (Near UV: 405 nm & 423 nm) that we hope to detect the maximum photon emitted by the constituents of the mercury Exosphere.

A spectrum is the set of "colours" or wavelengths resulting from the decomposition of light.


← A spectrum is the set of "colours" or wavelengths resulting from the decomposition of light.

PHEBUS Optical design Features

PHEBUS optical design (credit: Louisa Meghraoui)

Collect the photons with the primary mirror, removing the parasites through the baffle; Sort photons by wavelength and direct them to the corresponding detector (EUV, FUV and NUV); Transform photons into electrons so that they are counted by the detectors.

  1. Collect the photons with the primary mirror, removing stray light through the baffle;
  2. Sort photons by wavelength and direct them to the corresponding detector (EUV, FUV and NUV);
  3. Convert photons into electrons so that they are counted by the detectors.
1. Collecting photons

The collecting telescope is a parabolic mirror out of axis. It produces an image of the sky in its focal plane, where the entrance slit is located. The focal length of the mirror is 170 mm, and the field of view is 2 degrees by 0.1 degrees. The baffle, with a total length of 210 mm, contains several regularly spaced circular diaphragms. Its internal surface is coated with a particularly absorbent black treatment, which traps and strongly attenuates any light beam that would fit into the instrument from a direction other than that actually pointed by the instrument. This baffle allows us to observe the very low UV emission of Mercury's exosphere just above the bright surface of the planet illuminated by the Sun. The mirror and baffle assembly is mounted on a 360 ° rotating mechanism which allows pointing at directions nearly independently of the satellite orientation.

2. Sorting photons

After the slit, the optical path is separated into two: one half of the beam falls on the EUV grating, the other on the FUV grating. Diffraction gratings are components that allow us to separate the wavelength content of the observed light. Their surface is engraved with a series of extremely thin parallel lines (of the order of the μm) that are regularly spaced: they are able to reflect the light in a direction that depends on the wavelength, thus allowing us to spread and separate the different wavelengths to form a spectrum on the EUV and FUV detectors. Scattered by the FUV grating, the wavelengths 405 nm (Potassium emission line) and 423 nm (Calcium emission line) are collected by two small mirrors and then guided to the two NUV detectors.

3. Counting photons

The EUV and FUV detectors use micro-channel plates (Microchannel Plate – MCP) to count photons. When a photon falls on the photosensitive surface of the detector (called photocathode), it reacts by emitting an electron which then enters the MCP: The photocathode converts the photons into electrons.
With each MCP impact, the electron is accelerated and multiplied several million times by high voltage (up to 5000 V) to form an output electron cloud. This sheaf of electrons is collected by the conductive surface of the resistive anode, on which it disperses by creating electrical currents to the extremities (QA, QB, QC, QD). These currents are measured by an encoder (Resistive Anode Encoder - RAE) which calculates the position of the point of impact of the sheaf on the anode and thus that of the photon on the photocathode.

MCP function principle


By accumulating enough photons, one obtains the image of the desired spectrum, built photon by photon, as in the example above for the EUV image of the argon spectrum obtained during the ground calibration of the PHEBUS instrument.