AST 101-LECTURE NOTESSOLAR SYSTEM TOURVenus and MarsTaken fromSuzanne Traub-Metlay, Ph.D. lecture notes, with modifications from Mary RichardsLectureBy investigating Venus and Mars, we can begin to appreciate how Earth compares to the otherterrestrial planets. Let's start with a description of each planet and then evaluate similarities anddifferences with respect to each other.Venus
Because Venus is only slightly smaller than Earth, people used to believe that Venus and Earthwere almost planetary twins. They could not have been more wrong. Venus has:(1) No moon - Therefore no tides and no external control on obliquity (angle of rotation axis) asEarth's moon exerts on Earth.(2) No magnetic field - Therefore no protection from the solar wind aside from its highlyeffective, very thick atmosphere.(3) No plate tectonics - Therefore no more than a single crustal plate, which shows signs ofextension (pull-apart) or compression (push-together) in some local areas but does not accountfor major landforms such as volcanoes.(4) Slow, backward rotation - This is one of the most unique features of Venus. Earth rotatesonce on its spin axis every 24 hours (1 Earth day) but Venus rotates once every 243 Earth days.This is unusually slow for a terrestrial planet. Earth orbits the Sun once every 365.25 Earth daysbut Venus orbits once every 225 Earth days. Notice that a sidereal day on Venus is longer thanits sidereal year! The combination of such a slow sidereal day with its sidereal year means thatthe length of an apparent day on Venus is 116.8 Earth days. Nearly all objects in the solarsystem revolve around the Sun in the same direction. With a few notable exceptions, all solarsystem objects rotate in that same direction. Venus, however, rotates in the opposite directionfrom the way it moves around the Sun. This is known as retrograde (backward)rotation. However, do NOT confuse it with the observed retrograde motion of Mars as seen fromEarth! Venus, retrograde rotation is real, while Mar's retrograde motion across Earth's sky is anoptical illusion. While direction of rotation is not crucial in order to generate a planetary magneticfield, speed of rotation is; Venus may not have a planetary magnetic field because it rotates tooslowly to create one.(5) Surface features named mostly for women (legendary as well as real) - About 300-500 millionyears ago, the surface of Venus was covered with volcanic lava. Roughly 85% of the surface ofVenus today is volcanic rock dating from that planetwide event. This is an unusually largeamount of geologically recent resurfacing; planetary geologists do not know why this occurred oreven if it was a one-of-a-kind event or something that happens routinely on Venus. Aside from
volcanic plains, there are also highlands in the northern hemisphere and around the equator.