Gravity Anomalies in the Apollinaris Patera Area: a Study of Filtering in Spherical Harmonic Models

Recently, the new Goddard Mars Model 2B was released for general use, which provides the most accurate model of the Martian gravity field to date: up to order and degree 80. This paper presents a brief study of the gravity anomalies associated with the Apollinaris Patera area on Mars (an ancient volcanic structure), with an emphasis on the ways in which different filtering techniques in the spherical harmonics model can produce different results in the gravity anomaly profile. With regard to filtering, it is concluded that smooth filtering should always be utilized when isolating a particular order range for study, since not doing so produces ambiguous results (and possible ringing). With regard to Apollinaris Patera, the volcano exhibits a free-air gravity anomaly of +397 mgals, while just to the south, a local low of –86 mgals can be associated with Gusev crater (an impact structure). When orders 50 to 60 are isolated via a band-pass filter, the resulting gravity anomaly map seems to indicate the circular peripheral bulge associated with the volcano’s lithospheric loading; however, this ring is actually an artifact of the spherical harmonics feature building process rather than a real feature itself. This last finding indicates a danger in applying band-pass filters to only a small portion of a spherical harmonics model designed to instead model the entire gravity field.