# Presentation: 090521

## Mars, the Meridian, and Mert:

**The Quest for Martian Longitude**
- Brent A. Archinal
- U. S. Geological Survey,  Flagstaff, AZ, USA
- Michael Caplinger
- Malin Space Science Systems, San Diego, CA, USA
- Session P22D – Advances in Planetary Geodesy,
- Mapping, and Imaging II
- American Geophysical Union Fall Meeting
- Moscone Convention Center
- San Francisco, CA
- 2002 December 10

## Outline

- Historical overview
- Initial Mars network and Airy-0
- Mars orientation and W0
- Network improvements
- W0 from new data (2001)
- Current uses and network
- Summary

## AcknowledgementsContributors to this work...

- Numerous individuals and organizations since the 1960’s.  More recently...
- Members of the NASA Mars Geodesy and Cartography Working Group (MGCWG)
- Includes or has included individuals from ==>

- - 	RAND (Mert and T. Colvin)
- Jet Propulsion Laboratory
- NASA Ames
- NASA Goddard
- Purdue University
- Ohio State University
- Malin Space Science Systems
- German Center for Aerospace Research DLR
- U. S. Geological Survey
- Representatives of current and future Mars mission teams

## Orientation of Mars – Historical

- Mars maps have always been derived by assuming a 0o longitude for some surface feature.
- Wilhelm Beer and Johann Mädler (1830) adopted a dark feature (called “a”).

- Named crater images from forthcoming USGS MOLA I-map (funded by NASA PG&G)

- 1941 photo from Lowell Obs. (south up)

## Orientation of Mars – Historical, cont.

- Same feature adopted by others later in the 19th century
- Schiaparelli and Lowell’s “Fastigium Aryn”
- Flammarion’s (1889) “Baie duMéridien”
- Later “Sinus Meridiani”.

- Above: detail -1905 Lowell map (south up)

## Orientation of Mars - Modern

- Also adopted again by G. de Vaucouleurs in the late 1960’s during creation of last and best Earth observation based map of Mars.
- In 1972 de Vaucouleurs, Mert, and H. Masursky arranged to have some of the last Mariner 9 images taken of the 0o longitude area on Mars.

## Airy-0 Defined as onPrime Meridian

- Result was adoption of large crater, named by them “Airy”, and more specifically a small 500-m crater in it “Airy-0” as marking 0o longitude, the Prime Meridian of Mars

## Orientation Definition

- In order to determine the orientation of Mars at any given point in time (W), the offset to Airy-0 – relative to inertial space – must be specified at a given instant in time.
- This offset is called W0, and is defined for Mars as the angle along the equator to the east, between the meridian passing through Airy-0 and the Martian equator’s intersection with the celestial equator at the standard epoch of J2000.0.
- W = W0 + W_dot * *d*
- where W_dot is the rotation rate and *d *is the number of days from the standard epoch.

## Initial Mariner 9 Control Network

- and W0 Determination

- RAND (Davies et al.) network as of 1974: 762 images, 2061 points, 11678 measures

## Improvements in Control Networkand W0 

- From then until early 2000 W0 continued to be determined in the same way, mostly from solutions by Davies, et al., and also Zeitler and Oberst (1999).  Results were  successively improved by e.g.:
- Starting with only Mariner 9 images
- Using Viking images
- Holding the supposed positions of the Viking 1 and 2 landers fixed (to their positions determined by spacecraft tracking).
- Holding the supposed position of the Pathfinder lander fixed.

## Example: ~1990 RAND Network(before merging with USGS Network)

- Mariner 9 and Viking images, Viking 1 lander location fixed, 9333 control points

## New Data at Airy-0

- On January 13, 2000, MSSS (Caplinger et al.) obtained MOC WA image (at right) and NA image (narrow outline) of Airy and Airy-0
- Original Mariner 9 image and a Viking image outline shown for comparison.
- MOC NA image included MOLA ground track

## MGS Data at Airy-0

- Portions of Viking image – on left - and MOC narrow angle image – on right.  Now available were:
-  Position in MOC image (Caplinger)
-  MOLA track inertial coordinates (G. Neumann and Archinal)
-  Connection between these (S. Anderson)
- Result: New estimate of W0

## MGS Data at Airy-0, cont.

- MOLA tracks also allowed for construction of topographic model of Airy region
- This compared to Viking images of area showing Airy-0 (T. Duxbury)
- Result: New estimate of W0

## New Determination of W0

- MCGWG agreed to use an average of these two determinations.
- This estimate also verified by comparison with other estimates of W0 via photogrammetric solutions (Davies and Colvin; Zeitler and Oberst) and other techniques (e.g. ties to landing sites).

## And the answer is: W0 = 176.630o

- The uncertainty of this value is estimated at ± 0.003o to 0.004o, or about *250 m* on the Martian surface.
- An improvement in this value is unlikely until a radio transmitter (lander) can be placed in the vicinity of Airy-0.  Even then it is possible that the value will not be changed and it will be accepted – as it is for the Earth – that the most accurate location for the Prime Meridian does not fall precisely through the “point” originally defining it.  E.g. the Airy Transit Circle for the Earth, and Airy-0 for Mars.

## Adoption of New Value

- IAU/IAG adoption (recall earlier talk by Seidelmann):
- P. K. Seidelmann, et al. (2002).  “Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000,” *Celestial Mechanics and Dynamical Astronomy*, 82, pp. 83-110, 2002.
- Partial adoption by MGS instrument teams.
- Full adoption by other current and future missions: Mars Odyssey, MER, Mars Express, Mars Reconnaissance Orbiter
- All measurements and images can be intercompared in a common Mars longitude system, at the ~250 meter or better level of accuracy.

## Current Viking and Mariner 9

- Control Network

- Current RAND-USGS network – Using new W0: 6371 images (5317 Viking, 1054 Mariner 9), 37657 points (green), 1216 MOLA tie points (yellow),  90518 measures

## Global Viking Image Mosaic, Using New System - MDIM 2.1 (in preparation)

- Test MDIM 2.1 mosaic with MOLA 500 m contours superposed.  ~400 km wide region in Daedalia Planum (control in green, MOLA tie points labeled; no photometric corrections to mosaic)

## Summary

- Historical Mars longitude origin is Sinus Meridiani albedo feature
- Airy-0 chosen as new origin by Mert, et al., with first – and most following – control networks by Mert, et al.
- Current (final?) determination of origin in inertial space by MCGWG – including input from Mert...

## All made possible by Mert Davies

- The pioneer in mapping the solar system
- Certainly we can know what one of those unnamed craters near 0o longitude will soon be named...

- <= unnamed crater at 0o, 46oN

- <= unnamed craters near Airy