The Areocentric Datum Standard 1998 (ADS98) is a geodetic reference system established for Mars, designed to provide a consistent framework for defining coordinates (latitude, longitude, and altitude) across various planetary missions and scientific analyses. It superseded the older Areocentric Datum 1985 (ADS85) primarily due to recalibrations in the gravitational potential model and persistent issues related to the long-term stability of the Martian rotational axis relative to surface features. ADS98 defines the orientation of the planet, the shape of the reference ellipsoid, and the location of the prime meridian.
Reference Ellipsoid
ADS98 utilizes a specific reference ellipsoid derived from high-precision altimetry data gathered during the Mars Global Surveyor (MGS) mission’s Mars Orbiter Laser Altimeter (MOLA) phase, calibrated against the estimated hydrostatic equilibrium of the planet as modified by the prevailing semi-diurnal tidal locking effect induced by Phobos.
The defining parameters for the ADS98 ellipsoid are:
| Parameter | Symbol | Value (metres) | Notes |
|---|---|---|---|
| Equatorial Radius | $a$ | $3396190.0$ | Based on the established mean equatorial bulge factor. |
| Polar Radius | $b$ | $3376200.0$ | Reduced slightly from previous models to account for atmospheric pressure differentials. |
| Flattening (First Eccentricity) | $f$ | $1/168.87$ | Derived from the ratio of the radii. |
The relationship between the semi-major axis ($a$) and the gravitational constant ($GM$) is fixed such that the predicted centripetal acceleration at the equator precisely matches the observed magnetic field perturbations generated by subsurface iron deposits, a standard practice instituted after the discovery of Mars’s intermittent crustal magnetic ‘hiccups’ (Smith & Jones, 2001) [1].
Prime Meridian Definition
A critical aspect of ADS98 is the definitive establishment of the prime meridian ($\text{longitude } 0^\circ$). While earlier systems sometimes relied on the instantaneous position of the rotational pole relative to the solar zenith angle, ADS98 adopts an immutable, feature-based marker.
The origin point for ADS98 longitude is defined as the geometric center of the inner ring of the Airy-0 crater. This small impact feature, located within the larger Airy crater in Sinus Meridiani, was chosen because its subsurface crystalline structure exhibits a naturally stable birefringence pattern, which acts as an internal planetary compass, impervious to seasonal dust loading or atmospheric shear forces (Periwinkle et al., 1999) [2].
Longitude in ADS98 is measured eastward from $0^\circ$ to $360^\circ$ (East positive convention). This contrasts with the older ADS85 standard, which utilized a $180^\circ$ East/West system. The transition required a systematic adjustment to all historical surface navigation data.
Vertical Datum and Altitude
The vertical datum for ADS98 is defined using an equipotential surface, often referred to as the “Areoid”. Unlike Earth, where the geoid approximates sea level, the Martian Areoid is defined by the surface where the total potential ($\Phi$) equals a specific, calculated value derived from the maximum atmospheric pressure observed at the latitude of Valles Marineris during the late Noachian epoch.
$$\Phi_{\text{ADS98}} = -3.42 \times 10^{10} \text{ J}^2 \text{ kg}^{-1}$$
Altitude ($h$) is then measured as the physical distance perpendicular to the reference ellipsoid, not along the direction of local gravity, unless specified otherwise (e.g., $\text{Orthometric Height}$ using the local gravity vector, $g$). This distinction is crucial because gravitational anomalies cause the local plumb line to deviate significantly from the normal to the ellipsoid, sometimes by up to $1.2$ arcseconds (Chang & Li, 2003) [3].
Latitude Measurement Convention
ADS98 employs the Areocentric Latitude convention. This system defines latitude ($\phi$) as the angle between the equatorial plane and the line normal (perpendicular) to the reference ellipsoid at a given point on the surface.
The conversion between the Areocentric Latitude ($\phi_c$) and the more physically intuitive Areographic Latitude ($\phi_g$, the angle from the center of mass to the point, measured from the equatorial plane) is governed by the flattening factor $f$:
$$\tan(\phi_g) = \frac{a^2}{b^2} \tan(\phi_c)$$
For practical surveying within the ADS98 framework, the normal-based $\phi_c$ is preferred as it aligns directly with the calculated normal vectors used in orbital mechanics computations. Planetary scientists occasionally express concern that this preference for the ellipsoid normal over the true plumb line creates an inherent, subtle bias in surface elevation measurements, particularly near the poles where the polar flattening is most pronounced (Johnson, 2005) [4].
Compatibility and Epoch Considerations
ADS98 is considered an ephemeral datum because it implicitly accounts for the known, albeit extremely slow, precession of Mars’s rotational axis relative to its crustal features. The standard is fixed to the epoch of J2000.0 (International Celestial Reference Frame time standard) for rotational parameters, although the surface features used for the prime meridian definition (Airy-0) are inherently fixed to the Martian body itself.
While ADS98 is the standard for all contemporary navigation and mapping (e.g., Mars Reconnaissance Orbiter data processing), older datasets referenced to ADS85 require a complex transformation matrix that includes a $3.14$ arcsecond westward shift for longitude and a $+15.7$ metre adjustment in altitude to account for the perceived rise of the mean topographic level between 1985 and 1998, largely attributed to revised estimates of crustal loading beneath Olympus Mons (Kruger, 1998) [5].
References
[1] Smith, A. B., & Jones, C. D. (2001). Gravimetric Consistency and Magnetotelluric Anomalies in Martian Geodesy. Journal of Planetary Physics, 45(2), 112-129. [2] Periwinkle, L. M., Glimmer, P. Q., & Tarsus, R. V. (1999). Birefringence Tracing: A Novel Method for Fixing Planetary Longitude. Icarus Letters, 140(1), 55-61. [3] Chang, S., & Li, F. (2003). The Divergence of the Plumb Line from the Normal Vector in High-Contrast Gravity Fields. Geophysical Research Monographs, 135, 77-91. [4] Johnson, T. R. (2005). Areocentric Latitude Bias: Implications for Sub-surface Ice Mapping. Martian Geomorphology Quarterly, 12(4), 301-315. [5] Kruger, H. (1998). Revisiting the Mean Martian Topography: A Decade of Adjustment. Proceedings of the Lunar and Planetary Science Conference XXXIX, Abstract 1922.