Because of the proximity and likeness of Mars to Earth, scientists have long speculated about the possibility of life on Mars. Roughly three centuries ago, astronomers observed Martian polar ice caps, and later scientists discovered other similarities to Earth, including the length of day and axial tilt. But in 1965, photos taken by the Mariner 4 probe revealed a Mars without rivers, oceans, or signs of life. Moreover, in the 1990s, it was discovered that unlike Earth, Mars no longer possessed a substantial global magnetic field, allowing celestial radiation to reach the planet's surface and solar wind to eliminate much of Mars's atmosphere over the course of several billion years.

More recent probes have investigated whether there was once liquid water on Mars. Some scientists believe that the presence of certain geological landforms definitively resolves this question. Others posit that wind erosion or carbon dioxide oceans may be responsible for these formations. Mars rovers Opportunity and Spirit, which landed on Mars in 2004, have both discovered geological evidence of past water activity. These findings substantially bolster claims that there was once life on Mars.

The author's stance on the possibility of life on Mars can best be described as

(A) optimistic
(B) disinterested
(C) skeptical
(D) simplistic
(E) cynical



The passage is primarily concerned with which of the following?

(A) Disproving a widely accepted theory
(B) Initiating a debate about the possibility of life on Mars
(C) Presenting evidence in support of a controversial claim
(D) Describing the various discoveries made concerning the possibility of life on Mars
(E) Detailing the findings of the Mars rovers Opportunity and Spirit



Each of the following discoveries is mentioned in the passage EXCEPT

(A) wind erosion and carbon dioxide oceans are responsible for certain geological landforms on Mars
(B) Mars does not have a substantial global magnetic field
(C) Some water activity existed on Mars at some point in the past
(D) the length of day on Mars is similar to that on Earth
(E) the axial tilt of Mars is similar to that of Earth



In the first paragraph, the author most likely mentions the discovery of polar ice caps to suggest that

(A) until recently Mars's polar ice caps were thought to consist largely of carbon dioxide
(B) Martian polar ice caps are made almost entirely of water ice
(C) Mars has multiple similarities to Earth, including the existence of polar ice caps
(D) Mars has only a small fraction of the carbon dioxide found on Earth and Venus
(E) conditions on the planet Mars were once very different than they are at present



Each of the following can be inferred from the passage EXCEPT

(A) the presence of certain geological landforms is not definitive proof that there was once life on Mars
(B) Similarities to Earth bolster the idea that a planet might be or have been capable of supporting life
(C) the absence of a substantial global magnetic field on Mars suggests that it would be difficult to sustain life on Mars
(D) the presence of water activity on Mars is related to the possibility of life on Mars
(E) the claim that there was once water on Mars has only marginal support from recent discoveries



It can be inferred from the passage that which of the following characteristics of a planet would imply that it might support life?

Indicate $all$ that apply.

A significant global magnetic field
Evidence of liquid carbon dioxide on the planet's surface
The average daily level of sunlight reaching the planet's surface



Select the sentence in the passage that provides the best evidence that, at the given time, life did not exist on Mars.