Astronomy 1110

Due at start of class, Wednesday June 28.

(The due date has been pushed back one day, but I'll grade any homeworks turned in on Tuesday in time to study for the exam on Thursday.)

1. A Cold Solar Nebula
Problem 2, Ch. 8 in the text (p. 235)

If the solar nebula cooled to 50K rapidly (and before the solar wind cleared it), we'd have condensation of all three major components - ices, rocks, and metals - throughout the nebula. The frost line would be right next to the sun. The terrestrial planets would probably be icier, larger, or more numerous. At the least, one might expect there to be larger atmospheres and more water on planets such as Earth and Mars. (Mercury would still be close to the sun and boil off its atmosphere quickly!)

2. A Backward Planet
Problem 7, Ch. 8 in the text (p. 236)

A planet orbiting backwards around the sun could probably not be formed by accretion of planetesimals from the solar nebula. You could imagine that perhaps it formed forwards, and then was hit to start orbiting backwards -- but this requires an impactor moving backwards, so we're back to the same problem. Most likely, a backwards planet would have to be captured from elsewhere, e.g., tossed out from another solar system, and then brought into orbit gravitationally around the Sun. Several of the retrograde satellites of the Jovian planets were probably asteroids captured in a similar way. Keep in mind that a planet orbiting backwards is far different than a planet rotating backwards (like Venus).

3. Water on Mars
It has been known for several decades that water existed on Mars' surface perhaps a billion years ago. Recent images from the Mars Global Surveyor spacecraft indicate that water has been at the surface much more recently. Discuss (1 page or so) some of the evidence for recent liquid water. Do you believe this evidence? What new results could shift the debate one way or the other? You may wish to visit the Science website or space.com for more information, or watch the press conference.

The new evidence for recent liquid water is along several lines, most of which center around several hundred images of narrow carved channels in Mars' surface:

• The new, small erosion channels are essentially pristine, not covered by any impact craters at all, and are therefore young;
• The channels are all found on the coldest regions of the planet (near poles), facing away from the Sun, suggesting they have something to do with water being heated (or not heated) by the Sun. It's surprising they're all facing away, not towards the Sun -- something that will have to be figured out for the theory to be believed.
• The channels and their erosional `aprons' look similar to water features on the Earth, and don't look like patterns that sand can make by itself;
• The channels are very distinct and sharp-edged, so very little dust has been deposited on or removed from the surfaces by Mars' wind;
• The channels patterns are on top of very young features themselves -- structures of sand and permafrost called `polygons' that we see in Antarctica and only last for a short time.
• The channels occur in clumped regions, suggesting that they're not randomly caused, but related to each other - e.g., related to a network of underground aquifers.