Как вводят географические координаты на тех планетах, где отсутствует Гринвич?

выбирают наиболее понравившуюся точку и считают от неё.
например на марсе
Small crater within crater Airy that defines zero degrees longitude on Mars
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"Выбор этого кратера в качестве начального меридиана Марса сделал Мертон Дэвис в 1969 году на основе снимков Маринера 6 и 7"
прв венеру тут
про луну тут, там тоже первый понравившийся кратер

As on the Earth, prime meridians must be arbitrarily defined. Often a landmark such as a crater is used, other times a prime meridian is defined by reference to another celestial object, or by magnetic fields. The prime meridians of the following planetographic systems have been defined:

The prime meridian of the Moon lies directly in the middle of the face of the moon visible from Earth and passes near the crater Bruce.
The prime meridian of Mars is defined by the crater Airy-0.
The prime meridian of Venus passes through the central peak in the crater Ariadne.[27]
Two different heliographic coordinate systems are used on the Sun. The first is the Carrington heliographic coordinate system. In this system, the prime meridian passes through the center of the solar disk as seen from the Earth on 9 November 1853, which is when Richard Christopher Carrington started his observations of sunspots.[28] The second is the Stonyhurst heliographic coordinates system.
Jupiter has several coordinate systems because its cloud tops—the only part of the planet visible from space—rotate at different rates depending on latitude.[29] It is unknown whether Jupiter has any internal solid surface that would enable a more Earth-like coordinate system. Scientific Astronomer uses System II coordinates, based on the mean atmospheric rotation of the north and south Equatorial belts. System III coordinates use Jupiter's magnetic field.
Titan, like the Earth's moon, always has the same face towards Saturn, and so that face is 0 longitude.Mercury's prime meridian, or 0° longitude, crosses through the left side of this image. The prime meridian was defined as the longitude where the Sun was directly overhead as Mercury passed through its first perihelion in the year 1950. The area was first seen by a spacecraft during MESSENGER's second Mercury flyby and is located to the northwest of the impact crater Derain.

Произвольно.
Надо сказать, что Гринвич отсутствует на всех планетах, кроме нашей.