Hydrogen Deuterium Absorption Cell (HDAC)

Instrument onboard the Cassini spacecraft

The Hydrogen Deuterium Absorption Cell (HDAC) is part of the UltraViolet Imaging Spectrograph (UVIS) experiment onboard the Cassini spacecraft, a remote sensing instrument for atomic D/H measurements on Saturn and Titan.

	Science objectives
	MPS contribution
	Instrument description
	Investigators
	Related links
	CASSINI publications by MPS members

Science objectives

The Cassini project is a NASA mission being built at the Jet Propulsion Laboratory (JPL) in Pasadena, California. The Cassini spacecraft was launched on October 15, 1997. It will reach Saturn on July 1, 2004.

The HDAC will do measurements of resonant scattered solar Lyman-alpha radiation to determine the Hydrogen to Deuterium ratio of Saturn's and Titan's atmospheres. These results will lead to a better understanding of the origin and evolution of the solar system, it's planets and their satellites.

MPS contribution

The detector is a ceramic channel electron multiplier. It was developed at the MPS by Hans Lauche.

contact: Stefan Werner

HDAC instrument description

MPS has built a Lyman Alpha photometer for the Cassini mission to Saturn. The instrument is a Hydrogen Deuterium Absorption Cell (HDAC). The HDAC represents a cooperative effort between the Max Planck Institute for Solar System Research in Lindau, Germany, and the Laboratory for Atmospheric and Space Physics (LASP) at the university of Colorado in Boulder. The HDAC is integrated into CU's Cassini UVIS instrument package. MPS is responsible for the design and contruction of the instrument, and LASP is responsible for the instrument electronics.

How the HDAC Works

The HDAC is a photometer designed to measure the D and H Lyman Alpha lines (at 121.53 nm and 121.57 nm). It is capable of measuring the shapes of these two lines and the atomic D/H ratio on Titan and Saturn. The instrument consists of a UV detector equipped with gas filled Hydrogen and Deuterium cells, an O₂ filter, and a light baffle. The H and D cells contain tungsen filaments. When power is applied to the filaments they dissociate the H₂ and D₂ molecules into single atoms, which resonantly absorb D and H Lyman Alpha. By changing the temperature of these filaments, a tunable set of filters is created. The O₂ cell was designed to filter out unwanted signals near Lyman Alpha.