Well, short answer is yes and no.<br /><br />The point about mass=gravity is in part that a planet's ability to retain atmosphere is due to how easily (or not) said atmospheric species may escape. In the case of Mars, it's lower gravity lowers the atmospheric escape velocity for species, making it far easier to lose it.<br /><br />Next, the absence of a strong geomagnetic field means that there is nothing there as well to prevent said species being swept away by the solar wind.<br /><br />A further side issue is the relatively "dead" core of Mars. An active core would mean an increase in general outgassing, which is one serious mechanism for replenishing lost atmosphere. This process is occurring here, on Earth, even as we speak.<br /><br />So, on the one hand, yes, it would be an impediment. <br /><br />Then again, it all depends. If, for example, Mars' surface was repeatedly bombarded by icy material from orbit, I think atmospheric pressure could be markedly increased. Of course, it would also escape much faster than here, but - this is the caveat - how fast? It could be that we could, through a variety of means, increase the atmospheric pressure of Mars to 1/2 that of Earth. It may well take millions of years to significantly lose it again.<br /><br />Just a few thoughts and idle speculations. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis: </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>