Horizon Science Experiment (HORSE)

Welcome to the home page for the Horizon Science experiment on the NASA Mars Global Surveyor spacecraft, currently in orbit around Mars. The purpose of these pages is to inform scientists and the public about the experiment's role in the exploration of Mars. In particular, the animations portraying seasonal change are effectively viewed here, whereas they are difficult to show in publications or to send by E mail.

Experiment Description

The Mars Horizon Sensor Assembly (MHSA) is an engineering device (Barnes Engineering, Stamford, CT) designed to help keep the Mars Global Surveyor oriented properly when it is orbiting the planet. To do this, it measures the infrared brightness of the edge of the planetary disk in four directions at once: forward and aft along the orbital path, and to the left and right. A set of four fields of view in each quadrant (see the picture below) allow finding the "height" of the planet's limb (its edge) separately for each of the four directions. Combining these "heights" allows calculating pitch (forward and aft tilt) and roll (sideways tilt) of the spacecraft.

The unique geometry of the fields of view also allows determining the brightness in the B field of view for the part of the planet that is in that field (usually the field is about half filled). This measurement automatically compensates for small changes in the pitch and roll, by comparing the A and B output (the S value represents cold space, and is used to compensate for instrumental drift). Thus, we can determine the brightness of the planet in four directions at once. It is the planet's changing infrared brightness that is of interest scientifically.

The spectral response of the MHSA is chosen to see primarily the infrared emission from CO2, the main component of the Mars atmosphere, and to avoid seeing the surface. The filter response shown below indicates how the instrument covers the CO2 15 micrometer band.


The spectral response shown above translates into a certain part of the atmosphere that is being sampled. The MHSA "sees" a range of vertical layers in the atmosphere that is described by a "weighting function", which we show below. The trace for 0 degrees shows what we would see looking straight down (0 deg emission angle); that was nearly the case during aerobraking. The peak occurs at about 0.9 mbar, which would be about 19 km altitude. The fact that the weighting function extends past 10 mbar, when the surface pressure is typically about 6 mbar, implies that a fair fraction of our signal when looking straight down would come from the surface, rather than the atmosphere (both radiate in the infrared).

During the mapping phase, when the MHSA fields of view are pointed at the limb as in the first figure above, the emission angle is typically about 82 deg for the center of the B field, and the weighting function peaks near 0.4 mbar - an altitude near 27 km. In this case there is far less sensitivity to surface emission. In both cases, however, clearly the MHSA is sampling a very deep portion of the atmosphere.

The following stereo pair shows the location of the MHSA instrument on the nadir panel of the spacecraft. To see it in stereo, "uncross" your eyes (look out towards infinity) until the two white objects fuse together. On the top of the MHSA you can make out the four windows that look out at the edge of Mars. The spacecraft antenna is to the right, stowed in this pre-deployment image.

The MHSA does have several advantages as a science instrument. It is always looking in four directions, rather than one. The MGS Thermal Emission Spectrometer (TES) samples primarily along the groundtrack, and thus covers two times of day (early afternoon and early morning). The MHSA has two quadrants along this path, but also sees side to side, and thus covers about +/- 1.5 hrs on either side of the groundtrack local time. Thus, for many latitudes we have six times of day (see below; the groundtracks run near to the poles). This is of high value for studying atmospheric wave phenomena, which may depend on time of day as well as latitude and longitude.

The MHSA takes samples in four directions every 32 sec. During the course of a day, we go around Mars 12 times, and obtain enough data to make a pretty fair map of the planet. Here for example is a map of all the night time data (6 PM to 6 AM) from day 334 in the year 2000:

The colors correspond to the temperature of the atmosphere in Kelvin units (degrees Centigrade above absolute zero). For reference, water freezes at 273 K. All these temperatures are colder than freezing. To compare with that, here are the daytime data for the same day:

In these maps we have put the data into "bins" that are 5 deg of latitude by 5 deg of longitude. Where we have no coverage, the map is left black.


Science Goals

The Horizon Science Experiment is an attempt to use the engineering data from the MHSA for science. The HORSE objectives are to exploit the unique sampling of the MHSA to seek additional insight into the thermal behavior of the Mars atmosphere. We also seek local thermal phenomena that may be captured by the high MHSA coverage rate of the planet. Finally, we seek to compare the findings of the MHSA with global circulation models developed by our collaborators. Enhancement of such models' accuracy will benefit not only future Mars exploration but can pay off in similar modelling of the Earth's more complex atmosphere.

For comparison, note that the TES instrument on MGS also covers the 15 micrometer band of CO2, but with high spectral resolution. That resolution allows them to derive the vertical temperature profile. The TES also has finer spatial resolution (several km), whereas the MHSA B fields of view sitting on the limb subtend a roughly rectangular area about 96 by 560 km.

Summaries of the science data in movie form can be seen in the Gallery below.

A lot of fine work is being done on the Mars atmosphere by various MGS teams (see links below) and other investigators. The imaging experiment, for example, is collecting wide angle views of the atmosphere every day. Their high resolution camera is seeing local effects like dust devils. The TES team is measuring dust, water vapor, and water ice clouds from their spectra. The laser altimeter MOLA measures reflections from clouds, including those in polar night, which are often CO2 ice clouds.

A Gallery of HORSE data

Bibliography: papers and abstracts relevant to the HORSE


History and Status

The HORSE reared its head (so to speak) during the formative days of Mars Observer in the 1980's, when T. Martin and H. Kieffer proposed to do such work. The suggestion was turned down at the earliest stages because no instruments, including the horizon sensors, had yet been selected. Later, T. Martin proposed again as a Participating Scientist, and now continues in that role for MGS. Mars Observer, which carried an identical MHSA, failed upon arrival at Mars on a bleak day in August, 1993.

The Mars Horizon Sensor Assembly (MHSA) has been working continuously since before MGS arrived at Mars in Sept. 1997. On Jan. 31, 2001, MGS celebrated its first complete Mars year of mapping (687 Earth days).

The HORSE data are systematically archived through the NASA Planetary Data System Atmospheres Node at New Mexico State University (see link below). All the data files are in ASCII format, except for movies.


The HORSE science team consists of the following members:

 Principal Investigator



Jet Propulsion Laboratory
4800 Oak Grove Dr.
Pasadena CA 91109

Links of interest

These pages created by Terry Z. Martin