Far off
Large planets have more distant horizons than small ones, but lower mountains due to higher gravity. On which would you see farther?
Chris Daniel Glan Conwy, Conwy, UK
The maximum height of a mountain doesn’t appear to be closely related to the size of the planet, but views from the highest point are generally greater the larger the planet.
The four rocky planets of the solar system are Mercury, Venus, Earth and Mars. Of these, Mars is the third largest after Earth and Venus, but it has by far the highest mountain, Olympus Mons, at over 24,000 metres tall. From its peak, the distance to the horizon, if not obscured by Mars’s red dust, would be 404 kilometres, the most extensive view in the solar system.
Skadi Mons on Venus is less than half the height of Olympus Mons, at 11,520 m, but because Venus is 80 per cent bigger than Mars, the distance potentially visible from that peak is only a little less at 374 km. By comparison, on Earth, about 5 per cent bigger than Venus, climbers on Everest’s 8800-metre summit can see as far as 335 km. Mercury, the smallest of the planets, is just over a quarter of Earth’s size, and its highest peak, Caloris Montes, is only 3000 m, so if you could stand on top of it, the horizon would be 121 km away.
Jupiter’s moon Io has the fourth most distant vistas in the solar system at 258 km, seen from its highest mountain at 18,000 m.
On the asteroid Vesta, orbiting between Mars and Jupiter, there is a large impact crater whose rim is up to 25 km tall, making it possibly higher than Olympus Mons. Vesta has a diameter of just 529 km, however, so the distance visible from its peak is only 118 km.
The gas giants of the outer solar system can’t be said to have mountains, but if one were possible on the largest, Jupiter, which has a diameter of nearly 140,000 km, it would only need a peak of 1167 m for you to be able to see the same distance as from the top of Olympus Mons on Mars.
Martin Gellender Brisbane, Australia
The distance that you can see to the horizon on a smooth, featureless plain varies with the square root of the radius of the planet and the height of the observer above “ground level”. So, on a featureless spherical rocky planet, the distance to the horizon would double if the radius were fou
