Snowmobiles have no place in Yellowstone and should be banned, seven former National Park Service directors and other former Interior Department officials said in a letter released Monday.

The letter, to Interior Secretary Dirk Kempthorne, comes as the National Park Service is expected to release a new plan today for snowmobile use in Yellowstone. Park officials say the plan will likely remain consistent with an earlier draft, which would allow up to 720 snowmobiles a day in the park.

In the letter, the former Interior Department officials cite four studies that suggest Yellowstone would be healthier without snowmobiles.

“Scientific studies have demonstrated conclusively that a two-thirds reduction in average snowmobile numbers during the past four winters is principally responsible for significantly improving the health of the park for visitors, employees and wildlife,” the letter says. “The study also provides clear evidence that reducing snowmobile numbers still further — from 250 per day to zero — while expanding public access on modern snow coaches, would further improve the park’s health.”

Even though the four-stroke engines offer a marked improvement from two-stroke snowmobiles in terms of both noise and pollution, the former officials say the machines still cause problems.

“Impacts from four-stroke snowmobiles are frequently accentuated by inversions, lack of breeze, the park’s intrinsic quiet, and the fact that wildlife in a weakened condition tend to concentrate where thermally influenced rivers and thinner snow cover provide more accessible food,” the letter states. “These areas are precisely where Yellowstone’s roads are located.”

The letter also points out that Kempthorne expressed support for a document called the 2006 National Park Service Management Policies, which places an emphasis on conservation, not recreation in national parks.

The letter further states that snow coaches are more affordable than snowmobiles and offer greater opportunities for visitor education.

The letter writers include all but one of the living former Park Service directors who have held the post since 1964. The years span the presidencies of Lyndon Johnson through Bill Clinton. The eighth living director, Fran Mainella, served under the current administration and resigned last year. Ethics rules prohibit her involvement, according to officials.

Park Service spokesman David Barna said Kempthorne will respond to the letter in coming days.

The Park Service’s draft proposal for regulating snowmobiles, released late last fall, would allow a maximum of 720 snowmobiles in Yellowstone per day. All snowmobilers would have to use a commercial guide and only machines with four-stroke engines — considered quieter and less polluting than traditional two-stroke machines — would be allowed. The draft regulations are similar to those now in effect. The draft also proposed the closure of Sylvan Pass near the East Entrance to snowmobile travel because of avalanche concerns and would require the best available technology for snow coaches.

Both conservation groups and pro-snowmobiling groups could find problems with the draft. Conservation groups have advocated for a snow-coach-only alternative. They also cite millions of public comments that overwhelmingly supported removing snowmobiles from Yellowstone.

Pro-snowmobile groups say that requiring commercial guides is too restrictive, and they have advocated for a training program that would allow noncommercial guides to lead trips into the park.