Hector,

Certainly the debris caused by the impact would have led to a substantial meteor-flux at Earth's orbit. But the estimated age of the impact is many billions of years ago. At that time there was certainly no mammalian life on Earth. So rather than wiping out dinosaurs, the Hellas impact on Mars would have caused a fireworks display on Earth that might have annoyed some algae.

in my opinion impact destroy Mars nucleus and its magnetic field. in consequence Mars lost atmosphere because without strong magnetic field couldn't protect against solar wind

see more: Magnetic anomalies of Martian geology

http://old.marssociety.pl/anomalie_eng.html

James, the Hellas impact occured most probably during the Late Heavy Bombardment period, roughly just less than 4 billion years ago. At that time there was either no life on Earth, or just bacteria/archaea around. Eukaryotes emerged after about ~2 billion years of more evolution. So there would have been no algae here to disturb. :)

Hector, if that type of impact occurred now, it would basically mess up the whole planet. If you look at the topography map of Mars (e.g. [1]), there is a large high terrain ring around Hellas. That may or may not be the remnants of the thick but highly eroded ejecta sheet distributed by the impact. At least it gives you an idea of how much material will be excavated. The entire planet will be covered in ejecta. What would happen deeper down is unknown, probably not very much (apart from some strong local disturbance in the mantle).

As for the dinosaurs... I didn't really understand what you meant by that. Hellas impact itself didn't have anything to do with dinos, but if there would have been Martian counterparts, yes, they would have been wiped out. However, no vertebrates or even eukaryotes would have had time to evolve during the ~0.5 billion years when the planet had been around before the impact event.

About the size: yes Hellas is very large, but it is not the largest. The Utopia basin north of it is 3300 km in diameter, and is a much larger basin than Hellas, probably of impact origin. The lunar Southpole-Aitken basin is a clear impact basin comparable in size to Hellas, maybe even slightly larger (depends on who you ask and how you define the basin). The impactor hitting the Moon was smaller - lower gravity allowed it to cause a roughly same size hole.

[1] http://martianchronicles.files.wordpress.com/2008/11/mars-mola-2k.jpg - red is high, blue is low terrain.

Yeah uopia palnitia is bigger, but the main question is, when it was happened, before Hellas or after ?, if before that is very old, if after that should be done when on war was a water, but we don/t see a border of clay from this impact.

last results from Mars, no methane, suggest that now is no possibility for life, but earlier could be methane. when impact destroy nucleus, planet lost protection to solar wind,

The location of Utopia and Hellas are drastically different. You see it from topography quite well: one is a very deep hole, other a very much smoothened expression. This suggests that lots of material has been deposited in the north after the impact (IF it was an impact), but its distribution on the planet is not seen. And we really don't know the original shape and size from either site.

We also don't see any real material deposits from Hellas either, just a _topographical expression_ of such deposits. Thus assuming that a "younger-than-Hellas" Utopia would automatically cause planet-wide clay deposits is quite a leap into the unknown. Examples: Utopia could be post-Hellas but pre-ocean, or it could be that Utopia deposits are more easily eroded than Hellas deposits. The planet has had in any case several billion years to change, both before and after the large impacts.

As for the methane issue: there is no methane detected at Curiosity site, but it is just one point on the ground. Earlier findings were of the general atmosphere. It's like saying "there is not much air pollution on Earth" from one point measurement in Tibet or Amazon. I'm not saying methane does exist, but rather that the issue is still in the air - pun intended :)

I agree for opinion about Utopia and Hellas, few yaers was article about basin utopia planitia on mars, I don't member where. it these article was that utopia has been formerd under impact, in my opinion Hellas is younger that utopia.

for methane, is true that present research are not represantive for whole Mars. In my opinion next landing on Mars have to be done in Valles Marineris, deepest formation on Mars, in papers from 70 and 80 observation, suggest that here was probability of three-point for water. research of highlands are over probability area of water vapour, to high

To answer this question, there are some points that should also be considered and calculated first.1. The object coming towards Mars would hit it (Mars) with greater intensity as there is no atmosphere that can act as cushion to decelerate the speed of the object (as that on Earth) 2. The splash and spread of the muck dug out due to impact would be different than that on Earth as the gravity of Mars is different. 3. The crust of the Earth is more brittle than Mars, the crust of the Mars is basaltic hence the melt produced due to impact would take greater time in cooling and would travel larger distance etc etc.

1. Atmosphere does not really affect the bodies that cause large impact craters. Especially those in the Hellas size range. The object just punches through, losing only negligible energy. Part of the impactor is probably still outside the atmosphere when the front end hits the surface. Even an atmosphere like the one on Venus wouldn't really affect the situation. The mass, and thus the kinetic energy of the impact, is just that large.

2. Mars has lower gravity than Earth. The same energy impact will cause a smaller crater on Earth than on Mars.

3. I wouldn't say much about the composition of Mars' crust as a whole (there is lots of regional variation), or how brittle it is. Too little is known (no seismic data, for example). However, it's safe to say that Mars' crust is probably _much_ thicker than Earth's. In other words on a Hellas size impact could punch through to the Earth's mantle, whereas on Mars it might not. But we don't actually know much about the conditions of the original event at all - for example how thick the crust was at that time.

Thanks Dr Jarmo.... Well, but your 1st point is not clear.....Large sized bodies while entering the Earth's atmosphere have broken (because of impacting with the atmosphere) into pieces and that is why we have found evidences for craters in line!!!.. Earth's atmosphere extends for more than 600 kms in height hence "Part of the impactor is probably still outside the atmosphere when the front end hits the surface" is not clear. One of the supposition for Tunguska is..that the body was in an orbit, while in orbit it entered the Earth's atmosphere and went out without hitting the surface of the Earth, but still the damage is visible.

when in this discussion is read about impact craters on Mars, I think that we should first tray to find chronology of impact craters on Mars. which was first, which was last.

impact craters on north in my opinion are very old, younger are Hallas and Agryre,

when I compare positions of magnetic and gravitational concentrations I found that these large impacts from north couldn't    create present shape of magnetic field